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Old 09-23-2013, 11:00 AM
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Diagnosing a 7.3 no start

I have read and responded to several "No start" threads and i figured id bite the bullet and try to compile as many causes as i can for a no start in one thread so its easy to reference, maybe a mod could sticky it so its easy to find.

After a lengthy and frustrating no start situation with my OBS i had to forcefully learn all this stuff from several different sources so i figured id make it a bit easier for the next guy by putting it all here. I will also try to order them in the best order that makes the most sense money/time-wise but not necessarily.

If anyone has anything that they see is incorrect or extra info they are more than welcome to add it and i will correct as necessary.

First off there are 3 different types of "no starts"
Type 1-Cranking with smoke out of the tail pipe
Type 2-Cranking without smoke out of the tail pipe
Type 3-No cranking whatsoever
I will focus on the first 2 types on this thread because if you have the 3rd type generally its either your starter or wiring going from ignition to the starter. But i will call out on each symptom/test what type of no start it is for to make it easy to decode your specific no start.

Smoke out of the tailpipe indicates that there is fuel being injected into the cylinder and not being completely burned and the oil pressure is reaching a pressure that allows the injectors to fire fuel so most no starts are generally Type 2 because the problem is usually either oil or fuel related.

************************
Before you do anything the best thing to do of all is to check for codes with a Ford compatible scanner and that will help point you in the right direction more than anything else.

Type 1 or 2-First check your battery level. The PCM wont allow the truck to fire unless its seeing a certain voltage or if it has enough it may not want to fire because it didnt warm up the glow plugs enough depending on ambient temps and if it doesn't crank fast enough then its not gonna want to start.

Type 1 or 2-Next check your fuses both in cab and in the engine bay, it could be as simple as that. But not just a visual check, see if they are actually getting power with a test light or multimeter. At least on the OBS 7.3 the fuse for the PCM and the fuel bowl heater are on the same circuit so if the heater for some reason shorts out it blows the fuse for the PCM. This would cause the WTS light on the dash to not come on so if you have that symptom this is one major culprit.

Type 2-Next replace the CPS with a genuine motorcraft one(Genuine Ford 7.3L Diesel Cam Position Sensor (CPS) - Blue) or the international replacement for the T444 part number 1825899-C93(reference this engine or part number when ordering because international doesnt "know" what the 7.3 is.) Do not buy the parts store CPS because they are known to be bad right out of the box and arent as good as the ford/international one. This sensor going bad can cause a whole plethora of strange problems usually resulting in rough idle, shutting off suddenly for no reason at idle, or bucking while driving, and lastly no starts. For the 7.3 it is cheap/easy to change as a first measure to rule out. Worst case its not your problem and you have a spare for the glove box because eventually you WILL need one.

Type 2-Check your ICP(Injector control pressure) sensor. This is located on the drivers side head toward the front of the engine with a pigtail coming out of it, Often you will notice the pigtail has oil in it, usually signaling the sensor is going bad/is bad. The check for this is to unplug the pigtail from the sensor and start it up and that will cause the PCM to put in a default value of 500 psi which then will allow the injectors to fire. If it fires up with it unplugged then that is your problem and it needs to be replaced, do not drive with the ICP unplugged.

Type 1-Next check if your glow plug relay/glow plugs are getting power, Here is a good write up how to do that(Troubleshooting GPR & GP's) If your glow plugs arent getting power then even on a warm day it will be hard to start if it starts at all.

Type 2-Next check your fuel pressure, the truck will run with damn near 0 psi but will run incredibly rough, 65 psi is optimal for a stock truck. The way to test this on the OBS is to hook into the Schrader port on the right side of the fuel pressure regulator but because the engine lift hooks are in the way you have to have some sort of elbow to clear it that most test kits do not include. Have some one crank the truck while you read the fuel pressure or have a really long hose that extends to the cab so you can see it while you crank. On the super duty you have to hook into one of the special ports on the back of the fuel bowl with one of these(http://www.dieselorings.com/1999-200...-1-8-fnpt.html) and one of these (http://www.dieselorings.com/1999-200...-1-8-mnpt.html)

Type 2-Next find another working IDM from any 7.3 truck from 94-03 7.3, they are all 100% compatible then swap it in to see if it starts. That will tell you you need another IDM. They are somewhat costly new and about $75-$100 used so its better to see if you can swap real quick with someone else before you go and purchase one.

Type 1 or 2-Next check your UVCH(Under Valve Cover Harness.) The 7.3 likes to eat these up and melt them so unplug them and inspect for melting on either the plug or the pigtail and replace as necessary.

Type 2-Next rebuild or replace your IPR(Injector Pressure Regulator) they often stick which then doesnt allow enough oil to reach the injectors which then MAY cause a no start but usually just results in rough idle/low power so this MAY cause a no start but this would be one of the last things to try.

Type 2-Next would be to check your HPOP reservoir oil level. It should be within an inch from the top of the reservoir check port. If its not that would signal a weak LPOP. To test your LPOP get a real cheap gauge 1/8 npt pressure gauge and hose with a 1/4" nptm fitting on the end and install it on your hpop reservoir where the inspection port is and start the truck and watch the pressure while running, it should be ~50 psi at idle and up to 100 while revving. Then find the port down by the oil cooler, its right where the trans meets the engine, its a 4 sided male head plug(not the ones on the block with a female hole and not the one on the cooler directly) that is 1/8 npt. Move the just the gauge there and read that pressure while running and it should have about the same reading as the top side. Just be quick when removing that plug because that port will leak oil once its open so just be quick and you will be fine. If it is the same reading then yes chances are you have a bad LPOP but if they are different then you just maybe losing pressure somewhere.

Type 2- Test your HPOP pressure, best way to test is if you can get your hands on a scanner than can read live data from the truck. The HPOP needs to be around 500 psi to let the truck fire the injectors. If you cant get a scanner you can tie into the oil lines to the head but it would require a very high pressure(aka expensive) oil filled gauge and special fittings(#5 O-ring boss to NPT) to tie into. But for the price of that you can get this (
BAFX Products (TM) - PIC18F2480 Bluetooth OBD2 scan tool - For check engine light and other diagnostics - Android compatible : Amazon.com : Automotive BAFX Products (TM) - PIC18F2480 Bluetooth OBD2 scan tool - For check engine light and other diagnostics - Android compatible : Amazon.com : Automotive
) and download "Torque Pro" from the Google play store on an Android phone/tablet and it has the option to read HPOP pressure for both OBS and SD. Heres a thread that helps you get Torque set up (http://www.powerstroke.org/forum/99-03-7-3l-general-discussion/481106-torque-app-photos.html)

Type 1-Next check the compression of the cylinders. If they dont make the proper compression, depending on how bad, it will cause an incomplete burn and wont let the engine fire or if it does it will run very poorly. I know harbor freight sells a cheap diesel compression test kit but from the review it seems like the fitting isnt quite long enough to clear the injector but the may have fixed that by now but otherwise you will have to have a mechanic do it or buy a very expensive kit to test. IIRC it should be about 400 psi

Type 1, 2, or 3-The PCM itself could have just crapped out and the only way to really test it without extensive electrical knowledge is to find someone with the exact same PCM as yours which, unlike IDMs, are not all compatible. They have to have the same PCM code as yours so if you have access to this by all means try it, if not you can only buy a new one and try it out. Depending on how bad it went out it can cause all 3 types of no starts. These are a pain to diagnose.

Type 1 or 2-An injector(s) went out and it could be as simple as a o-ring is bad or the whole thing went bad but nevertheless personally this would be the last thing i would tear into because they can be costly but this is a good chance to upgrade to stage 1s.

Well there you go, there is an extensive list but by no means is it exhaustive so if anyone has anything to add ill update it. There are so many variables to every situation and your particular no start may be caused by multiple problems not just one but dont get discouraged, these engines run forever and take a lot to put one fully out of commission.

Last edited by Rammstein1224; 01-07-2014 at 11:35 AM.
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Old 10-06-2013, 05:08 AM
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very comprehensive. Thanks from all of us who will reference this in the future.
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Old 10-06-2013, 06:59 AM
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No problem its by no means every scenario but its generally the most common
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Old 10-06-2013, 08:46 AM
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I've heard that PCM modules have to be matched up to specific trucks. Why is this? Are there other differences on the engines (sensors, whatever) that require a specific PCM to run?
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Old 10-06-2013, 10:43 AM
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Quote:
Originally Posted by ttyR2 View Post
I've heard that PCM modules have to be matched up to specific trucks. Why is this? Are there other differences on the engines (sensors, whatever) that require a specific PCM to run?
the reason you need different PCMs is because each truck has different options on it that the PCM has to drive. Like ESOF, Manual tranny vs Auto etc.
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Old 10-10-2013, 07:20 AM
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That is awesome.ebp.ICP.gpr.IPR all check out good.hpop cks out good.now I put a pressure Gage on Schrader valve had wifey crank no pressure whatsoever. so I was told that if its air locked I won't build any
Pressure until I release all air from fuel lines.confused now?
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Old 10-10-2013, 07:35 AM
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Quote:
Originally Posted by jimbo263 View Post
That is awesome.ebp.ICP.gpr.IPR all check out good.hpop cks out good.now I put a pressure Gage on Schrader valve had wifey crank no pressure whatsoever. so I was told that if its air locked I won't build any
Pressure until I release all air from fuel lines.confused now?
Well theres really no way for it to get air locked. It just goes from the tank up to the upper chamber of the fuel pump then that fills the fuel bowl then from the fuel bowl it goes to the lower chamber of the fuel pump that pressurizes it then goes to the back of the head and through the fuel rails then out the front of the heads and to the fpr then back to the tank. Its an open system, really no where to air lock. You may have stirred up crap with the filter change and it clogged your fpr. you could take that off and clean the screen thats between the fpr and the filter housing be careful not to loose it when you take it off
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Old 10-10-2013, 07:40 AM
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Wow never thought of that. IMA try it when I get home.is that why the bowl doesn't fill? Cause I emptied bowl from valve and refilled it and no start.the bowl won't refill when cranked at all.
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Old 10-10-2013, 07:48 AM
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Quote:
Originally Posted by jimbo263 View Post
Wow never thought of that. IMA try it when I get home.is that why the bowl doesn't fill? Cause I emptied bowl from valve and refilled it and no start.the bowl won't refill when cranked at all.
Well actually that crap you stirred up may actually have killed your pump. Ill tell you when i looked at my fuel bowl it was really dirty at the bottom so that may be why it wont fill. But i suppose if the fpr is completely blocked then i suppose it COULD become air locked preventing the pump from filling the bowl but in that case when you crack the lid of the filter housing then there would be some pressure released.

You do know that the pump is mechanical and only fills the bowl while cranking? Not trying to sound like a D--- but i gotta ask because some people dont realize that
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Old 10-10-2013, 07:52 AM
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Quote:
Originally Posted by jimbo263 View Post
That is awesome.ebp.ICP.gpr.IPR all check out good.hpop cks out good.now I put a pressure Gage on Schrader valve had wifey crank no pressure whatsoever. so I was told that if its air locked I won't build any
Pressure until I release all air from fuel lines.confused now?
what yr SD you talkn about?
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