1978 Bus - Idle's But Dies With Throttle
- DurocShark
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1978 Bus - Idle's But Dies With Throttle
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?
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
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- DurocShark
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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.
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.
1/20/2013 end of an error
never owned a gun. have fired a few.
never owned a gun. have fired a few.
- DurocShark
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- Randy in Maine
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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.
1/20/2013 end of an error
never owned a gun. have fired a few.
never owned a gun. have fired a few.
- vwlover77
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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.
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.
Don
---------------------------
78 Westy
71 Super Beetle Convertible Autostick
"When we let our compassion go, we let go of whatever claim we have to the divine." - Bruce Springsteen
---------------------------
78 Westy
71 Super Beetle Convertible Autostick
"When we let our compassion go, we let go of whatever claim we have to the divine." - Bruce Springsteen
- bretski
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- DurocShark
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Is there a such thing as a stock tach on a bus? ;)vdubyah73 wrote:Is the tach stock, or is it an aftermarket one installed by you?
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.
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Don't know, just askin.DurocShark wrote:Is there a such thing as a stock tach on a bus? ;)vdubyah73 wrote:Is the tach stock, or is it an aftermarket one installed by you?
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...
1/20/2013 end of an error
never owned a gun. have fired a few.
never owned a gun. have fired a few.
- Amskeptic
- IAC "Help Desk"
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Re: 1978 Bus - Idle's But Dies With Throttle
This is interestingDurocShark wrote:When revved to 2k rpm and held, will only last 45 seconds or so.
- When it cuts out the tach drops to zero
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
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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???
Don
---------------------------
78 Westy
71 Super Beetle Convertible Autostick
"When we let our compassion go, we let go of whatever claim we have to the divine." - Bruce Springsteen
---------------------------
78 Westy
71 Super Beetle Convertible Autostick
"When we let our compassion go, we let go of whatever claim we have to the divine." - Bruce Springsteen
- DurocShark
- IAC Addict!
- Location: A Mickey Mouse Town
- Contact:
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That could be a volume issue.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???
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.