In the 3-4 months I have owned my '88 AT I have poured through the archives for ways to fix a rough hot idle. I began with testing every vacuum line I could find with a Mighty Vac. After that I tried the WD-40 squirt to fix the idle stablizer, followed by electrical contact cleaning, etc. I was about to move on to other potential sources when I received some new information.

The previous (original) owner emailed me to see if I am happy with the car. He is a good guy who took good care of the car (he had a stack of receipts, but he was not part of the Rennlist-OC-Big 3 community). When I told him of my quest to fix the idle, he wrote back that he had experienced the rough idle problems since sending the cams out to be ground at the same time he put an Autothority chip in (about 10 years ago). He had tried to find a bad vacuum line or other reasons, but gave up concluding that the new cams were the cause. He recommends now that I check to see if Autothority can program the chip to match the cams.

Elgin, in Redwood City CA, performed the grind. Autothority, curiously, is walking distance from my home. Other than the rough idle when hot, the car runs well (I think, I have never driven any other 928). I am well over my head on this challenge. I'd love to get some advice and recommendations for "next steps". Thanks very much.

 

No doubt the "regrind" on the S-4 cams was to higher lift and or duration and as a result it is NEVER going to idle that well.......the GT cams were not used in the GTS for that very reason.

 

Well "NEVER going to idle that well" sounds good to the part of me that is really tired of chasing this problem. It is not as bad as some other accounts I have read on Rennlist. Maybe I should just try to avoid idling....
Thanks, Jim.

 

If you get the spec of the cams from Elgin, Autothority should be able to help you out... If not, do nothing... idle is the devil's work...!

 

I do have the spec's scribbled on the receipt. Maybe it is worth a try - I was thinking of talking to Autothority about doing shocks and spring adjustment anyway.

 

It could be you have close to T spec cams with a slightly modded S4 Chip.. If the specs are close to GT specs, then you may be able to get a recalibrated GT program burned onto your chipset.. It would raise your idle abit, but it would idle smoother... If I recall correctly, GT's had an idle near to 1000rpm.. The DEVEK B1 cam grind I think needs almost a 1k rpm idle as well..

Whay are you adjusting the shock/springs..? Is the ride height off..? Or are they just worn..?

Good luck,

 

The car rides well on smooth surfaces, but is very jittery on road seams and rough spots. Rennlisters have convinced me to check the motor mounts as well as the shocks and spring height. I ordered the Jager speed mount for installation this weekend. If that helps, I will definitely put motor mounts on the list for the future and maybe forget about the shocks for a while. I don't know a way to check the condition of the shocks (original) other than the old "shove it down and see if it bounces more than once" test.

 

Have you dyno'd the car? It would be interesting to see what kind, and where, the power is being produced with the re-grinds. If the dyno shows the engine is down on power, then it would be relatively easy to get rid of the re-grinds and pop in some stock S4 cams.

No muss, no fuss...

 

I haven't dyno'd the car, but I could have it done at Autothority. That sounds like a good thing for me to do anyway. Is it really relatively easy to replace the cams? It sounded like a big/expensive job to me.

 

Depends on who is doing the work, and how much they charge.. If someone who really knows the car, and has the dial indicator to setup the cam timing, it should not be too difficult... I had my driver's side exhaust cam replaced in a few hrs... (less than 2 I think.)

Dyno it first... This can tell you a good deal on what's happening versus the stock cams.. Paul's right...

 

The reground cams would have no doubt been designed to make power at higher RPM than stock but with the resultant inefficiency at idle .....there are some who might like to trade stock cams for hotrod cams depending on what they really are

 

Jim.... Some of those people might trade a nice set of stock cams indeed..

Um, what specs are they again...? I'll be interested in zero overlap, but with more duration and not so much more lift....

I like my idle too, and for SC applications, overlap is not too nice....

However, guessing from the specs on the elgin site, you may have a set close to GT specs, unless it was a custom grind....

 

If the car is going to stay NA and a dyno reveals that the re-grinds sacrifice too much low-end, but does produce better peak horsepower (..as Jim stated), I would trade you my S4 cams for them (..perfect condition, no chipping).

With a centrifugal supercharger, I'm already making too much low end and am looking to attenuate low-end power production, and a longer duration cam does just that by bleeding off cylinder pressure through the longer-opened intake valve at lower RPMs.

Otherwise, if you're making good power everywhere, keep 'em and don't worry about the idle!

Just saw your post Richard; I'm definitely one of them.

 

I can't make sense out of the scribbles on the receipt. I will call Elgin to see if they have a record of the grind for my car. Dyno sounds like a good idea - would make sense to see how the engine is performing. Thanks for all the input.

 

Steve,
The idle speed on a GT is 775rpm. The idle speed on S4 is 675rpm.

Autothority should be able to reporgram the idle speed for you. Although you may find this is a bit of a pain with an auto.