1978 Bus - Idle's But Dies With Throttle

DurocShark · Post by **DurocShark** » Sun Jul 27, 2008 8:56 am

I know, I know. I haven't been around in a while. But I need help...

My bus has been down since January, but I have little time to troubleshoot. So in troubleshooting time, I've spent maybe a week on it.

I've isolated things (I think). Specs: 78 bus, stock FI and 2.0.

Symptoms:
Idles from cold start at least 30 minutes (I figure any longer was pointless)
If revved to 2k rpm and held, will only last 45 seconds or so.
- When it cuts out the tach drops to zero
- There are no warning signs leading up to this
- I can repeat this over and over. So it's not (or no longer) intermittent

Wiring has been checked and re-checked and re-re-checked. Coil
replaced. I've now swapped out 3 different coils and get the same
results. Points and condenser replaced. Distributor replaced. I even
put a compufire on it. Exact same results. Wire wiggling does nothing.

Vacuum leaks have all been sealed. The last one was yesterday, a fuel
injector seal wasn't seated properly which was probably from previous
troubleshooting.

Several dual relays tried. Injectors spray nicely when triggered,
don't leak when off.

Fuel pressure stays up. When idling and warm the vac is between 14 and
15, which is probably good for Colorado.

The only things I haven't tried are replacing the engine (heh),
the resistor pack, and the computer.

I'm leaning toward either computer or ignition due to the way the tach
just flat-lines like that, though I can't see how the computer would
affect the ignition?

Any thoughts? Things I haven't tried? Help?

spiffy · Post by **spiffy** » Sun Jul 27, 2008 9:08 am

Have you tried cleaning the contact surface in the AFM? I think Westy78 was chasing a gremlin like this a few years back and the AFM was the culprit. Replacement fixed the issue for him, maybe he'll chime in here as I forget his exact symptoms.

DurocShark · Post by **DurocShark** » Sun Jul 27, 2008 9:23 am

Hmm... I haven't tried cleaning it. The tracks don't cut through though so I didn't dig further. Gonna give it a shot right now.

vdubyah73 · Post by **vdubyah73** » Sun Jul 27, 2008 11:10 am

Your symptoms point to power to your coil cutting out. Try hot wiring your coil directly to the + post on your battery and see what happens. Your tach signal comes from the coil and it's hitting Zero before the engine stops spinning. Thinking as I type, try disconnecting the tach from the coil before you do anything. Maybe the green tach wire is chafed through somewhere and the movement of the engine on its rubber mounts cause it to short the coil?

EDIT: I don't think the resistor pack has anything to do with spark like it does on so many vintage American cars ( called a ballast resistor ). I think it is strictly for reducing the voltage for the injectors.

DurocShark · Post by **DurocShark** » Sun Jul 27, 2008 11:23 am

I thought I had listed that above. Yes, I've hotwired the coil. Nada.

And I agree about the resistor pack. Hence my not replacing it.

Randy in Maine · Post by **Randy in Maine** » Sun Jul 27, 2008 1:47 pm

Don, what kind of voltage are you putting out at those RPMs?

vdubyah73 · Post by **vdubyah73** » Sun Jul 27, 2008 2:43 pm

Do try disconnecting Tach wire from - side of the coil. It may be a short in the tach wire to the tach or in the tach itself. An old prank is to switch the reverse light wire from the + side of the coil to the - side. Every time the prank target puts the car in reverse, it cuts out as all the juice grounds to the reverse lights.

vwlover77 · Post by **vwlover77** » Sun Jul 27, 2008 2:49 pm

I don't think it's the AFM track. If it will rev to 2000 rpm and hold for 45 seconds, then the AFM track is working.

Have you performed the fuel pump VOLUME (not pressure) test to verify that your fuel pump can deliver enough fuel to the engine?

If you cannot get your hand on one locally, I have an ECU for a '78 California model that I can loan you to see if that's the problem. The ECU has to open the injectors for longer intervals at higher rpm and if a transistor in the output circuit to the injectors is marginal, it may shut off when it gets too hot.

Also, the computer is sensing engine rpm from the same lead of the coil that the tach uses. If something in that sensing circuit inside the computer is flaky, it could ground out the coil causing your tach to drop to zero.

The resistor pack is for the injectors. I don't think any VW ever used an external resistor for the coil.

bretski · Post by **bretski** » Sun Jul 27, 2008 8:23 pm

Hey Don,

Troy can probably spare an ECU to test with...Vandejo is out of commission at the moment. You should give him a call or send him an email.

-Bret

vdubyah73 · Post by **vdubyah73** » Mon Jul 28, 2008 3:35 am

Is the tach stock, or is it an aftermarket one installed by you?

DurocShark · Post by **DurocShark** » Mon Jul 28, 2008 4:20 am

vdubyah73 wrote:Is the tach stock, or is it an aftermarket one installed by you?

Is there a such thing as a stock tach on a bus? ;)

I get the same symptoms with both the dash mounted one and my handheld digital one.

I forgot Troy has a couple buses now. I'll drop him a note. I need to send his kids an invitation to my daughter's b-day party anyway.

vdubyah73 · Post by **vdubyah73** » Mon Jul 28, 2008 5:36 pm

DurocShark wrote:
vdubyah73 wrote:Is the tach stock, or is it an aftermarket one installed by you?
Is there a such thing as a stock tach on a bus? ;)

I get the same symptoms with both the dash mounted one and my handheld digital one.

I forgot Troy has a couple busses now. I'll drop him a note. I need to send his kids an invitation to my daughter's bday party anyway...

Don't know, just askin.

Post by **Amskeptic** » Thu Jul 31, 2008 10:58 am

DurocShark wrote:When revved to 2k rpm and held, will only last 45 seconds or so.
- When it cuts out the tach drops to zero

This is interesting

I would hook up a fuel pressure gauge and watch it as you replicate the symptoms. If fuel pressure plummets at failure, you can ditch the ignition system as a cause.

I would hook up a timing light and replicate the failure point and check to see if the stroboscopic flashing quits at failure, then you can rule out the fuel system.

Fuel pumps, having an electrical component, can act wonky based on the temperature of electrical componentry. This means that sometimes fuel pumps only die when the windings get operation-heated to some degree after working awhile. This is not to be confused with how damn hot it is outside, just the windings inside the fuel pump motor.
Colin

vwlover77 · Post by **vwlover77** » Thu Jul 31, 2008 11:58 am

The fuel pump works the same amount whether the engine is at idle or 2000 rpm, no? So why does it idle forever but die at speed???

DurocShark · Post by **DurocShark** » Thu Jul 31, 2008 2:21 pm

vwlover77 wrote:The fuel pump works the same amount whether the engine is at idle or 2000 rpm, no? So why does it idle forever but die at speed???

That could be a volume issue.

The pressure stays up (I have a gauge attached semi-permanently, at least since Feb.) even when shut off. Stays up for several hours at least, but I haven't measured beyond that.

The tach zeros BEFORE the engine fully dies.

Itinerant Air-Cooled

Itinerant Air-Cooled

1978 Bus - Idle's But Dies With Throttle

1978 Bus - Idle's But Dies With Throttle

Re: 1978 Bus - Idle's But Dies With Throttle