08 F250 (complete rebuild) lots of blowby - Ford Powerstroke Diesel Forum
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Old 06-19-2013, 12:43 PM
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08 F250 (complete rebuild) lots of blowby (solved)

we have a 2008 f250 6.4l we did a complete rebuild and now we are getting a lot of blowby whens it at idle there is a lot of pressure at the oil filler cap. and when running an you floor it will pop the glow plug connectors because of all the pressure.

I did a compression test and this are the numbers
Cylinder Compression
1 410
2 400
3 410
4 410
5 420
6 410
7 410
8 410

What do you guys thinks went wrong.

new pistons, rings, bearings, lifters, oil pump, water pump, injectors, injection pump, fuel pump, glow plugs everything brand new.

Last edited by ImperialA; 06-26-2013 at 02:22 PM.
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Old 06-19-2013, 01:03 PM
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6.4's have a lot of blow by from the factory, its just how they are. Your compression #'s don't look bad either, but you glow plug comment is a little confusing. Are you saying the glow plug its self its backing out or the harness is just popping off the glow plug? Either way neither should be happening, did you remove the glow plugs at all ?
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Old 06-19-2013, 01:10 PM
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When I floor it the connectors pop off not the glow plugs just the connectors. what I was think is that it my be pistons rings but the compression test numbers don't show a big difference on cylinder pressure.
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Old 06-19-2013, 01:21 PM
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How many miles/hours on the rebuild? Have the rings been broken in yet? Why are you flooring a brand new motor? Did you just get it finished?

If the rings haven't seated yet, which can take quite a few miles, then you will have blowby. Did you stagger the rings so the grooves don't line up on the initial build? Did you break it in properly at a high rpm so the rings and pistons can get the initial wear pattern? If you didn't do this right, the rings can scar the cylinder or fail to cause the hatching pattern that basically rotates the rings around the piston and keeps them from sticking.

A lot here we don't know to make a decision. Regarding the glow plug connector, that just doesn't make any sense. If pressure was blowing past the glow plug to somehow pop off the connector, your compression would show it. Is this on all the glow plugs or just one? Is it always the same glow plugs? What glow plug/cylinder is this happening on?

Please give us more info.

Regarding compression, 6.4 diesels compression, in terms of blow-by, changes with rpm/heat. They have a bit lower compression at idle than at a sustained rpm. This is due to several factors. The main assumed factor is that during regens, the temps get very high in the cylinder and will expand everything significantly more than normal operation. To compensate, the compression is lowered by increasing the gaps on the ring to cylinder wall. However, anything with a turbo will already have lower compression. To understand this better, there is the compression ratio, which does not change and is based on the bottom stroke cylinder volume and top stroke cylinder volume ratio (topvol/bottomvol). Then there is compression loss due to blow-by. At low rpms/temps this increases (blow-by). When you do a compression test, you are seeing some pressure reading based on many factors. Assuming all things equal, this should be some number "P". When you have higher blow-by you will have some number "Q". Your loss is P-Q. Look up the numbers and see what is normal and compare to what you have. Remember, compression testing a cold engine will show 'Q' to be lower than normal.

Last edited by Expirobo; 06-19-2013 at 01:33 PM.
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Old 06-19-2013, 01:34 PM
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Also, believe me when I say, I have failed on more than one engine build. That poor Chevy...
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Old 06-19-2013, 02:13 PM
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I am not 100% but I think that we have put around 80 miles since the build, we have done 5, 6.4l just in last couple of months and never had any problems. when it pops the glow plug connectors usually it will pop one whole bank. I was going up a bridge and I accelerated a bit hard and that's when it happened. we checked everything put the connectors back in place and went again to the same spot and did the same thing. I unscrew the oil filler cap and left it loose turn on the engine and after a few seconds of the truck being on it would push the cap off a little( I don't know how to explain it but the cap would dace a little).

Last edited by ImperialA; 06-19-2013 at 02:20 PM.
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Old 06-19-2013, 02:14 PM
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Oh, then you probably know what you're doing. LOL. Good luck.
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Old 06-19-2013, 03:01 PM
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Quote:
Originally Posted by Expirobo View Post
Oh, then you probably know what you're doing. LOL. Good luck.
I personally don't have a lot of experience with the 6.4L, I talk to one of my friends at the dealer and he said its going to be a piston ring problem but he didn't give me an explanation on how he got to that conclusion. so I am stuck, one of the other techs here who does have a lot of experience with the 6.4l cant figurate out. I don't want to start taking things apart with out first having an idea of what I am looking for. If you guys have any ideas or suggestions on what to check I would really appreciated. thanks
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Old 06-19-2013, 05:12 PM
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Well to properly check the piston rings and cylinder walls, you will have to tear into it. Before you install the pistons, you install the piston rings into the cylinder and check the ring gap. If the block was bored out or even honed, the rings could be out of speck. Also, the cylinder wall will need to be checked for the proper cross-hatch pattern. Could be an oblong cylinder if the block wasn't honed or bored. In that case, the rings will not seat all the way.

There's a chance it could be bad valve guide tolerances. That would yield some form of blow-by.

I guess the bigger question is, why did they rebuild it in the first place? What problems existed before the rebuild, how many miles, was it overheated, and what machining was done?
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Old 06-20-2013, 02:57 PM
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Quote:
Originally Posted by Expirobo View Post
Well to properly check the piston rings and cylinder walls, you will have to tear into it. Before you install the pistons, you install the piston rings into the cylinder and check the ring gap. If the block was bored out or even honed, the rings could be out of speck. Also, the cylinder wall will need to be checked for the proper cross-hatch pattern. Could be an oblong cylinder if the block wasn't honed or bored. In that case, the rings will not seat all the way.
We have a problem with two cylinders #2 and #4 tomorrow I get to take it apart again. I think that you are right and the two cylinders are not to spec, my first mistake was not checking them when I got the engine block back from the machine shop.
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