Surging and stalling but only when cold
Thread Starter
Track Day
Joined: Sep 2010
Posts: 16
Likes: 2
From: Waterford Ireland
Surging and stalling but only when cold
Hi folks, I'm looking for help with an idle issue with my 1989 928.
The car is U.K. spec r.h.d. non cat S4.
I’ve read through pages of idle issue problems on RL and tried a few fixes but to no avail.
The problem is that the car will not idle properly on the first start up from cold.
Once warm it idles fine at about 650rpm, but for the first start every morning (or whenever the engine is cold) it will fire up fine, but will surge up and down between about 800 and 400rpm.
If I don’t hold the gas pedal to keep it at 1000rpm or so, the motor will stall.
I can overcome the problem by holding her at about 1000rpm for about 3 or 4 mins and if I then let the pedal up slowly she will be o.k. and will idle steady at about 600 rpm. And as I said, once warm there’s no issue.
So far I’ve had the M.A.F. rebuilt by John Speak (Thanks John, top job!) which really transformed the cars performance but did not help with the idle issue.
I’ve adjusted the remote idle pot which on my car is above the CE panel in the footwell, to 382 ohms which made no difference and yesterday I removed the MAF and sprayed/blew wd 40 and carb cleaner, into a tube which I inserted into that hose elbow in the fitting below the M.A.F. which leads to the idle control valve and let it sit overnight, but this had no effect on the problem either.
It’s not a huge problem but there is an issue if I need my wife’s Audi 7 seater for the school run or whatever, and she has to use my car, she either revs the ****e out of it from cold
Or, keeps stalling and starting it over and over again.
She is putting pressure on me to get a “normal” car!
The car is U.K. spec r.h.d. non cat S4.
I’ve read through pages of idle issue problems on RL and tried a few fixes but to no avail.
The problem is that the car will not idle properly on the first start up from cold.
Once warm it idles fine at about 650rpm, but for the first start every morning (or whenever the engine is cold) it will fire up fine, but will surge up and down between about 800 and 400rpm.
If I don’t hold the gas pedal to keep it at 1000rpm or so, the motor will stall.
I can overcome the problem by holding her at about 1000rpm for about 3 or 4 mins and if I then let the pedal up slowly she will be o.k. and will idle steady at about 600 rpm. And as I said, once warm there’s no issue.
So far I’ve had the M.A.F. rebuilt by John Speak (Thanks John, top job!) which really transformed the cars performance but did not help with the idle issue.
I’ve adjusted the remote idle pot which on my car is above the CE panel in the footwell, to 382 ohms which made no difference and yesterday I removed the MAF and sprayed/blew wd 40 and carb cleaner, into a tube which I inserted into that hose elbow in the fitting below the M.A.F. which leads to the idle control valve and let it sit overnight, but this had no effect on the problem either.
It’s not a huge problem but there is an issue if I need my wife’s Audi 7 seater for the school run or whatever, and she has to use my car, she either revs the ****e out of it from cold
Or, keeps stalling and starting it over and over again. She is putting pressure on me to get a “normal” car!
Rennlist Member
Joined: Feb 2002
Posts: 7,060
Likes: 42
From: Cambridge England
If it's surging up and down, that 's a good sign as that means the LH ECU idle controller and the ISV are both working. But for some reason the loop can't reach equilibrium.
Does the car start easily ? If not, then the temp2 sensor may be out of spec.
For my rebuilt MAFs the CO pot should be set to more like 300 ohms. But the best way is to adjust the idle CO with a warm engine for a tailpipe CO of about 1.0 to 1.5%. The idle CO has to be correct for the car to idle correctly. But being as you had this problem prior to the MAF swap when the car was clearly running very weak, I doubt idle mixture is the fundamental problem..
Does the car start easily ? If not, then the temp2 sensor may be out of spec.
For my rebuilt MAFs the CO pot should be set to more like 300 ohms. But the best way is to adjust the idle CO with a warm engine for a tailpipe CO of about 1.0 to 1.5%. The idle CO has to be correct for the car to idle correctly. But being as you had this problem prior to the MAF swap when the car was clearly running very weak, I doubt idle mixture is the fundamental problem..
Instructor
Joined: Jun 2010
Posts: 169
Likes: 0
From: London, UK
Had the same issue for 2 years.
Swapped the MAF, replaced the Temp II sensor, swore a lot and made the pilgrimage to Cambridge to see Guru Speake. Interesting suggestions from John that didn't seem to resolve anything. He listened to the TPS and assured me it was making the right noises. He suggested the ISV was worth looking at but was doubtful that was the problem.
In desperation, I eventually took the bull by the horns and replaced both the ISV and the TPS. Problem solved!
I suspect the throttle position switch was at fault as, even though it was making the appropriate clicks, when cold it wasn't making proper contact.
Sorry about my unscientific account by I'm still learning about the mysteries of 928 technology.
Swapped the MAF, replaced the Temp II sensor, swore a lot and made the pilgrimage to Cambridge to see Guru Speake. Interesting suggestions from John that didn't seem to resolve anything. He listened to the TPS and assured me it was making the right noises. He suggested the ISV was worth looking at but was doubtful that was the problem.
In desperation, I eventually took the bull by the horns and replaced both the ISV and the TPS. Problem solved!
I suspect the throttle position switch was at fault as, even though it was making the appropriate clicks, when cold it wasn't making proper contact.
Sorry about my unscientific account by I'm still learning about the mysteries of 928 technology.
Rennlist Member
Joined: Feb 2002
Posts: 7,060
Likes: 42
From: Cambridge England
Hi Glyn,
Glad you fixed the problem. I'm assume we put it on a diagnostic tester to verify the idle switch was OK ?
At least when hot..... another case of the fractured solder joint syndrome I guess.
Glad you fixed the problem. I'm assume we put it on a diagnostic tester to verify the idle switch was OK ?
At least when hot..... another case of the fractured solder joint syndrome I guess.
Instructor
Joined: Jun 2010
Posts: 169
Likes: 0
From: London, UK
Precisely John. As you pointed out at the time, there wasn't a lot you could diagnose while the car was hot.
I seem to remember it was you who described the "desperation syndrome" where you eventually replace anything remotely connected to the problem. I guess it doesn't hurt to replace these parts anyway when the car is over 20 years old.
I seem to remember it was you who described the "desperation syndrome" where you eventually replace anything remotely connected to the problem. I guess it doesn't hurt to replace these parts anyway when the car is over 20 years old.
Thread Starter
Track Day
Joined: Sep 2010
Posts: 16
Likes: 2
From: Waterford Ireland
Does the car start easily ? If not, then the temp2 sensor may be out of spec.
I will try to get a local repair shop to test the CO for me, and I'll re set the idle pot to 300 ohms.
Other than listening for the click, is there another way to test the throttle position switch?
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Rennlist Member
Joined: Feb 2002
Posts: 7,060
Likes: 42
From: Cambridge England
Here's a scruffy picture of the LH ECU connector, you need to connect a continuity tester/buzzer across pins 2 & 5 with the throttle closed. You should get good continuity. Open the throttle slightly, it will go open circuit. Check closed and open ia few times more to make sure it is repeatable.
Good idea to get the ilde CO checked, it should be near enough with the 300 ohms setting of the idle pot that the MAF was sent to you with.
Good idea to get the ilde CO checked, it should be near enough with the 300 ohms setting of the idle pot that the MAF was sent to you with.
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Instructor
Joined: Jun 2010
Posts: 169
Likes: 0
From: London, UK
For what it's worth, I dug out the thread when I was going through this agony. Unfortunately, it only applies if you have a digital dash. If you don't, ignore what follows.
"My '90 S4 has demonstrated all the same symptoms for two years and the problem's particularly chronic in winter weather. Interestingly, if I start the car without touching the accelerator, it idles fine. As soon as the pedal is depressed the idle goes wild.
In response to the expert advice on this forum I've tested and replaced the usual suspects, to no avail. In fact, it's getting progressively worse.
Why does it only happen when the car's cold? What bits are sensitive to cold temperatures? I've checked and replaced the Temp II sensor twice with no discernible improvement. The MAF was replaced recently and there appear to be no vacuum leaks. The throttle mechanism seems well lubricated and working as expected.
The only things I haven't tested are the idle switch and the ISV. Didn't know how to without taking them apart, which would be beyond my capabilities.
Then I saw a posting from Wally Plumley on a simple test for the idle switch. Drive the car at speed, set the dash to Instant MPG and take foot off accelerator pedal. The dash should read 90 mpg.
Mine fluctuated between 9 and 21! When the car warmed up and the idle steadied the dash read 90.
Evidently, when the switch was warm it closed normally. Once opened when cold it would fail to close until up to working temperature."
Hope this helps.
"My '90 S4 has demonstrated all the same symptoms for two years and the problem's particularly chronic in winter weather. Interestingly, if I start the car without touching the accelerator, it idles fine. As soon as the pedal is depressed the idle goes wild.
In response to the expert advice on this forum I've tested and replaced the usual suspects, to no avail. In fact, it's getting progressively worse.
Why does it only happen when the car's cold? What bits are sensitive to cold temperatures? I've checked and replaced the Temp II sensor twice with no discernible improvement. The MAF was replaced recently and there appear to be no vacuum leaks. The throttle mechanism seems well lubricated and working as expected.
The only things I haven't tested are the idle switch and the ISV. Didn't know how to without taking them apart, which would be beyond my capabilities.
Then I saw a posting from Wally Plumley on a simple test for the idle switch. Drive the car at speed, set the dash to Instant MPG and take foot off accelerator pedal. The dash should read 90 mpg.
Mine fluctuated between 9 and 21! When the car warmed up and the idle steadied the dash read 90.
Evidently, when the switch was warm it closed normally. Once opened when cold it would fail to close until up to working temperature."
Hope this helps.
Rennlist Member
Joined: Feb 2002
Posts: 7,060
Likes: 42
From: Cambridge England
The idle speed is set electronically with the LH ECU controlling the ISV. To activate the idle control loop it is important that the throttle vables aren't set too tight or they will prevent the throttle closed switch operating. (it should click reliably as the throttle is opened from idle, and click again when the throttle is closed) It is that switch that activates the loop.
Thread Starter
Track Day
Joined: Sep 2010
Posts: 16
Likes: 2
From: Waterford Ireland
It seems the throttle position switch may be the problem.
I carried out the electrical test for the TPS on the LH harness plug and I'm getting an open circuit between contact 2 and 5 when the throttle is closed.
I can clearly hear the TPS itself "clicking" so I presume the unit itself is at fault.
Does the intake have to come off to get at the TPS?
I carried out the electrical test for the TPS on the LH harness plug and I'm getting an open circuit between contact 2 and 5 when the throttle is closed.
I can clearly hear the TPS itself "clicking" so I presume the unit itself is at fault.
Does the intake have to come off to get at the TPS?
Last edited by Hifive; Feb 4, 2013 at 12:57 PM. Reason: correction.
Rennlist Member
Joined: Feb 2002
Posts: 7,060
Likes: 42
From: Cambridge England
Suggest you do a sanity check to make sure you have the correct pins on the LH ECU plug. Check out the WOT switch using pins 3 & 5 (should be closed near full throttle)
This is because you do have to at least loosen the intake and lift it to access the switch.
This is because you do have to at least loosen the intake and lift it to access the switch.
Thread Starter
Track Day
Joined: Sep 2010
Posts: 16
Likes: 2
From: Waterford Ireland
Yeah, just reading Dwayne's article on his intake refresh and I see its kinda buried in there.
I should have known anyway as I had the intake off a couple of years ago and I now remember seeing the TPS at the time. I really don't want to go in there again!
I will definitely do a sanity check, but I did try it several times and got the same result, I didn't however try the wot switch on 3 and 5 so that gives me an excuse to have another go.
I see it can also be checked at the EZK plug on contacts 18 and 8, so I'll try that too.
Thanks for all the advice.
Mark.
I should have known anyway as I had the intake off a couple of years ago and I now remember seeing the TPS at the time. I really don't want to go in there again!
I will definitely do a sanity check, but I did try it several times and got the same result, I didn't however try the wot switch on 3 and 5 so that gives me an excuse to have another go.
I see it can also be checked at the EZK plug on contacts 18 and 8, so I'll try that too.
Thanks for all the advice.
Mark.