pureroundcircle

I'm trying to track down poor idle running/ poor hot start condition on my new to me 5000 CS quattro. I cannot seem to find any vacuum leaks but I'm reading 14"-16" vacuum at idle. This seems a bit low. But I do not remember what my previous 5000 used to run so I figured I'd ask what idle vacuum are you seeing?

This weekend I'm planning to look into the decel valve, possibly eliminate it.

I've checked the usual suspects; oil cap, dip stick, upper manifold hoses, lower wastegate hose, breather hoses.

I guess I should also point out that I've currently got the ISV unplugged and just have the bypass screw opened to a point where it will idle in the right range. The idle control was activating when it should not be and at random the car would idle at 3000 or die at idle. TPS is tested and the switches are working.

So many issues to fix...

Anyhow, what vacuum reading do you get at idle? I'm thinking I should be getting at least 18" maybe 20".

Thanks,
Paul

88A5TQ

In my 88 5000 TQ I get 13 psi vacuum at idle and as high as 21 PSI vacuum with clutch in when shifting. These are when the car is warmed up. Car does have a bypass valve installed but I don't think that will change anything. I don't think the decel valve can be eliminated as it is what brings the cis air flap back to cruising levels and prevents the rpms from lingering when you are off the gas and shifting. The way I understand it is that it introduced engine vacuum to the underside of the flap so that an equilibrium is created on both sides. Correct if I'm wrong but I hope this all helps.

pureroundcircle

You have the general function of the decel valve, but it was added to basically allow a controlled vacuum leak which would allow the CIS plate to drop so you would not keep injecting fuel while decelerating. It can be eliminated, the car doesn't really run much differently but they have been known to leak and stick open. From what I understand there was even a service bulletin to cap it off at one point.

The bypass valve being opened or closed doesn't matter as far air metering goes because both sides of the bypass are downstream of the CIS metering plate.

88A5TQ

I like your explanation better hah. How old are your coolant and cold start valve sensors? Maybe the csv sensor is eff'd. Did you check the idle control valve for sticking or tears in the diaphragm? Is your exhaust smoking like its too rich when its not running well? Maybe throttle body is sticking?

pureroundcircle

Thanks for the suggestions. I'm currently running with the ISV disabled, and I've tested that it does not leak while unplugged. I'm not 100% sure what is wrong with the ISV circuit yet but the car likes to adjust the ISV all over the spectrum. I've checked the switches in the throttle position sensor. So its either a wiring issue, malfunctioning isv or maybe a bad idle control box? Anyhow I've got it temporarily disabled and I'm just using the idle adjustment screw bypass for now.

My coolant temp senders are operating, the cold start valve is working. Though my first day of real work on the car was the hottest day in like 5 years here, it was 107F out. So the CSV was probably not working while I was testing and it was running very lean as I later figured out.

I never did find a vacuum leak, but it seemed to run very lean and would misfire and want to die. I adjusted the CO per a VW CIS adjustment technique (not the correct method outlined on SJM). I used the alternate method as my ISV is bypassed. Basically I adjusted for greatest vacuum before noticeably rich running. You can also hear the frequency valve adjusting and can tell when it starts to adjust for a rich mixture.

So I adjusted it and went from 14-16" vacuum to 20" of vacuum, warm idling at 800rpm. It runs much better now. There was no vacuum leak after all!

I don't intend to leave it as such without further adjustment, but I needed to get it running and verify there weren't more serious issues before I decide to drop some actual cash into the car to correct all (or some of..) the lack of proper maintenance issues shes got.