Idle stumble / throttle hesitation

[Freshdub]

Hey all, just finished a 3000km road trip in my new to me westy. Worked well!
After a long 12hour day, pulled into the hotel and turned her off.
Next morning, an unusual throttle lagginess existed. Not a problem on the highway at all and turning the fridge off 12v power helped a bit.
It idles OK, a little bit of stumbling but nothing major i kind of expect it with an old engine. Its rougher than it was though and when you touch the gas it actually kind of starts to die or struggle for a split second before revving up. Smooth through the entire range after that first little bit.

Heres a video to show what im talking about 1.9L '84
https://youtu.be/X8Xeo7Tlob4

The weirdest part that i only noticed once i was back in the garage trying to take a look was that the coolant light flickers very very faintly when it is doing the stumble...

It felt like fuel to me, but ive replaced the pressure regulator the lines and put a new prefilter on about 100km ago. After searching on here im turned all around..

Has new basic maintenance items, (oil, filters, coolant, plugs) new coolant temp II and dash coolant sensors. Recently cleaned all my grounds. Turned CO screw 2 turns in both directions and no difference, returned to where it was when i bought. Adjusted the idle adjustment screw, it was spot on. Tested the switches and both had proper resistance

Have a new coil, ignition control module and distcap to put on still

Anyone have a place to start testing? Im fine with buying new parts for the van, but love to get a few opinions if im along the right track before throwing parts/money at it.

New idle control module? I plugged the 2 wires together and it wouldnt even run... Back into the existing unit and problem still exists. Is it early signs of a failing fuel pump?

Thanks!

[ewalt98]

This may not help much for you, but I had a similar situation once on my air cooled, and it turned out to be a vacuum leak. One of the larger hoses came off, and it caused poor idle, lumpiness, and loss of power.
Might be a good place to start looking.
_________________
Eric
82 Westfalia Camper (Currently getting a little bit fixed up)
85 944
72 GT6
88 F350
03 F150

[Merian]

Has new basic maintenance items - specifically what and how long ago?

- did it do this before you did that? (just to keep in the pronoun only theme)

[Freshdub]

Oil, oil filter, air filter, fuel filter, coolant and plugs. No, it only started randomly one morning after a loooong day of highway driving

[Bassyaks]

Remove the cover and wipe down the wiper contact on the Air Flow Meter, look for any worn spots then weak the wipers if needed

[Freshdub]

Afm wiper has been cleaned, shows little signs of wear, voltage test shows nothing wrong and ive done the capacitor mod to it found here
http://www.thesamba.com/vw/forum/viewtopic.php?t=377466&highlight=capacitor

Confident afm is working as its supposed to

[Merian]

replace, rotor, cap & all wires - report back

[Bassyaks]

He said he did all that!

[Tom Powell]

[hdenter]

I can't offer you a solution, but I can say that once the engine is at full operating temperature, it should be able to start and idle with the idle control disconnected and the plugs plugged into each other. Was that long haul hard uphill driving? Did you change elevation significantly? After replacing the ignition components mentioned I would want to check the timing (I prefer 10cent's method). I would next check the actual fuel pressure.

Hans
_________________
'79 triple white convertible bug
'84 sunroof vanagon
'85 weekender

[Freshdub]

Oh ok i didnt know it had to be operating at temp for that to work. Just unplug while its running right?
Long haul, widely varying altitude through the rockies.

Have new rotor cap wires and o2 sensor in the garage, gotta do them anyways so ill do all of those hopefully tomorrow and report.

Ill do a search for 10cents timing method. Appreciate the thought hans!
Can anyone tell me specifically which lines are vacuum? Im not sure which ones to test... Cheers!

[madspaniard]

Borrow a good working AFM and switch it with yours. This solved my hesitation problem. Also, adjust your TPS as per Bentley.
_________________
1991 Westy auto w/ Peloquin TBD

"The only difference between me and a madman is that I'm not mad” - Salvador Dali

[hdenter]

No, It will probably die if you unplug it while running. Warm it up, then shut it off, connect the plugs and start it again. It should work cold, as well I suppose, but that brings in more parts and systems to the equation (aux air valve...) Unless this problem is isolated to the warm up period and goes away once warm, do your diagnostics with the engine hot.

Hans
_________________
'79 triple white convertible bug
'84 sunroof vanagon
'85 weekender

[Wildthings]

Your coolant light blinking would indicate an electrical problem to me. I would go around and check voltages at various locations to see what they are doing. Grounds are always suspect when things get weird.

[Merian]

the first things to do are things that should be done periodically as regular maint. - i.e. a full tune up

you can next do things that are either cheap to buy or easy to do (low labor)

Lots of possible things could cause your problem - another bet is you may a worn throttle body causing this, but do a full tuneup first - you then know the motor ought to run right and something else is wrong

make a list & check it twice - save the list for next Dec.

[hdenter]

Well,... once you have done a full tune up and checked a few things like the timing and fuel pressure, use the digijet pro training manual to test components at the source and at the ECU plug. Don't just start throwing cheap parts at it. My money is on either the throttle switches, a timing issue or fuel delivery. Maybe you have a gunned up injector that can't flow enough at idle and Is giving you a lean miss there as well as a flat spot off idle. You can do a flow test.

Hans
_________________
'79 triple white convertible bug
'84 sunroof vanagon
'85 weekender

[Howesight]

Hey Freshdub:

You have a transitional throttle response problem, as is evident from the video you uploaded. (By the way, such videos are very helpful for us folks trying to assist in diagnosing running problems over the internet).

Here's my theory:

You have a significant vacuum leak. That's it. This leak reduces the air flow signal that the vane flap in the AFM normally would receive, meaning that the "accelerator pump" function of the ECU is not adding enough fuel when the mass air flow initially increases when the throttle is opened. (The combination of the flap vane movement and the air temperature in the AFM at the air temp sensor allows the ECU to determine mass air flow and mass air flow changes from steady-state.)

This has knock-on effects. The lambda system reacts quickly enough to the temporarily lean mixture to add fuel, but not as quickly as the AFM, when working properly, would do.

To test my theory, drive your van with very slow changes in throttle position. If it now drives as it always did, this indicates that my theory about a vacuum leak is correct, and/or that your AFM is not functioning properly. By the way, in this mode of avoiding quick throttle position changes, the lambda system is correcting for the mixture problems created by the vacuum leak(s).

The main culprits for vacuum leaks on these old engines, in order of likelihood and seriousness, are:

1. Cracked boot from AFM to Throttle body (you must remove it to thoroughly check for cracks), or loose connection of boot;

2. Cracked/disintegrated connector hose between intake plenum and intake manifold runners;
3. Ruptured diaphragm in crankcase vent tower;
4. Very worn throttle body;
5. Seals at the injectors;
6. intake-manifold-to-head gasket failure.

Check these out and let us know.
_________________
'86 Syncro Westy SVX

[Freshdub]

Perfect, seriously can't explain my gratitude to the community here.
After work today I pulled all the plugs to check for fouling, everything was good. Swapped the alt belt and all spark wires.
I've got such a large list of parts in the garage I only want to change one or two at a time to try and isolate the cause.
Replaced injector seals when doing the lines, it does react fine to very minute changes in throttle, the faster and more abrupt the throttle body opens the more it bogs.

Made a slight difference, HOWEVER(!!), I noticed when removing the air box & AFM that the passage was very greasy on the boot and inside the AFM.. Dark and oily... I cleaned it out, definitely thinking the boot may be cracked and causing a leak/getting crap into the engine.
When reinstalling the air box and I hit the snap clip holding it in place to the car, it seemed to pull the air box away from the boot at the bottom. Looks sealed properly from the top, but definitely slips away at the bottom.
Who knows, after a long hot drive it could definitely cause a slight gap between the boot and AFM which caused all this.
Seems to be running OK now, I'm sure its from getting the intake box sealed on better (not the wires)
Will replace the boot, thoroughly clean the AFM inside the vane, and get a new filter clamp and report back. Would defs be an easy fix to all this!
The list of vacuum spots is very helpful, I'm still learning my way around the engine. There's some things that need to be in the sticky!

[ohhorob]

[Wildthings]

You need a good seal between the S-boot and the AFM and between the S-boot and the throttle body. There should be screw clamps on both of these joints.