Bucking and stalling, only when using cruise control

[ogelneb]

Greetings,

It's been quite a journey getting my Vanagon up to snuff these past couple of years. I've finally sorted out 95% of the mechanical problems and only have one outstanding issue.

As soon as I engage the cruise control at highway speeds (can take 3-4 minutes after it's engaged), the van starts bucking and loses power, sometimes even stalls outright. Other times once I disengage cruise it will continue to buck / lose power for a few minutes and then the issue will go away. It only happens with cruise control on, I have zero issues otherwise. It idles fine, accelerates well, and runs well at speed. I (mostly) understand how the cruise control mechanism functions, and can't figure out what could be causing the bucking and stalling issue.

It's a 1989 Vanagon GL Westy, automatic with the 2.1L engine, all stock. Over the last 2 years in order to address other problems I replaced the ignition coil, spark plugs, fuel lines, TPS, throttle body and gasket, fuel injection relay, water temp sensor, thermostat, rad fan sensor and many other unrelated parts. I also did the tantalum cap piggyback mod, and repositioned the AFM carbon track.

Any thoughts on what could cause the issue and what can be done to investigate/remediate would be greatly appreciated!

[Silverghost500]

Are there any vacuum lines to the cruse control unit in the engine bay? If so there may be a line that has a break or has come off.
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1986 Volkswagen Vanagon Westfalia "Von Westy"
Honda K24A converversion-Road Testing Phase
3-Speed Automatic

Von Westy YouTube Channel
https://www.youtube.com/channel/UCbhyZYxJEQrUuLnMbLleeIA

[DanHoug]

the cruise control is an independent mechanical system, it takes NO vacuum from the engine as it has its own pump, and the electrical inputs to the brainbox behind the glovebox are speedo, brake, and clutch switches. that's it. no ECU, no coil input, no nuthin'. these inputs are tied to the brainbox that regulates when the vac pump runs and when the purge valves open, mechanically sucking on a vacuum actuator that moves the throttle.

my point being, your drivability issues are not caused by the CC system. you likely have cruise issues (vac switches) going on if there is a long lag between engagement and actuation, mine is instant, but there is not a tie-in that causes driveability issues other than possibly steady state throttle which rests the AFM in a worn spot on the track.

so. start by doing a voltage AFM sweep and look for any dropouts.
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-dan
60% of what you find on the internet is wrong, including this post.

'87 Westy & '89 Westy both 2.1 4spd

Past projects can be found at--
www.thefixitworkshop.com

[ogelneb]

[ogelneb]

[Howesight]

I agree with Dan Houg that the Vanagon CC system is not connected to manifold vacuum, but not that it is 100% mechanical. In trying to think about a link between using your CC and the symptoms you describe, here's what occurred in my fertile imagination:

1. Remember that the location of the CC vacuum actuator and the twin rod that connects the rubber diaphragm to the throttle pull is a very busy bit of real estate. With the engine off and lid off, use a vacuum pump and check to see if any part of the actuator is fouling against any wiring.

2. Somebody may have mistakenly plumbed the vacuum hoses for the actuator to connect to vacuum hoses connected to the intake manifold. Check for that.

3. The electrical connector for the CC pump may have been incorrectly wired in the junction box at the front left corner of the engine compartment.

4. If your throttle body is worn, (most are, but it's a matter of degree) then it is possible that the CC actuator, pulling at a different angle than the throttle cable, is affecting the Throttle Position Switch, either switching on the "idle" switch or the "enrichment" switch. (The TPS is one unit with two switches in the unit). If the "idle" switch is closed as a result of this movement due to a loose throttle plate shaft, then the ECU goes into "decel fuel cut" mode, which will definitely cause bucking.

I can see how this might occur because you normally apply the CC switch and take your foot off the accelerator,(waiting for CC to engage), but when releasing your foot from the accelerator, the throttle switch "idle" portion is triggered. To test this possible problem, try the CC again on the road while driving, but this time, hold the accelerator pedal down until you are confident that the CC has engaged. In this way, the throttle will be open sufficiently to rule out the "idle" throttle switch from going closed-circuit.

After determining whether the above is the cause (or is ruled out), it will still be wise to check and adjust if necessary the Throttle Position Switch.


Having said the above, the most common cause for a Vanagon CC operating with lots of delay is a vacuum leak at the CC pedal switch/vacuum valve. They are a combination electrical switch and vacuum valve which can be easily rehabbed. Sometimes, a vacuum hose is frayed near the nipple it connects to. Cut off the frayed bit and re-attach or replace the hose with new. Sometimes, (but not often), the vacuum pump itself is worn and can't develop vacuum quickly.

Regarding re-habbing the vacuum switches, see this video:

https://www.youtube.com/watch?v=KcIL_K9H8zQ
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'86 Syncro Westy SVX

[1988M5]

[ogelneb]

[ogelneb]

Started working my way through possible causes, and haven't pinpointed the problem yet. I disconnected and took apart the brake pedal switch, and cleaned / lubricated. It works as it should and still seals well. Visually inspected all hoses and they check out, but I haven't pressure-tested them yet (need to buy a hand pump gun). Will also swap the TB with a spare I have to see if there's any difference.

Will keep providing updates in case this thread is useful to someone else in the future.