My 1986 560SL is driving me crazy. The problem is that it has good performance when the engine is cold, but as soon as it warms up the performance is extremely poor. When the engine is cold the intake manifold vacuum is at 10 inhg and the enrichment ma is in tolerance, as the engine warms up my vacuum falls to 7-6 inhg and the enrichment falls to well below tolerances. All fuel pressures look good.
I have replaced the temperature sensor, ignition control module, airflow pot, fuel pressure regulator, fuel distributor, fuel pump, fuel filter, distributor, all new fuel injectors, exhaust system, o2 sensor, and overload protection relay. I also changed out the K-jetronic control module

My question is where do I go now?

David
1986, 560SL

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Dave

Try a leak down test to pinpoint the problem!

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1988 560SL
Black Pearl/Palamino

That's not much vacuum. If it started suddenly, I would look for a broken hose, fitting not connected, or maybe a problem in the brake booster.

What is the vac guage in the instrument cluster reading?

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe

the intake manifold vacuum should be 15-19" at hot idle in neutral. Something is seriously wrong--either cam timing is off (bad chain guides allowed chain to jump a tooth?), ignition timing way late, or engine is seriously ill. I am presuming you have checked for vacuum leaks--it would be a big one to drop the vacuum this low.

Run a compression test--both dry and, if low then with 1 teaspoon of oil added to each cylinder in turn, cranked 5-sec and then read the pressure.

Thanks for the help. The dry compression test is as follows:
1-115, 2-120, 3-120, 4-115, 5-130, 6-105, 7-120, 8-120.

Vacuum readings from manifold.

Idel 700RPM 7 then stablises at 12
open throttle 4 , very unstable
at 400 RPM vaccum goes to 20 and runs smooth

Thanks for any and all advise

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Dave

Pretty hard to evaluate the problem with absolutely no testing having been done.

Since money is no object why not do it the cheap way and pay a professional.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Steve, Thanks for your helpful remarks. I am working with a MB Indy that has years of experance. Now the question is he a pro. or just stumped on a difficult problem. He has completed a number of test and problem still points to low Vacuum, 6-10 at 700RPM. Question is what test to run now.

Thanks again for your help.
David
86, 560SL

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Dave

I hate to disparage another professional, but I get suspicious when one of my techs claims more than one source for any problem. You know: "it wasn't the fuel distributor but it needed it" type conversation.

"temperature sensor, ignition control module, airflow pot, fuel pressure regulator, fuel distributor, fuel pump, fuel filter, distributor, all new fuel injectors, exhaust system, o2 sensor, and overload protection relay. I also changed out the K-jetronic control module"

I'd give my tech a big "F" on that report card.

Manifold vacuum is a result of engine performance. Other than cam timing it isn't something one can add to other than adding to what is missing. I would add fuel, subtract fuel and see what happens to man vac. I would add timing and subtract timing and see how it effects man vac. (this requires subtrfuse as the timing isn't directly adjustable). I would measure ignition timing to see if its in specs. I have seen a couple cars that ran poorly recently due to the flywheel being installed incorectly after a trans repair. The timing was way off. Cam timing is easily verifiable. Since you have replaced everything concerned except the motor, I would look at how it is assembled.

When I diagnose a problem I first wish to determine what is happening, is it fuel, ignition, or compression. I would seriously worry about your compression if it were taken with any of the gauges I have ever used. That motor should have 150-180psi compression. I didn't mention it because MB v8s never have compression problems. It is probably a gauge issue. The way you get compression problems that far off, uniformly, is with a uniform cam timing issue. Hard to happen with two cams, but might if the crank spun on the chain somehow or if the balancer was on wrong and both cams timed to it.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Has there been any major work recently? Like a valve job. I have seen where the base adjustment of the lifters are way out after a valve job and the problem looks like tight valves when running.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Steve, Thanks for the great advice. As far as I know the car has not had any head work and the transmission has not been removed. This is info I recieved from the former owner.

When I purchased the car, it was not running due to the former owner trying to convert to a carb. to solve what he throught was a fuel problem. Hence my replacing many of the parts lisited.

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Dave

So--this is the way the car has always been for YOU. The compression is very very low--but uniform. Did you have the throttle blocked open when you did the test? all the plugs out? . The compression readings you provided, if verified, indicate most likely cause is cam timing is way off on both sides. Highly unlikely other mechanical problems (valves or rings) would give such uniform readings.

Hi there,
With the new information you divulged concerning the carburation attempt, it seems the intake manifold must have been removed. I'm wondering if it was reinstalled correctly, proper gaskets installed, etc. Also, does this engine have an EGR valve? If so, is it sealing properly? If the cam timing is correct, you just about have to have a vacuum leak. When it's running, try removing the oil filler cap - does it immediately quit? If so, maybe you have a vacuum leak into the crankcase area from the intake manifold. Whatever it is, it has to be fairly obvious to get the huge reduction in vacuum that you have. Maybe you should take Stevebfl's advice and go to a real pro!

Regards,
Richard Wooldridge
'82 300D/4.3LV6

Hi again,
Here's another thought - once I had a Nissan 280Z that had a similar problem, and it turned out that the intake manifold had corroded through the passage between the EGR circuit and the intake manifold itself. There was absolutely no visible external indication of the problem. Just a thought...

Cheers,
Richard Wooldridge
'82 300D/4.3LV6