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  #1  
Old 06-08-2004, 08:16 AM
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wide spread in compression test

I love my 1990 300ce and enjoy this forum almost as much!

What do people think of these dry compression readings?

171
175
190!
171
150!
170

The car runs pretty well but I would like it to be quieter, smoother and use less petrol.

David

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  #2  
Old 06-08-2004, 09:01 AM
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If I remember correctly, at least for American cars the spread between highest and lowest should be no more than 20%. You have 27%.

If the wet compression readings are significantly different from dry readings, this would indicate a problem with the bottom end (rings). Otherwise, this variance may be attributed to the valves.

Are there any other issues with the engine? How many miles?
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  #3  
Old 06-08-2004, 11:21 PM
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300ce compression

Thanks for your reply Kestas

The car has 123,000 kilometers. I've had it about 2 months.

The general condition is good. The engine is powerful but a bit noisy and fuel consumption is very high. It's running rich at 3000rpm and lean at idle?! I had the fuel pressure valve changed last month.

The car has a well documented service history - dealer then good indy for past 3 years.

Gasket and valve guides were done three years ago.
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Last edited by Columbo; 06-08-2004 at 11:42 PM.
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  #4  
Old 06-09-2004, 09:31 AM
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I wouldn't expect so much work done on this engine with such relatively few miles and a good pedigree. Perhaps the question now is whether the valve seats were ground or lapped when the head was reconditioned.

Unless somebody can recommend other checks or potential problems, I'd recommend letting it go further until you HAVE to address this issue.
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  #5  
Old 06-09-2004, 10:36 AM
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make sure the car is at normal temp. before test is done,and hold throttle wide oped during test
rich
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  #6  
Old 06-09-2004, 11:10 PM
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I was concerned about that work too, but not enough not to buy the car! Now I am begining to wonder...

The first owner probably did a lot of short trips and maybe flogged it when cold too. I hope not! The car is tight in other respects, so my first assessment was that it had been well treated.

What sort of problems would this uneven compression be likely to cause?

Also, is there anything I should do to stop it getting worse?
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Old 06-14-2004, 11:25 AM
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Are you sure the compression test is correct? How come you have no wet readings? Was the motor warm when you did the test? How do you know that it's running rich at rpm?

If it was running really rich for a long time you could have carboned up your rings.

Test to see if you are actualy running in closed circuit mode and not just on mechanical. When I bought mine the plugs were carbon fouled and the O2 sensor was dead. It still ran fine on mechanical injection but would eventualy have created problems for itself.

Those are really low km's to be worried about serious problems.
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  #8  
Old 06-14-2004, 10:57 PM
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Hi 89-300CE. Unfortunately, I can't do any testing myself. No tools; no garage.

This was the only test the mechanic did and he says the compression is 'excellent'!?

So, relaying what he told me, the car is running very rich. All the plugs are carbon fouled. The O2 sensor is showing an error.

He replaced the fuel regulator and says there is no point replacing the 02 sensor till he is sure it is not something else that is causing the car to run rich. He asked me to run a few more tanks through the car to clean it out. Fuel consumption is 4 km per litre so that won't take long!!

I am begining to think I should get a second opinion - and full compression test - from another garage.

How do you test for closed circuit mode?

Also, do you know what the factory compression spec and range is for the M104 - I guess yours is the M103. 190 seems high. I need to go buy a manual.
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  #9  
Old 06-15-2004, 10:35 AM
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Maybe someone here more familiar with the m104 could comment but on my system the O2 sensor fine tunes the mixture so to me it makes no sense looking for additional problems when the primary controler of fuel mixture is known to be defective.
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  #10  
Old 06-17-2004, 09:23 PM
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I agree with you and said as much to service manager. He said that O2 sensors are often replaced when they don't need to be and he needs to identify the underlying cause. He wants me to bring the car back when he can test it with an ECU borrowed from another M104 (is this common practices in shops?). In the meantime I am doing my bit to accelerate global warming!

I work in IT and the idea of a mechanic swapping computers between cars makes me nervous (albeit the box in the M104 seems to be pretty basic). Not sure I would be happy about my car being the donor of a test ECU! What do you think?

This whole experience is making me think about my wheels way more than I am accustomed to. Am I becoming a car nut?
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  #11  
Old 06-18-2004, 01:09 AM
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It's not a "computer" like a PC. It's a digital ECU - faily simple hardware, firmware, and a simple run time OS. Swapping in a known good one is a good way to isolate whether your ECU is part of the problem.

Your service manager is saying the right things. Diagnostics should be the approach rather than just guessing and throwing parts at it on your credit card!

Duke
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  #12  
Old 06-18-2004, 03:48 AM
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There is nothing wrong with becoming a car nut,nut ,nut, nut, nut, nut, nut, nut,nut, nut!!!!!

William Rogers..........

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