I have a recently acquired 1993 C4 and since buying it I have been having an issue with the car returning to idle after a rev or when putting in the clutch. The car starts fine hot or cold and has no issues finding/holding idle. The issue only presents itself after the engine is revved or when driving and you clutch in (as if you’re coming to a stop) and the rpms have to drop to idle. Basically when the motor speed is coming back down toward idle it will dip below idle almost to the point of stalling and then catch itself by surging and then retiring to idle rpm. I have taken a couple videos to demonstrate the issue.

Here’s what I know about the car from the PO (all work was done ~7,000 miles ago):

• MAHLE Cylinders, rods, and pistons replaced to increase to a 3.8
• Webb cams
• Steve Wong Chip
• Secondary muffler delete

In searching the forum I know this can be a pretty common issue, especially for cars without the dual mass flywheel but I don’t have any documentation suggesting it isn’t the stock flywheel.

So far I have cleaned and tested the idle control valve which seems to be in working order as well as performed a vacuum leak test without and visual sign of a vacuum leak.

I was hoping someone could help point me in the direction of additional things to check or experience with this issue. Appreciate all the help!

 

Adding links to the videos here as well:

 

If this occurs both when the engine is cold or up to temp, then an easy first check is to remove and clean (and check for function) the Idle Stabilisation Valve (ISV). If unfamiliar with this then plenty of threads to plough through - quick search will produce many results.

If the idle seems 'wayward' at any temp, or inconsistent, then the above plus remove the dizzy caps, pull the rotor arms and check for excessive pitting/wear. Again, another easy and cheap fix - instantly solved an erratic idle I had. These two easy DIY suggestions before all the more lengthy other component checks. At least you can tick these off the elimination list (of later avoidable erratic behaviours).

 

@LM964 Thanks for the suggestions! It does occur both when the motor is hot or cold. I did remove and clean the ISV, that was the first thing I checked. I also tested it while it was out and the valve would operate when I applied 12v so I'm assuming it's functional.

The idle itself is rock solid, it it doesn't fluctuate and is very consistent. This issue is only when the car is returning to idle from a rev but then is solid once it settles out. This is why I didn't think the dizzy caps or any associated components were an issue but I can check those as well.

 

Interesting... You've had this issue from day one of buying the car? Did you notice this on the test drive, or not something you would have consciously paid attention to?

Has it had a lightweight clutch/flywheel fitted recently?

 

@LM964 honestly it’s not something I paid attention to so I missed it before picking it up. It honestly wasn’t even something I noticed right away during the first couple drives, it wasn’t until I was revving the car stationary to listen for something that I really picked up on it.

I have seen that this is common with light flywheel cars but I looked through the prior service records and modification receipts and I don’t see any evidence of a lightweight flywheel being installed. The clutch was changed but nothing to suggest a lightweight flywheel.

 

Few possible other pointers to try out (following some similar digging around, plus again being fairly easy and cheap); Call/ask prev owner if there is any knowledge of e lightweight flywheel being fitted (can then 100% rule that one out). Next try swapping out the DME/fuel R41 relay and check for any rev change. Also might be an idea to start from scratch (as a new owner) a DME Control Unit reset - via the diag tool ('Bosch hammer') and/or disconnect the battery for a few minutes or more (DME loses some of it's programming from previous driver use). Reconnect battery and take out for a good 15-0 min drive. However, not all DME's units respond/clear-down from a battery disconnect, hence the workshop diag tool needed to do this.

Some other suggestions look at the air flow meter internal cleaning (as a possible result of oil overfill). A bit more time needed and starting to follow a 'more what's left to do?' investigative path. Hopefully others will chip in soon with their own experiences and ideas.

 

Before you go down the DME route, check your mechanical connections at the intake. Any slack in the accelerator cable. Weak return spring, worn bearing, or even a small vacuum leak can contribute. Flywheel would have been my first suggestion too.

 

Thank you both for the suggestions so far! I have reached out to the PO and will see if he can definitively rule out a lightweight flywheel. Looking back at the records, the car does have a custom chip to account for the increase in displacement, cam, and secondary muffler delete but not a lightweight flywheel (if the car does in fact have one).

Speaking of fuel relay, I did see in the records that it looks like a solid state DME relay was installed (https://www.ftech9.com/new-products/...kxZ5iXfgYZKPWE). I don't know much about these and if there are any known issues. I haven't yet checked out that relay, I just came across it when looking through the records.

I did look over the mechanical connections at the intake when I was trying my initial diagnosis and didn't really come across anything that looked alarming. The return spring seems strong and throttle returns to idle position without issue, cable appears to be okay, but I will look over everything again. I also did perform a smoke test for vacuum leak and nothing popped up that I could see.

 

The RS flywheel causes chatter at slow speeds. That’s an easy way to identify if the car has one. What you are experiencing is pretty much what I had when I had a RS flywheel.

 

Thanks for your input! I will have to listen a little more intently next time I am out with the car but I didn't notice a chatter. I have an E36 M3 with a lightweight flywheel and that chatter is much easier to pick up on so I'm not sure how noticeable it is on these cars.

I am waiting to hear back from the PO on whether he has any records of a lightweight flywheel being installed as this was my initial thought after reading through the forums. If it isn't the case then I will need to keep digging.

@John McM what did you do to fix the issue when you has the RS flywheel? Custom chip or did you switch back to the dual mass?

 

I put in a Steve Wong chip designed with the flywheel in mind. That fixed it.

 

I also run with web cams in my 964, along with a LWF. Both of these can wreak havoc on idle stabilization. Steve Wong makes a customized chip that largely solved the problem for me. I would reach out to Steve, but you should first determine whether or not you have a LWF.

Cleaning out the ISV is also good advice, and worth the time and effort.

 

Here’s how my car was idling before the custom Wong Chip. Does this seem familiar?

 

I’m not experiencing the same issue you have shown in this video. The car idles rock solid, it doesn’t search for idle. The issue is only when the car returns to idle after a rev. When the RPMs are coming back down they dip below idle RPM to the point of almost stalling but then catches itself, surges, and then settles at idle RPM and then it’s fine just idling.

The car currently has a custom Wong chip however it doesn’t take into account a lightweight flywheel (if the car does in fact have one). My leading theory is I need a new chip to take the lightweight flywheel into account but I have to confirm whether the car actually has one installed.