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  #1  
Old 02-22-2010, 07:17 PM
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Location: Washington, DC
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126 -- high idle

I have a fully-restored '88 420SEL which hasn't been driven in about 7 months and I am starting to use it again. The battery has been recharged and I have run two tanks of gas through with Techron at the suggestion of my mechanic, including a DC to Philadelphia highway drive.

It cranks immediately when cold (with an initial idle at 800 RPM or so -- goes up to 1,000 when first starting, then settles down to 800) and runs normally except that:

(a) the in-gear idle is about 800 RPM (in drive or reverse),

(b) when put in park or neutral, the idle pegs at 1500 RPM and stays there (it goes back to 800 when put in gear),

(c) the oil pressure gauge also pegs at "3" and stays there during driving, whether going 80 MPH or idling at a stop light (my recollection is that it previously was always very sensitive and would normally vary appropriately between 1 and 3 according to RPM and accelerator pressure, going down when stopped at idle and increasing when running), and

(d) when you turn off the engine (parking to run an errand, etc.) and recrank it, you have to give it gas to get it recranked and then run it for a minute or so at a higher idle before driving to avoid a bit of stumbling when first starting out again.

Interestingly, this (d) phenomenon does not occur from a cold start -- you do not need to push the accelerator and give it gas to do a cold crank.

Would appreciate any insights -- fuel delivery issue (keep using the Techron)? Idle control valve/idle control unit? OVP relay?

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  #2  
Old 02-22-2010, 08:36 PM
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Join Date: Jul 2008
Location: New Orleans, LA
Posts: 27
possibly the throttle position sensor? i had a similar problem with my 190e. Look slightly behind the ignition distributor and along the drivers side cylinder head for a 3 pronged plug. Unplug the connector and get your multimeter set to continuity. Put one probe on the center pin and the other on one of the outer pins. With the throttle closed, one pin should have continuity. With the throttle wide open, the other outer pin should have continuity with the center pin. Without this signal, the computer does not enter the idle circuit and thus, a high idle will result.
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  #3  
Old 02-26-2010, 10:08 PM
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Get a can of gumout carb cleaner. Spray it around the intake manifold while running. If you get a change in speed, you have vacuum leaks in your air distribution hoses. About a hundred bucks for everything and a few hours to change them.

Could be worse too - may need to change round intake manifold seals. Big job.

If that is not the problem:

While running, pull the ovp. If idle goes up further, not the ovp.

While running, pull the idle speed control located under the passenger side carpet on the metal piece attached to the floor, near the top, small black box. If idle goes up, not the control.

All that's really left then is the idle control valve, located on top of the engine just behind where the thermostat is.

A two wire connector is attached to it, and a 10mm head nut holds it in place.

With wires disconnected, should read about 4-5 ohms. Remove the valve. Shake it, it should rattle a bit. If not, clean the internal parts with that gumout you used earlier. Will take quite a bit of cleaning.

If that doesn't fix it, then find a known good valve and swap it out, before you spend a couple of hundred bucks on a new one.
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  #4  
Old 02-28-2010, 11:39 PM
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Quote:
Originally Posted by waybomb View Post
Get a can of gumout carb cleaner. Spray it around the intake manifold while running. If you get a change in speed, you have vacuum leaks in your air distribution hoses. About a hundred bucks for everything and a few hours to change them.

Could be worse too - may need to change round intake manifold seals. Big job.

If that is not the problem:

While running, pull the ovp. If idle goes up further, not the ovp.

While running, pull the idle speed control located under the passenger side carpet on the metal piece attached to the floor, near the top, small black box. If idle goes up, not the control.

All that's really left then is the idle control valve, located on top of the engine just behind where the thermostat is.

A two wire connector is attached to it, and a 10mm head nut holds it in place.

With wires disconnected, should read about 4-5 ohms. Remove the valve. Shake it, it should rattle a bit. If not, clean the internal parts with that gumout you used earlier. Will take quite a bit of cleaning.

If that doesn't fix it, then find a known good valve and swap it out, before you spend a couple of hundred bucks on a new one.
I had this problem and I gave my idle control valve a good cleaning with some carb cleaner and it settled my idle down.
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  #5  
Old 03-01-2010, 12:54 AM
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you know i have just the opposite problem,my car idles on the low side when warmed up,then the idle surges,up and down,so low that the engine stalls while turning the wheel at real low idle.the idle control valve is brand new.also i replaced all the vacum line to the fuel pressure regulator.i also replaced a cracked valve cover air breather.what else could be causing the idle to race up then down?thanks everyone.
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  #6  
Old 03-01-2010, 01:10 AM
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Posts: 347
quick note the electro hydraulic actuator is defective and unplugged,will this effect the idle? i do know its only used for cold starting.

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