byrdman454

The streets finally thawed out after a couple weeks and was able to get my '88 S4 out to drive. I have been having some surging problems a couple times this year, but today it seemed worse. Here are the symptoms:

When I started it up I noticed it idling about 1500 rpm...much higher than normal. I pulled the car out of the garage and let it idle a while. The car then started surging. It would be idling at 1500 for a few seconds, then drop to the correct idle for about 1 second, then back up. It was a very quick rpm change as if you turned the ignition off and back on again. It kept doing it even after it started warming up. There was no color to the exhaust at 1500 rpm, but every time it dropped and went up again it puffed some white smoke. The air temperature outside was in the high 30s/low 40s.

Normally I would take it out when this happens and floor it and the problem would go away. It seemed to keep coming back today but only when my foot was not on the gas pedal.

I checked the TEMP II sensor a week ago and it was at about 4.6 KOhms at about 40 degrees. Wally P. said this seemed reasonable.

Does anyone else have any suggestions???

Jay Wellwood

Could be a couple of other things to look at-

Idle Stabilizer - might be gummed up and needs to be cleaned/replaced.

LH Unit - might be starting to go out. From what I've read - it appears that the Idle circuit on the LH is the first part to go south before the whole unit dies.

hth-

byrdman454

Where is the idle stabilizer located? How do I test it?

Where is the LH unit located? How do I test it?

Thanks

928ssss

sounds exactly like what happens to my 88 when it gets warm. It runs great then I'll do a warm start and it'll surge up and down a few times and die. Sometimes it'll go away if I can keep throttle on it, otherwise it dies at stops.

Since it's not happening all the time, I'm now thinking maybe it the LH? Keep me posted. thanks.

Stan
928ssss

John Speake

yes, I'm afriad Jay is dead right on this one. Could be classic LH ECU failure.

I hope it's just that your car has been laid up, and the idle stabiliser is sticking.

You can't easily test the LH yourself. Try to swap out the unit with a known good one. If you can get to a garage that has a Bosch Hammer, they will be able to check out both the LH and the idle stabiliser itself quickly.

Jim bailey - 928 International

If you are not maintaining a stabile idle speed with any 87-95 928 it is quite often a faulty idle stabilizer valve no longer stabilizing the idle speed (85-86 usa is similar ). It is buried deep under the intake manifold and when the L H Brain tells it to it adjusts the amount of air bypassing the throttle buttery fly to hold a constant idle speed . Sometimes they get sticky "gummed up ?" and shooting some solvent WD 40 etc into a breather hose has freed them up . Other times the brushes which make the valve turn open to closed wear and fail and it must be replaced . It is also possible for the LH brain to have a problem with the idle control circuit BUT the stabilizer valve is far more likely to fail and no longer be able to maintain the correct constant idle .

Nicole

When my engine had its erratic idle problem - it was like randomly dropping cylinders - surging up and down was one of the secondary symptoms. It was like a macho man pounding the gas pedal at the traffic light, but without any action from my end.

However, the engine already had a new idle valve, ne knock sensors, new TPS, and all kinds of other new stuff. Swapping MAF or brain did not make a difference either. The whole thing was worse whenever the car sat for a few days, but it never really disappeared either.

Then Wally suggested I might have some "sticky valves". I was skeptical, and had a compression test performed. The cylinders were all over the place - huge differences. So the first thing I did was run heavy doses of Lubri Moly Valve cleaner. That brought the compression into a perfect range, and gave me a temporary improvement - until the car sat in the garage for 2 weeks without being started.

Following Wally's and Max's suggestions and recommendations, I had a Rislone treatment and oil change done. Since then, my idle is almost as smooth as butta - but it does get somewhat erratic again when I don't drive the car regularly. Then I have to do an Italian tune-up...

Hope this helps.

jwbeck17

I hope you don't mind, but I a resurrecting this thread, as it seems to match my current issues.

My car starts strongly and when left at idle, it does so fairly smoothly and comfortably. However, there are times when the throttle goes to about 1500 RPM is when the RPM surge and bounce with no change in the throttle position/pedal. If I let off the pedal, the car is likely to stall as it drops below idle and can't recover. I was able to record a video of the RPM bouncing when I had the throttle steady.

As I have redone and covered a lot of electrical work in a separate thread, I am thinking the possible culprits are:

• Throttle Position Sensor (has not been replaced yet)
• Idle Control Valve (has not been replaced yet)
• Faulty LH (unlikely as this was rebuilt and was tested in other cars, but driven around or tested at throttle)

I plan to do some ignition system tests from the Factory Manual that focus on the TPS and the ISV to see what I get. I also may want to test the LH in another car to see if it actually handles the throttle correctly.

Any other thoughts or suggestions of areas to check?

GregBBRD

Engine surge is generally a rich condition, an air leak, or a idle stabilizer.

Check the voltage output from the O2 sensor and see if it is fluctuating between .2 and .8 volts (hot and connected.)
Check the dampers and the FPR for fuel on the vacuum side. Use a small allen wrench as a "dipstick", as unless the diaphragms are super "blown", they won't gush fuel.
Smoke test for air leaks, works best, if you know where the potential "hidden leaks" are located.
You can rev the engine in neutral and let the throttle snap close. Generally you can hear the idle stabilizer "sucking" air and then hear the decrease of the amount of air, as it compensates for the sudden demand for idle air.

jwbeck17

Thank you for the list of checks to run, Greg. I do believe it is running a little rich due to some of the excess popping and crackling I am getting. My car no longer has the catalytic converter, so the burnt gas smell is pretty heavy from the exhaust. Will run the checks (I may have questions) and will let you know what I find.

jwbeck17

I want to make sure I am doing this test right, as I am not getting the desired result, which may be my issue. Here is image from the factory manual that shows how to test the Q2 sensor circuit:



So I have warmed up the car and have unplugged the O2 sensor below the CE Panel. Since this test is testing the sensor itself, I am testing the plug end closest to the center console (left plug), as that leads to the O2 sensor itself. The other end is part of the ignition harness and leads to the LH/EZK.

I placed one clip on the #1 pin on the O2 sensor connector and another pin to a ground point above the CE Panel and measured AC volts at idle and at throttle. I was reading 0.001 to 0.000 V no matter what I did.

Did I perform the test correctly?

Note that disconnecting the O2 sensor had no effect on the engine performance. Also note that the car has the catalytic converter removed, however I believe the O2 sensor is before the cats, so it should still operate correctly.

Let me know your thoughts.

jcorenman

You are doing it wrong, the O2-sensor signal is DC Volts. Also, do the test with the O2-sensor connected, otherwise LH will not be getting the signal and will be running open loop. The connector is rubber, just stick the meter probe into where the wire comes out.

If disconnecting the sensor had no effect on how the engine ran, then that is an indication that the O2-sensor is not working properly. Unless it is new or known good, replace it. It is a Bosch 13048, not expensive.

But first, as I mentioned in your other thread, if you haven't already done so then disconnect the battery for a moment to reset the LH. It may have adapted itself to the previous issue. Running on less than 8 cylinders puts a lot of extra O2 into the exhaust stream, which fools the O2-sensor into reporting that the mixture is quite lean. (Excess-O2 is only an indication of air/fuel ratio when the mixture is actually getting burned). The LH will then add buckets of fuel and remembers that it did so.

Also, I am not sure that appending a new question onto a decade-old thread is helpful. But maybe that's just me.

jwbeck17

I thought I was off, so thank you for confirming.

I had recently done a long battery disconnect on the battery after getting the cylinders working, but just did it again to be sure

I pulled back the rubber on the O2 sensor side to send a backprobe into where the black voltage wire is, and connected the other probe to ground. had the O2 sensor connected and car running measuring DC volts, but was getting odd readings between mV and V indicating that I wasn't getting a good signal, or that the sensor was shot. I then decided to backprobe LH pin 24 for the O2 sensor and had the other probe grounded. My measurements there were approx. 0.7-0.8V at idle, then it was unreadable at throttle, switching from V to mV again, like getting a poor signal.

Maybe i should just replace the O2 sensor then and recheck?

jwbeck17

I was able to take it for a drive which is a first. The RPM jumping that I had at throttle seems to have disappeared due to the PB Blaster spray into the ISV intake tube trick. (I get that I may need to replace that shortly.) At idle the car bounces between around 630 RPM to 730 RPM. That could be a faulty O2 sensor playing a part, so I'm going to replace that and see what happens.

jcorenman

The output from the sensor will be around 0.2 vdc when the mixture is lean and around 0.8 vdc when the mixture is rich. There is a nice graph here showing sensor voltage and also the relationship between mixture and emissions. So the 0.7-0.8v reading that you saw at idle would be rich, consistent with a stinky fuel smell in the exhaust. The switching between mV and Volts is your meter auto-ranging. You need to lock the meter in one range or the other.

The actual voltage is unimportant, all the LH cares about is more than 0.5V (rich), or less (lean). With the sensor connected, and everything is working normally, you should see the sensor output switching between 0.2 and 0.8V as the LH adjusts fuel up and down in response to the rich/lean reading from the sensor.

If the O2-sensor is not connected then the LH just calculates fuel from the maps with no fiddling.