Idle Speed is too high, plus Poor Gas Mileage. - Mercedes-Benz Forum
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post #1 of 2 Old 10-16-2005, 02:32 PM Thread Starter
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Idle Speed is too high, plus Poor Gas Mileage.

I just bought a 1989 190E 2.6L, 95K miles, auto trans. Its my first Benz, but hopefully won't be my last.

I'm in the process of giving it a good tune-up. The dealer changed all the fluids, but that's about it. First off, I'm changing all the fuses from the crappy ceramic OEM type to the newer glass-capsule type. Next, I put in a new air filter and sprayed out the air intake passages with cleaner.
Next on the list is new spark plugs, but I haven't gotten to that yet.

Here's the problem: My idle speed is about 2000 with a cold engine, car in neutral. Occasionally, it even races up to close to 3000. After a very long warm-up, it will drop to 1200 with trans in neutral, 750 in gear.

Also, black soot around my tail-pipe. Idle mixture is too high, right? Hopefully, cleaning the air intake will have cured this. Another few weeks of driving will tell. At least the exhaust smells better now that I've changed the filter.

The engine starts and runs smoothly, there's more than enough power for all driving situations, but I think this high idle is eating away at my mpg.

I haven't done an exact calculation, but my gas mileage seems to run about 16mpg on 92 octane, mostly around town driving so far. I feel I should be getting somewhere in the low 20's around town, and the high 20's for highway driving.

A sticker under the hood says the idle should be 750 with a warm engine and trans in neutral. Mine alway hovers around 1200 in neutral, and drops to 750 in gear, but the trans always SLAMS into gear, both D and R. Once in gear, it shifts smoothly at all speeds under all conditions.

The sticker also tells me that idle speed is electronically set and cannot and should not be adjusted. If so, what is controlling it, and how can I check that this unit is operating properly? Please don't tell me I need a new computer. I can't afford that. I suspect the problem is something simple and easy to repair, but finding it is the issue.

Are there vacuum hoses I can look at to see if they are pinched or loose?
Is there a wire loose somewhere, or an electrical connection that's become corroded? When I cleaned out the air intake chamber, was there something else I should have cleaned at the same time?

Ideas, anyone? I'm sure this is a simple fix, but again, finding it is the real problem.

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post #2 of 2 Old 10-17-2005, 08:10 AM Thread Starter
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Update: Never mind about fixing the idle speed.

Apparently, after I changed the Air Filter, all it took was a day and a half of driving and about 50 normal miles for the computer control to reset itself.

The cold idle is now about 1200 in Neutral, the warm idle is now about 800 in Neutral, and about 750 in gear ... just like the sticker says it should be. The engine warms up a lot faster, too, although I can't figure out why. The trans no longer SLAMS into gear, and it purrs like a kitten. The exhaust smells a lot better, too.

And I learned a valuable lesson in the process.
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