An attack of the Vapours - in Death Valley...

[fxr]

On our recent road trip to the Grand Canyon and back, our Westy behaved impeccably - except once. It was a situation I'd never met before in any vehicle, but with a bit of thought it was pretty evident what had happened (if anyone has a different diagnosis, I'm all ears!).

History: We had just completed a very long hard slog over 12 miles up a road with 6% to 9% grades - no let-up. We did it by going about 20-25mph in 2nd gear - no over-revving or labouring, and the fan only came on low once. However, the downhill afterwards was 9 miles at a pretty constant 9%, and I kept it in 3rd and less than 50mph, using the brakes as little as possible, but didn't touch the throttle for all the time it took (12 minutes?). When it levelled out and I tried to accelerate, the van really didn't want to know. It just struggled, so I slowed down and pulled over. The idle was very low and then the engine died.

There were no warning lights - yes I double-checked the oil pressure light still worked! - and no funny smells. It wasn't a sharp cut-out, as an electrical problem might have been, but a gradual disinclination to run. This made me think of fuel - yes we had loads on board. Then it clicked - I'd not touched the throttle for 12 minutes or so, just using as much engine braking as was possible - perhaps in this circumstance the fuel pump wouldn't be doing much, and the hot engine could allow a vapour trap to develop. I'd have expected this to be dissipated as soon as the pump began to work - but maybe that's not how things happen in reality.

We decided to let the whole rig cool down for 20 minutes - and then she started up just fine, and gave no similar trouble the rest of the journey. We encountered about five more long hard grades over the next two days, but none of the subsequent downhills needed more than a minute or two of zero throttle.

I'm sticking with my vapour lock diagnosis - unless anyone can suggest another logical reason for the 'Vapours'?

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Jim Crowther
1984 1.9l EJ22 Westy Wolfsburg Edition
Vespa GTS 300

[crazyvwvanman]

The fuel pump only has 1 speed, full blast. It runs full blast at all times when the engine is running. Most of the fuel it pumps goes through the fuel lines, out the pressure regulator and right back to the tank. The heavier the engine load the less fuel goes back to the tank since the engine is using more of it.

So going down hill with your foot off the gas there is actually the most fuel circulating through the system.

Mark

[snwbrdr435]

How full was your tank?

[slave1pilot]

It's funny, but I think I know exactly where that pic was taken. It's down on the flatlands just before the steep grade up Towne pass , right?

I test cars for Hyundai/Kia out in Death Valley and we go out there at least one week a month between May and October
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[fxr]

Tank was about half full I'm reminded we filled up at Stovepipe Wells, just a few miles before the pass - would a 9% grade put the outlet above the level in the tank? It might have done with a bit of braking going on as well, and caused the pump to be spinning in air.

Point taken about the pump being on all the time - I somehow imagined it'd shut off if there was no demand - but that's silly as at zero throttle the idle microswitch is on.

Yes, it was coming down Towne pass heading west that caused the problem. We camped at Panamint Springs a couple of miles further on - a very clean and cheap ($15) private campsite.
_________________
Jim Crowther
1984 1.9l EJ22 Westy Wolfsburg Edition
Vespa GTS 300

[djkeev]

The TPS tells the ECU to not pulse the injectors much....... As mentioned cold fuel is circulating at full speed ahead.

Check your TPS for a sticking situation? They get gummy inside.......

Dave
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http://www.thesamba.com/vw/forum/viewtopic.php?t=392473

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http://www.thesamba.com/vw/forum/viewtopic.php?p=6315537#6315537

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https://www.thesamba.com/vw/forum/viewtopic.php?t=482968&highlight=74+super+vert

[slave1pilot]

[Howesight]

When driving with a trailing throttle (throttle closed, engine on, transmission turning the engine), the "fuel cut" feature of the ECU programming stops all injector firing. This is to save fuel and also for cleaner emissions. This feature starts to inject fuel again once the engine goes below a set rpm. I forget what that rpm is on the Vanagon, but most OEMs use between 2,000 and 2,500 rpm.

Your descent lasted about 10 to 12 minutes. During that time, the cylinders were cooling due to no combustion occurring. The oxygen sensor became completely non-functioning because it needs the heat of combustion to create its voltage signal to the ECU. The engine coolant temperature would not have cooled enough in that 10 to 12 minutes to significantly enrich the fuel mixture.

The engine faced a temporary lean condition because, once the injectors started firing again, the cylinders were cold enough for fuel to condense on the cylinder walls and the pistons, and the oxygen sensor was not hot enough to sense and correct the lean mixture resulting from that fuel condensation.

Since your fuel pump never shut off, the system could not suffer vapour lock. In an EFI system, vapour lock can only occur if your engine is hot, fuel pump off, and the residual fuel pressure is lost due to, say, a failing/leaking check valve or an internally leaking fuel pressure regulator.

That's my guess.
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'86 Syncro Westy SVX

[fxr]

[tschroeder0]

also don't rule out chunks of the cat breaking off and moving around and momentarily blocking the exhaust outlet...low power or killing the engine. In general it would then stay plugged, but just putting it out there for another idea if it occurs again.

[Wildthings]

If your deceleration fuel cutoff isn't working your engine may flood on a long downhill and be way too rich as you first open the throttle again.


In normal driving you should be able to feel when the deceleration fuel cut off kicks in and out. If it isn't working you need to check and adjust your Throttle Position Switch.

[jberger]

I tend to think that while in a long decent, pulling high vacuum coupled with no valve seals and a failed Crank Case vent tower, your cylinders became temporarily fouled with oil.. I have experienced this on many vans in the bay area with all of our lovely hills. The fuel cut off feature goes away the instant the throttle switch is engaged and the o2 sensor never actually stops working as it is a heated type. Good rings, intake valve seals and proper crank vent control makes this event a thing of the past.

[fxr]

[Howesight]

Now that I hear your oil consumption was a quart per 500 miles, I agree with Jberger that oiling of the spark plugs probably caused the symptoms you described. However, I don't agree with JBerger on the oxygen sensor issue because the 1983.5 through 1985 (Digijet) 1.9 engines did not have an electrically-heated oxygen sensor. That feature was only added in 1986 with the introduction of the 2.1 Digifant engines.
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'86 Syncro Westy SVX

[jberger]