This is my first gas vehicle in over 10 years. It is a 1988 300E in very nice shape, body wise and interior wise. Yet it failed emissions so was put up for sale. Older lady owned it and put maybe 200 miles on it a year.. for the last few years.. The tank has been drained and refilled with fresh gas, and a injector cleaner.

I've done a tune up, consisting of a cap, rotor, plugs, wires, IAC valve hoses, and tested the other small hoses connected to the intake manifold.

Runs ok at speed, but stoplights are not much fun. HOT idle is about 600RPM in neutral. Vacuum is about 13 inches, and at the moment the car is at 5600 feet above sea level.

Headed out to pull the Over Voltage Protection Relay, and test for vacuum leaks ... as I am telling myself that low vacuum is part of the problem.. (I have all ready ruled out the brake booster, and 5 vacuum ports on the intake manifold) Testing the fuel injectors next.

Guessing there is no idle speed setting on this motor.. and that the low idle speed is ether vacuum related, or something else is out of whack.

I am open to suggestions, on this particular issue, as well as general instruction on how to come up to speed on the CIS fuel injection system/engine management...

The most common cause of a rough idle in a gasser is a vacuum leak somewhere, but you really didn't say anything about being rough. In a fuel injected car, I believe that it can cause a low, but not necessarily rough, idle. Don't forget all of the vacuum under the dash, maybe you can pinch off the supply to the dash and see if it helps. good luck...Mark

Oops... yes it has a somewhat rough idle. it does not usually stall, but acts as if it would... I had the vacuum disconnected to the cabin and did not find any improvement.

Just followed along here PeachPartsWiki: Isolating Vacuum Leaks

That is a really neat way to find vacuum leaks... I doubt I would ever have found it any other way!!!

And found the big funnel shaped rubber, or square gasket at the bottom of the (what is the throttle plate assembly called by Mercedes?) funnel where it connects to the intake manifold runners.... That is gonna be a bit of a job getting it all apart.... anyway when squirting some ether there, it takes a moment then the O2 sensor jumps from about .15 =>.9 and sometimes over 1 volt..

Does that sound like I have found the culprit? After lunch I will re test..

Considering that I am now likely taking the whole fuel distribution assembly off, are there other things I should do to while I am in there?

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'83 240D
'85 300D
'86 300SDL
'88 300E

Drat! Still have low vacuum, and a low idle. The results I posted earlier were a false positive, as squirting some ether on the intake funnel did nothing to EGR readings when the air cleaner housing was on the throttle body....

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'83 240D
'85 300D
'86 300SDL
'88 300E

Blue may be temp sensor is faulty .Telling computer its hot, and in doing so its weakning out the mixture.Making it run weak. Also it may be fuel related .So a fuel pressure test is needed .Low revs at the lights could be down to bad fuel filter .Have you dhecked the idle control valve .It sits under the filter box .Remove it to look .It as 2 hoses going off from it , these can split ,and a plug on the rear .Remove it from the engine and clean it out with white spirits.Dont sink it in the liquid as it holds electrical items.This can change the tick over if gummed up inside..But if your sure its vacuum then its teasting time ,dont forget the vacuum hose in the front lights.

Blue ,This is the part to look for . Its the one with sadle over it and 2 bolts to hold it in there. Just right of the dip stick.Ican see you have done the hose .Did you clean it out inside? What plugs have you in there must be resistor less

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Thanks for the suggestions, it helps me conceptualize where I am in the troubleshooting process.

IAC valve is clean. Will test it today by unplugging and noticing the change in idle speed. Also toying with the idea of applying 12 volts momentarily to see if there is a "click"

VACUUM to the headlights? I can only find vacuum ports going to the transmission, the vacuum gage, door locks and one to the ignition system.

I am really toying with the idea of changing the intake manifold boot, and gasket... I do not want to... and I would really enjoy having an engine that gets the best fuel economy, and shifts smoothly.

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'83 240D
'85 300D
'86 300SDL
'88 300E

FWIW, I had a similar issue with my 82 R107 that I chased for months until I did a smoke test and found 7 large cracks in the Air Chamber which was letting way too much air into the system and telling the analog CPU to add more fuel. The part was cheap (under $90) but was about 4.5 hours to tear down and replace. This was after replacing the OVP, O2 sensor, oil temp sending unit, and temp sending unit, and a few other un-needed part replacements.

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82 380SL 41K miles sold, 98 SL500 44K miles, 13 C300 4Matic, 14 CLA-250 4Matic

Yes, a vacuum leak is a problem... It may not be my only problem, but it won't be right until that is fixed at the least...

Where that vacuum leak is coming from, I do not know yet..


It would be nice to be able to rent a smoke machine..

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'83 240D
'85 300D
'86 300SDL
'88 300E

I thought long and hard about purchasing a smoke machine which runs about $900 to $1,000. If you can get in on a trailer most shops will charge $100.00 for the test. much cheaper than buying one...

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82 380SL 41K miles sold, 98 SL500 44K miles, 13 C300 4Matic, 14 CLA-250 4Matic

I think I am gonna pony up for the intake manifold boot and put some time in this weekend installing it.. IF I am still chasing my tail by next week I will spend some $ on a smoke machine diagnostic job.

Once the vacuum leak is resolved, then I can focus on what is left.. if there is any thing left to be resolved.

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'83 240D
'85 300D
'86 300SDL
'88 300E

Cheap smoke machine: 1 cigar. Blow in smoke through one of the lines and see where the leak is.

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1990 190E 3.0L

will this actually work, or are you kidding me?

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'83 240D
'85 300D
'86 300SDL
'88 300E

Of course it will. It's doing essentially the same thing. https://youtu.be/zMok2y05jNE

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1990 190E 3.0L

Look at this: DIY smoke machine - diagnose vacuum and evap leaks : General Chat

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'83 240D
'85 300D
'86 300SDL
'88 300E