It seems once the weather got real hot and humid here in Iowa it started to run like this and the clock started turning on and off. So far I have changed the spark plugs and the thermostat, adjusted the tps, cleaned the bac valve and I can't find any vacuum leaks. The car is a 88 s4 na no mods


Video is of a warm start. On cold starts it idles alright until it is warm, then this happens

 

Exorcism?

100% sure the BAC is working properly? What about your cold start thing? It is attached to the throttle body, has coolant running around it. Maybe it is failing? Just throwing ideas out there.

My first thought was BAC. But if that is fine.... How is it when you drive? Pull strong, etc?

 

id say a ground for the clock issue, is the clock flashing like that when the rpms change?

 

The clocks ALWAYS fail. If it's worked this long, consider yourself lucky. The solder joints to the board are notoriously cold. It's not related to your idle problem.

For the idle, have you tested the resistance values on the TPS, or just set it by voltage or the two-light method? Are they within spec, and are there any dead spots in the sweep? (http://howto.globalvicinity.com/gv_w...i=58&co=1&vi=1) It's clear the ECU is correcting for whatever the problem is, which is what's causing the idle speed to jump around. I'll bet if you unplug the BAC, while it's idling, it'll probably stall. The changes are random enough that I'd say a vacuum leak is not too likely; I've always gotten a rhythmic bounce in idle speed with big leaks. If not the TPS, the BAC or maybe even the AWS solenoid may be getting triggered when they're not supposed to. See if you can eliminate things by unplugging, then checking the result.

 

I did the voltage check on the tps. I will try the sweep tomorrow
and what is the aws solenoid?

thanks

 

The AWS solenoid is also called the air bypass solenoid, and it's the gold colored solenoid on the dynamic chamber, next to the BAC. It has a white plug with 1 green & 1 black wire I think. It's also got rubber hoses going in/out.

 

thanks everyone, it was the bac valve. i cleaned it again just to make sure and everything seems to be fine now

thanks again

 

What is the BAC valve and where is it located??? I have the same issue.

 

nice vid... thanks for the share...

 

Download the factory service manual (FAQ at top of forum). There are pictures of the BAC and testing information in the fuel and emissions section.

 

OK, I must be blind because I cant find it. Can someone post a link?

 

I think its far more advantageous and easier to diagnose some issues when you can post a vid of what's going on with the car. Glad you fixed the problem!

 

I have the same problem right now but mine jumps a little more. Going to clean the BAC valve tomorrow. Curious tho, you said it idles alright till its warm. Mine is like the opposite, lumpy idle still its fully warm. Then its pretty steady. Cleaning the BAC valve should fix this? Im pretty sure i have my TPS adjusted correctly and im certain i have no vac leaks. - Nick

EDIT: From the link you posted my TPS maybe bad.