Hm...that's weird...maybe a stuck float??

[old jeep]

You are running out of gas under load. The fuel filter is fairly new. Change the fuel pump.

 

[MDJEEPER]

Gas has a low vapor pressure(thats why it feels cold on your hand) and so does ethanol which is getting mixed into your gas in alot of places. Low vapor pressure means lower boiling point. One way to rais the boiling point is to put a substance under pressure. Your mechanical pump does at most 5 psi and modern cars run pumps that push 60-90+ PSI. So your jeep is passing fuel through your mesh screen in the tank, through your lines, through your fuel pump, through your fuel filter and into your carb. Now since fuel companies don't really have to tailor to carbed vehicles since they are going extinct little by little(Damn you cash for clunkers!) the fuel is designed to work in the higher PSI system which you do not have and is more likely to be susceptible to vapor lock.....

If your jeep is working at low rpms and when its really cool then I wouldn't suspect any clogs.

That mechanical pump is right on the block and therefore heats up with the block. The more load you put on your engine, i.e. gassing it up a steep hill or holding a high RPM the hotter your block gets. Your gas enters your fuel pump which is most likely getting hotter than the boiling point of your fuel and then begins to boil. Since your AMC 258 i6 / 4.2l doesn't get as hot as my 360 I would be willing to bet that some of your fuel is boiling off in the line and some of it is getting pushed through. So your carb isn't getting the enough fuel to the bowl to hold those high rpms. When you let your foot off the gas and let it coast you arent demanding as much fuel and your fuel bowl can fill back up until you gas it again and so on and what not.

Another solution is to install a carb spacer. But I don't think this will help you since you have an I6 and the 258s run coolant through the intake to help bring the temp down.

I hope that maybe helps you understand why a little better.....

All that is just my opinion so if some one wishes to correct me then feel free. That is just how I understand it.

Oh and if you do decide to install the fuel pump I would like to suggest that you put it across the front of the tank just above the diff instead of on the frame under the passenger seat. It looks real clean there and there are already bolts for holding the skid plate you could mount to. Also if you mount above the lip between the floor and the x-member you have some protection already for the pump, and you will be in line with the top of the tank as well as close as possible to the outlet and your pump will have less trouble priming itself and therefore have more life.

I can get you a pic of the one I installed in the CJ for the MPI kit if you want. It is a high pressure one but should be the same install procedure as any low pressure one.

Wow! This is an EXCELLENT description! Thanks!!

I'm curious...Did you run new hard fuel lines, soft fuel lines, or did you use the existing lines? And what did you do with the hole in the block where the mechanical fuel pump was? Or, did you just leave the pump on the block but disconnected and plugged?

 

[MDJEEPER]

You are running out of gas under load. The fuel filter is fairly new. Change the fuel pump.

Thanks! That seems to be the consensus from everyone...BUT would you just go with another mechanical pump, or switch to an electric one??

 

[Old Dog]

Thanks! That seems to be the consensus from everyone...BUT would you just go with another mechanical pump, or switch to an electric one??

IMO I would go with another mechanical one.

 

[ThisGuyUKnow]

Wow! This is an EXCELLENT description! Thanks!!

I'm curious...Did you run new hard fuel lines, soft fuel lines, or did you use the existing lines? And what did you do with the hole in the block where the mechanical fuel pump was? Or, did you just leave the pump on the block but disconnected and plugged?

I cut a section of hardline out just beyond the where it leaves the tank and then and then put the inline fuel filter connected with regular fuel hose then the pump then some more fuel hose. Pretty easy just kinda time consuming if there is alot of crud inthere.

I would go with an electric one over mechanical just because I am very happy with the one I went with, mech fuel pumps are relatively cheap so you could try that first.

If you went electric you can either just leave the pump there(did on the 360) or go to the parts store and order a block off plate (did this for the AMC 258 i6 / 4.2l ).

 

[BusaDave9]

I think at least part of the problem is vapor lock. The combination of new fuel and an old mechanical pump will make vapor lock more likely.
In the 1980's gas had higher lead. This helped prevent vapor lock. The lead also increased the octane rating. In the 1930's gas usually had an octane rating of only 40. This is because there wasn't any lead added. For details check out this link:
Gasoline - Wikipedia, the free encyclopedia
Then they added lead to prevent detonation. The lead also helped engine wear. In the '80s they tried to eradicate lead from gas because of pollution. Now detonation was a problem. Engines and gas were engineered to prevent detonation.
Vapor lock happens when gas in the fuel line vaporizes. It happens in hot weather. Gas is now made with Vapor Pressure Specifications. These specifications change from summer to winter. Winter gas is much more likely to experience vapor lock on a hot day. But that isn't an issue unless you buy your gas in the winter but don't drive until summer. For details check out this link and scroll down to "MYTH: E-10 causes vapor lock"
Ethanol Myths
New cars have a fuel pump in the tank. This electric pump pushes gas to the fuel injectors or carburetor. Even if the fuel vaporizes it will still get pushed to the engine.
On the other hand older cars had a mechanical pump. This pump on the engine would try to suck gas from the tank. This would work fine if the fuel was all liquid from tank to engine. But if there is any vapor in the line then the gas can't be sucked. Sucking on hot gas would only produce more vapor.
The best place for any pump is in or near the tank. Pushing is always better than sucking (ask any pimp). So if you get an electric pump follow the suggestion of This Guy U Know and put the pump near the tank.
Now if you get a mechanical pump I bet it will still make it much better. A new pump may be able to suck from the tank with very little problem. But I don't know if it will fix it for good.
Now, why would vapor lock be a bigger problem now as opposed to when the jeep was new? In part because of the changes that have been made to gas over the years (I covered that above) but also the changes to the jeep. Older vehicles run hotter than when they were new. My jeep emits a tremendous amount of heat through the floor. The gas must travel under that floor and into the engine compartment. If the gas line is anywhere close to the exhaust then that will heat up the gas tremendously. It all adds up.

 

[radman]

as an older rver, i have experienced the lovely carburated 454v8 chevy!
because of HORRIBLE engineering, the pump pushed/pulled fuel past every hot spot everywhere. it vapor locked... pump in tank area could not keep pressure high enuf to compensate for heat! ! unless your cj fuel lines have been moved, you might consider putting electric pump nearer the carb as oppossed to the tank. gas under pressure will lose temperature (supra) and will, by my experience, pump better. my experience....

radman


(supra=above)

 

[MDJEEPER]

Thanks to everyone!!! All of you have outstanding ideas!!

I guess the basic question is whether to try another mechanical pump first, or go with an electric pump. If I go with the electric, will any "universal" electric pump work, or would it have to be a specific model??

Seems like any frame mounted model would be OK since any of the electrical pumps is almost certainly going to generate more pressure than the mechanical one. BUT could there be too much pressure??

 

[ThisGuyUKnow]

Go to advance or carquest or whatever shop and ask for a low pressure fuel pump they should be under 10 PSI.

I would go with the electric pump but that is my preference. Old Dog suggested just replacing the mechanical, its just up to you.

 

[MDJEEPER]

Go to advance or carquest or whatever shop and ask for a low pressure fuel pump they should be under 10 PSI.

I would go with the electric pump but that is my preference. Old Dog suggested just replacing the mechanical, its just up to you.

Right...I should have been more clear. If I understand what you are saying...Any brand of LOW pressure electric fuel pump would work since there's going to have to be some modifications to get it to connect with the Jeep's existing fuel lines.

I might try the mechanical pump first just because it's a cheap, easy, two bolt solution. However, the electrical pump is probably the long term solution....assuming my problem is the pump and not the carb, the fuel lines, or something messed up in the tank!!

 

[ThisGuyUKnow]

Try taking the air blower attachment for a compressed air setup. Remove the feed line to the mechanical pump and blow into it. You should hear the tank bubbling pretty good. If you have anythng clogging the tankthen this should blow it out.

 

[ThisGuyUKnow]

Oh and don't forget to pull the Gas cap off otherwise you will get a face full of gas.

 

[dslywalker]

I was reading this pretty quick but did anyone mention a bad gas cap.Also talk to a guy at a tank farm,the amount of water in gas these days is scary.

 

[MDJEEPER]

I was reading this pretty quick but did anyone mention a bad gas cap.Also talk to a guy at a tank farm,the amount of water in gas these days is scary.

Right! A bad gas cap is definitely another potential cause of vapor lock! Probably something else I should just replace.

 

[MDJEEPER]

Try taking the air blower attachment for a compressed air setup. Remove the feed line to the mechanical pump and blow into it. You should hear the tank bubbling pretty good. If you have anythng clogging the tankthen this should blow it out.

Good idea! I hadn't thought of trying to blow out any debris backwards from the pump toward the tank!

 

[wardly258]

mine did the same thing when i had first gotten it turned out that after buying the $12 carb kit was a good investment at advanced auto. also check your plugs you never know. the sea foam suggestion is also a good idea

 

[ThisGuyUKnow]

mine did the same thing when i had first gotten it turned out that after buying the $12 carb kit was a good investment at advanced auto. also check your plugs you never know. the sea foam suggestion is also a good idea

You should go to a pick and pull find a xj cherokee pull out the ECM harness intake and such and fuel inject the rig

 

[MDJEEPER]

So, I was digging through a pile of various garbage that I have from the PO and I found a receipt for a fuel pump going back to 2004. I'll probably still try replacing it (+ gas cap) since a new one is not very expensive, but it does make me wonder now if I am barking up the wrong tree??

 

[BusaDave9]

A fuel pump should last longer than that.
If it is vapor lock then completely removing the gas cap and that should help. You could try that as a test. Then when it hot try to recreate the problem by accelerating up a hill.
A OEM replacement pump is $20 and is a simple swap.
An electric pump will cost about $80 (+ fittings) and is more involved to install.
Take a look at the fuel lines to make sure they aren't too close to the exhaust.

 

[ThisGuyUKnow]

The electric one I got at carquest was only like 54 bucks if your pump is only six years old it should still be good IMO the electric is slightly more complicated But well worth the effort.

If you spend twenty bucks on a new mech pump and it doesn't help then you have to pay for an electric one too so then you are out 100.

The pump I got came with the fittings included I just had to buy a foot of carb hose so it was under 65 bucks