Good ideas - I've also been trying to think of things that could cause the tank to be pressurized...
- LDP : I believe the LDP does pressurize the tank and then does a leak-down test. I believe it also closes the vent line and N80 valve, to test a fully closed system. With the vent and N80closed, vapors can not pass through the canister to be adsorbed or be burnt through the purge system. Could the LDP get stuck in "test" mode? That might explain everything, though my van has no emissions codes, so seems unlikely.
- pressure back flowing from the N80 purge valve? If somehow the intake manifold was not at vacuum, but under pressure, then the N80 opening could allow pressure back into the fuel tank. I'm pretty sure this is not possible, because (A) if the intake manifold has positive pressure and your engine is not turbocharged, things are going very wrong, and (B) I think the N80 flows only one direction, and I think there's a check valve between the carbon canister and the tank.
- Pressure / altitude : I used to think this was mainly a heat issue, but my recent experience says otherwise: everything was fine with a very hot but gradual trip up to about 4000', but only when I headed up to 6000' then 7500' did things go wrong, even though ambient temperatures were going down by then. Also, when I measured the gas/gas tank it was only 120F, which doesn't seem that hot to me.
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On Jul 13, 2023, at 5:53 PM, Stephen Jackson <register@...> wrote:
Round trip is normal for this era of vehicle. It’s only in the last decade or so since they quit using a return line
I can’t remember if the leak detection pump draws a vacuum or pressurizes the tank
But I suspect the problem is heat is getting into the fuel lines. From where and how is the question
In the hot rod world we had a problem very similar to this. People were using high volume fuel pumps, that combined with small tanks and summer heat, could boil the gas in the tank, as the pumps were circulating it too quick. The solution was to use a PCM (pulse controlled) pump. Not a solution for us
Back to the tank pressure, isn’t that supposed to vent out through the carbon canister?
Stephen
On Jul 13, 2023, at 6:11 PM, Michael Diehr <md03@...> wrote:
?This seems possible. I believe that the fuel pump not only pumps fuel to the engine, but it also flows back to the fuel tank. Is there some reason it does a loop rather than just a one-way trip? And if is doing a round trip, could there be some failure mode in this system?
On Jul 13, 2023, at 3:23 PM, Alex L <amlevy@...> wrote:
Armchair mechanic opinion - gas lines are being heated to/from fuel rail via engine bay, causes vaporization which then gets deposited in the tank on the return line. Try heat wrap on the fuel lines? Or heat shield around exhaust/engine block?
A
On Jul 13, 2023, at 14:43, Jerome B Dwight <jeromebdwight@...> wrote:
That sounds dangerous to me. A friend was incinerated when he carried gas in his car on a hot day. With the right fuel/air mixture the dome light can ignite the fumes.
On Jul 13, 2023, at 4:57 PM, Michael Diehr <md03@...> wrote:
?I repeated my usual drive which triggers the problem: up the 395 through Bishop (roughly 4000' elevation) then up the steep climb towards Mammoth. (7000' elevation). It's a roughly 3000' elevation gain in about 30 miles. Temperatures were hot but not exceptional (95F or so).
As I got above 6000' I noticed gas fumes while driving. Pulled over, turned off the engine. Got out of the van, could not locate the source but it seemed to be near the tank (not in the engine compartment, not the tailpipe).
I opened the gas cap and vented the tank until it stopped outgassing. It was a tremendous amount of gas. Once all pressure was gone, I continued on to my destination which is over 7500'.
I was no longer smelling fumes at that level, but after I stopped, I did a test. The engine was still running.
I opened the gas cap. Again, there was tons of gas escaping, as well as some drops of liquid gasoline.
- with an IR thermometer, I measured the temperatures of the gas tank exterior and gas coming out of the tank: about 120F. Warm, but really not hot enough to account for this great volume of vapor.
Here's where it gets interesting:
- I stopped the engine, and the vapor quickly stopped.
- I started the engine again, and within seconds, the vapor is flowing out of the gas tank again.
- I stopped the engine, and the vapor stops.
This feels like an important clue: why did stopping the engine stop the vapor?
Mike
