I just purchased a 1990 E300 (2.6L) with a M103 engine with 176K miles and I noticed that the car runs rough especially when I put it into gear. I did a leak down test and it seems that cylinder #6 has low compression. The car does burn oil but does not smoke. Could this be a burnt valve problem or is it just the valve seals that need to be replaced? Can this problem be fixed without removing the head? If not, is the head removal pretty straight forward. Any special tools needed? I am trying to decide whether to keep the car or sell it. Any help is greatly appreciated. Thanks.

If you did a Leak -Down test, what did the results of the test show ????????

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A Dalton

The test showed that cylinder #6 had low compression compared to the other cylinders. The number was much lower compared to cylinders #1-5. Any thoughts?

You have done a Compression Test...not a Leak-Down test.
Now that you have deducted you have a low compression cylinder, you now do a leak-down Test on that cylinder and that test will tell you why it has failed the compression test...
Leak test is not a Compression test..it is a leak detection test used to diagnose the fault/cause of a low compression cylinder .
So, it is a follow-up test to pin-point why you have low compression.

Search 'Leak Down Test'

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A Dalton

Both a compression and leak down test were performed and the leak down test gave a reading of a high percentage lose. A local mechanic did the test for me and they indicated a possible burnt valve. Does that sound right?

Yes ..that is what the leakdown test would tell him. It would also tell him if it was an intake or exhaust valve problem. Might want to ask him that.

A leak test can verify ring, intake valve , exhaust valve, head gasket, etc.
So, he has determined you have a burnt valve, but it could also be a stuck valve or bad quide. Alarge % loss does indicate a burnt or open valve and the leak test would have told him not only a valve was leaking, but it would also tell him which valve [ int/exh]..that is why we do a leak test right after a compression test. Narrow down the diagnosis right then and there..........

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A Dalton

Criminies! You squirt some oil in the cylinder and re-test. A significant improvement in compression tells you it's rings, no improvement tells you it's valves or other.

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Gone to the dark side

- Jeff

Given that you have no history on the car, I would give it a round or two of top-end cleaner and some tanks of Chevron to help rule out carbon deposits interfering with valve operation. In the meantime you can plan out your work.

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Prost!

oil change , top end cleaners and chevron all very sound advise . a leaking lifter can sort itself too with a oil change.

Thanks. Any recommendation on a good top end cleaner? What type of grade for the oil should I use? Should I also put a quart of Mystery Oil when I do the oil change? I guess if after the oil change and cleaner the problem still exsists, then I will need to do the valve job.

Be certain that your suspected weak cylinder as indicated by leakage is also the weak cylinder detected with a cylinder balance or scope test. Why? I have been fooled by the aforementioned carbon blocking the valves open. The leakage at the intake valve could be noticed with a vacuum gauge as it's needle might vibrate with pulses of combustion gas escaping into the intake. Got Backfire? Exhaust valve leakage may be heard at the tailpipe with engine running as a "putt putt" sound. Or cylinder leak air could be heard at tailpipe also as a"$$$$$$$$$ing" sound.
Peace

I spoke to the mechanic that performed the compression and leak down test. He indicated that cylinder #6 had low compression compared to the other cylinders. The leak down test reading was over 30% in cylinder #6 and indicated both a leak in the intake and exhaust valve. The plugs are not fouled and no blue smoke comes out of the tailpipe.

Also, I found spark plugs in the trunk of the car and I am thinking about replacing the plugs. I found Bosch Platinum +4 spark plugs and NGK V-Power Copper Resistor type. Which type are better plugs? Thanks.

GM has a potent cleaner that you spray or pour into the intake while the engin is running, then let sit awhile and then drive around smoking up the neighborhood. I haven't used it in years, but the part number is either gm12378552 or 1052626.

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Prost!

Neither.

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Gone to the dark side

- Jeff