Vapor Lock, Help!!!

[Jeepman85]

I have a 85 CJ7 with the AMC 258 i6 / 4.2l and Mopar fuel injection setup. It has an electric fuel pump with an inline filter that only has the one output port. I believe the fuel tank is the 20 gallon bigger one.

A few months back i topped off the jeep with fuel before a long wheelin' trip. After doing so, the jeep ran and idled like it would if i was in high altitude (i wasn't) with a carb. I got it up to speed and the syptoms stopped even when the engine was just at idle. This has happened a couple of times and is only after the jeep is filled with gas. Im thinking vapor lock.....?

A couple days ago i was playing with the jeep and decided to put some insulation around the fuel pump because it is soooo loud that its almost unbareable. After i did this the pump got quieter but i developed a new problem. The jeep fires up and idles perfect, but dies almost immediately after eccelerating!!! I have no clue. Due to the previous problems with the vapor lock, and the fact that the jeep is 25 years old, im just gonna replace all the rubber fuel lines on top of the tank due to the age.

I don't know if the dying upon acceleration is due to something i did, or if the old vapor locking problem has just gotten worst (luck that i was messing with the fuel pump at the time??). I did start with the easiest fix first and replaced the gas cap (didn't fix). FYI, it only has a couple gallons of gas in it when it started dying at acceleration.

Thanks in advance for any assistance!

 

[Sasquach]

My Mopar system fuel pump can not be heard with the engine running, only when you turn the key on and for a second or 2 does it make any sound at all. The tank pick up goes to the filter then to the fuel pump where there is a pressure line to the rail and one return to the tank, this is the updated system I believe.
My engine also sounds exactly the same at whatever altitude I'm at, -257 to +10,000 feet have been seen.

 

[73CJ]

Sounds like a fuel pressure problem. The motor is probably starving for fuel.

Have you changed the fuel filter lately? It maybe time. Older jeeps fuel tank rust inside a lot.

Check the fuel pressure in the rail with a gauge.

 

[dave123]

I have a mechanical fuel pump and not familiar with your setup, but I'm guessing your electric fuel pump is far enough away from heat so that should not be an issue.

Vapor lock is caused by fuel getting hot and boiling (turning to vapor). This happens when the fuel line (or pump) is too close to something hot.

So, if I were you, I would make sure the fuel line is insulated/ routed to avoid heat. I would check to make sure the fuel pump is getting at least 12 volts and is properly grounded. You can also take off the fuel line and hold it over a measuring cup and run the engine a second to see pump output. You should be able to see if it is not putting out enough fuel this way.

Also, if the dying problem started when you wrapped the pump, I would unwrap it (and check all connections) to see if that helps.

just my .02 Good luck

 

[73CJ]

Vapor lock is caused by fuel getting hot and boiling (turning to vapor). This happens when the fuel line (or pump) is too close to something hot.

Vapor lock is usually found when re-starting a hot motor with a manual pump.

Motors with FI are not commonly affected by it.

With the fuel pump being noisy, it may be weak and generating too much heat.

Filter or fuel pump are bad. A pressure test will tell the story.

 

[ballzdeep]

I had an issue with mine....AMC 304 with a fan that was too small. Upgraded fan #1. I also checked the fuel lines and they were very close to the headers and manifold. Re routed lines and wrapped them with thermo-tec heat insulating tape. Havent had an issue since. The tape was only 15 dollars from summit. Worth a shot for! Good Luck

 

[hole]

Jeepman, after changing all the rubber hoses, take a good look at the section of steel line from the tank to the pump. My 8 with the same set up would just up and die from time to time, so I thought I would replace all the rubber fuel lines to see if that was the problem. Ended up being a pin hole in the steel line before the pump causing it to suck air and die. Also,I second everyone's advise to get a fuel pressure gauge.