91 7.3 Compression Test/ Warm or cold?
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From: Bothell, WA
91 7.3 Compression Test/ Warm or cold?
My Haynes diesel manual says some Ford engines compression test cold and some warm and that I should consult the factory service manual. If I had the factory manuals I wouldn't have been reading the Haynes 
but can anyone tell me if I should test the 91 7.3 cold or if I need to have the engine warmed up? Thanks!
but can anyone tell me if I should test the 91 7.3 cold or if I need to have the engine warmed up? Thanks!
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From: houston
I never heard of running one hot or cold. I have heard of running them wet or dry. Just a thought, if your problem shows up when the engine is cold, run the test cold. Most mechanics don't like working on a hot engine, so my guess is that the test is going to be run cold, or warm at best, if you took it to ford. The most important thing you will learn from the test is if all the cylinders are close to each other in reading. Good luck.
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Thanks, I thought to solve the problem by doing the engine warm and then repeating the test with it cold but by the time it cooled enough to work on and I got that tough to reach plug under the turbo out it was 11:30 last night and the engine was cold. I'll run the test cold this morning. I already plan to pull the heads hoping that the pressure problem in my cooling system, which only occurs when the engine is cold, is the head gasket. The motor is running fine. Seems to have as much power as ever on the hard pulls just wanted to check the rest of the engine as best as possible while I have things apart.
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From: Manitoba
You should do the test warm, never do our 6.9-7.3 diesels wet there is insufficient clearance for any liquid in the cylinders and they will hydrolock and bend a rod at best.
The worst cylinder should be no lower than 75% of the best cylinder IE 300/400. Do 5-6 rotations try and note when the best compression is obtained and which rotation, this will give a cylinder/valve condition check. For instance if 1 cylinder takes 4 rotations to get to 425 and another takes 3 to get to 450 there is difference to be noted. Probably just UFI but ford wants you to see the differences.
The worst cylinder should be no lower than 75% of the best cylinder IE 300/400. Do 5-6 rotations try and note when the best compression is obtained and which rotation, this will give a cylinder/valve condition check. For instance if 1 cylinder takes 4 rotations to get to 425 and another takes 3 to get to 450 there is difference to be noted. Probably just UFI but ford wants you to see the differences.
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Thanks for sharing the information. I'll do the test warm. Now that I've had that last glow plug out once it should go faster the next time. Worst case is if I'm too slow I'll have to warm the engine back up to complete the test.
Post Fiend
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From: Manitoba
EDIT........
Book states normal operating temp engine, I dislike burning my fingers too so leave BH plugged for 3 hours and when doing the test.
Book states normal operating temp engine, I dislike burning my fingers too so leave BH plugged for 3 hours and when doing the test.
Last edited by PLC7.3; Dec 5, 2004 at 12:07 PM.
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Compression Test Results
I've never plugged the heater in when it was warm before. In my heated shop the block got just to the point where you didn't want to leave your hand on it for very long. Bad luck on the test results though. #5 & #8 were both 340. The rest varied from 440 to 480 with 3 very uniform at 460. Should I rule out the blown headgasket theory and start thinking "cracked heads"? This engine did get really hot 3 years ago. Could #5 & #8 have been bad that long and just now developed the coolant pressure symptoms?
Last edited by tdford; Dec 5, 2004 at 07:04 PM. Reason: Add information
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From: Bothell, WA
Edit:
Forgot to add this information in the last post. The cylinders all came up on the first 3 strokes. 6 were over 400 by the 3rd stroke and then went up a little bit more on the last few strokes. #5 & #3 both went to 340 on the 3rd stroke and stopped there. I tested all cylinders at least twice and ran the test 3 times on the low cylinders. Hope you can help me intrepret what I'm looking at.
Forgot to add this information in the last post. The cylinders all came up on the first 3 strokes. 6 were over 400 by the 3rd stroke and then went up a little bit more on the last few strokes. #5 & #3 both went to 340 on the 3rd stroke and stopped there. I tested all cylinders at least twice and ran the test 3 times on the low cylinders. Hope you can help me intrepret what I'm looking at.
Post Fiend
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From: Manitoba
We need a major correction look at your last posts is it 8 and 5 or 3 and 5. If 8 and 5 they are on different heads but 3 and 5 are next to each other which are also in the middle of the head. This would make me lean toward a head gasket has blown.
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Sorry about that mis-typing. The low cylinders are 5 & 8 on opposite heads. I wrote them down as I was testing each one. Don't know where that 3 came from.
I checked the GP's just a few minutes ago as I remembered one of them was discolored when I removed them. There is a rust colored stain on the slick metal section just under the threads of #5. I labeled the plugs as I pulled them out so I'm certain the stained GP came from cylinder #5.
Thanks for taking the time to answer and sharing your knowlege with us newbies.
I checked the GP's just a few minutes ago as I remembered one of them was discolored when I removed them. There is a rust colored stain on the slick metal section just under the threads of #5. I labeled the plugs as I pulled them out so I'm certain the stained GP came from cylinder #5.
Thanks for taking the time to answer and sharing your knowlege with us newbies.
Last edited by tdford; Dec 6, 2004 at 12:10 AM. Reason: add information
Post Fiend
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From: Manitoba
Well ya got me now, I honestly do not know what to tell you. Had there been one really low cylinder it would have made things easy. The numbers are really just outside the ball park required if you use 480 and 340.
What condition is the rad in internally.......clean or scaley. I do know there is a product that you test the coolant for exhaust gasses, not sure if it would work on a diesel, but I can't see why not. Does anyone know of this product by name??? and where it's available.
What condition is the rad in internally.......clean or scaley. I do know there is a product that you test the coolant for exhaust gasses, not sure if it would work on a diesel, but I can't see why not. Does anyone know of this product by name??? and where it's available.
Post Fiend
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From: Manitoba
tdford
I have done some info getting from parts guys and mechanics and we have come to the conclusion the problem of you coolant back flushing itself, could be soley due to a blocked check ball in the manifold or a bad Tstat. This would be the cheapest attempt at a fix right now, couple hours to check this out could save many$$$$$.
The way we figure the tstat is not open, as the engine heats the check ball is blocked so air is trapped in the heads the temps rise causes steam, dumps the fluid out. After you refill it the tstat is open enough to allow air out and refill enough to run ok until next time................what say you.
I have done some info getting from parts guys and mechanics and we have come to the conclusion the problem of you coolant back flushing itself, could be soley due to a blocked check ball in the manifold or a bad Tstat. This would be the cheapest attempt at a fix right now, couple hours to check this out could save many$$$$$.
The way we figure the tstat is not open, as the engine heats the check ball is blocked so air is trapped in the heads the temps rise causes steam, dumps the fluid out. After you refill it the tstat is open enough to allow air out and refill enough to run ok until next time................what say you.
Last edited by PLC7.3; Dec 6, 2004 at 11:53 AM.