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Volkswagen Jetta, Golf, GTI: 1985-1992 (A2) 91 Jetta Digifant 1 Hesitation Under WOT


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Last Post: Jan 18, 2014 9:14 PM Last Post By: Jeff_Kimo
boschmann

Posts: 41
Registered: 01/06/04
91 Jetta Digifant 1 Hesitation Under WOT
Posted: Feb 18, 2004 9:26 PM
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Hello all, I've come across a problem on a '91 Jetta that is about to have me totally stumped. I work on alot of VW's and currently use the Mitchell On Demand system (sorry). It's a Digifant 1 California Emmision 1.8 litre (engine family MFV), of which I have only seen few since I'm on the East coast. It's manual trans. and appears to have about 150K miles on the engine; new cap, rotor, wires & plugs before I got it in. This gets fairly complicated so sorry in advance for the length of this post. The problem only occurs once the engine begins to warm. When started cold it will idle and rev up fine, once warm it will still idle fine but will severely hesitate, backfire (small & usually through the intake) and die if you open the throttle too quickly and hold open. If you slowly open the throttle and are very careful you can get it to rev without problem, but as it gets warmer it gets more difficult. It sounds like it's leaning out as I get a split second of spark knock under the rapid throttle oppening. So far I checked fuel pressures (36psi running 42psi w/regulator disconnected), but found the fuel pressure regulator was not holding pressure after shutting down & Idle air valve was leaking after it warmed, so they were replaced. There are no engine codes being stored and when I cause a fault it will flash out most codes correctly. I have checked wiring connections, grounds and vacuum hoses. Coolant temp sensor checks good. I noticed one peculiarity that if I disconnect the air flow meter it will die, but will not set the appropriate codes (no codes at all). During my investigation I have found that it has the same part number as several Digifant II meters I have (even as a very different Vanagon one too). Does digi 1 use a different number, maybe it was replaced? When I check it's resisttance I am getting about 520 Ohms between pins 3 & 4 and it will fluctuate Ohms between pins 2 & 3 when I move the flap. I noticed fairly different values on the potentiometer side when I compare the four meters I have. The one I feel is most "correct" reads about 550 closed goes up to about 1900 at mid-point and will drop back to about 600 at WOT. As I have no spec values other than "fluctuates as flap is moved" does this go along with your experiences? Again, with the Throttle Position Sensor it will set the code properly, but I am unable to get actual resistance values. I'm reading about 7000 closed to 1500 open. Since it will run at times under all RPM ranges I'm not focused on the Hall sender unit. If it had an external Hall Control Unit I would try replacing it with a spare, but it continues to spark while it is sputtering out so I'm not ready to try an ECM (if I can even find one) yet. Also, I don't have detailed info on knock sensor checks and fuel volumes from the injectors. Or, perhaps you've seen this before and can save me some time. For now I will back up & check some more basics, regroup and hope to get some suggestions. Thanks for reading all this,
-Joe
JoeBean

Posts: 4,697
Registered: 02/14/02
Re: 91 Jetta Digifant 1 Hesitation Under WOT
Posted: Feb 19, 2004 12:27 PM   in response to: boschmann in response to: boschmann
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1. The airflow sensor on Digifant I and Digifant II are the same.

2. The actual resistance values between 2 and 3 are not as important as the fact tha tit changes smoothly, without any sudden drop outs.

3. These vehicles have notorious ground problems. If the grounding to the control unit is poor it will almost always destroy the control unit. Check all ground connections.

4. You can check the intake air temp sensor between pins 1 and 4 of the airflow sensor connector. It should read as the coolant temp sensor does.

5. VW doesn't seem to specify fuel volume delivery rates for Digifant fuel injectors. THe resistance of each injector should be 15-20 ohms.

6. Make sure nobody has tried to set the idle with the screw on the airflow sensor.

7. Make sure the knock sensor is torqued correctly (11-18ft-lbs). There is no resistance value given for Digifant I knock sensors.

8. Here's one more test you may not have ran - for the timing advance. Make sure ignition timing is set correctly. Then, with the engine at normal operating temperature, disconnect the coolant temp sensor. Raise the engine speed to a steady 2300 RPM and record the basic ignition timing value. Reconnect the coolant temp sensor. Recheck the timing at 4500 RPM. It should be at least 30 degrees more advanced. If not, there is a fault in the airflow sensor or the control unit.

Terry

P.S. I just read what you wrote about the throttle valve potentiometer. You SHOULD be getting resistance values between terminals 1 and 2. And the value should vary continuously and smoothly as you open the throttle. If not, replace the throttle valve potentiometer.

Message was edited by: JoeBean

boschmann

Posts: 41
Registered: 01/06/04
Re: 91 Jetta Digifant 1 Hesitation Under WOT
Posted: Feb 19, 2004 4:45 PM   in response to: JoeBean in response to: JoeBean
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Thanks for the suggestions Terry. I dont get any bad spots in either of the potentiometers as they move. As for the ECM ground, I got the car from another VW shop, who got it from another shop. It's had alot of hacking on the harness, etc... it was driven to me with the ground strap from coil to head off and the ECM was loose (strap on bracket) but ECM not connected to the bracket by the screws on the metal portion, just to the plastic back. So, I've been skeptical of it's abilities from the start. The problem I have is that when I've encountered a fried ECM from loss of ground it is very dead & even if the engine will run it's super rich or floods. I've had one that was somewhat similar in that it would run for about 5-10 min, then randomly start running very rich & then flood. This car runs fine cold, and will idle forever hot. If you are very slow with the throttle it will rev fine to 6K and stay there indefinitely. But after one minute from startup, if you are too quick with the trhottle it will cut out with a bit of spark knock, and unless you let it go it will die like you turned off the ignition. It will start right back up again.

Intake air temp reads fine and it doesn't appear to have had the plug tampered with. Base timing is at 7 BTDC, timing @ 2300 w/ECT disconnected is about 15 (jumps quite a bit), timing at @4500 is 25, timing @6000 is 30. I reset the ECT code before checking @4500 & 6000. Mitchell says total advance is 27-30 @ 6000, so I'm not sure how that relates to your test, but the timing seems to increase pretty well. Knock sensor seemed tight (might only be that it hadn't been removed in a long time) & was retorqued. I wish I could find some sort of test for it, how about on Digi II?

I still am perplexed by lack of some codes setting. No 2322 or 2323 if I disconnect air flow meter. No 2141 if I disconnect knock (even if I short the yellow & black together). No 1111 to suggest an ECM problem. No 2342 if I disconnect O2. It always gives a 4444. I notice there is no 2113 for Hall sensor available.

I noticed I have an extra water sensor/switch at the left side of the head where the heater hose goes in. Its a white sensor w/2 green wires. One goes to a connector where the A/C compressor used to be and the other goes to a piont above the P/S reservoir beside the battery where there are severall disconnected wires as if a relay had been removed. I think it's all for a dealer installed A/C system that was removed as it appears to be a temp cut-out switch for the compressor.

One last note, while idling every couple of minutes it will cut out a little but the idle control will kick in and prevent stalling. I notice the timing jumps a little during this event. If I have on a timing light it doesn't seem to be related (even if you go WOT until it dies the light continues to flash). Only once or twice did I notice a stutter in the flashes, but it's hard to tell if it's just the light. It also jumps around alot when you are causing it to cut out by too quickly opening the throttle. When you open the throttle slowly the light flashes steady and the timing advances without jumping at all. Have you ever seen a Hall sender cause similar problems?

I keep going back to why I get some codes (ECT & TPS) and not others & suspecting the ECM has fried in a very peculiar way. I just dont want to got to the hassle & expense of finding one & it not fixing the problem (I've already spent way more time on this car than I will ever recoup & the owner will have a heart attack if I add an ECM w/o fixing it). I don't feel it's the ECM, but I lack experience with this one that controls the ignition too.

Sorry for these expansive posts.

-Joe

Art,

If you're following this;
I have a stack of the older Bentley manuals, but in the mid 90's decided to go with Chilton, then Mitchell CD-ROM systems. I haven't been impressed with their content after being spoiled with your coverage, but as I do alot of Domestic & Japanese in addition to European so I had to pick something. Have you guys thought about offering an all inclusive European DVD or CD set? Or just by manufacturer? I know about e-Bahn's ability to activate by mfg, but was burned by Chilton & haven't been into the subscription scene since. Basically, I'm just asking if you have a discounted rate for us w/shops who need a lot of your seperate systems, but was looking for a better deal than your individual retail prices.

JoeBean

Posts: 4,697
Registered: 02/14/02
Re: 91 Jetta Digifant 1 Hesitation Under WOT
Posted: Feb 19, 2004 8:30 PM   in response to: boschmann in response to: boschmann
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The ignition timing is not advancing enough according to those numbers. That coupled with the other problems you're describing leads me to suspect ECM damage as well.

To relate a brief story, I had a CIS-E Motronic vehicle once (which has the same ignition and fuel system control together) which had a bad ECM. The only indicators, though, was an intermittant starting problem where the car would start and immediately stall, and a constant fault code for the knock sensor. Swapped ECMs and immediately the problem went away. So it doesn't always have to cause billowing smoke and gunshot backfires :).

Terry
boschmann

Posts: 41
Registered: 01/06/04
Re: 91 Jetta Digifant 1 Hesitation Under WOT
Posted: Feb 20, 2004 4:05 PM   in response to: JoeBean in response to: JoeBean
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I finally have given in to accepting it's got to be the ECM. I've been avoiding that conclusion because I haven't been able to locate a good match. The car has Bosch # 0 261 200 554, VW # 037 906 023. I have found one that is Bosch # 0 261 200 284, VW # 025 906 022D. I believe it came from a Vanagon & seems a liitle too difierent (number wise). I suspect it would work, but it will probably have some fairly different programming characteristics. I was thinking something out of an A2, Cabrio or Corrado. Before just crossing my fingers & trying it (and burning something out) I was wondering if you had any experiences or advise. This one is cheap, the car is not much to look at & the owner doesn't want to spend any money on it (like a new ECM), but as long as the car is basically driveable she will be happy.
JoeBean

Posts: 4,697
Registered: 02/14/02
Re: 91 Jetta Digifant 1 Hesitation Under WOT
Posted: Feb 21, 2004 12:40 PM   in response to: boschmann in response to: boschmann
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It is a different ECM. You'll likely have some problems, how severe I don't know. As a note, part 037 906 023 has been superceded by 037 906 023 M.

Have you tried www.car-part.com for the ECM? They have some there for Cali RV Digifant I engines for as little as $60.

Terry
boschmann

Posts: 41
Registered: 01/06/04
Re: 91 Jetta Digifant 1 Hesitation Under WOT
Posted: Mar 2, 2004 10:46 PM   in response to: JoeBean in response to: JoeBean
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It seems there are alot of different varieties of Digi 1 ECM's, and alot of those on car-part.com were incorrectly identified. I did find one in New Jersey, an 023M, and tried it. Unfortunately, the problem persists. Again, once warm it will idle fine but will act like you turned off the ignition switch when you depress the throttle. If you slowly rev the engine it will rev fine. I plan to try a distributor & knock sensor even though they seem fine. Any other suggestions?
boschmann

Posts: 41
Registered: 01/06/04
Re: 91 Jetta Digifant 1 Hesitation Under WOT
Posted: Mar 5, 2004 12:53 PM   in response to: boschmann in response to: boschmann
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I just wanted to post an update that replacing the distributor fixed the problem. I'm still not sure exactly what was wrong with it. Thanks for your suggestions.
golfist

Posts: 4
Registered: 05/16/04
Re: 91 Jetta Digifant 1 Hesitation Under WOT
Posted: May 17, 2004 6:05 AM   in response to: boschmann in response to: boschmann
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Hi,
I have Passat B3 model, with same problem.
For me, after many tryes, solution was simple.
In air filter compartment is a small piece that controls vacuum depend of air temperature.
This piece was wrong for me, so, when air was hot, the vacuum decrease, and, the relief valve pressure have not enough vacuum level to compensate idle to run transition enrichment.
So, first i take off the vacuum hose engine-air filter, i obturate the vacuum tube, so the vacuum is not temperature dependant and my engine runs allllrigt all day.
Maybe this works for you too.
wowman

Posts: 18
Registered: 07/17/04
Re: 91 Jetta Digifant 1 Hesitation Under WOT
Posted: Jul 17, 2004 11:33 PM   in response to: boschmann in response to: boschmann
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I've got the same problem with my 89 Golf, Digi 2. What was wrong with the distributor? Can it be cleaned? Thanks!
Jeff_Kimo

Posts: 1
Registered: 01/18/14
Re: 91 Jetta Digifant 1 Hesitation Under WOT
Posted: Jan 18, 2014 9:14 PM   in response to: wowman in response to: wowman
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I followed the previous thread very closely, since it best described the symptoms of my 92 Cabriolet. Specifically the engine would idled normally, run well without major load or acceleration, but with firm acceleration the engine would stammer and hesitate. Similar to a cold engine with a lean fuel mixture, and this got progressively worse over a few weeks. I checked fuel pressure, all fuel injection connections and grounds, distributor eta. But chasing all the logical sources of trouble yield nothing. There were hints that the ignition / ECU could be the problem, but the spark was strong and timing spot on.

The breakthrough was in changing the generic spark plug cables to new Bosch set. Plugs, cap and rotor had previously be been replaced with minor improvement. The old cables (graphite core?) had extremely high resistance, and I assume were causing arching within the distributor cap. I would speculate this was interfering with the Hall sensor. Long and short after weeks of checking out everything under the hood, it was the spark cables. Now runs like a charm, hope this helps.


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