1988 Audi 90 idle oscillation
 

Greetings all, I am investigating the following:

Vehicle: 1988 Audi 90 4 cylinder. New Plugs, Plug Wire set, Distributor cap. No (apparent) vacuum leaks, good gas mileage, Clean ISV, most connectors disassembled, cleaned.

When car is warmed up, idle oscillates between 800 and 1200 RPMs about once a second. When this occurs, the control voltage to the ISV drops from about 4.0 (average DC reading, actual signal is AC) to zero. This allows the ISV to mechanically oscillate from closed to open resulting in the idle oscillation. I have thoroughly cleaned the ISV, and it works perfectly until after warm up. This suggests a bad sensor signal/connection telling the ECU to stop controlling idle ??? The temperature dependence has less than 100% correlation, but close.

Any insights, similar experience data would be helpful and most appreciated.


Thanks, Ian.

RE: 1988 Audi 90 idle oscillation
 

Did you try pinching one of the ISV hoses or disconnecting the electrical connector?

RE: 1988 Audi 90 idle oscillation
 

Pinching ISV hoses corrects problem, as this simulates a "closed" ISV condition, (4 volts results in a closed ISV which is OK with engine at operating temperature).

Yes, Electrical connector removal immediately results in this oscillation again as this simulates in a zero volts ISV control reading which I obtain during the oscillation.

I am looking for the answer as to why the ISV control voltage drops to zero... still a mystery...

Thanks Offramp.

RE: 1988 Audi 90 idle oscillation
 

What troubles me is the initial diagnosis this is very unusual and not easy to accomplish even when playing around with the much bigger throttle valve. The ISV has a much smaller opening running through a long hose and controls a much smaller volume of air. I think that the ISV valve is not the problem.

The ISV valve works on a square wave, and what you should measure is % duty cycle, or "on" state vs. "off" state. 100% duty cycle means it is always "on" in a single pulse width, a 50% duty cycle means that the square wave pulse is 50% in the "on" sate and 50% in the "off" state, etc.

I have seen Bosch ISV valves come in two flavors, three wire (older systems) and two wire. A two wire ISV should have a spring that keeps the valve closed (or open depending on the design.) I am not sure which one you have on your Audi. If you disconnect the electrical connector to the ISV and bypass the valve, the idle should be steady and higher. It shouldn't fluctuate/drift by more than ±50RPM. If you still have wild fluctuations then I doubt the problem is with the ISV. From my experience I have noticed that disconnecting the valve either causes the idle to drop, rise or no change at all.

RE: 1988 Audi 90 idle oscillation
 

Is the throttle switch working OK?

Do you still get 0V when you turn on the lights, AC, etc. or when the engine is cold?

RE: 1988 Audi 90 idle oscillation
 

Yes, I understand the ISV Signal is a square wave, not scoped it yet, however, 4.1 Vdc reading seems to close the valve entirely implying a 100% duty cycle, less seems to open it. - I will likely scope it on the weekend. As far as this ISV having a small maximum flow rate, with no control voltage, I can breathe through it virtually unimpeded, but fully closed, virtually no air passage... lots of air flow...

This ISV is two wire.

My latest theory is that the ISV is worn out, spring fatigue allowing the diaphragm to oscillate with no control voltage - I agree, research indicates that the removal of ISV control power should result in some stable idle state, higher or lower depending on engine temp.

Problem still remains that control voltage suddenly drops to zero (0% duty cycle)... in this state we have full unimpeded air flow through the ISV. Go figure???

Next test wil be to run the ISV from a variable power supply, establish idle, warm up, see if problem is gone. If so, we are back to failing control voltage.

RE: 1988 Audi 90 idle oscillation
 

Sorry, just saw your next question - no, all is fine and stable when engine is cold, lights have no effect. Points to bad sensor/heat sensitive connection???

Interesting point you bring up re the throttle (idle) switch. Although it clicks just off idle, I have yet to ascertain its stability, perhaps it is intermittent, this may cause the ISV control voltage to disappear, I will check it, thanks again.

New data as of this morning... Started car, ran perfectly from cold up to operating temperature, did not exhibit oscillation. Checked ISV control signal parameters, car warmed up:
Frequency - about 100 Hz
Duty cycle (Positive reference) - 30% to 40% (load dependant)
Calculated Pulse width (Positive reference) 3ms - 4ms (load dependant)

I then removed the ISV connector, oscillation started (actually, a more accurate description is 1000 - 1100+ RPMs, twice per second...). I replaced the ISV connector, oscillation continued. New ISV control signal parameters during this oscillation:
Frequency - about 200 Hz
Duty cycle - 70% to 75% (NOT load dependant)
Calculated Pulse width (Positive reference) 3.5ms - 3.75ms (NOT load dependant)

Next interesting fact - when oscillation is present, it can be reset by quickly turning ignition off, then back on (rapidly enough so that engine does not stall), This "resets" whatever state the ECU / ISV control voltage is in, car once again idles perfectly.

Finally, during normal idle control operation, moving the throttle off idle (= Idle switch open) does NOT cause the control voltage to change significantly or drop out...

So, the quest for idle goes on...

RE: 1988 Audi 90 idle oscillation
 

Final though for today:

Would it not be safe to assume that my ISV is indeed faulty, as with no control voltage, it should be able to hold a static idle speed, and does not?

Trouble is, I priced a new ISV, about $600.00 CDN... expensive mistake if I am wrong.

More research...

RE: 1988 Audi 90 idle oscillation
 

Any shorts between ISV terminals and housing? What is the resistance between the ISV terminals? For Part Number 034·133·455·B (Bosch 0208·140·512) it should be between 7-11 Ohms.

RockAuto.com has ISV for US$196 that probably works out to be about CD$300. Don't know if they ship to Canada. I know that my local salvage yard probably sells them for about $30 or less. However I doubt that ISV is the problem.

My experience with ISV has been a more common sticking rotary valve due to carbon and oil deposits. It is possible that the ISV is defective in some peculiar way and throwing the ECU into a chaotic state. However my gut feeling is that it is not the valve but an electrical problem because of the on/off/on condition. Could be an oxidized connector or bad ground point. What is the voltage at the connector going to the ECU? I have seen too many electrical connection problems on older vehicles where people replace a bunch of $$$ parts and never solve the problem. Right now I experiencing what suspect is a bad voltage regulator in my ’91 100Q and this problem sometimes can be corrected with the on/off/on trick! When I start my car after a long cooling period the voltmeter reads 11.5V and the motor starts and idles fine. The voltage will stay bellow 12V even if I race the engine to 3000+ RPM or warm the car for a few minutes. If I try to drive while the gauge shows less then 12V the voltage drops even further as the engine comes under load and I have very little go power. To keep it from stalling I have to step on the clutch, at that instant the voltmeter jumps up to 13.5+ volts and the idle speed drops slightly but now I if try to drive, the car has power. This problem doesn’t repeat if the car has not been driven for 1-3 hours.

RE: 1988 Audi 90 idle oscillation
 

No, no shorts to ISV housing. Resistance is about 8-9 ohms.

I tend to agree with the electrical ground/connector problem. Difficulty is, this is my son's car and hence not always available to me. we at present have no manual, so tracing electrical connections is a pain. I am considering purchasing a manual, but have not been able to locate one at my standard sources, there is always Bentley Publishers on the net, but seems expensive... still, I can't go much further efficiently without a manual, so I better get one soon.

For instance, do you know where the ECU can be found in this car, that would be my next source of investigation...?

... Interesting Voltage Regulator problem ... as you say, if it sometimes can be cured by on/off/on, sounds like an electrical connection problem - is the physical actuation of the clutch shaking some bad connection up? Also, on/off/on may in some ways reset ECU parameters. Does this problem reoccur immediately if you leave the battery disconnected for several minutes and then re-connect? That may reset any "learned" parameters to factory settings?

By the way, many thanks for your inputs, they are much appreciated.

Ian.