# Digifant II 8V - stumbling after start and during warm up, tons of troubleshooting...



## Legoguy (Sep 18, 2002)

Okay, so normally I am able to research all of the internets, my Bentley manual, etc, and find a good answer and resolve these kinds of problems based on these resources, but this one has eluded me. 

tl;dr: I did quite a bit of troubleshooting and replaced quite a few parts but my symptoms still exist. Read on if you'd like to help out.

Vehicle: 1992 VW Cabriolet Wolfsburg, 8V, Digifant II, manual, with an ECU containing the vw_pilot (DigiFAST) chip. I still have the original chips.

*Symptom #1:*
(Engine is cold, or has been sitting for some time after running. Does not apply to hot starts.)

Car fires up awesome, idles perfectly for half a minute, maybe a little longer (this seems to be variable).
After said period of time, the idle drops to 400 (estimated), and the ECU attempts to compensate by way of the idle stabilizer valve, at which point the engine really misfires until it "catches" and revs high (near 2000 rpm sometimes).
Eventually the engine idle figures itself out, and I proceed to try to get her moving...
Upon pressing the throttle (lightly, heavily, does not make a difference) the engine hesitates slightly, revs up, I begin to engage the clutch...
Under the slightest load, the engine bogs down, by this point I have disengaged the clutch to prevent bucking, RPM drops to 400 (approx) and misfires quite badly. 
Engine recovers idle speed after a few seconds, I try again. Rinse and repeat a few times. Finally, after some time (usually about 3-4 minutes after starting the car) the engine has enough stability to apply power to the transmission, and I begin my drive, but I have to be careful with it.
As the engine warms up, there is hesitation when revving and while accelerating.
 Once warm, nearly everything pans out and the car runs fairly well. 

*Symptom #2:*
Rough transition on- and off- throttle while in gear. Maybe normal. Maybe not. This applies at all times.

*Symptom #3:*
Combustion in the exhaust pipe on engine overrun / deceleration; this is occasional only: it maybe happens twice or thrice in a half-hour period of city driving.

*Symptom #4:*
Backfiring in the intake when the engine is cold and the throttle is abruptly opened.

*Steps taken so far:*

Replaced Blue CTS with a genuine VW unit (this is seemingly the #1 failure point of Digifant...). No change observed.
Replaced the Oxygen sensor, again with a genuine Bosch unit with factory harness. Better fuel milage observed, overall performance seemed to increase, original issue still at hand, however.
Replaced fuel filter. No change observed.
Checked and adjusted Idle & Full-Throttle Switches. The Full-Throttle switch bracket was bent, and was engaging at half of the throttle travel. I bent it back, adjusted according to the manual, and I noticed another improvement in behavior, but listed symptoms still exist.
Timing was set and adjusted @2000 RPM as instructed by the Bentley after the bottom end was rebuilt about a month ago.
Checked for vacuum leaks; found several:

Long ago, the intake duct (AFM to TB) had a hole near the inlet of the PCV valve that I had "sealed" with a piece of copper tube and some glue. This repair had failed. Repaired again with some heater hose and black RTV, taking care to clean out all the oil and residue, and to get the RTV into the cracks of the crumbling rubber really well. You don't want to know how much a new one of these tubes is from VW Classic Parts...
Found a worn out vacuum elbow for the intake preheat flap system on the airbox. Replaced with a spare I had in my Box'o'Junk. Likely was preventing intake air preheat from functioning, as well.
Replaced most worn-out cloth-covered vacuum hoses with genuine Meyle hose of the same type.
Discovered that the o-ring around the idle adjustment screw was brittle and not sealing anymore, also allowing the screw to self-adjust (resulting in self-raising idle speed). Replaced screw with one from another engine that I had laying around.

After repairing all of the vacuum leaks and performing preventative maintenance on the smaller vacuum lines, yet another improvement of the general performance and idling was observed, but yet again, the main symptoms existed.

Checked air filter. Practically new, no contaminants (or even dust, for that matter).
Checked AFM wiper arm. Removed black cover, gently wiped traces clean with a pencil eraser, cleaned with contact cleaner. Checked operation with a DVOM and found no issues. Reassembled & Reinstalled. No change observed. This thing looks pretty sweet when you run it without the cover! Nevermind that it's a bad idea. Only temporary, of course.
Cleaned ISV & reinstalled. No change noted. ISV does "hum" when ignition is on.
Bought & hooked up fuel pressure tester to rail. Found that Fuel Pressure Regulator had failed in such a way that it was providing too much pressure, and not keeping pressure when the engine was shut off. Replaced with a new Bosch unit, retested and confirmed correct pressures and check-valve behavior. Improvement to starting and running in general as a result, but somehow the main symptoms remain...
Inspected inside of Digifant power relay for cold solder joints. None found, replaced with a different (matching part number) relay from my Box'o'Relays. No change noted.
Along the way I ended up cleaning my alternator's positive post after it got hot enough to melt the plastic protective cap off of it. It was loose after my bottom end rebuild, oops. Voltage throughout the car increased almost 2.5V. No difference in engine operation noted.
Made sure the ground straps were all clean and tight. No problematic ground straps found.
[NOTE: After nearly every one of these troubleshooting steps, I disconnected the Blue CTS and did the 3K/3 times reset procedure to reset the ECU adjustments]


Is there anything I'm missing that could cause this behavior? I have a whole list of things I _haven't_ checked yet, as well, but I'm hoping someone has a suggestion I can follow.


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## Legoguy (Sep 18, 2002)

After everything said and done in the previous post, I finally believe I have resolved this issue. I will post the solution here for those with similar issues.

I further investigated all aspects of the intake pre-heat system.
I found that the thermostatic valve (the thing with the two small vacuum ports [one brass, one plastic] sticking out of the side of the airbox) was installed upside-down.

This was based upon the information contained in VW's TSB for Digifant II vehicles in Canada (regarding possible icing). This is the only place I ever found documentation regarding the orientation of this part. It states that the thermostatic valve must be installed with the brass port towards the air filter, mine was the other way around (plastic port towards air filter).

I removed the valve from the airbox top, then found that the valve's gasket was old and not sealing. The effect of this is likely negligible, it would only allow unfiltered air into the airbox, which would still be metered. It would not be considered a vacuum "leak" due to this. Regardless, I replaced the gasket with another used piece, and reinstalled the valve in the correct orientation. I then confirmed visually that the intake pre-heat system is now functional.

This resolved this issue. Cold starts are no longer an issue. Car runs very well in all conditions, and has all of the pull that it should.

For experimentation's sake, I switched the vacuum lines on the valve back to how they were prior to flipping the valve. It disabled the intake preheat system entirely, and the symptoms reappeared. It is critical that vacuum from the throttle body is routed to the plastic vacuum port, and the line running to the hot air flap actuator is routed to the brass port, otherwise the system is effectively disabled. I doubt the actual orientation of the part has much effect on the function of the car; it's more important to make sure the vacuum is routed to the correct ports.


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## Fat Rabbit (Nov 18, 2001)

Nice troubleshooting. It always amazed me that the TB icing TSB was only issued in Canada and not in the US. Living in Northern Wisconsin I experienced several occurances of TB icing and even ISV icing over the years apparently related to temperature and humidity. FR


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