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Misfire with bad leak down numbers

Tiny
PSPARKY
  • MEMBER
  • 1985 AUDI 4000
  • 2.2L
  • 5 CYL
  • 4WD
  • MANUAL
  • 70,000 MILES
This car sat in a garage for several years prior to trying to fix the problem, when this car was parked it ran perfect. The car would start and misfire on several cylinders. All injectors were changed and they have good spray patterns now, A compression test was performed and all seemed normal, a leak-down test was performed and two have poor numbers, one is 60/90 and one is 75/90. The mechanic feels that it probably has some stuck piston rings. I realize that this is not good but would poor leak-down cause a misfire when it has good compression?
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Thursday, August 24th, 2017 AT 3:58 AM

9 Replies

Tiny
STEVE W.
  • MECHANIC
  • 13,020 POSTS
I would suspect stuck rings or valves that have corroded seats. When you did the leak down test the air would leak out at the various problem areas.
First thing I would do is repeat the test and listen for the leaks. What were the actual compression numbers?
Next do a wet compression test and see if the numbers go up, they should if it is sticky rings. (Use ATF and as it leaks past the rings it may help free them if they are stuck)
Go through the ignition system and check for issues there as well. This engine also has a timing belt so be sure it is okay, if it is dry rotted it could easily slip and cause timing issues that cause misfires.

Setting can really do bad things to vehicles.
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Thursday, August 24th, 2017 AT 4:12 AM
Tiny
PSPARKY
  • MEMBER
  • 5 POSTS
Steve W, thanks for a quick reply, I think all you thoughts are good and I will forward ideas to my mechanic. I think he told me compression was about 140 in all cylinders and did not see need to do a wet test as they were so good and then he moved onto the leak-down test. He is on the fence about it being a valve problem or a ring problem but we decided to soak the misfiring cylinders down with solvent for a few days (until Friday) and see if it improves. He did not say if he could hear air leaking out of intake, exhaust or crankcase.
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Thursday, August 24th, 2017 AT 6:39 AM
Tiny
STEVE W.
  • MECHANIC
  • 13,020 POSTS
140 is a good number if they are even. The plan of soaking them and testing again is good. Sounds like he knows his stuff.
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+1
Thursday, August 24th, 2017 AT 9:57 AM
Tiny
PSPARKY
  • MEMBER
  • 5 POSTS
Steve, do you have a preference? It seems people have tried Marvel Mystery Oil, B12 Chemtool and Seafoam, and mix of acetone and Marvel Mystery Oil.
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Thursday, August 24th, 2017 AT 10:08 AM
Tiny
STEVE W.
  • MECHANIC
  • 13,020 POSTS
I use Seafoam and B12 both in a running engine. Added to both oil and gas.
For an engine with unknown issues I use ATF as it is thicker than the other two so it stays above the rings longer and because of the amounts used.

MMO is good in things like lawnmowers and air cooled engines, it helps keep them running with the "gasoline" we have these days.
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Thursday, August 24th, 2017 AT 11:13 AM
Tiny
PSPARKY
  • MEMBER
  • 5 POSTS
Brainstorming here, what about an intake manifold air leak?
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Thursday, August 24th, 2017 AT 1:27 PM
Tiny
PSPARKY
  • MEMBER
  • 5 POSTS
Steve W, some good news, the leakage problem got much better. Mechanic put O2 sensor in tailpipe and it is running lean, 18 to 1. Propane was then hooked up to intake manifold and it smoothed out. So we are now trying to remove the airflow meter but everything is rusted.
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Thursday, August 24th, 2017 AT 5:13 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 43,110 POSTS
MAF sensors can cause a bunch of troubles.

Here is a guide that can help you see what you are in for when changing the sensor out:

https://www.2carpros.com/articles/how-to-replace-a-mass-air-flow-sensor-maf

Here is what it will be like on your car (below):

Please let us know what happens.

Cheers, Ken
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Friday, August 25th, 2017 AT 12:48 PM
Tiny
STEVE W.
  • MECHANIC
  • 13,020 POSTS
Rust is such fun. I buy PBlaster and Kroil in case lots.

Setting that long could cause a list of problems. MAF could be damaged. Vacuum lines dry rot, gaskets crack. Adding extra fuel could be a sign that the fuel pump has an issue. Take the propane and run it around the engine near the vacuum lines and intake and see if you get a sudden change in the engines rpm's. If you do you have a leak nearby.
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Friday, August 25th, 2017 AT 9:02 PM

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