Was wondering if anyone out there knows how to solve a vapor lock problem. I've done a search and everyone knows what it is but no one really knows how to solve it. I have a 1985 Z28 with 305 TPI. New fuel filter, new plugs and wires, new cap and rotor, new HEI coil, fuel pump was replaced a year ago, new stant vented fuel cap. My car always has an incredible amount of pressure in the fuel tank when its hot out. My old fix was to just drill a small hole in the fuel cap and the car had no problems at all. The bad thing is, with a full tank, fuel gets all over the side of my car. Can anyone give me any ways to stop the vapor locking?

 

You do not have vapor lock. Vapor lock consists of fuel in the line vaporizing, which happens on the suction side of a block-mounted fuel pump. You don't have a block-mounted fuel pump, instead you have 40 psi of fuel pressure in the line, so you don't have vapor.

There is supposed to be pressure in the tank. This is NOT a malfunction.

There is no reason your car shouldn't work right with all the parts put back like they belong; no holes in anything, etc.

 

Why then, does my car stall when its warm out and the regular vented fuel cap is on? At night when its cooler out, it runs fine. I can drive the car all night long and not have a problem, but when I try to drive it in the afternoon, it always wants to stall? Also, during the spring and fall seasons, it runs fine. There is also way too much pressure in the tank. When I take the cap off after it has stalled, it almost shoots off of the fill neck and the air escaping takes about 10 sec. to completely escape. After the pressure is relieved, i can put the cap back on and the same thing happens, car stalls and there is a lot of pressure in the tank.

 

That is normal. It's supposed to do that. It's designed that way. It did that when it was new. Alot of cars will light the Check Engine light and set a code if it DOESN"T do that. In fact, in places with emissions testing (among places I've lived, this includes California and Ohio), one of the few things they actually physically check, is to make sure your gas cap holds pressure. They have a machine for doing this automatically at inspection stations in both of those states as a matter of fact. Your car will fail inspection if the cap doesn't hold pressure and you will not be able to license it.

I'd suspect that the reason your car has this trouble, is that there's something wrong with it. Destroying some additional part to defeat its purpose and thereby "hide" the result of the first broken part, is not the same as repairing the car.

First thing to do is to figure out why your car "stalls"; specifically, is it for a rich condition, or a lean condition.

I'm guessing it's for a rich condition (the opposite of "vapor lock"). That would be caused by gasoline being forced through something it's not supposed to go through, and getting into the motor. Hacking up the gas cap doesn't "fix" it; it just appears to band-aid the real problem, whatever that is.

 

also if anything having pressure in the tank would help stop vapor lock.
vapor lock is when for when you have a block mounted fuel pump that tries to suck the fuel out of the tank
since there is quite a bit of fuel in the lines that gets put under vacuum and with the heat it turns into a vapor

putting a pump in the tank helps solve this because now the fuel isn't under vacuum it's under pressure

part of the same reason why radiator caps are pressurised.
with the pressure given by the radiator cap it keeps the boiling point of the coolant higher and prevents it from getting bubbles in it.

my rx7 does the same thing with gas being blown out same with my honda though it's normal.



you have something else going on in your fuel system that is not vapor lock
I'll be honest and say I'm sure what it might be
only thing I'm able to think of is maybe the charcol can has become flooded with fuel but then again I don't know I'm just taking a guess and that is not something I would want to mess with either cause if you just remove it you now have raw gas going into the engine bay.

 

Thanks for the replies, if you can think of anything else let me know. FYI, I don't use the old cap with the hole in it, I just tried that to see if relieving the pressure in the tank would help out. I'm going to take the tank back down and check the actual vent line to see if its plugged anywhere. I might try a new pump too, maybe I got one that isn't working properly. Again, if anyone has any ideas, please let me know.

 

I think the charcoal canister is a good suggestion for a first place to look. Check it out, and see if it's saturated with fuel.

I doubt it's anything to do with the tank itself, or the fuel pump. I would certainly not recommend going to THAT MUCH TROUBLE right off the bat.

I'm not real sure how the vent system in your car is plumbed. There should be some sort of pressure relief valve or metering orifice or something in the vent line from the tank to the charcoal canister, but I don't know exactly where it is. Whether it's in the canister itself, or at the tank, or somewhere in between, or what.

No doubt when it tries to stall, it runs bad for a little while before it reaches its full severity. Try to determine during that time, whether it's running rich or lean. If you have a scan tool, you can watch the pulse width to the injectors. If it's rich, you should see the ECM try its best to compensate by shortening the pulses; if it's lean, you should see the opposite.

 

i have a similar problem with my v6 3.1 SFI. I was driving 80 on the interstate, 220 engine temp, a/c was on, less than 1/2 tank of gas. It was extremely hot outside, probably 100 degrees. Engine just started running like crap and i couldnt accelerate anymore. It died after i pulled over. Got it started again, i could only get up to 3000 rpm, drop it in first gear and only get to 10mph for 10 sec then it would die again. After doing this a couple times, i let it sit for 5 minutes. After that it ran fine. My engine is running hot (220 F) with A/C because i think my fan is out, but i haven't checked it out yet. And i have a lot of pressure when i remove my fuel cap when it is hot out. It will hiss for a few seconds. I had this happen to the V6 2 summers ago. Just kind of forgot about it and now its happening again.