Horrible vapor lock issue or something else?

adventureboss · Jul 13, 2026

This will be my first summer out in Arizona, and I know vapor lock is an issue on these vehicles but this is really bad. Not an issue when its under 105, but when its over that and I drive my Jeep, then let it sit for 20-30 minutes, it will not start when everything is heatsoaked. It eventually will after a lot of cranking, but it takes a long time. Yesterday it took me 9 tries, then it idled rough for 20 seconds, and then it was fine. No issues starting up in the morning ever or when its cooler out. I have also noticed that when this is the case, I cannot fill up the gas tank without it spitting everything out.

I've already tried insulating the injectors and adding heatshields, I had my mechanic check fuel pressure on the fuel pump and it's right on the mark. Tried replacing the purge valve too. No change. Is this seriously typical? I'm wondering if the fuel pump could be acting up anyways only when its hot. But that doesn't explain why I can't fill the tank when its like this either.

Chris · Jul 13, 2026

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adventureboss said:
This will be my first summer out in Arizona, and I know vapor lock is an issue on these vehicles but this is really bad. Not an issue when its under 105, but when its over that and I drive my Jeep, then let it sit for 20-30 minutes, it will not start when everything is heatsoaked. It eventually will after a lot of cranking, but it takes a long time. Yesterday it took me 9 tries, then it idled rough for 20 seconds, and then it was fine. No issues starting up in the morning ever or when its cooler out. I have also noticed that when this is the case, I cannot fill up the gas tank without it spitting everything out.

I've already tried insulating the injectors and adding heatshields, I had my mechanic check fuel pressure on the fuel pump and it's right on the mark. Tried replacing the purge valve too. No change. Is this seriously typical? I'm wondering if the fuel pump could be acting up anyways only when its hot. But that doesn't explain why I can't fill the tank when its like this either.

I suppose it’s possible that the fuel pump could be losing its prime when it gets too hot. It might be worth a second opinion on the fuel pump’s performance at elevated temperatures or considering a replacement if it’s been in there for a while. Did the mechanic check the fuel pressure during a time where it was really hot and had been running for a while?

The trouble with filling the tank sounds like there might be an issue with your tank venting system. I wonder if it's related to this: Wrangler TJ Gas Overflow Fix

Using high-quality fuel can also have an impact on vapor lock. Sometimes, lower-grade fuels can vaporize more easily at high temps, making symptoms worse. Just something worth throwing out there that many overlook.

hummerbk · Jul 13, 2026

My 2003 does the same thing. It will start after sitting and heat soaking for half an hour or so but it will feel like it's only running on two or three cylinders.
If I can get it to rev up a couple of times it clears itself out and runs fine after. It can take 30 or 40 seconds of babying the gas pedal to keep it running. I can get some popping thru the intake manifold although it could be coming from the tailpipe. I assumed it was a vapor lock and needed cool gas in the fuel rail to clear itself.
I put new heat shields on the intake manifold and installed hood louvres to reduce under hood temperatures, to no avail. 10 or 15 minutes of heat soaking won't cause it. It takes 20 to 30 minutes. As long as I can get it running I don't worry about it but I would like to know why.

GoldenGorilla · Jul 14, 2026

After changing my fuel rail to a nice billet aluminum unit, without using a damper, I've had 2 experiences similar to yours. Driving in 100°F plus temps. I had not previously had any such issues in 22 years with it.

I'd recommend you turn the key to on for 5-10 seconds and let the fuel pump prime the line. The boiling point of gasoline increases with pressure.

There is alot more than that going on with mine. Installed aluminum head with compression ratio increased to 9.5:1, 195 thermostat that is working properly, basalt fiber intake heat shield, an issue with exhaust leak at the header-precat joint...
At the end of the day, the aluminum fuel rail just absorbs the heat better than steel.

The extreme end would be a fuel return system.

macleanflood · Jul 14, 2026

Texas Jeep Products ala @Jezza makes a fuel return system.

I'd argue all of your issues are related to fuel pressure and I would be monitoring it through your testing process.

-Mac

GoldenGorilla · Jul 14, 2026

I'd start with that (monitoring fuel pressure). My aluminum fuel rail was extremely hot to the touch. I think I saw it at 155 with the IR thermometer, or something close to that. Without fuel flowing through it, very hot. Don't believe I have a fuel pump or regulator issue.

adventureboss · Jul 15, 2026

You know I ran a tank of ethanol free fuel through and haven't had issues with hot restarts since. So I think I'll just keep an eye on it.

macleanflood · Jul 15, 2026

Interesting. Ethanol as an ingredient to gasoline acts like an octane booster and helps with premature detonation.

I would think removing the ethanol would have the opposite affect.

But then you're going from 87 to 92.

Try regular ethanol gas and add an octane booster.

-Mac

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Horrible vapor lock issue or something else?

adventureboss

TJ Enthusiast

Chris

Administrator

hummerbk

Member

GoldenGorilla

Everybody thought it was crazy. We all know

macleanflood

I break sh!t