vapour lock

[geoff]

hi all
I just got my (2000 injection) spider back on the road following a radiator rebuild and she performed lovely.
It has been a hot day here (33°C) and the temp gauage stayed right in the middle. That is the good part......................
After parking for about 15 minutes, she refused to start. Seemed like a fuel problem so I suspected vapour lock.
I poured some water over the throttle heating junction and she started fine.
I beleive this was caused by the fuel vapourising whilst the car was parked, hot and for just a short time.
Question is - although 30°C is hot for me (and lately for my part of France) it is not that hot - and my car came from Sedona in Arizona where 30°C is nothing.
what do you think, does anyone else have this problem?

[Dougieb]

Not exactly sure on what your particular problem is but I can add a couple of things.

Vapor lock is usually due to low fuel pressure after the car has shut off.
Usually in fuel injected systems, there is a fuel checkvalve to keep pressure in the system.
Without this pressure, fuel in the lines are no longer moving to keep drawing cold fuel from the tank.
The stagnant fuel will "boil" from residual heat from surrounding components and create problems.
I have had this in both carb cars and fuel injected ones.
Carb cars usually have a spacer between the carb and the intake manifold to help keep the heat from transfering into the carb.
Many people have had problems only to find out their spacer is not there for some reason.
Porsche built a checkvalve in their electric fuel pumps just for this reason.
(Don't ask me how I know this. ha ha)

Recently, I had to add a fuel checkvalve in my '72 Spider.
Even though this was a carb car, it aided in this problem.
If it was left sitting for 30 minutes or so, it would take a long time to start.

The only other reason for this type of fuel problem is if you have negative pressure in your fuel tank.
Without proper ventilation, it will slowly suck the gas in the fuel line back into the tank when left sitting.
Either way, the problem is low fuel pressure in the system and can be helped by a check valve.

I am not sure what may be going on with the throttle heating junction or why the water helped.
Just adding my $0.02

Good luck.

[geoff]

thanks for this. I just cooled the throttle body so I assumed this stopped the fuel evapourating as it entered under pressure of the fuel pump. It certainly worked.
The check valve idea sounds like it is worth investigating. This is an Arizona car and has the typical USA emmission controls and so I *think* there is some negative pressure designed into the system so that fuel vapour does not come out of the filler cap when it is removed. If anyone can verify this please, it would be most helpful.
Regards
geoff

[majicwrench]

To be honest, you really have no idea why car didn't start. Pouring water on throttle body would not cool fuel, there is no fuel in throttle body. It may have worked, but kicking the tires may have worked too. Unless you can repeat this success several times, it means nothing.

Fuel Injected cars ALREADY have a fuel check valve in the fuel pump.

When/if this happens again you need to test some things and see what the problem truly is. Do not assume.

[geoff]

wise words - thanks.
I did call it the "throttle body" but perhaps this was incorrect.
All I know for sure is that there is a union where the hot water enters, to (I thought) warm the accelerator assembly, and it is on this that I poured the cold water. To my mind, this would have cooled the surrounding area and allowed the pressurised fuel to pass without evapourating. And then the car started.
This seems logical to me ?

[Kevin1]

Look below and next to your intake manifold and you will see the fuel rail. From there trace four small hoses. Those lead to the injectors. Each injector also has a wire leading to it. That is where fuel enters the manifold - well after the throttle body.

Any vapor lock, if that is what is actually happening, is happening well away from the throttle body or throttle body heater.

[azruss]

Sedona is up there is elevation, so doesnt suffer from the torrid temps we get in Phoenix. My 80FI has had no start issues during the summer, so the cars can take the heat okay. I agree with others the dumping water was just coincidence. The first thing i would check would be the vent hoses in the trunk and the gizmo that the hoses go into. The other possibility is the injector/injectors are leaking a little and flooding the motor.

[courtenay]

My '80 FI starts fine when cold, but not when hot. When cold, when I turn the ignition to run (but not start) and move the AFM flap, the fuel pump turns on. When hot do the same thing but the fuel pump does not run. Now waiting for delivery of a new dual relay which will hopefully solve the problem.

[76was124]

wise words - thanks.
I did call it the "throttle body" but perhaps this was incorrect.
All I know for sure is that there is a union where the hot water enters, to (I thought) warm the accelerator assembly, and it is on this that I poured the cold water. To my mind, this would have cooled the surrounding area and allowed the pressurised fuel to pass without evapourating. And then the car started.
This seems logical to me ?

Problem sounds more like the coolant temp switch that is screwed into the front of the water inlet T coming off the front of the head by the radiator. Try cleaning up the connector and see if happens again.

Also, you could unscrew the gas cap and listen for a sucking sound which may be a sign of a vent line problem.

Lastly I try to avoid putting water on anything hot on the engine as it could create bigger problems.

[geoff]

yes, Azrus and dougieb said about the fuel line negative pressure possibility.
Al though now in France, my car is a US model and it does have some emmissions control which I don't understand.
I thought that there is supposed to be a sucking sound when the fuel cap is released in order to prevent the emmission of fuel vapour ?

[RRoller123]

Sealed system, and there should be no sucking sound when you pull the gas cap on an FI engine. The vapors are controlled by passing tank make up air through the charcoal canister.

[geoff]

thanks for that schematic. It is very useful - but do you know if there is a description of how it functions anywhere?
I have just checked and on my car there is a very definate suction when I remove the filler cap - and as I said, I thought this was designed in so as to prevent any vapour emission. Can you just confirm again please that there should be no sucking sound when the cap is released ,
thanks
geoff

[RRoller123]

I too have been trying to get a description of how the system works for 3 years. But nobody on the forum seems to know, so I just have my own analysis (fwiw) from examining the diagram: The only way make-up air gets into the tank is through the cap on top of the canister, down through the canister (intentionally picking up the gas vapors from the charcoal) and on through the tubing and 2 way valve, the liquid/vapor separator, and hence into the tank through door # 1,2 or 3. (Why they made 3 lines coming out of the separator remains a mystery to me). So if you have vacuum in the tank, something along that line from the canister back to the tank is blocked. The engine should actually slowly die out, unless there is another source of make-up air. (I think I have read that POs with this problem went and drilled small holes in their sealed gas caps to try to "fix" the problem). I tested my system, with the engine idling, by removing the small plastic cap on the top of the canister, holding my thumb over the hole to block the flow of air, and seeing the engine slowly die and eventually it would have stalled. Remove thumb and it runs correctly again. Likewise, when you gas up the tank, the air has to go somewhere. It either leaks past the fuel nozzle and pollutes our beautiful planet with unburned HCs, or it goes back through these same lines and through the canister, where presumably the charcoal captures major portion of the HCs. The part I haven't figured out is how it regulates the flow of air from the canister cap through the canister and up in to the intake manifold. I believe that this is where the 2-way valve comes in, regulating the flow by balancing pressure and intake manifold vacuum. Otherwise, this line to the intake manifold just looks like a big vacuum leak. So this part I still ponder. Maybe someone else knows more on this?

[majicwrench]

Quit worrying about a sucking sound. Lack of venting in the tank system did not cause your issue. Tank venting can certainly cause problems, but the problems will show up when driving. After sitting 15 minutes, not very likely.
Many cars run a neg pressure in tank. Don't worry about it unless you know it is a problem.
Keith

[geoff]

that is indeed a fair comment. the car runs very well andonly a couple of times have I experienced this refusal to re start when very hot - so I am not unduely worried. I do however like to understand things - and if it is "not correct" to have a vacuum in the tank then I would like to rectify it - or at least know exactly why it is occuring.
Best regards
geoff