Car idles rough in park but drives fine - Mercedes-Benz Forum
 
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post #1 of 9 (permalink) Old 06-25-2019, 07:15 PM Thread Starter
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Car idles rough in park but drives fine

So my car idles like absolute crap and the revs go up and down while in park but it drives fine when going 30 MPH. I replaced the injector seals, manifold gaskets, all the rubber hoses, caps and rotors, and some vacuum lines. The throttle body boot is going to replaced along with the throttle body as a CEL came on for it. I wanted to ask if the car continues to run like crap what could the other issues be?
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post #2 of 9 (permalink) Old 06-25-2019, 08:54 PM
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I wanted to ask if the car continues to run like crap what could the other issues be?
I just don't get it.

When I post to a forum seeking help I take the time to include all information that might elicit helpful, accurate replies to my problem. In this case you haven't even bothered to identify the year and model of your 129!

Mileage is often helpful too.

Suppose the complaint is an engine seems weak on power. If the vehicle has 250,000 miles suggesting that the owner borrow a gauge to perform a compression test might be prudent, whereas on an engine with 50,000 it would be far less so.
rorypff and FATHERTIME like this.
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post #3 of 9 (permalink) Old 06-25-2019, 10:44 PM Thread Starter
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Ok that’s a valid point it’s a 91 500SL with around 120K Miles
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post #4 of 9 (permalink) Old 06-26-2019, 08:14 AM
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The throttle body boot is going to replaced along with the throttle body as a CEL came on for it.
Looking to inform a reply with knowledge of the code you read I looked here at the list of possible codes I presume you might have read, and I can't guess which one suggests replacing a throttle body. So you'll need to supply still more information unless U.K. Rob or someone can suss this out.
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post #5 of 9 (permalink) Old 06-26-2019, 10:52 AM Thread Starter
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I checked the onboard diagnostics and the #2 code came up which says there’s an issue with ththat throttle body
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post #6 of 9 (permalink) Old 06-26-2019, 05:11 PM
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Code 2 is for the throttle position switch. It has two contacts: one closes when the throttle is closed, the other when it is wide open.

Before you go to the effort of replacing what could be a perfectly-working switch I would check its switching action at its connector (picture). Among the three male connector pins the brown wire is chassis ground and common to the two switches.
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post #7 of 9 (permalink) Old 06-26-2019, 06:52 PM Thread Starter
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So if I replace it will it fix the issue? I already have a whole new throttle body
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post #8 of 9 (permalink) Old 06-27-2019, 01:43 AM
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Idle speed contact switch is a common issue on early Cis-e V8s. With everything else fully operational, a faulty switch would not cause revs to rise and fall at idle though. Typical symptom would be high idle when hot after revving over 2k rpm.

So maybe there is another underlying issue.

A leaking throttle body might be the cause of the erratic idle, but you should also consider other possibilities.

An oscillating idle on Cis-e V8 can be caused either by
1) uneven combustion between cylinders due to a difference in the air/mixture formulation in the inlets leading to each cylinder. Typically caused by air leak / broken /displaced vacuum line to one of the inlet tracts. (Not normally a defective manifold gasket, unless something catastrophic/significant in engine history, like engine overheating or itís been messed around with).
2) Defective idle speed valve (ISV). Default (unenergised) position is wide open. This would confuse Lambda control and cause oscillating idle.

Before replacing more parts I suggest you consolidate your efforts to just diagnosis before you disturb anything else and compound your problems. Check over the vacuum lines and rubber elbow connectors to manifold and transmission. Do a smoke test.

With engine off, unplug the EHA and restart engine. Any difference? Is the erratic idle still there?

Unplug ISV. Does this have any effect? Check the current to the ISV while engine running and compare to spec values.

Check the duty cycle. For this purpose I suggest you acquire a multimeter capable of reading duty cycle (%).
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post #9 of 9 (permalink) Old 06-27-2019, 04:32 AM
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So if I replace it will it fix the issue?
Here is a quick and easy way to find out.

Erase code #2. Then unplug the connector shown in my prior post. At the female connector use a paper clip or wire to jumper together the brown wire with the yellow/brown wire. Start the engine and observe the idle.
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