# 88 Jetta Digifant II Idling problem



## Perratzo (Jun 1, 2007)

Hi, 
This topic is a bit old but I want to tell you that I have lived with symptom #2 for most of the life in my 88 jetta 8v. I reproduce it below from Mike Ocksbigg (10/26/2007)







:
"Symptom #2:
At idle, the car will advance the timing on it's own(verified with a timing light), stumble, the idle speed will drop to ~600rpm, then it kicks over to retarded timing(roughly 2 degrees ATDC), the idle speed jumps, and it returns to normal idle. This cycle continues constantly, occurring every 5-10 seconds. It idles perfectly with the CTS disconnected. "

I keep tinkering with the timing to correct it but I always end up having the same problem. On idle, while hot, morning, afternoon or night. The car idles in accelerated/decelerated cycles. The RPM's go under 1000 and then jump up to 3000 in the worst cases.
I have taken it down to a minimum by adjusting the throttle flap screw back a few turns and adjusting the accelerator position screw in order to leave the car accelerated around 1250 rpms, where the problem is not as bothersome. Still, the engine cycles within 900 to 1450 rpms.
I have replaced the coolant sensor twice trying to ax this glitch. I recently found out that when you replace the white (two-wire) coolant sensor, you have to reset the fault code form the computer by disconnecting the blue oil temp sensor and revving the engine past 3000rpms (allowing the throttle flap to completely close) three times before plugging the blue wire back in.
I just did this yesterday (after replacing the coolant sensor five or six months ago) and experienced no sensible difference in my glitch. 
Like Mike, if I disconnect the coolant sensor, the car idles smoothly and does not hesitate when the light turns green at intersections. I have run the car like this a couple of times but I sense the gas mileage goes up significantly.
In fact this problem got me rear-ended once because I could not get past this hesitation (doesn't buck, but starts to build power really slowly) in time to avoid the jerk behind me who thought I had started going already.
Now I am in the habit of revving it past 2000 before I let go of the clutch at stop signs.
My ISV seems to work well. All filters replaced (airbox has a clean K&N). New cables and plugs (platinums after coppers after platinums, and so on). New distributor cap and rotor. Timed it right with a light and with the coolant sensor disconnected, as recommended by Haynes manual.
Mornings are always hopeful because the car starts right up and revs steady at 1000 or a bit below, but only for a couple of minutes where it starts to stall after dropping revs. It will keep on doing it if I don't tap the gas to avoid the stall or just push the gas to 2500 rpms for a solid minute or more, where it stabilizes into the cumbersome cycle I described. At this time of the day, the tailpipe smells a little rich and at times it has left some blackish hue on the driveway under it.
Upon listening to the tailpipe for a moment I hear:
rrrrrrrrrrrrrrrrthrrrrrrrrrthrrrrrrthrrrrrrrrrrrrrrrrrrrthrrrrrrrrrrrrrrrrrrrrrth
The "r" being normal firing combustion and the "th" I suspect misfire. It is long-intermittent and not constant. It happens with and without the coolant sensor connected. This "misfire" is greatly reduced everytime I do the plugs and cables/rotor and cap. But not completely gone. I also believe that the "th" are exactly the times where the revs drop in my "problem" cycle.
I am still to trace grounding of the system to the bolt on top of the tranny, but I think I will not be doing anything to make a difference. I also suspect a fuel pressure problem but I fear it in that these are expensive components (two fuel pumps and FPR). Why bother with something I am out of reach of?
I will post this as a new topic as well to get more feedback.


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## sholford (May 21, 2007)

Leave the CTS plugged in and disconnect the O2 sensor harness. Then start the car.
If the idle remains constant, but rich, check the ground wires for the O2 sensor at the driver's side of the engine at the water outlet. If they're broken or corroded, that's most likely the problem. If the grounds are good, it's probably a bad O2 sensor.
Don't connect or disconnect the O2 harness while the engine is running. It can supposedly damage the ECU according to the Bentley.


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## funkwerx (Oct 2, 2008)

*Re: (sholford)*

im also having very similar symptoms with my 89 jetta. i have replaced the O2 sensor with no change and it also idles stable when i disconnect the coolant temp sensor. i changed all the vacuum lines with mute hope but still no change.
is there a writeup somewhere on how to clear and read codes on this machine? im stumped.


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## Perratzo (Jun 1, 2007)

*Re: (sholford)*

Water outlet?
I'll look into it. thanks for the advice. I was already looking into buying







a three-wire universal O2 sensor, they seem less expensive.


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## Perratzo (Jun 1, 2007)

*Re: 88 Jetta Digifant II Idling problem (Perratzo)*

I verified the timing marks (TDC in the flywheel and the cam sprocket against the correct dizzy position). Did the timing-light-less procedure I found on another thread in here: All sensors connected, revved it up to 2500 RPM (in full normal engine temp) and turned the dizzy clockwise to reduce rpms, then came back slowly to increase rpms to the point where no change would happen (the alleged "sweet spot"). 
Also inspected the K&N filter, it was freeflowing and almost free of dirt. Closed it and took it for a spin.
Response improved when shifting to 2nd gear (where it usually stumbled a bit). Idling stabilized (stays within 900-1100 rpms, up and down still) but increased the sound of "misfires" at the tailpipe and it doesnt smell as rich as before. I also noticed rattling upon increasing load (punching the gas in high revs before shifting up). Car also seems a bit more peppy.
The above results have happened before, I always timed the car to reduce or eliminate the rattling (which I always suspected as mis-timing) but I always ended up with the erratic idling.
I will try a timing light as soon as possible. As well as the ground check on the o2 sensy-thingy.
I think I made progress.


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## Perratzo (Jun 1, 2007)

*Re: 88 Jetta Digifant II Idling problem (Perratzo)*

Here's Fritz by the way


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## MaDDWrenches (Oct 2, 2003)

I had all these problems and it turned out to be the idle stabilizer. I unplugged it and had to raise the idle screw a little. I have to give it a little pedal when its cold. I havent had abouncy idle since, and the car hasnt had a isv for a while now


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## Perratzo (Jun 1, 2007)

*Re: 88 Jetta Digifant II Idling problem (Perratzo)*

bump


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## Perratzo (Jun 1, 2007)

*Re: (MaDDWrenches)*

I'm suspecting a bad o2 sensor. I am trying to find a cheap one on ebay.


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## Perratzo (Jun 1, 2007)

*Re: 88 Jetta Digifant II Idling problem (Perratzo)*

Does it have anything to do with this wire?
http://forums.vwvortex.com/zerothread?id=4093420


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## Perratzo (Jun 1, 2007)

*Re: 88 Jetta Digifant II Idling problem (Perratzo)*

bump


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## sholford (May 21, 2007)

I picked up an '88 Jetta (digi2) from a guy that believed the car had a fuel injection problem.
Symptoms it had:
-Started but quickly fouled the plugs.
I found the O2 sensor wire broken, discovered the car had 1 1/2 year old gas in it, and a bad ignition module.
After changing the module, adding good gas & octane booster, and fixing the broken ground; the car idles bad with the CTS plugged in, but great without it....but now it runs. BE SURE TO CHECK THE RESISTANCE OF THE CTS. I don't know the specs off the top of my head, but roughly 35-40 deg fahrenheit is 5K ohms, op temp was 200 ohms. Unplugging it doesn't determine if that's the problem. My CTS checks good, but I really think the O2 sensor is just bad. Running without the O2 sensor creates a rich condition, resulting in so-so idle, great top end power, but will eventually foul the plugs out if you continously run the hell out of it.
Pull your plugs, if they're black or wet you have a fuel mixture or delivery issue.
I'm heading back to the garage and I'll post the results when I pinpoint the problem.


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## sholford (May 21, 2007)

*Re: (sholford)*

Ok......found the problem and I suggest using this method anytime you run into digifant issues. You do need a Bentley manual and a multimeter. I'll post some pics & charts for those that don't have a Bentley.
The problem was a shorted ground for the O2 sensor. The PO attached an extra wire to the O2 sensor harness, and grounded it at the valve cover. I never would've thought that it created a problem....more grounds are better right? Not necessarily.
For those who want to argue, I checked the corresponding pins at the ECU plug and there was continuity, in which there shouldn't have been. I cut the extra wire the PO installed and the problem went away just like that.
ECU plug pin layout:








Ignition module/control unit pins:








Test chart:
Digifant II Electrical Tests 
The charts weren't big enough, so follow this link.
http://www.iol.ie/~kilkennyhoc....html
THIS APPLIES TO DIGIFANT II only and it was taken directly from the Bentley.
1. Disconnect the harness connector from the Digifant control unit 
2. Using an ohmmeter or a multimeter set to the 20k scale, check continuity between terminals 2 and 13 of the Digifant control unit harness connector. *There should be no continuity. If there is, there is a short to ground in the oxygen sensor wiring. *
3. Disconnect the oxygen sensor harness connector. Connect a jumper wire between ground and the green wire (terminal 1) on the harness side of the connector. There should be continuity. If not, there is a break in the oxygen sensor wiring between the harness connectors.


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## Perratzo (Jun 1, 2007)

*Re: (sholford)*

Excellent post. I'll do my checkup before replacing the o2 sensor. http://****************.com/smile/emthup.gif http://****************.com/smile/emthup.gif http://****************.com/smile/emthup.gif


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## sholford (May 21, 2007)

One other thing to add to this. I tried the 20K range on the ohm meter like the Bentley suggests, a number blipped, and the reading went blank. This indicates that the reading was beyond the range of the meter. If you have an audible meter that tests continuity it will work that way as well.


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## Perratzo (Jun 1, 2007)

*Re: 88 Jetta Digifant II Idling problem (Perratzo)*

I've been running with advanced timing for a while and it seemed to reduce the problem to an almost unnoticeable minimum. But it experienced a little rattling under load, so I retarded it a bit. It went back to its usual variation. I'll keep digging.


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## babarber (Nov 3, 2008)

*Re: 88 Jetta Digifant II Idling problem (Perratzo)*

why didn't ne body know this. if your timing is changing or bad u have a problem w/ knock senor located under the 2nd plug wire from the left.
if u r idling erratically than it is probably the idle stabilizer valve. the oxygen sensor only controls how long the injectors r open and can't exactly mess w/ the idle. if u have a problem gas mileage then go ahead and replace that o2 sensor. by the way in ne 8v d2 if u unplug the cts it picks up the idle and has nothing to do w/ ur problem. 
here's what u do:
1) unplug the idle stabilizer
2)adjust the idle bypass screw until the car revs at about 925 rpms
3)check and replace or clean the idle stabilizer
4)hook the idle stabilizer up
5)check the torque rating on the knock sensor it should be 15-18 in/lb
6) if the torque rating is fine replace the sensor
7) after u finish tuning and u still have ne questions or problems email me @ [email protected]com


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## sholford (May 21, 2007)

*Re: 88 Jetta Digifant II Idling problem (babarber)*


_Quote, originally posted by *babarber* »_why didn't ne body know this. if your timing is changing or bad u have a problem w/ knock senor located under the 2nd plug wire from the left.
if u r idling erratically than it is probably the idle stabilizer valve. the oxygen sensor only controls how long the injectors r open and can't exactly mess w/ the idle. if u have a problem gas mileage then go ahead and replace that o2 sensor. by the way in ne 8v d2 if u unplug the cts it picks up the idle and has nothing to do w/ ur problem. 
here's what u do:
1) unplug the idle stabilizer
2)adjust the idle bypass screw until the car revs at about 925 rpms
3)check and replace or clean the idle stabilizer
4)hook the idle stabilizer up
5)check the torque rating on the knock sensor it should be 15-18 in/lb
6) if the torque rating is fine replace the sensor
7) after u finish tuning and u still have ne questions or problems email me @ [email protected]

Not trying to start a pissing match here, but the ISV is overrrated for being the problem. Cleaning doesn't hurt, but their main function is during cold starts and a/c operation. Plenty of people run without it and deal with the cold start issue.
I can agree with the knock sensor making the timing fluctuate, but that's only under load, not idle. The CTS has a more profound affect at idle than the knock sensor because it adds extra fuel to a cold engine. If you hear pinging during acceleration, suspect the knock sensor or timing and check it for frayed wiring and deterioration at the connector.
I've dealt with plenty of digifant cars and most of the problems are wiring corrosion, not sensors. If you have a Bentley, do the tests at the ECU connector to pinpoint the problem. You might be surprised.


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## babarber (Nov 3, 2008)

*Re: 88 Jetta Digifant II Idling problem (sholford)*

i have 4 digifant 2 jettas and 3 of the 4 had a combination of problems w/ the isv, knock sensor and o2 sensor and had very similar symptoms to this guy's. o2 sensor just caused it to get poor mpg. 
the sensor all had been left on there from factory installment and were in very bad shape. changed them and fixed a couple of bad grounds and the cars ran perfect.


_Modified by babarber at 3:36 AM 11-10-2008_


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## Perratzo (Jun 1, 2007)

bump


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## Perratzo (Jun 1, 2007)

*Re: 88 Jetta Digifant II Idling problem (sholford)*

First of all, thanks for the great input. 
I've had no time available to run ground checks. What I did do this sunday before church was to remove the idle screw and found it almost clogged with dirt so I cleaned it with my shop rag and put it back in. I also RETARDed my dizzy a bit from where I had it and the idling stabilized further when fully warm. I could not hear the engine rattling under load either.
I will do my best to make some time to clean all grounds. I don't have a Bentley but I do have a Haynes and I am certain that it is not as thorough but it has helped before. I want to carb clean all intake parts and plug any vac leaks I may find. There's so much good info here that I want to use.
I really appreciate everybody bearing with a procrastinator. I used to be a lot more fun to my previous Jetta. I have turned into a DD owner more prone to quick fixes than "quality time" TLC for my current coupe. I wonder if all DD's are like me.
Anyway, I pro-mise that I will apply all input supplied here and even post pics if I get my hands on a camera.
Many thanks to all once again







http://****************.com/smile/emthup.gif http://****************.com/smile/emthup.gif http://****************.com/smile/emthup.gif


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## babarber (Nov 3, 2008)

*Re: 88 Jetta Digifant II Idling problem (Perratzo)*

driving w/ a bad knock sensor is like driving w/ a brick under the gas pedal...


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## Perratzo (Jun 1, 2007)

*Re: 88 Jetta Digifant II Idling problem (Perratzo)*

I did the cheapest thing I could afford.
But I will first tell you that I found parts and prices for everything suggested here. I found the OS for $50 (BOSCH with original plug), for instance.
Since I am close to broke I went out and got the cheapest thing: A can of Carb cleaner/ degunkyfier.
I reeally went after the ISV and cleaned the crap out of it. I later tested plugged it in to verify functionality and it hums ever so nicely opening consistently. I later cleaned the pcv/breather, its connecting hose, the throttle body, and finally the idle screw. Messy deal.
I checked for bad hoses (air intake was fine, vac hoses, fine), checked all sensor and ECU plugs to be clean.
I sanded the two connecting grounds from the negative side of the battery (one right next to the battery and the other on a bolt on top of the tranny).
I started the car and it idled without spikes for the first time ever. It was revving above 1500 though, since I had meddled with the throttle flap screw before. I set it back to the paper-pressing adjustment and went on to the idle air screw: I had screwed it all the way in and was surprised that the car idled so low without dying. The needle was floating at 800 (yay!!!).
There was a slight "pfhrrrpt-pt-pt" feel in the idle so I jiggled with the distributor to advance it a bit and I almost got rid of it.
I adjusted the idle air to 900 rpms.
One thing I noticed was a screw (allen head) on the airflow meter there it is normally plugged with a lead cap. I unscrewed it out and it is a clean aluminum screw. I figured it is a limiter of some sort so I tested different levels and noticed no change in idle or acceleration. What does it do? I screwed it in all the way and came back 4 turns. If anyone knows the position of this screw and its function, pleas tell me.
I idled Fritz for 15 minutes this cold morning and it ran fine. One thing that did not go away was the smell at the exhaust, kinda irritating in the nose. But then again, I don't have a cat installed.
The car now stays below 1000. Mission accomplished?

Please send some input if I missed anything.


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## Perratzo (Jun 1, 2007)

*Re: 88 Jetta Digifant II Idling problem (Perratzo)*

Gonna take it to a sniffer and play with the air mix screw.


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## Perratzo (Jun 1, 2007)

*Re: 88 Jetta Digifant II Idling problem (Perratzo)*

Idle improved but smells rich as hell at both ends of the car. Can I do something before taking it to the sniffer?


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## Perratzo (Jun 1, 2007)

*Re: 88 Jetta Digifant II Idling problem (Perratzo)*

I went five more turns to the left on the mixture screw. Throttle response from dead stop improved.
Morning smell decreased ever so slightly.


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## babarber (Nov 3, 2008)

*Re: 88 Jetta Digifant II Idling problem (Perratzo)*

your cat may be a little clogged. and u might want to check the sensors that directly control the fuel. if u don't know what those are get a manual or u can just ask me and what u should get on an ohmmeter. 


_Modified by babarber at 9:04 PM 12-5-2008_


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## Perratzo (Jun 1, 2007)

*Re: 88 Jetta Digifant II Idling problem (babarber)*

Thanks man but the cat´s gone. I think I have to get another one.


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