Last year my parents bought me a 1977 Bus for my birthday/graduation. My father drove it the 130 miles home without a problem. A couple of weeks later though, I noticed a slight hesitation here and there. It quickly became worse. It would surge even though my foot was steady on the gas pedal. Then it would eventually stall and die. When it cooled down again, the Bus would run fine until it got warmed up. A friend of mine and I spent some time with it. We replaced the plugs, wires, condenser, distributor cap, fuel lines, cleaned the injectors, and found that it passed all of the amp, volt, and compression tests we could throw at it. We did notice, however, that it drops significantly in fuel pressure when it warms up and that when the pedal is a a certain angle, the Bus will literally shudder down the road. I’ve brought it to the local Volkswagen dealership and they replaced the fuel pump twice, cleaned the tank, replaced the airbox and head sensor, and have done an overall tune-up but nothing has helped. It will idle all day without a problem, but as soon as it is put under a load and it is warmed up, it hesitates and dies. Whats wrong with it?
Okay, firstly, you’re about 20 years too late to be taking it to the VW dealership. Dealer techs are typically younger and probably have no experiance with the old style VW fuel injection, plus they charge a lot more than you should be paying. If you’re lucky, you live in a town that has an independant VW specialist-- they usually have old timers that have the know-how to really diagnose this thing.
Also, really the only way that owning an air-cooled VW is a reasonable proposition in this day and age is if you do the majority of your own work. There’s two books you should probably have, the first being the Bentley Publishing Co.'s service manual for your model, which will contain the information you absolutely need to work on this beast. The other one would be the Muir “VW Repair for the Compleat Idiot” book, which is less useful technically, but is an excellent read nonetheless. Also, the board over at http://www.thesamba.com will have people with more expertise on your van.
It’s been a long time since I worked on a old pancake motor, but it sounds to me like you may be having a distributor advance issue. There’s some sort of wierd hall-effect thingee that I recall being problematic which may be part of your problem. Hooking it up to a timing light and watching how and if the spark advances would be a good start. Also, I don’t know what this thing has for a fuel pressure regulator, but that would be suspect due to the drop in fuel pressure, although the drop in fuel pressure could be due to fuel injectors opening as a result of normal operation. Sorry I can’t be of more help!
EDIT: Also, was a valve adjustment part of the complete tune up? Air-cooled VW’s are very sensitive to misadjusted valves and they need to be checked every other oil change at least.
I agree that you need to find a good shop that works on air-cooled VWs, the dealer will have no experience with these. The fuel injection on these was always a little flaky and you really need someone who understands it. On my 914/4 (type IV VW engine) I replaced the fuel injection with a couple of weber carburetors and made my life much simpler.
The bus uses AFC injection and off the top of my head I do not remember various specs, etc. but here are the most likely suspects to check since the airbox has been replaced.
Make SURE there are no air leaks in the intake system - from air cleaner to cylinder head it must be airtight.
Check the throttle switch.
Check the intake air sensor.
Check temp sensor II that is located in the intake tract.
Either a manual on AFC would be very helpful to have around or you might find the check procedures with a net search.
In your post I see no mention of having changed the fuel filter. I am sure the dealership mechanic would have done that. One thing to consider is that the fuel filter may be clogged again. I once worked on a 280Z that had drooping fuel pump pressure. Changing the fuel filter would cure the problem but the stumbling and loss of power would return. After the fuel tank was steam cleaned and a new filter installed, the problem went away; didn’t come back; and the Z ran like a champ.
Hope your solution is the same. Let us know the final solution to this problem.