1978 Bus - Idle's But Dies With Throttle

Post by **Amskeptic** » Fri Aug 01, 2008 8:26 am

If indeed the fuel pressure is up at drop-dead, then we can go elsewhere.

There is a "circuit breaker" in the ignition circuit. I know not the details, but I do know if you short an ignition supply wire (choke heater wires love to fall off and ground against metal), the engine will stall then start right back up.

Is this a high rpm die off only?
Colin

DurocShark · Post by **DurocShark** » Fri Aug 01, 2008 10:21 am

Amskeptic wrote:If indeed the fuel pressure is up at drop-dead, then we can go elsewhere.

There is a "circuit breaker" in the ignition circuit. I know not the details, but I do know if you short an ignition supply wire (choke heater wires love to fall off and ground against metal), the engine will stall then start right back up.

Is this a high rpm die off only?
Colin

It would appear so. I haven't tried other than around 2k. Idle works fine. At least for the several hours I've had it idling.

vdubyah73 · Post by **vdubyah73** » Fri Aug 01, 2008 4:48 pm

Do you have an aftermarket tach? If you do, go lookin' for a chafed wire. If not, look for a chafe in the - wire from the coil to the ECU. The engine does move around on it's mounts when it is revved or when it is being driven, possibly causing said wire to short. I mentioned this in an earlier post.

DurocShark · Post by **DurocShark** » Fri Aug 01, 2008 7:57 pm

Yes and it's clear.

DurocShark · Post by **DurocShark** » Sat Aug 02, 2008 9:36 am

Another troubleshooting step taken. I don't think this eliminates the ECU, but I was really hoping for an obvious failure.

I pulled the ECU out and removed the cover. No obvious blown diodes or resistors. All the standard diodes had zero continuity one way, near zero resistance the other way. I don't have any way to test the zeners but they were all clear. I grounded the housing and fired up the bus. I held the rpms to around 2k and watched the computer. No diode flash when the engine died at the usual, a little under a minute, point. Fuel pressure stayed up with no significant fluctuation.

:(

vwlover77 · Post by **vwlover77** » Sat Aug 02, 2008 10:41 am

I would still swap out the ECU....

.... and do that fuel pump volume test!

Post by **Amskeptic** » Sat Aug 02, 2008 11:21 am

DurocShark wrote:No diode flash when the engine died at the usual, a little under a minute, point. Fuel pressure stayed up with no significant fluctuation.

:(

Let's get esoteric. Bosch had issues with cooling down the engine too much on closed throttle overrun fuel cut-off (this was an economizing step to shut off fuel while coasting/engine braking). The cylinder walls would cool down so rapidly that there were HC spikes when the fuel was turned back on. This switching program was originally designed to cut off the fuel any time the throttle switch contacts were closed and the engine speed wire was signaling above idle at operating temperature. It was bumped up to 1,750 rpm on later units. Cut-off would end if the RPM's dropped below 1,750 while coasting.

Now let's say you have this amazingly peculiar engine death issue after a couple of minutes of high rpm only. What if the fuel cut-off circuit was supposed to be only telling the ECU that it is now available to do fuel cut-off, but the ECU is inadvertently assuming that the engine is coasting and it shuts off the injectors?

To test:
Run the engine with an accurate tachometer at no more than 1,600 rpm.
Does it die at usual time?
Run the engine at 1,800 rpm. Does it die at usual time?
If no at 16 and yes at 18, new ecu if you do not have the throttle switch under the throttle body. (that TS was 1976-ish, no?)
ColinThisISInteresting

DurocShark · Post by **DurocShark** » Sun Aug 03, 2008 12:13 am

I have another ECU in hand. (Thanks Troy!) I'm going to take the easy route and swap it in first. If it works, I know I need a new ECU. If not, well...

;)

DurocShark · Post by **DurocShark** » Sun Aug 03, 2008 9:20 am

New ECU made no difference.

HOWEVER
I did some wiring harness rerouting and that particular symptom SEEMS to have stopped. My tach signal is still crap, dropping and pegging randomly, but the test drive was mostly a success.

I'm not feeling good.

Who was it that was selling new wiring harnesses?

bretski · Post by **bretski** » Sun Aug 03, 2008 9:35 am

DurocShark wrote:New ECU made no difference.

HOWEVER
I did some wiring harness rerouting and that particular symptom SEEMS to have stopped. My tach signal is still crap, dropping and pegging randomly, but the test drive was mostly a success.

I'm not feeling good.

Who was it that was selling new wiring harnesses?

Kyle is your man...can't remember his user-handle on IAC, though.

http://www.kyleautomotivespecialties.com/home

DurocShark · Post by **DurocShark** » Sun Aug 03, 2008 10:01 am

That was it. Thanks.

What sucks is I don't think that'll fix the bucking/misfiring problem, only prevent the "die after 45 seconds" problem.

:(

DurocShark · Post by **DurocShark** » Sun Aug 03, 2008 10:09 am

Interesting observation on the ECUs. The one that came with the Guac had pink Bosch stickers and a Bosch part number ending in AH. Troys, also from a 78, had yellow stickers and ended in AG. I wonder what the differences were?

bretski · Post by **bretski** » Sun Aug 03, 2008 10:22 am

AH = '78 Federal (automatic) ECU
AG = '78 Federal (manual) ECU

AF and AE are '78 California models for auto and manual, respectively.

As far as your bucking problem is concerned, who knows? Maybe you have another bad wire grounding out, and the new harness will fix it...one can always hope.

-Bret

DurocShark · Post by **DurocShark** » Sun Aug 03, 2008 10:23 am

Took it for another test drive. Light misfires on the way to the gas station. I got on the freeway and bad bucking like it was doing in Jan when I parked it. Off the freeway the same thing happened.

So puttering around town the symptoms are minimal, but once I load it up on the freeway they're bad until I shut it down and let it cool off.

And I've got emails off to Kyle and Chris. Troy said Chris may have a good used harness.

DurocShark · Post by **DurocShark** » Sun Aug 03, 2008 10:24 am

bretski wrote:AH = '78 Federal (automatic) ECU
AG = '78 Federal (manual) ECU

AF and AE are '78 California models for auto and manual, respectively.

As far as your bucking problem is concerned, who knows? Maybe you have another bad wire grounding out, and the new harness will fix it...one can always hope.

-Bret

Cool, thanks. So the Guac has an auto ecu? Odd. Musta been a PO replacement.

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1978 Bus - Idle's But Dies With Throttle