M271 Engine Problem - low compression but perfect cylinder leakdown results.

[Rogerfl]

i need help diagnosing a 2005 C230 Kompressor (M271 Engine) that is hard to start and will not idle. It runs ok with reasonable power when the RPMs are over 1500. It dies if you let it idle when cold. Will idle but very rough when warm. Starting requires a shot or stater fluid spray.

It is throwing codes P0171 (lean trim adjust limit) and P0300 (misfire) all the time and occasional throwing codes P0304 (RPM) and P0444 (Evap purge circuit open).

Cold compression test results:
dry: 95-100 PSI all cylinders
wet: approximately 110 PSI all cylinders

Cold cylinder leakdown test shows minimal or no leakage all cylinders.

I have used an inspection camera through the plug hole to inspect the inside of the cylinders and it looks good inside. No signs of cylinder scaring and no signs that the valves are contacted the pistons.

I have removed the valve covers and inspected the timing chain that is tight and most likely replaced. I have also placed cylinder 1 at TDC and observed the timing marks on both camshafts. Although not perfect, the timing error appears to be less than a single link so the valve timing appears to be correct. I ulilized the inspection camera to look at the lower timing chain gear and observed minimal wear.

Just purchased the car, so do not know the history. 140,000 miles. The body/interior indicates the car was well taken care of.

I'm at a loss to explain the low compression with no cylinder leakage? Valve timing? Would the compression test be affected by low boost pressure?

any help is greatly appreciated.

- Roger

 

[wrdcc01]

Did you have all the spark plugs out when you did the leak down test?

 

[Rogerfl]

No. That is an interesting question. I assume you are thinking that maybe there is a head gasket leak between cylinders. Correct?

 

[Rogerfl]

I repeated the cylinder leakdown test with all plugs removed. The result is a consistent 5% on all cylinders. Almost less than the leakdown tester measures.

 

[Stocious]

The difference between dry and wet compression test is not what should be expected, you say it has low compression a wet test should show a huge improvement. What’s blow by like?

 

[190]

I confess I've never done a compression test on a modern engine. In the old days we used to open the throttle during a compression test to ensure the engine could draw sufficient air to compress. If it was a CV carb we would open the throttle and raise the piston.

How do yo ensure a modern engine gets enough air during a compression test. Is it just a case of opening the throttle.

 

[markjay]

As long as the compression readings are consistent across all cylinders, there's nothing to worry about.

As for the pressure increase when wet, this is expected. A significant increase indicates engine wear as you suggested, but again if it is equal across all cylinders, then it is what it is - normal engine wear. At some point the engine will start consuming more oil, and at some point it will need an engine rebuild - but that's just the normal life cycle of an engine.

 

[markjay]

As for the compression figures themselves.... yes we used to get 180+ psi in the olden days, but the figures are meaningless.

Firstly, as 190 said, me too I am not sure how you achieve full throttle with a Drive-By-Wire accelerator pedal (throttle position sensor).

Then, charged engines have low compression ratio by design, to accommodate for the pressure boost from the charger. Your W230 engine produces (from memory) 196bhp, which is 53bhp more than the C180 that has a similar block. This is done by using a larger turbocharger and lowering the compression ratio to accommodate for the higher charger boost. So your W230 will have the lowest native compression ratio of all M271 series engines.

Either way, I wouldn't worry about the compression test figures, as long as all cylinders are (roughly) equal.

 

[markjay]

More to the point.... the typical M271 faults are split vacuum hose under the air filter box, and blocked crankcase breather. There's also the oil-in-loom issue (where oil leaks from the camshaft position sensors and travels up the wiring loom into the ECU), and the timing (the chain might be tight now, but check the wear on the camshaft sprocket wheels teeth, to prevent future failure).

In your case I would say it's a vacuum leak somewhere? The fact the the engine runs fine above 1500 rpm eliminates many other things... fuel system issues for example only get worse with rpm increase, not better.

 

[Rogerfl]

Thank you all for your responses. I agree, the leakdown test is good and I don't think the compression test is the problem. I originally thought that perhaps valve timing was off causing the low compression, but timing marks are spot on.

It seems like a large vacuum leak, but a smoke test using my homemade smoke tester showed no leaks.

It may be starving for fuel, so I will check the pressure on the fuel rail next.

 

[markjay]

Thank you all for your responses. I agree, the leakdown test is good and I don't think the compression test is the problem. I originally thought that perhaps valve timing was off causing the low compression, but timing marks are spot on.

It seems like a large vacuum leak, but a smoke test using my homemade smoke tester showed no leaks.

It may be starving for fuel, so I will check the pressure on the fuel rail next.

Fuel starvation issues usually get worse with RPM... not better.

How do you use a smoke tester to locate a vacuum leak?

Also, some of the vacuum hoses are under the air cleaner box and not easy to get to.

 

[Rogerfl]

Perhaps the "it runs ok above 1500 RPMs" is not really true. Actually it dies if you don't keep the RPMs up, but it has issues at higher RPMs as well, it is not really responsive when pressing on the accelerator, but it catches up and with the higher RPM it does not die. I'm thinking low fuel pressure is not out of the real of possibilities.

To perform the smoke test I removed the air cleaner box and sealed the large hose coupling where the air cleaner box connects. I then connected the smoke generator through a vacuum hose which pushed smoke through out the intake area. The idea is that you will be able to identify any leak as you see the smoke escaping through the leak.

Will let you know results of the fuel pressure check.

- Roger

 

[Stocious]

Blocked exhaust possibly

 

[Dickster]

Have you checked the simple stuff like plugs and coil packs ?

 

[Rogerfl]

Fuel pressure is ok. It stayed around 55 PSI at the fuel rail even when revving the engine.

This is my first time to trobule shoot an engine with a super charger. I expect a vacuum leak to be a little different. e.x. a leak between the MAF and the super charger would "suck" in air causing lien codes, but a leak between the supercharger and the intake valves would "leak" air (as a result of boost pressure) and cause a rich mixture. Correct?

Could a clogged Catalytic Converter cause my symptoms? I would expect the mixture to run rich not lien. How would I check?

What about clogged fuel injectors?

 

[Dickster]

It helps to know that basic things like plugs and coil packs are ok to start with, 140k and no history?

 

[Rogerfl]

Agree. No history because I purchased the care in its current condition.

I have done a visual inspection of the plugs and they seem good. When revving, there are no obvious misfires, so I'm assuming the coil packs are ok.

 

[Dickster]

I wouldn’t assume anything. If you don’t know you can’t rule it out. Plugs are a service item. And coil packs don’t last forever. I haven’t had the SL long but on my BMW motorcycle they go at around 40k.. and cause strange running and missing issues.

 

[Jigglyjohnson]

Fuel pressure is ok. It stayed around 55 PSI at the fuel rail even when revving the engine.

This is my first time to trobule shoot an engine with a super charger. I expect a vacuum leak to be a little different. e.x. a leak between the MAF and the super charger would "suck" in air causing lien codes, but a leak between the supercharger and the intake valves would "leak" air (as a result of boost pressure) and cause a rich mixture. Correct?

Could a clogged Catalytic Converter cause my symptoms? I would expect the mixture to run rich not lien. How would I check?

What about clogged fuel injectors?

Hi

We have seen a few blocked cat converters over the years and seem to through some weird symptoms up

Have you tried un bolting the exhaust from the manifold and running it and see how it performs. If the cat /exhaust is blocked the back pressure would be to high at idle and then even higher at high rpm. We had a car that would rev up then bog down under acceleration and try and stall this was down to the exhaust being blocked. I would unbolt as suggested and then drive may be a big noisy but it's a test that would cost nothing and rule out the exhaust side

 

[wrdcc01]

Hi Roger, how about looking more closely at the purge valve as this can give you a rough idle and hard starting. I know the code has only come up a couple of times but I think this is the real cause of your issues.
Dean