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High Idle

Post here for mechanical and engine topics such as fuel issues, transmission problems, rough idle, exhaust, electrical issues, etc

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angad22
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Location: Newark CA

High Idle

Post by angad22 » Tue Feb 20, 2018 2:55 am

I would like some advice regarding my euro 1980 635csi M90. The car idles high and when I adjust the throttle bypass to lower the idle it stalls after you accelerate and leave the pedal. There is a drop in RPM from 1200 to 800 and then it stalls. I have changed all vacuum hoses, boots, replaced all ignition parts including a new TPS, AFM, fuel pump, filter, FPR. I have adjusted the TPS for continuity and just don't get it what it could be. When my mechanic got it to pass smog he blocked the FPR hose as he mentioned that fuel pressure was low. Looking around I also noticed that the distributor vacuum advance and retire hoses were in the wrong place so swapped them to the correct location and then plugged them and checked the ignition timing to correct spec. Any suggestions would be helpful.
Angad

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Brucey
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Re: High Idle

Post by Brucey » Tue Feb 20, 2018 7:34 am

you may have multiple problems there. However the #1 thing to do is to reset the throttle body so that there is a 0.020" clearance under the butterfly on the idle stop. (I bet it is less than that at present.) Then reset the TPS so that the idle contact is made when the throttle is fully against the stop screw and not before.

The symptoms you describe are exactly like an engine where the ICV has to open nearly full to make idle (because the TB is badly set), and there isn't enough time for the ICV to open when instructed so the engine stalls. However there may also be a faulty ICV that isn't opening fast/fully enough, or bad air leaks that afflict some cylinders and not others, leading to poor fuelling at idle.

If you have low fuel pressure then you have other problems (check pump(s) filters, FPR); blocking the compensation hose to the FPR is a right bodge and the basic fuelling will be way off, only saved by the O2 sensor closed loop when the engine is hot. The fuelling could be undriveably bad until the engine warms up.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

ron
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Re: High Idle

Post by ron » Tue Feb 20, 2018 10:06 am

angad22 wrote:
Tue Feb 20, 2018 2:55 am
Any suggestions would be helpful.
Put your location in your profile as requested by admin.
They are ALWAYS rustier than you thought!!!!!!
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'85 M #207
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GRNSHRK
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Re: High Idle

Post by GRNSHRK » Tue Feb 20, 2018 9:14 pm

Not to highjack this thread, but I do have a question for Brucey, more of a clarification :-?

here's the quote [-(
However the #1 thing to do is to reset the throttle body so that there is a 0.020" clearance under the butterfly on the idle stop.
I think this infers that the TB butterfly plate still needs to close a little, in order for the TPS idle switch to turn on, true :-k

Also, if this is a stock M90, ain't gonna be no O2 sensor [-X

angad22, does this problem go away once the car is warmed up :-"

If yes, sounds like an issue with the aux air valve, perhaps it's sticking or something when you blip the throttle and let off, causing the engine to stall :-&

Since you didn't mention changing this part out, it's gotta be the reason #-o
:mrgreen:
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Brucey
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Re: High Idle

Post by Brucey » Wed Feb 21, 2018 1:55 am

GRNSHRK wrote:
Tue Feb 20, 2018 9:14 pm
Not to highjack this thread, but I do have a question for Brucey, more of a clarification :-?

here's the quote [-(
However the #1 thing to do is to reset the throttle body so that there is a 0.020" clearance under the butterfly on the idle stop.
I think this infers that the TB butterfly plate still needs to close a little, in order for the TPS idle switch to turn on, true :-k
Not really. The TB linkage wears over time so that the butterfly ends up almost fully closed. This creates all kinds of running/idle issues even if the TPS is adjusted to suit. The correct setup procedure is to set the throttle stop to give a 0.020" gap between the butterfly and the bore, then to adjust the TPS to suit that.

Adjusting the TB to suit the current position of the TPS is putting the cart before the horse, as it were.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

angad22
Posts: 10
Joined: Sun Nov 29, 2015 12:43 am
Location: Newark CA

Re: High Idle

Post by angad22 » Wed Feb 21, 2018 6:13 pm

I adjusted the butterfly as suggested and checked the gap between the bore and butterfly with a spark plug feeler gauge. It started up with a slightly high idle so I adjusted the throttle bypass screw to lower it a bit. After it warmed up it started to idle well so further adjusted the bypass screw to get it at 1000 RPM. The acceleration was good and it did not stall when I left the pedal. I turned off the car and the started it up again and everything was great. Then I let the car sit for about 5 minutes and then started it again and this time it did not start without a tap on the gas pedal. I tried this few times and same results. Here are some questions that i have:
- Is the bypass screw adjusted to low means should I open it up to let more air pass?
- Is it related to the fuel pressure (as the mechanic blocked the FPR vacuum hose to increase the pressure so does that mean that there is too much fuel at the injectors)?

The FPR, Additional Air Slide Valve and all injectors have also been replaced with the correct matching parts.
Angad

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hornhospital
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Re: High Idle

Post by hornhospital » Wed Feb 21, 2018 8:34 pm

If the FPR has been replaced there's no reason not to connect the vacuum line to it. It needs the FPR to be working for the ECU to balance the air/fuel mix. When the FPR has no vacuum it defaults to full pressure in the rail, and that will cause a rich condition the ECU cannot overcome.
Ken Kanne
'84 633CSi "Sylvia"; '85 635CSi "Katja";'87 325is "Odette"; '93 325is "Elvira"; '95 M3 "Ashlyn"

angad22
Posts: 10
Joined: Sun Nov 29, 2015 12:43 am
Location: Newark CA

Re: High Idle

Post by angad22 » Thu Feb 22, 2018 8:40 pm

I installed new hose on FPR today. During cold start everything was fine. I switched the car off and started it again works great. Left it for few minutes and then same issues like before. I need to tap on the accelerator pedal to get it going.

Then I removed the connector on the TPS and now the car starts up right away. When I connected the connector back the car has same issue - need to tap on the pedal to get it going. I also tested by installing the old TPS that I removed from the car and got same results so looks like the TPS is not bad so what could it be. Anyone experienced similar issue before?
Angad

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Brucey
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Re: High Idle

Post by Brucey » Fri Feb 23, 2018 6:34 am

a 1980 635CSi could either have L-jet or Motronic fuelling. From your mention of a distributor with a vacuum advance line I take it that you have L-jet fuelling. The stuff to do with the TB and the FPR is common to both systems but after that there are major differences.

One special feature is that the fuel pump is only activated by the AFM flap moving. If the flap is sticky or there is a bad air leak that bypasses the AFM flap then the fuel pump may not run on cranking.

If there is a leak in the fuel line or there is a fault in fuel delivery line (eg check valves that are not working) then there might not be liquid fuel in the fuel rail on a hot restart.

In addition to the above there are many devices (and variations thereon) that let more or less air into the engine when the idle contact is made, to stabilise idle. These are likely to be closed when the engine is hot, thus the engine may genuinely be pulling less air through the AFM on a hot closed throttle restart, and this may make the difference as to whether the fuel pump runs or not.

The vagaries of L-jet are give in more detail on the firstfives website; if you have not looked at this then it is worth a read.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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