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Old 10-17-2002, 11:59 PM
Registered User
Join Date: May 2000
Posts: 376
Unstable idle on a 16V. Where to look?

My wife`s 87 190e 16V has issues. Starts immediately when cold and idles fine while warming up.

(I believe the mixture specs are different than what Stevebfl describes on the 300e as when I turn the mixture screw clockwise to richen, the duty cycle number increases (?). I currently have it set at 55% and am seeing a zero to negative two millamps at the EHA which I think would correspond with a slightly rich mixture (does this make sense?)

Anyways after I adjust it when its hot, it idles at a nice steady 900 rpm. Go for a drive around the block and return and its idling at 1200 rpm. Turn it off and restart, and it almost always idles near 900 rpm. Drive around the block and return....back to 1200 rpm? It also has some moments when you come to a stop where it nearly stalls or stalls. But once it starts to idle at 1200 rpm, it won`t stall which is understandable.

Any ideas? The decell switch is ok, throttle position swith is making contacts a idle and full, air idle valve is new, OVP is fine since I am seeing ma at the EHA. NO vacuum leaks. valves adjusted. Air flow position sensor appears to be within spec. with a nice linear increase to approximately 5v. I know these have been problems with these car and are prone to causing high idles. Should I focus my attention there? What else could cause the idle to change???

Thoughts? Condolences? Prayers?

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Old 10-18-2002, 02:49 PM
Dan Rotigel
Posts: n/a
possible reason, no solution

i have been told that a high idle such as you described (one that mysteriously goes away when the key is turned off) is caused by a component going bad as the car warms up. The ECU gets the message that the unit is FUBAR'd, and uses the last known 'good' value from that component. This value says that the idle should be at 1200 rpms. BUT when you kill the power to the ECU (by turning the car off), it resets itself and the process starts over when the component again sends a bad signal and the ECU uses the last known good value... I think this is called fixed operational mode. Check out this thread

Fast idle when warm, 2.3-16v

where benzmac tells me to send the car to a shop. Rather against the purpose of asking the board, but still. My problem actually went away after a time, so I haven't followed it up. Perhaps check the components WHILE the car is idling poorly?

Best of Luck

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Old 10-18-2002, 05:17 PM
Join Date: Mar 1999
Location: Tucker, Ga USA
Posts: 12,153
A defective air flow sensor will send the fuel ECU into fixed mode FOM. Try search on this subject. Benzmac had that problem on his personal car!!
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Old 10-19-2002, 11:20 AM
Registered User
Join Date: May 2000
Posts: 376
Dan and Doc,

Thanks for the insight. I am going to have to get the car the behave poorly and recheck the main component values and compare.

I drove it yesterday and it is stalling quite a bit. Did not idle at 1200 rpm??? When coming to a stop the idle drops to 600 rpm and slowly drops to zero. Sometimes it catches itself and jumps to a normal idle, but not often.

I have checked the decel switch and its working. I also checked the air flow meter both when it was idling at less than 600 rpm and normally. Its sending about 1.0 to 1.1 volt to the computer under both instances.

I guess its time for more investigating.

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