Cold Start Flat Spot Issue

[Vwman55]

I have tried to fix this flat spot when engine is cold for a while now, need some help. What happens is, when its cold press the gas pedal, no response. It will die if you don't let up on the pedal. I thought that it was the TPS but, after testing it numberous times all the way back to the ECU I believe it's OK. Set at .004 opens. Opens and closes as it should at idle and WOT. Temp sensor was replaced, the one I took out was ok. It also checks out back to ECU. A few things I did, don't know it means anything, when cold I unplugged idle stabilizer, wouldn't start, with idle stabilizer plugged in and TPS disco'ed, idle rough. This is kind of intermittent. I replaced the fuel lines and found 2 possible vacuum leaks that I fixed. Breather tower O ring and had a hole in one of the valve covers. It had the flat spot before and after. It runs just fine after warmed up. It devoloped the problem a few years after I bought it. Anything else I can check? I going camping later next month and would like to fix it.
'91 2.1L Auto.

[Ahwahnee]

Have you cleaned the track on the AFM? If it is dirty it can cause faults that change with temperature. Only takes a moment with a Q-Tip and lube/cleaner (once you get the lid off).

[Vwman55]

[tencentlife]

Don't look a gift horse in the mouth, you asked for advice and he's on the right track. But test fuel pressures first, always, without this info everything else you do is possibly futile. THEN test AFM with a voltmeter. But don't go straight to cleaning the track and wondering if it fixed anything when it probably won't, get a VOM and perform the test shown here:

http://www.thesamba.com/vw/forum/viewtopic.php?t=369815

and you will KNOW whether the signal is clean or not. If dropouts are indicated THEN you should open the AFM and deal with the track, try cleaning with electronics contact cleaner and rstest, then repositioning the board and retest.
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[Vwman55]

Fuel Pressure @ idle 32psi w/vacuum_38psi w/vacuum disconnected
Residual Fuel Pressure 30psi more than 10 minutes
AFM Test: Pin 3-4 = 562Ω, 1-4 = 2000Ω @ 85º, 2-3 = varied with flap movement
tencentlife test = no drop outs.
I noiticed after starting that at WOT it stops at appox 2500rpm and surges/labors like it wants to rev high but its limited.

[tencentlife]

Your fuel pressures are a bit high, but the spread is about half a bar so it could be gauge error.

Just for grins, try disconnecting the throttle switch and see how it drives.

And make sure all the intake tract is tight, no unmetered air leaks. Check the S-boot clamps, you could even take it off and check for cracks.
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[Vwman55]

Disconnected the TPS idles goes up. What I need to know is where the brake booster vacuum hose goes when it disapears in the front. I found it won't hold vacuum from the check valve forward(used vacuum pump). I can see from air intake: short house, check valve, short molded hose, connector, long soilid hose goes up by spare tire, and disappears. From inside I can see what appears to be another molded hose from the booster that dispears near shifter.
Is there another connection underneath? Where? Need to fix the vacuum leak for sure.

[tencentlife]

[Vwman55]

[Vwman55]

Tencentlife: Fixed the vacuum leak in the brake booster line. Bad check valve. Improved running. Drove with the TPS disco'd. Can't really say it made much difference in running. Only thing I noticed was I stopped for about 45-50 minutes and after a bit of a cool down, had a small miss/backfire off idle from a complete stop. It dissapeared after it warmed back up. I also cleaned the temp sensor plug. I think that's what was causing the limited RPM when cold.
After reconnecting everything I'd say the only complainant is it has a hesitation off idle from a complete stop. No pop, miss, backfire, just hesitates before accelerating. If I floor it, it's really noticeable, if I ease on the gas not as bad.
ThanksΩ

[Ahwahnee]

[Vwman55]

Ahwahnee, No didn't clean the track yet. Read above what Tencent suggested. I take that as if it doesn't drop out then cleaning isn't going to help. I'll keep on the list.
ThanksΩ

[Ahwahnee]

In my case I could not recreate the drop out while I was poised at the engine with the analog voltmeter and the cover off the AFM.

I just cleaned it because it was there & I was there and I figured it certainly wouldn't hurt.

My hesitation coming off of idle did not happen until the engine got truly warmed up. It's possible that by removing the engine cover and then the cover from the AFM (for testing) I introduced some variable (e.g. cooler air) that changed the result.

Good luck.

[tencentlife]

If you do the AFM voltage test the caveat is you have to remove the AFM from the airbox and push on the airvane with some kind of tool. If you work the wiper from above with the cover off you change the pressure on the wiper and may mask a dropout that is actually there, I found this true with some of the units I've tested.

I was suggesting doing the test first since it sounded like the OP's AFM had not yet been opened up, testing tells you what you need to know without having to break the cover seal. If it's already open, then it can't hurt to clean the board, so long as you use an appropriate product, but then again it's more likely to need cleaning once the original seal has been broken, they stay pretty clean otherwise. I would think trouble due to a film causing resistance at the wiper contact point would be much more prevalent in urban areas with more air pollution.

In my experience the most common causes of off-idle stumble, cold or hot, are the throttle switch first and AFM second. You rule out the TBS by unplugging it and driving it under the conditions where the complaint occurs, most will idle but not exactly the same and some may not idle well or at all, because the ECU is not being signalled to run the idle program and on late models the ISCU is also not activated, but the tip-in should be smoother. Temp changes can cause TBS or AFM problems to vary, the TBS is especially affected by temp changes.
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[Vwman55]

AFM cover has never been removed. I tested the AFM by pushing on the flap as shown in your video. What's next?

[Vwman55]

Tencentlife, after the van set for a few days, backed out of driveway and pressed on the gas, WOT, and won't go pass 2000 RPM. Pulled over, left running, disconnected TPS and problem was gone.
Does that indicate the TPS is bad or sticking?
If so, the only replacement I've seen is Vancafe's version. Have you or anyone used one of them? More important does it work correctly? Any other alternatives?
Thanks º

[tencentlife]

Yeah that was the test I wanted to see what happened with. That does indicate the TBS microswitch is sticking closed. You can get a 5V-rated microswitch at Radio Shack and adapt that to it simply, I've done that and it's pretty easy, or get the replacement parts from VC, they've basically done the same for you. I also have had the vacuum-actuated module on offer that supercedes the TBS, it's really the ultimate solution, but the guy who builds them for me doesn't seem very keen on producing and they've been unavailable for months, so I can't help with that right now.
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Shop for unique and useful Vanagon accessories at the Vanistan shop:
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Please don't PM here, I will not reply.

Experience is death to doctrine.