Paddcomp

Not sure if the title correctly describes the issue, so here's a longer description.
On my 88 S4 auto RHD UK version, engine starts first time and idles at 900 rpm when cold (okay with this). After warming up fully, 10+ miles the idle doesn't settle to the 775 rpm it should. I've seen as high as 2600, but usually 1100 ish.
I've completed an intake refresh as per Dwayne's guide. Went back in and changed the ISV, as I didn't do it 1st time. Also checked the throttle body for free movement and a good spring back, tick. No improvement, went back in and changed the TPS, it measured ok 1st and 2nd time . Also added more slack in the quadrant cables as I wasn't getting TPS closed all of the time.
Idle was now 1000rpm, after a heat soak of 30 mins. Still high but not as bad as it was.
Went to Liverpool Cars and Coffee this morning (1hr+ each way), idle was fine 775 rpm until nearly there and then started rising to 1200rpm.
On the way home I thought I'd full throttle it for about a minute, engine performed well but on slowing down the idle started to increase rapidly. Getting to 3000rpm and making parking dangerous.

As I changed much of the idle control and full throttle seems to make it worse I'm thinking it's the cables.
Any other ideas?

Bertrand Daoust

+1 on the cables.

Make sure everything moves freely everywhere.
From the gas pedal to quadrant to throttle body.

Mrmerlin

based on your info the throttle cable is hanging up ,
with someone in the seat ,
remove the throttle cable ball from the quadrant and have them move the pedal. to full throttle then slowly release see how much drag the cable has.
if its not free then replace it.

NOTE also check the quadrant for freedom of movement ,
and make sure that the wheel turns free,
add a few drops of oil to the bearing of the wheel

Paddcomp

Hi Merlin,
I did this today, disconnected the cable from the pedal and to the throttle. No.1 son on pedal duty, me acting as return spring, no feeling of snaging, the cable quadrant also appears free.
Sqirted some WD40 down the cable end for good measure too.

What's the routing for the pedal cable, it goes through the bulkhead near the master cylinder and then where?

Bertrand Daoust

Have you checked that the cable is well aligned on the wheel under the MAF?
Like Stan said, also make sure that the wheel is moving freely.

bureau13

On mine, my cruise control cable was hanging up and causing tension, which prevented me from getting back down to idle. This was on an S3, so not sure what all is different, but my cruise cable is disconnected temporarily, until I get around to figuring that out.

jheis

WD40 = water dispersant 40 (40th attempt to formulate a water dispersant).

WD40 gets used for lots of things it was not designed to do - it's not a very effective lubricant or penetrant, nor was it meant to be... Use something else to lubricate the cable. WD40 is pretty much useless for this purpose.

GregBBRD

^^^^Very common on S4. The plastic end that retains it into the throttle quadrant breaks off its little retaining tabs and the cable pops out when the throttle is opened past about 1/2 throttle. The left hand drive cruise control cable (not sure if there is a different cable for right hand drive) has been NLA, so I retain the plastic piece into the quadrant with tie wraps.

Paddcomp

Confirmed all above.

Paddcomp

I did check the cruise cable for secure attachment, but will disconnect the ball to be sure it's not pulling unknowingly.

I'll try to replicate high idle and physically check the TPS at the ECUs to know where the throttle valve is. Then it's mechanical not ECU related.

the flyin' scotsman

there are 4 cables on the quadrant; anyone or a combo could be cause of the symptom.

Id troubleshoot by removing all cables and verify free operation of the quadrant then re-install the quadrant to throttle body, check operation. A gummed up throttle body, the wheel Bertrand mentions and/or the cable could be suspect.

Then in order one cable at a time; reinstall the pedal to quadrant, transmission to quadrant then crusie control to quadrant.

pwestover

mine is an OB, but had a similar problem. sometimes the problem turns out to be very low tech. In my case, the original floor mats would push forward as I drove and cause the gas pedal to be partially pushed. I thought I had a sticking throttle cable or cruise control problem. turns out I had a case of sliding carpet that due to the weight of the 70's ****, could put enough pressure on the pedal to cause a high idle or secondary cruise control. On one trip, it held the RPM to just under 2000. Now that i know what it is, I just pull the carpet back once a trip and have not had a problem since.

mark kibort

this sounds like a intake air leak , but that could definitely be a possibility, and certainly easy to check.

Paddcomp

I've checked and checked again, but don't have the equipment to pressurise the intake. Wouldn't a leak be more consistent and there all the time. I've checked the vacuum at idle as -20hg.

Brian

mark kibort

the best way to check sometimes, is to spray brake cleaner around the critical areas. if the RPM goes up or down during this, you got a leak. your problem sounds strange. if its not the linkage binding, then im at a loss...