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  #1 (permalink)  
Old 21 Aug 2012, 03:21 pm
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Exclamation Combustion Test Readings

My 2007 PT Cruiser, Touring-non turbo 2.4 Eng., after replacing timing belt-Stalls on idler and missing up on hard acceleration. When checked codes (P0300, P0016) came on. I performed a combustion test and read as followed, (dry test) Cyl. 1- 150 psi, 2- 141, 3- 139, 4- 141 - (wet test) Cyl. 1- 186 psi, 2- 170, 3- 158, 4- 185. I like to know if this are good numbers to determine if there are some dad valves.
Note: After compression test, checked codes again and there was only code P0016, (crank/camshaft sensors). Checked wiring and replaced both sensors. Code P0016 still on. I'm thinking to recheck timing marks. Please advise. Thank you all that are trying to help.
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Old 21 Aug 2012, 03:27 pm
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Default Re: Combustion Test Readings

Quote:
Originally Posted by 1sanlando2 View Post
I'm thinking to recheck timing marks.
Indeed. This should be your first course of action.
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Old 21 Aug 2012, 03:49 pm
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Default Re: Combustion Test Readings

I agree also. You have the timing off.
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Old 21 Aug 2012, 04:05 pm
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Default Re: Combustion Test Readings

You did have the throttle completely open during your testing?
Never the less the percentage difference and accounting for the likelihood of not doing a correct WOT compression test then a leak down would be in order to give a better assessment of the condition of your engine specifically the head. Compression wise generally more then a 10% difference between the highest cylinder is cause to examine the engines condition further.But these day compression test are sorta antiquated and the results unreliable. Most of us grew up first learning to do them so they still are the first procedure most of will perform when checking the condition of an engine.If the leak down test is between 12-18% on a high miles street engine it may not be something to worry to much about. But you would need to post all other observations for more internet guess work from some of us. The art of evaluating compression test and leak down test has changed over the year with better quality control of factory assembled engines and parts. The old rules usually no longer apply in many cases.

The operating condition you describe really sounds as if cam timing is incorrect, so I am 3rd to agree to double check that rather then to continue seeing that your engine is slightly worn.
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Last edited by NitroPT; 21 Aug 2012 at 04:14 pm.
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Old 23 Aug 2012, 10:23 am
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Default Re: Combustion Test Readings

I agree with everyone else. Your dry test shows less than 10% difference between your highest and lowest numbers, and your wet test is 15%. That sounds perfectly fine to me.

Continue looking toward your timing marks to solve your problem.
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Old 23 Aug 2012, 12:03 pm
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Default Re: Combustion Test Readings



i'd be interested to see the numbers you get after you get it right
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Last edited by rob342; 23 Aug 2012 at 12:16 pm.
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Old 23 Aug 2012, 12:41 pm
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Default Re: Combustion Test Readings

Wow 25% is a lot. I have always heard 10% like said above.
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Old 23 Aug 2012, 01:08 pm
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Default All these response to a new member who has only posted once?

Rob342 another good information post by you.
All Data is an information "guide" for qualified Trained Technicians to use. This compression information gives a broad interpretation of what should be a sound engine. The 170 - 225 as the base line for a new engine with a variance of 55psi is in itself high. This information gives a Diagnostic Technician an idea of how much more testing may need to be done. If you search service information on rebuilt 2.4 engines the acceptable psi is 165-170psi +/- 5-7psi before break in as good, and after service run it 3-5psi total variance.
Personally if an engine with aprox. 90k miles is in good mechanical order if all the cranking compression is at least 160psi +/-5psi between the highest reading and no indications of oil use,oil on the spark plug threads,and no apparent carbon build up in the combustion chambers or piston tops, then this would be a good engine.
The 25% reference is measured between two cylinders next to each other. Many of us from time to time find questionable generalization information with All data and report to them which they are great at correcting. But most often many searches are grouped as general mechanical information even under specific models. However even if the complete engine tested for cranking psi were in the 123-130psi it would give me cause for concern.Compression testing engines are becoming a thing of the past. The better and more accurate way of diagnosing engine problems is testing an engine using a cylinder leak-down test followed by Bore scoping. The use of a compression tester/gauge is a fast way only to see the cranking compression of a cylinder. All to often most DIY home mechanic's perform an incorrect procedure with doing the compression test resulting in unusual results.
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Last edited by NitroPT; 23 Aug 2012 at 07:48 pm. Reason: added the intro line
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Old 23 Aug 2012, 02:45 pm
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Default Re: Combustion Test Readings

If you're getting a code that says timing might be off, directly after timing belt replacement-
There's no further need to do compression (or other tests.

All compression tests do is cause worry. DON'T conduct them unless you're itchin' for an excuse to tear motor apart.

Does it run OK, have acceptable performance and acceptable oil consumption?

It so, ANY compression test results are good
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