Audi A6 / 100 idle-deceleration problem

**wysiwyg73** · 06-07-2007,02:46 PM

I have run my 1994 Audi 100 (A6) 2.6 V6 (ABC engine) for 10 years with virtually no major problems, but a while ago it started having intermittent running problems. When warm it often won't idle at less than 1100rpm, sometimes as high as 1400rpm. From cold, sometimes it just cuts out and I have to restart, keep the revs up and drive until it gets some heat into it. I can cope with these instances, but there is an associated problem when I decelerate - there is a time lag which means the power is still on after I've lifted the throttle. This makes the car jerk as it comes off suddenly and this erratic behaviour is not pleasant in traffic.

I have had the car tested by a local garage and they say nothing shows up on a diagnostic check. They seem to think that it is a problem associated with the throttle body and after they cleaned this the car ran well for a whole week! The problem then returned. I am not convinced that a repeat clean will cure the problem and feel that they may have just accidentally effected a temporary cure by carrying out the work.

Any ideas / comments would be much appreciated.

**Lex Luthor** · 06-07-2007,03:18 PM

A "diagnostic Check" that just looks for fault codes is only half a job.
You need to have the various sensors checked for correct readings while the engine is running or even being driven.
Find a local forum member who will hook-up their VAGCOM and check it properly.
My money would be on the coolant temperature sensor...but then again I always say that when the engine will not idle properly....

**wysiwyg73** · 06-07-2007,05:35 PM

Thanks for the suggestion. I asked for the thermostat to be replaced when it was in for this fault as it was running a bit warm (90), but they didn't do it advising me that this was quite normal. They claim to have checked the sensor, it being "within acceptable limits". The test was apparently carried out with the engine running. At least changing both stat and sensor would eliminate them from the investigation.

I have a feeling this may be a long process and I can't understand why it was okay for a week after it was worked on, but since the problem returned it has occurred daily. Just call me Mr Confused!

**Lex Luthor** · 06-07-2007,06:36 PM

Originally Posted by wysiwyg73

At least changing both stat and sensor would eliminate them from the investigation.

Don't poke and hope...it's a waste of money and effort.
The sensor will only have a negative effect if it is way out. If they checked it then it's probably ok.

Post your location in your profile and you might receive offers of help from forum members.

**s6sputnik** · 07-07-2007,04:29 AM

Originally Posted by wysiwyg73

Thanks for the suggestion. I asked for the thermostat to be replaced when it was in for this fault as it was running a bit warm (90), but they didn't do it advising me that this was quite normal. They claim to have checked the sensor, it being "within acceptable limits". The test was apparently carried out with the engine running. At least changing both stat and sensor would eliminate them from the investigation.

I have a feeling this may be a long process and I can't understand why it was okay for a week after it was worked on, but since the problem returned it has occurred daily. Just call me Mr Confused!

You gotta tell me how coolant can make a motor run poorly? I would think that fuel, spark, or O2 not H20 would be your most likely issue(s) assuming your idle is proper and the car is not running the A/C.

Generally when the A/C is running the idle increases to keep the motor from stalling..so when you are testing this stuff ..make sure to shut off the A/C

hope that helps,

**Lex Luthor** · 07-07-2007,08:46 AM

Originally Posted by s6sputnik

You gotta tell me how coolant can make a motor run poorly?

If the CTS is stuck on full cold then the ECU will fuel the engine too rich.
If the CTS is stuck on full hot then it will be difficult to start as the ECU thinks you are doing a warm start.

Generally when the A/C is running the idle increases to keep the motor from stalling

.
This engine has a fully computer controlled idle using the servoed throttle body to control the idle so it should idle at the same RPM no matter what the load.

**s6sputnik** · 07-07-2007,05:04 PM

Originally Posted by Lex Luthor

This engine has a fully computer controlled idle using the servoed throttle body to control the idle so it should idle at the same RPM no matter what the load.

On a '94 with a small 6? Are you sure? My S6 (same body style different motor) does idle differently when the A/C is on. And you also feel and see it when the compressor comes on.

FWIW I just checked...Without A/C idle is about 875rpm with A/C running 950rpm so either my car is different than his or my car is sick.

**s6sputnik** · 07-07-2007,07:26 PM

Originally Posted by wysiwyg73

I They seem to think that it is a problem associated with the throttle body and after they cleaned this the car ran well for a whole week! The problem then returned. I am not convinced that a repeat clean will cure the problem and feel that they may have just accidentally effected a temporary cure by carrying out the work.

Any ideas / comments would be much appreciated.

Maybe replace the throttle body and cable...the cable is 13 years old and might be rusty on the inside

**Lex Luthor** · 07-07-2007,10:41 PM

Originally Posted by s6sputnik

On a '94 with a small 6? Are you sure?

Yes your right it's too early for that.
The throttle body does have an idle switch that would be worth checking and there is a separate idle stabilisation valve that may benefit from cleaning.

**s6sputnik** · 07-07-2007,10:52 PM

Originally Posted by Lex Luthor

The throttle body does have an idle switch that would be worth checking and there is a separate idle stabilisation valve that may benefit from cleaning.

I think that is where i would look...Is the guy who needs this even looking any more?

Luther