Question about 1999 Audi A4

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I have a Audi A4 1998 1.8 Automatic, when i turn on the air conditioning the car rpm surge up and down, it also does this when i first start the car , the rest of the time the car is fine.

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Hi, it sounds like your throttle valve is having trouble keeping the idle. First check for a clogged PCV valve, which may not allow sufficient auxiliary flow for the throttle controller to manage the idle. The valve is near the throttle as shown in the pic below. Loosen the clamps, take out the valve, and shake it to make sure it isn't stuck closed. If the PCV valve is OK, the problem may be with the feedback (throttle position circuit) circuit in the throttle valve controller. Testing the throttle valve controller requires special test equipment that your Audi/Volkswagen service center should have on hand. Please let me know if you have more questions, and thanks for using FixYa.

I have a Audi A4 - jturcotte_1783.gif

Posted on May 29, 2011

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As a car gets over 10 to 12 years old, it tends to like to be warmed up before being driven or anything like that. With age, the engine wears in to running at a normal temperature -- and as metal heats to this temperature it expands a bit. This happens more with age. Be patient with your car and let it heat up to normal running temperature before you do anything (like driving or turning on the AC unit, etc.). When it gets up to normal temp, is it still having the revving issue at idle (without the AC)? If not, then your engine parts have stretched over time -- it happens with all older vehicles -- just give it some time to warm up before you drive it and you will extend the life of your car by quite a bit.
Hopefully, that will resolve the issue of the jumping RPMs at startup ... so lets move on to the AC RPM jump. The AC unit takes a lot of power from the engine (generally, it uses the full HP and torque of one cylinder of your engine). This means that when you have the system turned on, the compressor motor will eat about 1/4 of your car's running power. Due to this fact, smaller engines (like a 1.8 L, 4 cylinder engine) will need to work a lot harder to run the AC than a V6 or a V8. If you listen to your engine closely, is there a click just before the engine starts to increase RPMs when you turn on the AC? This click should be the AC compressor motor turning on to cool the air. Just before the RPMs drop back to normal, there should be a similar clicking sound -- the compressor turning off. If you hear these clicks at the beginning and end of the compressor's run cycle, then your RPM jump is actually normal. You wouldn't notice it while driving because the engine is already running fast enough to run the AC ... but at idle you will notice the difference because the AC unit takes SO much power. The engine should not jump more than 750 to 1000 RPMs to run the AC unit, and should only do so when the compressor is running. In order to save fuel and make your AC unit more efficient, the compressor does not run constantly, so when you turn it on you should have a 15 to 45 second cycle -- click REV .... click idle ... click REV ... click idle.
This rev cycle should not be a problem (and is actually normal for small engines) unless it is not consistent or if it revs way too high. If you still believe that it is a problem, have a mechanic look at the car (especially the AC system) to give you a proper diagnostic (without actually seeing and hearing the car it is difficult to be sure that this is normal ... but I have owned a lot of 1.8L 4 cylinder cars -- mostly Toyotas -- and they ALL do this rev cycle with the AC on at idle). Best of luck!

Posted on May 29, 2011

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1 Answer

Why does idle speed surge?


Thread: 6.0 V12 XJS High RPM When Warm - Jaguar Forum.co.uk

www.jaguarforum.com/showthread.php?t=54266
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Car surging


Is the car equipped with the Automatic Transmission?
If so, read on.
Otherwise skip to the end and answer some questions.

POSSIBLE SCENARIO:
I have observed a condition where my car surges slightly when the torque-converter clutch (TCC) cycles between lock and unlock when driving on an uphill grade.
First some basics and history that will explain why the TCC is used.

Engine, Torque Converter, TCC, and Transmission relationship--
The TCC allows for a solid connection between the engine and transmission which allows the input to the transmission to rotate at the same speed as the engine.
Without a TCC, there is slippage between the engine and automatic transmission. The slippage is greatest at low engine RPM. That is what allows the engine to run with the automatic transmission in gear, like when you first shift into gear or stop at a stop sign. When the throttle pedal is depressed, the engine RPM begins to increase and the torque converter begins to slip less and less the more the engine RPM increases. The car moves. But even at cruising speeds the torque converter slips slightly. Engine RPM is greater than transmission input RPM, which is realized as slight decrease in fuel efficiency.
When acceleration is complete and a constant speed is being maintained, the engine power output is reduced to the point where the TCC can engage and eliminate any slippage between the engine and transmission. If the car has a tachometer the engagement of the TCC can be verified when a slight reduction in engine RPM observed without a corresponding change in vehicle speed.
One method used to test the operation of the TCC is as follows:
Find a flat section of road where it is safe to perform the test.
Reach a steady speed and keep the gas pedal depressed with one foot. While observing the tachometer (or listening for an increase in engine RPM), with the other foot depress the brake pedal enough to activate the break light switch but not enough to engage the brakes. When the brake light switch activates, the TCC receives a signal to disengage. With the gas pedal being held steady, release the brake pedal and the engine RPM should decrease when the TCC engages.
Old cars with Automatic Transmissions did not use a TCC. I believe the TCC was put in use in an attempt to increase fuel economy.

MY EXPERIENCE WITH SIMILAR SYMPTOMS
The condition that causes that issue on my car is this:
- A slight uphill grade increases the load on the engine.
The car tends to gradually slow and it is necessary to depress the gas pedal to maintain speed.
- Depressing the throttle pedal (manually, or automatically with cruise control engaged) signals the torque converter clutch to unlock when the load increases slightly. (A more drastic load increase would signal the Transmission to downshift to a lower gear.) The corresponding increase in engine RPM and output is enough to compensate for the reduction in speed. When the vehicle speed, engine RPM, and throttle position stabilize to the point that the TCC will engage and the engine RPM will reduce in correspondence with TCC engagement. Now, if the road conditions have not changed, power output is not enough to maintain vehicle speed. With the increased load caused by full engagement between engine and transmission, and the cycle (surging) repeats itself until the road conditions change.

Does that help?
If not:

QUESTIONS
Please define the symptoms.
What are the road conditions when the surge occurs? (A slight uphill grade?)
What is the frequency of the surge?
Does the engine power output have a noticeable surge?
Is there a speed change related to the surge?
Does the tachometer move up and down with little or no change in vehicle speed?
Are all instrument indication in the normal range?
What else has changed?

Good luck!

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1 Answer

When engine warms up it start to surge


Engine stalls cold or RPM fluctuates at idle up and down or dies near idle.

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1 Answer

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READ THIS (happy ending so far...):
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1 Answer

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1 Answer

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