| Re: [GMCnet] Vapor Lock (yes, again) [message #88186] |
Sat, 12 June 2010 07:36 |
Gary Casey
Messages: 448
Registered: September 2009
Karma:
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Senior Member |
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Yes, it does sound like vapor lock, but there is another issue that seems to be unique to the Q-jet, and that is what we used to call "spewing". The root cause is that the float bowl is very small and the fuel has very little exposed surface area. In fact there is a plastic spacer in there to reduce it to the minimum. Works well most of the time, but if the carburetor heats up and then is provided with a fresh batch of fuel (long idle period or short hot soak) the light ends of the fuel will vaporize. There isn't enough volume to allow the vapor to escape, so the fuel goes into foam and "spews" out the vent at the top. Symptoms are similar to vapor lock, but the engine flames out rich instead of lean. Something to consider.
Gary
I'm up on the mountain with the coach, and had some interesting vapor lock problems on the trip up here. Thought I'd pass it along to add data points to the collective consciousness.
First incident: After having driven about 75 miles on a hilly I-81, speeds 65-70, I exited in Harrisonburg and came to a stop at a traffic signal. After about 45 seconds of waiting, the light turned green. The road climbed from the intersection and I gave it fairly hefty throttle. After about 200-300 feet, I lost power in classic fuel starvation symptoms and had to back the throttle way off to keep the engine from dying. After about 15 seconds, it was fine again. No problems with other signals on that route.
But then, I stopped at a grocery store and idled for several minutes while making a radio contact. Second incident: While talking on the radio, the engine died. It would not restart on either tank. I went in and did my shopping, wondering if I'd run out of gas (which didn't seem possible--I figured I had at least 20 gallons in the tank). Shopped for about half an hour, came back out, and the engine fired right up without the slightest issue. Drove it one block to a gas station and put in 31 gallons--just as expected (my tank capacity is 54 gallons).
Fuel was from last fall, bought in September and probably still a summer blend.
Drove it up the mountain, which is a climb of about 3500 feet in 15 miles, including the last 500 feet of climb on a rocky forest service road at just above crawl speed, and had no further issue, except maybe a bit of a burble when starting up after a pause at the start of the dirt road to raise the rear.
On the fan clutch front (and it may be related), temperature would not maintain at 180 on the highway with the AC going. The clutch cycled in every little while, and just as before, the coolant temperature immediately starting dropping and the fan would go off less than two minutes later with the coolant temp at 178. Coolant temps when the fan came on ranged from 188 (gradual warmup) to 200 (fast warmup on a steep climb).
The clutch ran much less often on the big climb--I had turned the AC off. The AC is definitely adding to the problem, as one would expect.
Ambient temps were in the 90-degree range on the Interstate, and probably in the upper 70's on the mountaintop. It was warm.
Rick "deeply annoyed by this vapor lock thing" Denney
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'73 Glacier 230 "Jaws"
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