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Hello and good day everyone!

I'm reaching out for some help here with my wife's F350. I have a low ECT/EOT issue that I cannot get sorted out despite doing all that I know how to do, and so I am hoping somebody out there might be able to kick a couple ideas my way.

Driving the truck, I cannot get ECT/EOT above 130-150, 150 at 70mph uphill, downhill drops to mid/upper 130's... here are some details.

(2010 350/ 130,000mi on odo)

About a month ago we got a p0128 code. I pulled the thermostats and the rubber seal around both thermostats was gone entirely, so I thought this was a slam dunk, boy was I wrong.

I drained the system from lower radiator hose, as well as block (driver side only) I replaced the thermostats with Mishimoto 200, as well as adding a coolant filter to the system, refilled the coolant utilizing a vac fill procedure and there has been no difference in its operating Temps.

  • pulled a 25psi vac on the system for refill, it'll hold at 25psi but after about 10 minutes it goes to 23psi, but then it holds there. I only know that because I got sidetracked with my daughter as I was pulling a vacuum, and when I came back thats what the gauge settled at.
  • monitoring Temps through the obdII port
  • ambient air temp 52 degrees
  • ect/eot at 54/56 degrees (at first start)
  • during warmup, the ect/eot will generally stay with 4-8 degrees of one another. The eot might jump ahead of the ect, but then the ect will jump up. Granted, the software I'm using ect doesn't seem to update as fast as eot.
  • heater blowing warm(ish) air, not getting any intermittent issues with cold/warm
  • thermostat housing looks okay (to me) in terms of the thermostat sealing surface, no pitting or cavitation craters. There is some texture to it, but it looks like its just its casting, I will try to post a picture
  • if I "simulate" a load on the engine, i.e., ride the brakes (yeah, not the smartest thing ever) with some heavy acceleration, I can get eot/ect up to 192 +/- but it'll drop right back down.
  • after 25 miles at 70mph up a few 6% grades, if I crack the reservoir cap the coolant level will rise from between the cold fill lines to the upper cold fill line. Without cracking the cap, coolant level does not go up or down. (Red flag?) Then again, its not at full temp either, so I'm not too sure about that.
  • i checked the Mishimoto thermostats, now this is hardly scientifically conclusive, but in a pot of water, they start to open just a bit before boiling temp. (No thermometer at the time) but figuring water boils at 212 or so, and these started opening just before I "***-umed" an opening temp of somewhere around 190.
  • after a good long run, I am not seeing any coolant leaks

Well, I think I have included everything, but I will be more than willing to provide additional details (if I can) to assist with troubleshooting. I would like to extend my thanks in advance for any suggestions that you may have. With Temps now being consistently in the 30's in the am, I am starting to pucker up ;)
771082

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So whats the average temp while cruising? Im thinking you got a mix up from mishimoto, getting a lower temp thermostat instead of a higher one. Imo i wouldnt put in a mishimoto one in general. The oem is best. But you are already here.

and also confirming you put the thermostat housing on in the correct orientation?
 

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Yeah, the truck has always been a little bit on the cold side since we bought it, but I'm saying 180-185 or so. I thought this might have been a good way to keep those temps up since where we now live gets quite a bit colder in the winter.

- Avg Temp Cruising, 147* +/- with air temperature (outside not intake) at 59*.

- Housing, with "B" shape pointing towards front/grille, per the stamping in the plate. (Top/front)

As of an hour ago, I did end up finding an air leak at the reservoir hose snap connection. It would hold vacuum, but if it was jiggled it would leak air a bit. I replaced the o-ring, vacuum is holding dead flat and after a 50 mile jaunt, same story.

I am thinking it has to be fluid somehow getting past the thermostats, I don't think it takes much for the engine to cool itself below normal op temp. The reason I'm thinking that, is that when the engine is at 150 or whatever, I can squeeze the upper hose pretty easily, not feeling much heat or resistance. When I put a harsh load on it, and force Temps above 200, you can see the hose full up and it's quite a bit hotter, as well as more rigid.

At this point, I think that I need to pull the thermostats again, (I'm damn good at it now, haha) change them out with oem, or another brand just to eliminate variables. Our Ford dealership here can't get them for about a week or two (which i question a bit, but I digress) so I may need to grab something from an Autozone or Oreillys. May not be optimal, but at least I know the wife and kids won't have issues while I'm at work.

I dunno, completely stumped on this thing. I don't think I've ever had this much trouble with a vehicle cooling system...
 

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I forgot to mention, I verified the part numbers of the thermostats per the shipping slip and box, not on the parts themselves. That may be worth looking into, but if I recall, those open at 190 right? So I should still be seeing mid/upper 180's, or so I would think. Let's remember that I "thought" I had this thing licked when I saw the original thermostats were in bad shape :)
 

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I forgot to mention, I verified the part numbers of the thermostats per the shipping slip and box, not on the parts themselves. That may be worth looking into, but if I recall, those open at 190 right? So I should still be seeing mid/upper 180's, or so I would think. Let's remember that I "thought" I had this thing licked when I saw the original thermostats were in bad shape :)
I’m thinking a bad sensor, if one sensor is out, it will use the other sensors parameters to make up it’s own. IE, computer with read voltage of all other sensors including maf map and eot or ect and tell the computer to “make up” a temperature for the “other”
Check voltage at the sensors if possible and or monitor with a scan tool, do you have any codes? If your thermos at 200* and opened just under they’re fine. Stock opens at 180.
If your guage is pegged at C and it still shows 0128 it’s either bad thermostats or bad sensor theres no question about it. It can’t be more than those other than wiring issues. OR is your fan clutch stuck engaged? That could also be a culprit.


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Hmm, excellent points, thanks!

With regards to the sensors, that is what I started to question as well, however please have your tickets ready, we are now departing my area of knowledge! I'm not entirely certain where they (it?) are at. Not even sure if there is one or two. I started to turn from that thought after trying to read as much as I could from other folks having these types of issues and since mine "seem" like they are working according to advice on their topics. Of course, theirs usually ended with "replaced thermostats and all is well now" :p (see below)

Fan clutch, I don't think so, but double check me. During warmup, the fan will cycle on/off once or twice, it lasts about 10-15 seconds then cycles off. Where I'm going with this is that when the fan engages it is a pretty decerbable noise. (See below)

After clearing codes, the dash gauge will (depending on how I'm driving it) move up to the first line past the C, and if I'm romping on it uphill, will go a bit further, thats about the 140-150 range according to my obdII scanning tool. Its only when it is about in there and after about 10-15 minutes will p0128 pop back up.

So, here are my questions pertaining to your suggestions... its important to me that it is understood that my questions are absolutely not questioning the knowledge of more experience than my own, I'm trying to learn more about this thing with help :)

1. If a sensor is going haywire, would the ECT/EOT be so close to outside temperatures prior to starting and then close together during warmup? I know very little of how these computers do their job. I know "mechanical part broken, replace mechanical part"

2. With regards to the fan, piggybacking my comment above if the fan clutch was stuck engaged, would one hear it going all the time? Asking to be sure, these fans definitely seem to let you know they're running.
 

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Shoot, what i meant to add and didn't, is that when I am really working to push the truck into the 190 ect/eot temp, the trans temp is at about 190-205. During driving it holds at about 170-180 or so, fuel rail temp is at 125. FR is more or less irrelevant, on a separate cooler but I thought I would throw that in there anyways.
 

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Hmm, excellent points, thanks!

With regards to the sensors, that is what I started to question as well, however please have your tickets ready, we are now departing my area of knowledge! I'm not entirely certain where they (it?) are at. Not even sure if there is one or two. I started to turn from that thought after trying to read as much as I could from other folks having these types of issues and since mine "seem" like they are working according to advice on their topics. Of course, theirs usually ended with "replaced thermostats and all is well now" :p (see below)

Fan clutch, I don't think so, but double check me. During warmup, the fan will cycle on/off once or twice, it lasts about 10-15 seconds then cycles off. Where I'm going with this is that when the fan engages it is a pretty decerbable noise. (See below)

After clearing codes, the dash gauge will (depending on how I'm driving it) move up to the first line past the C, and if I'm romping on it uphill, will go a bit further, thats about the 140-150 range according to my obdII scanning tool. Its only when it is about in there and after about 10-15 minutes will p0128 pop back up.

So, here are my questions pertaining to your suggestions... its important to me that it is understood that my questions are absolutely not questioning the knowledge of more experience than my own, I'm trying to learn more about this thing with help :)

1. If a sensor is going haywire, would the ECT/EOT be so close to outside temperatures prior to starting and then close together during warmup? I know very little of how these computers do their job. I know "mechanical part broken, replace mechanical part"

2. With regards to the fan, piggybacking my comment above if the fan clutch was stuck engaged, would one hear it going all the time? Asking to be sure, these fans definitely seem to let you know they're running.
1.) The ect sensor is located in the front cover house front right top up the ft cover
Oil temp sensor is in the oil cooler housing
If ect sensor is bad and or open it will automatically think the engine is overheating and run the clutch fan, not sure about the cycling but when i forgot to plug in the ect sensor on a durango the fan ran constant and wouldnt let it heat up.

2.) how is your coolant level, any residue on the overflow tank? the warm ish’ heat could just be a blocked/clogged heater core totally separate from your cause. The fan clutch running on startup is a definite issue,

3.) what codes did you pull?

4.) probably bad thermostats especially if you’re computer is programmed to start the fan clutch at the desired temp it was programmed too and you put in a thermostat that’s higher than normal temp without calibrating the system it will trip and send it into overheat mode.


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Be very mindful of forums and also what “everyone else” posted there fix was.

When did you start having this issue? After replacing thermostats? After what? When and how long have you owned it since the problem arose.


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1.) The ect sensor is located in the front cover house front right top up the ft cover
Oil temp sensor is in the oil cooler housing
If ect sensor is bad and or open it will automatically think the engine is overheating and run the clutch fan, not sure about the cycling but when i forgot to plug in the ect sensor on a durango the fan ran constant and wouldnt let it heat up.

2.) how is your coolant level, any residue on the overflow tank? the warm ish’ heat could just be a blocked/clogged heater core totally separate from your cause. The fan clutch running on startup is a definite issue,

3.) what codes did you pull?

4.) probably bad thermostats especially if you’re computer is programmed to start the fan clutch at the desired temp it was programmed too and you put in a thermostat that’s higher than normal temp without calibrating the system it will trip and send it into overheat mode.


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Man, now I'm getting worried... ugh.

1. Thanks!

2. No residue on the bottle, but its brand new. When I pulled the old one out to get to the therm housing, I dropped it and it busted the overflow connection right off. The old one does have some discoloration, and there is a little bit of schmootz in there, but nothing that I would throw my arms up at. Relative to the engine temp, heater is blowing warm air just fine. Not getting any cold/warm/hot/cold, it's pretty consistent. When its reading 190 (for the very little amount of time I can keep it there), its blowing heat like the day we bought it.

3. I've only had two codes with this truck since 2010, P0128 (now) and a... wait for it... HPFP pressure too high. Can't remember the code exactly, that was about 6 months ago, had the pcm reflashed and my fingers are still crossed for that one...

4. I'm not quite understanding this, but I think you are saying it will put it into limp mode? Haven't had that happen, can't even get the temp that high. I may be misunderstanding though.

Let me ask you this, I have vac filled the system a few times now, and I don't suspect that I have any air in the system based upon my fluid level in the reservoir, and in conjunction with the heater and temp readings being consistent albeit way low. But let's assume for a moment that I do. My Jeep would have erratic temps until it burped the air out, but its cooling system is far, far simpler than this. Do these diesels exhibit the same behavior with an air pocket, or is this a symptom of air being stuck in there?

Regardless, in the am I'm going to pull these thermostats and put in a crap set i got from Oreillys today. I'm not keeping them in there long term, but it may answer some questions. If so, I'll get some oem ones ordered. If not, I guess it'll be back to the drawing board. I also think I should look at the housing again once I have it out, it seemed okay to me at the time, but now I'm filled with some doubt. I may be just wishing on a star here too.

I'll also do the best I can at going for that sensor, going to start (again) with different thermostas and looking at that housing then I'll move onto electrical gremlins.
 

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Man, now I'm getting worried... ugh.

1. Thanks!

2. No residue on the bottle, but its brand new. When I pulled the old one out to get to the therm housing, I dropped it and it busted the overflow connection right off. The old one does have some discoloration, and there is a little bit of schmootz in there, but nothing that I would throw my arms up at. Relative to the engine temp, heater is blowing warm air just fine. Not getting any cold/warm/hot/cold, it's pretty consistent. When its reading 190 (for the very little amount of time I can keep it there), its blowing heat like the day we bought it.

3. I've only had two codes with this truck since 2010, P0128 (now) and a... wait for it... HPFP pressure too high. Can't remember the code exactly, that was about 6 months ago, had the pcm reflashed and my fingers are still crossed for that one...

4. I'm not quite understanding this, but I think you are saying it will put it into limp mode? Haven't had that happen, can't even get the temp that high. I may be misunderstanding though.

Let me ask you this, I have vac filled the system a few times now, and I don't suspect that I have any air in the system based upon my fluid level in the reservoir, and in conjunction with the heater and temp readings being consistent albeit way low. But let's assume for a moment that I do. My Jeep would have erratic temps until it burped the air out, but its cooling system is far, far simpler than this. Do these diesels exhibit the same behavior with an air pocket, or is this a symptom of air being stuck in there?

Regardless, in the am I'm going to pull these thermostats and put in a crap set i got from Oreillys today. I'm not keeping them in there long term, but it may answer some questions. If so, I'll get some oem ones ordered. If not, I guess it'll be back to the drawing board. I also think I should look at the housing again once I have it out, it seemed okay to me at the time, but now I'm filled with some doubt. I may be just wishing on a star here too.

I'll also do the best I can at going for that sensor, going to start (again) with different thermostas and looking at that housing then I'll move onto electrical gremlins.
Well you said “warm-ish” not it’s “just fine” which one is it? Good heat is good heat, my 6.4 blows hot air so hot if i put my hand infront of the vent I think it’ll burn my hand. Now THATS good heat for a diesel,

Yes air pockets will effect ect temp but if air is getting by a temp sensor it will read very hot since theres no cooling capacity... if you’re getting a p0088 haha well your issues got issues LOL. Usually that’s a pump.. USUALLY. Can be a frp sensor, hpfp wiring (job 1) chafes on the pump etc.. i’m not there to diagnose it. Im giving you probable causes.

You still never answered when these problems arose.....if after the tstats change or what. And since you had it. If you’ve had any issues....??????

The system vac filled..w.e i’ve filled up multiple diesels without and it’s been fine. If you had “smudge or w.e” you call it in the old reservoir, i’d look at a ft cover cavitating. That can slime up the ect sensor and F it up.

SO. Change the tstats “once again” report back, any 0128 i’ve found is “change tstats” ALSO. At a Ford dealer.. any 0128 code the flow chart literally says
CHANGE THERMOSTAT. Not check wiring not nothing.
Or it would throw a open/short circuit code for a sensor...

What scanner are you using?
These engines don’t like ANYTHING. But factory equipment if you can get some Stant or Oem tstats then try it and report back..

Also.. make sure none of your connections are loose... allowing air to enter the system. The highest point for cooling in these trucks is the heater core, if you hold steady heat..GOOD heat, from cold to warm driving 30min to idling.. fine but just make sure you cross your T’s and dot your i’s that’s all I can say.

Id say bad Tstats but I have NO idea what the history and or when the problems occured since you still haven’t disclaimed that

GL OP!


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Well you said “warm-ish” not it’s “just fine” which one is it? Good heat is good heat, my 6.4 blows hot air so hot if i put my hand infront of the vent I think it’ll burn my hand. Now THATS good heat for a diesel,

Yes air pockets will effect ect temp but if air is getting by a temp sensor it will read very hot since theres no cooling capacity... if you’re getting a p0088 haha well your issues got issues LOL. Usually that’s a pump.. USUALLY. Can be a frp sensor, hpfp wiring (job 1) chafes on the pump etc.. i’m not there to diagnose it. Im giving you probable causes.

You still never answered when these problems arose.....if after the tstats change or what. And since you had it. If you’ve had any issues....??????

The system vac filled..w.e i’ve filled up multiple diesels without and it’s been fine. If you had “smudge or w.e” you call it in the old reservoir, i’d look at a ft cover cavitating. That can slime up the ect sensor and F it up.

SO. Change the tstats “once again” report back, any 0128 i’ve found is “change tstats” ALSO. At a Ford dealer.. any 0128 code the flow chart literally says
CHANGE THERMOSTAT. Not check wiring not nothing.
Or it would throw a open/short circuit code for a sensor...

What scanner are you using?
These engines don’t like ANYTHING. But factory equipment if you can get some Stant or Oem tstats then try it and report back..

Also.. make sure none of your connections are loose... allowing air to enter the system. The highest point for cooling in these trucks is the heater core, if you hold steady heat..GOOD heat, from cold to warm driving 30min to idling.. fine but just make sure you cross your T’s and dot your i’s that’s all I can say.

Id say bad Tstats but I have NO idea what the history and or when the problems occured since you still haven’t disclaimed that

GL OP!


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Thanks bud, I will report back tamale. My apologies if there was ambiguity in my prior statements, what I was attempting to convey was that relative to the engine heat, the heater is blowing a consistent air temperature. At 147, its not blowing fire, at 190 it feels like it should as you described. I suppose I was trying to say that I don't believe I have an air pocket in the heater core.

The problems arose out of the blue about a month ago, I think I described it in my first post, but maybe I didn't do a good job. One day, running just like it should, next day, this. And that is what is perplexing me, I'm attaching a pic of the old thermostat which after looking at it, I'm sure you would agree that this was a zip bang boom sure bet as to what was wrong.

Truck is 100% bone stock save some better brake pads. I'm using a Bluetooth obd scanner with a Ford diesel specific software on my phone.
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Fan clutch wouldnt have anything to do with it imo. If one argues that it is cooling the coolant than that means the thermostat is open. If the thermostat is closed than the extra air wont have an effect. I also dont think its a sensor because your oil and coolant are close temperature wise. The odds of two sensors failing simultaneously are zilch. What i am also concerned with is the rubber material from the oem. It is still in your system probably and when you got the coolant heated up, perhaps it got wedged between the thermostat and the sealing surface causing coolant to flow and bypass the t-stat.

with that said, i concur with changing the t-stats again with oem. Also check amazon, you may get them faster.
 

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Fan clutch wouldnt have anything to do with it imo. If one argues that it is cooling the coolant than that means the thermostat is open. If the thermostat is closed than the extra air wont have an effect. I also dont think its a sensor because your oil and coolant are close temperature wise. The odds of two sensors failing simultaneously are zilch. What i am also concerned with is the rubber material from the oem. It is still in your system probably and when you got the coolant heated up, perhaps it got wedged between the thermostat and the sealing surface causing coolant to flow and bypass the t-stat.

with that said, i concur with changing the t-stats again with oem. Also check amazon, you may get them faster.
That is a very valid point, and requires looking into. I flushed the engine 4 times, had some particulate matter in there, but truth be told it was pretty clean. We had the system flushed around 60k when we had a coolant leak due to the gen1 hoses, and had them replaced so that is probably why the fluid wasn't too bad. I never saw any rubber pieces which was concerning, I thought for sure that a new oil cooler was going to be part of this process. Either we got ridiculously lucky, or its just the other shoe waiting to drop. It's 6am where I am, once the sun comes up over the mountains I will be ringing the bell for round 7.
 

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Well gee golly God d*n f-ing gosh. I wonder if this could be the issue? What in the actual f** here.

Apologies guys, fairly frustrated.
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And, I can tell you with absolute certainty, when I did mount the thermostat prior to installing, it was NOT sitting like this.

What you're looking at is the housing with a light shining through the hose end, looks like the upper t-stat is also stuck open. Guess in this day and age you don't get what you pay for.

Well, it would seem that I didn't apply KISS to the situation, and having new parts was a hill I was almost willing to die on. My father in law, (45 year heavy equipment mechanic) would slap me across the back of my head if he was here, and I'd deserve it.

If any of you are near NorCal/Shasta Lake someday, come on by. You can slap me across the head too for wasting your time. What a d###head. (Me)

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20 minutes and 15 miles later. In all seriousness, thank you guys and I mean that. This was my screwup 100%.

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That is a very valid point, and requires looking into. I flushed the engine 4 times, had some particulate matter in there, but truth be told it was pretty clean. We had the system flushed around 60k when we had a coolant leak due to the gen1 hoses, and had them replaced so that is probably why the fluid wasn't too bad. I never saw any rubber pieces which was concerning, I thought for sure that a new oil cooler was going to be part of this process. Either we got ridiculously lucky, or its just the other shoe waiting to drop. It's 6am where I am, once the sun comes up over the mountains I will be ringing the bell for round 7.
If you didn’t use VC-9 and pulled the block plugs you didn’t **** S***
And the thermos have to be sprung down and twisted in the cap chances are ya didn’t do it properly,

And obviously I was saying if the fan clutch is stuck on with a bad thermostat you will NEVER build heat which is exactly what you said when you said it’s one “tick past C”

And two sensors failing together maybe not.

But if one sensor fails lets say ECT it will get an automatic ready of what the PCM THINKS. It should be. So ya it can be “close to each other” doesn’t mean that’s what it actually is. It’s a fake number made up by ambient air temp, maf temp and iat temp along with oil temp then the pcm says.. “hmm” well it should be pretty close let’s just say it’s 180 when the eot is at 185
“Close enough...”


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Do you have a link to that info where the pcm calculates a number?

i would figure that if the coolant sensor was out and the pcm was adjusting it, it would throw one of the 4-5 coolant sensor codes and op would see that. Doubt the pcm would edit something like that and not throw a code for it.

And to the op, it happens man! Thats why i mentioned the mishimotos in the first few posts, most items from them are really good, but ive heard alot of Negitive about the thermostats they offer here. Glad you got it figured out. If you are ever down in sonoma county stop by so i can take you up on your offer
 

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Do you have a link to that info where the pcm calculates a number?

i would figure that if the coolant sensor was out and the pcm was adjusting it, it would throw one of the 4-5 coolant sensor codes and op would see that. Doubt the pcm would edit something like that and not throw a code for it.

And to the op, it happens man! Thats why i mentioned the mishimotos in the first few posts, most items from them are really good, but ive heard alot of Negitive about the thermostats they offer here. Glad you got it figured out. If you are ever down in sonoma county stop by so i can take you up on your offer
It’s in flow charts for that Code and or oil temp high, i’ll get it from ALLDATA tomorrow at work. And many sensors do, google it you’ll find it.


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