PeachParts Mercedes-Benz Forum - Rough Idle Update on 300TE....I'm totally out of ideas

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He has an '88, no EGR valve.

M103 engine ignition timing is all CPU and sensor controlled. MB sets up the timing for the US based on the fuel octane, quality and emission controls.

Quote:

Originally Posted by JonL (Post 2163569)

But you said you have slower performance... how would you even know if you only have a lack of power at idle? At idle you are using no power!

OK, I don't know your model in particular, but I've been a mechanic in a previous life and I'm a mechanical engineer in the engine development business... so I'll stick in my .02.

Idle vacuum should be very high. It sounds like yours is low. The fluctuation I wouldn't worry about just yet... if the engine is idling poorly, the fluctuating idle speed will cause a fluctuating vacuum reading. Low vacuum... it isn't because of a leak (although you might still have one). It's because the engine is idling with more throttle than it should require. It's doing this to take in more air and fuel because it is running inefficiently. Why is it running inefficiently? That will be the key to solving everything. Possibilities:
Incorrect A:F ratio. Probably not, because you've got on O2 sensor, no fault codes, and you've been through the fuel system. Still could be a subtle cause like bad intake valve guides, or a faulty PCV system (that's why someone posted about the dipstick and oil cap). Possibly something with the evaporative emission system, but I have no clue how that works on your car.
Exhaust restriction. You say you have checked the cat, also it would manifest more as running out of power with a slight lag as you accelerate.
Retarded or erratic ignition timing. I haven't seen that you've checked this. Put a timing light on it, see where it is at idle. See if it is stable. See if it advances sharply and cleanly when the engine is revved up. Erratic and retarded timing would cause all your problems.
Poor compression. Your engine looks beautiful. It could still have screwed up cam timing which is essentially like having poor compression. Especially possible because the system has obviously been molested. (No reflection on your mechanical capabilities, but a realization that something that has been touched is more likely to have a problem.) Retrace your steps. Why was the head off? Are any of your valves bent or sticking? Weak or broken valve springs? Tight valves anyplace? Bad hydraulic tappet(s) making a valve stay open?

Thank you.

I have low power at the bottom end, I didn't mean idle, sorry. Along with very poor MPG. My last fillup was 12.1mpg.

To this post and others, the 1988 model does not have an EGR valve.
Compression is 190psi all cylinders.

I had the entire head rebuilt including new valve guides + seals. Head pressure tested and skimmed. I had the head off because I was losing a lot of oil and coolant and was told by a local indy that the head gasket was leaking. It was a great opportunity to really get to know the engine (my first rebuild) and it didn't cost me a whole lot. Experience was well worth it. Had some help from my dad of course...but he is stumped on the rough idle too. He goes by "Merky" on this forum, but rarely posts. There were a few other items such as the water pump that needed replacing. The valves looked great.
I have a large thread with tons of pictures that I took throughout the whole thing here:
http://www.peachparts.com/shopforum/tech-help/241701-my-300te-head-gasket-thread-yes-pics.html

When I rebuilt the top end, I set the cam timing as it shows in the MB manual to perfection. Should I need to get a light to still check it?

EDIT: Here is a video on youtube
http://www.youtube.com/watch?v=kMAVAjyVSTE
You can see the idle economy gauge and rpm fluctuations. Maybe this will give you some ideas.

I would love to take it to the dealer and have it fixed....but I A) expect a huge bill B) Expect several expensive items to be fixed before the real solution is found :(

Interesting in the video. It really does look like either the idle control valve is fluctuating/hunting........possibly due to the Air Flow Potentiometer not being in spec......or there's a bad vacuum leak somewhere.

12.1 Mpg would point to massive overfueling.....does the exhaust smell like it? Is it clean? I wonder if the timing might be slightly off somehow?

Quote:

Originally Posted by ps2cho (Post 2163787)

I would love to take it to the dealer and have it fixed....but I A) expect a huge bill

No guarantee taking it to a dealer will be any good. The local M.B. factory operated stealerships near me have "technicians" instead of mechanics. Many of these technicians are not much older than my M103 engine. They are all geared up to operate on the computer controls of the newer models. - The diagnose and replace system.
When I need advice from them (extremely rarely) they need to call their "old guy" or do a search from their on line manuals. Sometimes they even point me in the direction of an indie specialist - often an old dealership mechanic out on his own.

You would need to find an Indie specialist.

For injection problems I take my cars to a franchise Bosch injection specialist. Even the dealerships send their cars to these guys.

Quote:

Originally Posted by pawoSD (Post 2163166)

The US spec cars do not have the vacuum adjustable headlamps.

I see that Wagon has Euro headlights. Are these not converted to level control?

Quote:

Originally Posted by Ivanerrol (Post 2163854)

I see that Wagon has Euro headlights. Are these not converted to level control?

Not usually....my dad's car has euros without the level control as well....

I take it back, he does not have them "without" level control, the level control hookups are present, but it is not used. Someone just put little stubs of plugged vacuum line on them to keep dirt out. They are true Bosch Euros...

Quote:

Originally Posted by pawoSD (Post 2163823)

It is clean, yes...but as for the smell...I have a brand new cat in there so that's hard to tell...
I checked the AFP with a DMM and it showed .7v which is on spec. I have replaced every idle air hose in the engine...I just don't get it :confused:
I have sprayed carb cleaner all over the engine at every hose and it makes no change.

Is there a way to check for crankcase pressure? Is there a spec for it or something?

Quote:

Originally Posted by Ivanerrol (Post 2163848)

That's what I have read from a number of people.
I took the car to a specialist in Tarzana "Enrique"...and he replaced a hose that he said was leaking and said the car was perfect. It was far from it when I got it back...It was horribly slow and still had a rough idle. He must have adjusted the EHA because I had to turn it richer to get the lost power back. It was horrendously slow. I won't take it back to him again that's for sure.

Quote:

Originally Posted by Ivanerrol (Post 2163854)

I see that Wagon has Euro headlights. Are these not converted to level control?

They are just DEPO Euros. I have not done any conversions. The stock lights didn't have it either.

Quote:

Originally Posted by ps2cho (Post 2163878)

I wonder if a whole new air meter/distributor assembly off of another car would make it run any different. Then you'd be swapping the air meter, Potentiometer, EHA, and Fuel distributor/pressure regulator all at the same time.

Quote:

Originally Posted by pawoSD (Post 2163929)

I have one...so I could. The potentiometer wire is cut off though. Got it one day at the junkyard and went the checkout window with the female who looked new....Charged me like $25 for the whole thing ;)

...but I'd really like to figure out WHY there is an uneven flow of air in the intake manifold. It shouldn't be fluctuating like that...it should be rock steady, right?

But for reference, I have swapped the EHA, fuel pressure regulator, and fuel distributor before....

Quote:

Originally Posted by ps2cho (Post 2163787)

Thank you.

When I rebuilt the top end, I set the cam timing as it shows in the MB manual to perfection. Should I need to get a light to still check it?

The light is to check ignition timing, and yes you should do it. I realize that it is all set by computer, but there may be a problem in the ignition timing nonetheless. If you find that the timing is off, then you know what systems to start chasing. Things like crank position sensor, cam position sensor (if present), knock sensor, temp sensors, etc.

Does the air meter move smoothly? On my 300E it was crooked and got stuck in the opening when coming back to the rest position.

I think that the Potentiometer is more likely to cause a hunting idle syndrome than the EHA.....

Yes. The picture in the first post shows the whole mixture unit removed. I adjusted the sensor plate so its perfect.

Even if the potentiometer checks out voltage-wise...could it still be bad?

Quote:

Originally Posted by JonL (Post 2164101)

Gotcha. That will be first thing to do from now.

Quote:

Originally Posted by ps2cho (Post 2164119)

Yes. The picture in the first post shows the whole mixture unit removed. I adjusted the sensor plate so its perfect.

Even if the potentiometer checks out voltage-wise...could it still be bad?

Gotcha. That will be first thing to do from now.

Even if it tests in spec with the car off, that doesn't tell you if its in good shape in a linear way.....I learned from a great mechanic local to me that the only way to truly test the Potentiometer is with an analog voltage meter, so you can see if the voltage change is perfectly linear. A digital meter has too slow a refresh rate to display accurate changes in the voltage.

If the computer is trying to adjust the idle with the idle control valve but the Potentiometer has a bad spot on it around idle, it will confuse the computer and cause it to keep adjusting the valve back and forth trying to find a good idle.

Yes, you would need to check that with an analog meter for best results. Digital meters won't show dead spots, dips, or other non-linearities nearly as well as a needle meter.

Forgive me if you've already answered this question but, have you checked to make sure it's running in closed-loop?

Also - Somebody posted a link to a PDF file with a complete theoretical description and diagrams of the Bosch CIS system. It's well worth the read. I think the post is maybe a month old? I highly recommend checking it out.