Compression test

[Blk04MM]

Had a compression rest done on the MM today. Good and bad result.


Good: The left side (passenger side) checked out great. Each were 190 or a tad bit over With the throttle closed. With it open they each were about 230

The driver side is the issue. Each cyl with throttle closed was registering at about 90 but wouldn't hold compression.
With the throttle cracked open a bit it revved super high and compression was over 200 but still dropped. And the #1 had oil in the plug hole.

Plugs and coils looked good. Car runs. Very very little smoke out the tails when you get on it real good.


So looks like I'm in need of a rebuild or new motor.

Anyone can chime in with info or where to start? The driver side is confusing on how it's acting.

[SpartaPerformance]

I'd do more testing and try to isolate the problem better, it's odd to have one side compression be bad and say its the engine. Theoretically the engine will lose compression on both side being all cylinders moving equal amounts of time therefore wearing evenly. One side may be cylinder head.

BTW: passenger side is the right side, cylinder 1 is there, the first cylinder on driver side is cyl 5.

[LANDY]

It could be the valve is not seating all the way..

[Blk04MM]

I'd do more testing and try to isolate the problem better, it's odd to have one side compression be bad and say its the engine. Theoretically the engine will lose compression on both side being all cylinders moving equal amounts of time therefore wearing evenly. One side may be cylinder head.

BTW: passenger side is the right side, cylinder 1 is there, the first cylinder on driver side is cyl 5.

Ok thanks for the cylinder # correction.

So #5 has taken on oil. It's not flooded very bad but there is oil on the plug. It didn't file it down. Performance is there but I know oil in the plug isn't normal. It's the ring correct?

[Blk04MM]

It could be the valve is not seating all the way..

That allow oil in the plug area? Or that's making the driver side act funny?

What type of job am I looking at to reseat the valves?

[Blk04MM]

How hard is it to reseat the valves or change the piston rings???

Definitely looking to get this done ASAP so car can be put up for sale. It's the only issue I'm just over the car entirely. Power wise etc...

[Krytin]

I'm not sure who is doing your compression test, but you DON'T take readings w/throttle closed - results are completely meaningless.

Driver's side bank reving higher when cranking - if it is the same for all four cylinders it would indicate a common flaw. All I could think of for that would be a major head gasket failure effecting all of the cylinders.

The plugs have to be out of ALL the cylinders when doing a compression test - the only cylinder to see compression when you are cranking it over for the test is the one w/the gauge in it. All of the others have to be open/no spark plug. Throttle blocked open for the duration of the testing. Compression gauges are captive - they (are supposed to) hold the highest pressure made while cranking over. Normally I crank the motor over until I get 10 compression cycles/strokes on the gauge. Then I record the reading. Release the pressure from the gauge and repeat for the rest of the cylinders. If your readings are dropping while you do the test or after you stop cranking - the tester is no good.

If you want to find out how long the cylinder will hold pressure you do a "leak down test" - entirely different test rig and procedure and NOT done w/the same gauge.

If you are really concerned, find someone who really knows how to do compression & leakdown testing and re-test it. The driver's side appears to have a problem but I'm not sure about the overall testing procedure so I couldn't recomend major repairs yet.

[Blk04MM]

I'm not sure who is doing your compression test, but you DON'T take readings w/throttle closed - results are completely meaningless.

Driver's side bank reving higher when cranking - if it is the same for all four cylinders it would indicate a common flaw. All I could think of for that would be a major head gasket failure effecting all of the cylinders.

The plugs have to be out of ALL the cylinders when doing a compression test - the only cylinder to see compression when you are cranking it over for the test is the one w/the gauge in it. All of the others have to be open/no spark plug. Throttle blocked open for the duration of the testing. Compression gauges are captive - they (are supposed to) hold the highest pressure made while cranking over. Normally I crank the motor over until I get 10 compression cycles/strokes on the gauge. Then I record the reading. Release the pressure from the gauge and repeat for the rest of the cylinders. If your readings are dropping while you do the test or after you stop cranking - the tester is no good.

If you want to find out how long the cylinder will hold pressure you do a "leak down test" - entirely different test rig and procedure and NOT done w/the same gauge.

If you are really concerned, find someone who really knows how to do compression & leakdown testing and re-test it. The driver's side appears to have a problem but I'm not sure about the overall testing procedure so I couldn't recomend major repairs yet.


The compression test was done right. Throttle was open. Plugs were out. I'll pull the cover off on the passenger side, reseal then recheck. Won't hurt to eliminate the seal.

[boatmangc]

#1 on the Leakdown test, find out where the air is going before you tear things apart

[Blk04MM]

#1 on the Leakdown test, find out where the air is going before you tear things apart

Ok.,.,.,.,.,.,.,

[Blk04MM]

What's the mm motor and trans worth north of 150k in good running condition?

[Blk04MM]

Bump for last post ....^^^

[Blk04MM]

After much research via google I think I've narrowed it down the issue to being a seal, gasket, guide or valve seating issue. The oil is only on the threads of the plug. The porcelain part of the plug is untouched and the coil is dry. There is no misfire either. So I suspect the oil rings to be fine. Some day it's an issue to fix some say its not much of a pull your hair out issue because of how good the other cylinders checked out compression wise. I'm thinking pulling the driver side valve cover off and seeing what I can see visually. The best price on a leak down is $200 I've heard. I'm mechanically inclined so it won't hurt for me to inspect and possibly put that $200 into a simple part to fix the issue.

Or

I can sell my motor and drop a low mile in her. 159k on the clock what's she worth??

[bill64]

Ive never seen a head gasket cause a compression leak on a entire bank of a v-8 engine. Also, it would be rare to have valve or ring issues on all four cyl's on the same bank...sure the test was done right? one thing to look for if the pressure's is in fact low for the entire bank is a camshaft that jumped time, why did you have a compression test done?

[ctrlraven]

Maybe do a leakdown test, that can give you a better idea of where air is escaping to.

[Vortech347]

Ive never seen a head gasket cause a compression leak on a entire bank of a v-8 engine. Also, it would be rare to have valve or ring issues on all four cyl's on the same bank...sure the test was done right? one thing to look for if the pressure's is in fact low for the entire bank is a camshaft that jumped time, why did you have a compression test done?

Bingo. 10 characters

[Blk04MM]

Ive never seen a head gasket cause a compression leak on a entire bank of a v-8 engine. Also, it would be rare to have valve or ring issues on all four cyl's on the same bank...sure the test was done right? one thing to look for if the pressure's is in fact low for the entire bank is a camshaft that jumped time, why did you have a compression test done?

Was going to spray it ...wanted an idea of what shape the motor was in. The test was done right. The tester was faulty at first then another one was used. All plugs were out and throttle was open. Maybe I'll give it another shot. Either way the threads of the plug still had oil on it. Something is faulty. I'm trying to narrow it all down. I know it's weird for a whole bank to act how it acted. I've driven it every day since then and it's ran perfect. That night I pushed it hard and no failures. I'm confused. What's a sure way to know if the cam has jumped time?


Maybe do a leakdown test, that can give you a better idea of where air is escaping to.

Yeah I'm trying to avoid blowing $200 on a quick test

[bill64]

I wouldent bother on a 200$ leak down test. If the engine runs good @150k miliage Id leave it alone. Im not so sure it jumped time, probably not, but that would require a partial dis assembly of the engine to check the timing gears and chains. If its using a little oil at that mileage so be it. I would not loose any sleep over it. If you want to be pro active then pull the engine and go through it!