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  #1  
Old 05-13-2003, 12:54 PM
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Return of idle problems - '91 190E 2.3

Idle problems saga continues - thought I'd taken care of all the problems, apparently not. Behavior had been normal for about 2 mo. since last time idle system worked on, before new problems appeared.

Symptom - for several days, engine will start and either go to 1500-2500 rpm and stay there, or start up at 500 rpm. When at the high idle, turning engine off then on may cause the same behavior, or a return to normal fast idle of 1000 rpm, dropping to 700 in a few seconds. Note no 'blipping' throttle or anything other than turning off then on affects the problem, even after driving off. Then, for several days, startup behavior may be normal.

Repairs - All idle circuit hoses replaced, as well as bottom of airflow meter plenum. All vacuum hoses inspected - no leaks found. Idle air valve cleaned - alcohol. OVP replaced. Fuses and holders burnished. Not sure where to turn next:

- Possibly the idle air valve is sticking and needs more potent cleaning? I would guess I could check voltage to the valve - if it doesn't drop low when the high idle occurs, I imagine the valve is not the problem.

- Bad NEW OVP? Anyone run into this?

- Bad ECU? I sure hope not!

- Bad air flow meter pot? I would think this could screw with the mixture, but isn't idle speed determined by engine rpm sensing in closed loop idle control?

- Bad rpm sensor? Wouldn't this likely cause problems at all speeds?

Any sage advice would be appreciated.

Steve
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  #2  
Old 05-13-2003, 05:22 PM
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bump..
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  #3  
Old 05-13-2003, 05:37 PM
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Steve

I remember all the work you did before .... I had hoped you would not be posting on this topic again!

First question: Is it at all related to outside temps? Does it change in the evening verses during the day .... type of thing?

Haasman
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Old 05-13-2003, 05:53 PM
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Hi, Haasman - when I originally had the problem, I thought it was temperature related, but I don't think so anymore. It can happen first thing in the morning, or when I'm ready to head home at 5 pm. It is more likely to happen when the engine has been off for a bit, though. It is behaving just the way it did when replacing the OVP stopped the symptoms. That was only a couple of months ago though.

Steve
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Old 05-14-2003, 11:15 AM
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Any suggestions?
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  #6  
Old 05-14-2003, 01:15 PM
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I did a search using the words "2.3 idle" and came up with quite a few postings with the idle control valve as being mentioned a lot.

Haasman
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  #7  
Old 05-14-2003, 10:56 PM
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Did you check the two idle signal switches for proper operation - the microswitch on the linkage and the throttle position sensor on the butterfly valve (buried under the air flow sensor housing)?
The latter has a three pin connector on about a six inch pigtail, with the connector easily accessible in a harness channel above the inlet manifold. One circuit is ground, one should show continuity at idle, and the other at WOT. If this switch does not show continuity between two of the pins with the throttle pedal in the idle positon, the engine will not idle properly because the ECU is not receiving the proper idle signal.

About a year ago when my idle behavior went haywire, I found that the TPS was not giving the idle signal and solved the problem with a thorough maintenance of the throttle linkage -took it partially apart, cleaned, lubricated, and adjusted. Mercedes calls for lubricating the linkage with some specific hydraulic oil with every oil change. I did this, but gave the visible roller joints a quick shot of WD-40 in lieu of their esoteric lubricant recommendation. The ball and socket joints further down are virtually impossible to lubricate without taking them apart, so time and dirt take their toll on them and eventually the linkage misbehaves.

Mercedes deserves a lifetime "Rube Goldberg" award for taking a function as simple as throttle valve control and designing as complicated a linkage as could be imagined to control it. I thought the bellcrank on an old FI Sting Ray was a PIA, but Mercedes takes the cake!

Duke
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  #8  
Old 05-15-2003, 08:46 AM
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Steve,

Have you checked your AFM pot? If it's showing infinity at idle or wild resistance variations at idle that could be the problem.

Also make sure you check the zero position of the AFM plate - key on. Exactly top of the tiny 2mm vertical cylinder in the AFM, position to check is furthermost from fuel distributor.

After that depress the plate to basic position (should be approx 0.1-2mm free travel - must have free travel) where you should feel the resistance increase as it contacts the control plunger. During this free travel you should see very little change in resistance at the AFM pot.

If the zero position is out it will affect mixture and the idle can do VERY weird things - trust me I know. To tune mine I set zero position, lean her out till basic is at the very bottom of the vertical cylinder (2mm), then richen her up (works out about .5mm on mine - or 1.5mm down the cyl) till she will start, then I being tuning her via Duty Cycle/EHA current.

As Duke mentioned check the throttle switch and the deccel microswitch for correct operation.

Cheers,
Neil
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Old 05-15-2003, 11:17 AM
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Duke, I checked the throttle linkage microswitch, and it's action seems consistent. I did not check the TPS - that much disassembly will require more time than I'll have for awhile unless the car becomes undriveable.

However, I didn't think these would be the problem. My reasoning is this: without touching the accelerator, the idle problem ultimately responds only to one thing I can pinpoint - turning the engine off then back on, which should not move the throttle linkage. It specifically does NOT respond to 'blipping' the throttle linkage. My reasoning could be faulty - current surges through switches could be temporarily cleaning contacts, or other such weird issues. Still, I have put this possibility below the prime suspects.

Neil, I did check the pot, and behavior is less than ideal at small displacements. My ohmmeter shows that contact may be poor, but not to the extent that it goes 'open'. Still, I would think that a high idle would be a symptom of too much airflow, and the flow meter should only affect mixture. I know that with other rpm monitoring idle systems you can adjust the mixture over a wide range without affecting idle speed, and my impression is that is true too of these CIS systems.

Too much airflow points me to the idle air control valve, but I had cleaned it previously, and the problem that corrected (rough idle and occasional stalling) is quite different from the current symptoms. It also seems unlikely turning the engine off then on would be the only thing that affects the problem if it were this device, or any other air 'leak'.

Also, I am factoring in the fact that after extensive futzing and replacing parts, the idle had returned to normal - for a month or so. The last thing I replaced was the OVP, and the symptoms now tell me I have a confused ECU. It is getting an incorrect signal for engine rpm OR it is trying to control engine speed but can't (air leak, sticky idle air valve) OR the brain is scrambled (bad new OVP or bad ECU). Am I offbase in this line of reasoning?

Thanks to those who have responded, and I WILL eventually be checking more into the airflow meter and throttle position sensor, even if they are not the current problem.

Steve
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  #10  
Old 05-15-2003, 11:56 AM
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Quote:
Originally posted by sbourg
I did not check the TPS - that much disassembly will require more time than I'll have for awhile unless the car becomes undriveable.

No disassembly is required to check the TPS other than disconecting an easily accessible connector to run the continutity check. Sounds like you've thrown a lot of time and money at the problem, and your symtoms could be caused or certainly contributed to by a misbehaving TPS.

Duke
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  #11  
Old 05-15-2003, 08:03 PM
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Hi Steve,

Have you checked the zero position with the key on? It's like a 1 minute job to remove the air filter and see where the plate is. AFAIK if the plate is too high - which is VERY common it seems - the resistance from the pot is still the same _above_ the correct zero position as to when it is in the correct zero position. The same resistance should be shown all the way down to the basic position - the bottom of the vertical cylinder..

Now, if the plate is high and above the top of the vertical cylinder - the correct zero position - IMO there will be additional, unmetered airflow entering the engine. This (again IMO) will result in a high erratic idle. The AFM being calibrated to expect a certain leak when the throttle plate is in the zero-basic position only.

FWIW you can test the throttle position switch at the harness pickup located on the intake manifold runners near cyl 1 - you can't actually check it at the throttle body itself, it's sealed - ie no plug.

HTH
Neil
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  #12  
Old 05-19-2003, 11:25 AM
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Neil, if I understand you correctly, I don't think the rest position of the airflow meter should be significant. Even at idle, airflow should cause a significant deflection of the plate due to pressure differential between the top and bottom surface. Of course, there could still be a mechanical misadjustment of the plate position relative to the plunger into the fuel distributor - or the connection to the feedback pot. In my case, though, the key datum is that turning the engine off then back on usually restores normal performance - or in any event it seems to be the only thing that will. I don't see that affecting the mechanics of the sensor plate, nor - in the case of Duke's suggestion - the TPS.

I returned to the issue last night after a weekend away camping, and I removed the new OVP relay. I disassembled it and checked for problems. The relay contacts are hidden inside an internal case, but I would think they should not yet have problems. Several solder connections looked suspicious, though - problems on the assembly line. I reflowed these with fresh solder, repackaged, and reinstalled. I will report back in a week or so on my conclusion, but a short errand in the evening and the drive to work this morning proceeded with completely normal operation.

Steve
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