So my sons car 1990 300e m103. Uses a lot of oil leaks everywhere as well as fouls the plugs. So recently it started running rough and we removed the plugs and cleaned them. As well as inspected the cap and rotor. Then it ran good again for a while and now its running rough again. Could be the change in weather but who knows. So I pulled it into the shop today and started trying to figure something out to fix. I bought a spark tester and checked the spark at all the plug wires and they all had good spark all the way to 20mm. Then I did a compression check, but the engine was cold and I did not hold the throttle open. all 6 cylinders were 125-130 ish. I added some oil to each one but unfortunately I did not use equal oil lol Some went up to 175 some went to 150. Then I hooked air up to the cylinders this is were I got really lost. I took the valve cover off. Then I had all the plugs out and I would rotate the crank to where the valves looked close and then open the air up. The air would then either come out the intake breather hole or come out another spark plug hole. So then I put the plugs back in and did the test. I think the intake valve was opening from the air pressure so I would turn the crank with the air on until it stopped. Then the cylinders were holding air with no leak. Not once did I hear air coming from the dipstick tube. Did I make the mistake of doing this test after I put oil in the cylinders? Should I put some acetone/atf mix in the cylinders and let it sit? At this point the valve seals look like a really easy job, but I dont want to just replace things for no reason either.

Compression tests should be done with the engine "hot", and the throttle should be blocked wide open, so I suggest you redo it properly. The absolute numbers are not as important as the variation between cylinders.

I don't understand what your "air" test is. A conventional leakdown test is probably what you want to do. Leak down tests can be more informative than compression tests, but it's not a bad idea to do both.

Also, it would help if you would expand on "running rough". Cold idle? Hot idle? Cruise? Acceleration?

Duke

Acceleration is terrible it just stumbles. Also the air test is like a leakdown test. I have a leakdown tester but its worthless. Either I dont know how to do it or it just doesn't work well. As far as holding the throttle open do I need to disconnect the fuel lines?

It's probably okay to let the fuel system work, but to be safe remove the fuel pump relay... much easier than messing with fuel lines.

Still not much to go on regarding symptoms. Doing a compression test is a good idea, but if all are within 20 psi or so, the problem is likely in the ignition or KE fuel system.

Duke

Do a proper compression test if you're not comfortable with your leakdown tester, if the results are good don't bother with a leakdown test. If the results are not good a leakdown test should be done to locate the leak e.g., rings, pistons and/or valves.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

I know the fuel system has some kind of problem. But what I want to do here is see if I should replace the valve seals and see if I can get rid of some of this oil leakage. I get tons of oil out of the oil cap and valve cover. And I have replaced the gaskets on both. I'm gonna put some marvel mystery oil in the cylinders too. Cant hurt.

If the spark plugs have deposits building up on one side of the ceramic (this is the side facing the intake valve) or are generally oil fouled, I'd suggest replacing the valve seals if the compression is good.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Yes plenty of build up. I ordered some valve seals and new plugs. Lets see how that goes.

When doing a leak down test, the piston must be at the top and crankshaft exactly at top also otherwise the sir pressure will rotate the engine.

Have you replaced valve seals before? Be careful, it is very easy to pull the threads out of the rocker mounting holes, use an inch pound torque wrench and only tighten them when the cam is on the base circle for the cylinder you are working on.

Good luck!!!

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Yeah I will rotate the crank so that 2 pistons are at the spot so both valves are closed then do them ones and so on.

Ok got my parts in today. I they didnt come with those little sleeves for the top of the valve and I also dont have seal pliers. I plan on using some tape on the top of the valve to not damage the seal. But how hard is it going to be to pull the old seals?

going to revive this thread in the hopes I can get some more help. I was able to change the seals. Also resealed the timing cover hopefully and changed the plugs. The car still runs a little rough and will not accelerate from a stop without stumbling. I finally figured out how to use my code reader and read the codes and then cleared them all and drove it again and read them again. The codes with * are the codes that came back. I didnt bother checking socket 6 though as I dont think they pertain to this issue i am having.
socket 3
11- Air injection system
16- Exhaust gas recirculation
* 17- Oxygen sensor is shorted to positive or ground
31- Intermittent contact in engine coolant temperature sensor circuit
socket 6
5- Driver seat beat buckle
6- Front passenger seat belt buckle

8- Circuit 15R, Voltage supply
3- SRS - Driver air bag
socket 14
2- Fuel pump relay (circuit 87) not functioning
* 3- TN/TD signal (RPM) interrupted

* 6- Output for kickdown switch control defective
* 12- Output for A/C compressor control defective
* 4- Output for oxygen sensor heater control defective
* 5- Output for air injection pump control defective

I would replace the O2 sensor first with the short and the heater fault. Be sure to use the OEM O2 sensor on that car. O2 sensor is not a place to cheap out!
With that out of the way next I would adjust the mixture on that M103. There is an excellent article on the Bosch KE3 Jetronic Mixture adjustment. The website with the instructions is www.landiss.com/mixture.htm . You just need a volt meter and a calculator unless your good with doing math in your head. Its a great article! It has bailed me out many times on my 88 300E M103 . I keep a printed copy in a folder for the car. Good Luck John

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88 W124 3.0
96 R129 119eng
06 ML350
98 ML320 sold
02 CLK55 amg

hanly2, I agree that replacing the Oxygen sensor should cure the acceleration stumbling. Get a crow's foot removal tool if you don't already have one. Need to remove the front passenger side carpet piece, unplug Oxygen sensor lead (likely a blue cable) and thread through driveshaft tunnel grommet. There should be Never Sieze on the threads of the new Oxygen sensor, if not apply a light coating before installing.

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Fred Hoelzle