fuel psi issue when activating the brakes

[echdaviem]

i've recently ran into a very odd problem. my truck started running kinda rough the other day and i've since checked all i could check as far as ignition and fuel and compression and stuff like that. so i decided to hook up a fuel pressure gauge to the fuel rail. at idle it runs fine and when you drive easily it seems to be ok but the gauge vibrates alot. as soon as you step on the brakes it settles down to about 32 psi at idle, and about 38-39 when on the gas, it runs great this way. i even drove home with my one foot on the brake slightly to get it to run good. anyone have any ideas? i'm all confused as to what the issue is.

the other question i had was: there is a ceramic block looking thing on the driver side engine bay with two wires going to it both orange with black strip, what is that thing? i would take a pic but i'm at work right now, maybe later when i get home.

oh yeah this is an 89 4.0 poo joe 231

[88pioneer]

the white thing is the ballast resistor, it sends electrical signals to the fuel pump

[echdaviem]

oh ok, that thing is very hot. do you think that is my issue?

[Pete M]

it's a resistor designed to lessen the voltage getting to the pump and so lessening the noise of the pump. It eats electricity and will get warm. But there's a limit on how hot it's supposed to get. A quick way to check it's condition is to pull the wires and connect them directly together (you won't hurt anything, 86/87s don't even have it). If there's any change in the truck, you can replace the resistor (they are cheap)

[echdaviem]

thanks pete, i will try this as soon asap. where can i get one? does advance or auto zone have it? what would they call it?

[kastein]

They'll call it a fuel pump ballast resistor probably. It is a 1 ohm 20 or 30 (forget) watt power resistor, if you can't get one from them, buy one from an electronics supply house or just splice the two wires together and forget about it like I did. So far I've done approx 20 thousand miles and counting without it and everything works great.

 

Have you checked for vacuum leaks? My only guess is maybe your fuel pressure regulator isn't getting a good solid vacuum unless your brake booster is pulling some vacuum as well, not sure though, this is me thinking out loud / talking from you know where.

[Wildman]

I'll second on the vacuum leak.

 

Sounds like you brake booster has a leak, and when your riding the brakes, you changing the position of the diaphragm, allowing more vacuum for the fuel pressure regulator.

 

If your talking about a 4.0 Renix era engine, you should be reading 31psi at the test port with the pressure regulator connected, and 39psi with the vacuum line disconnected.

[aemsee]

If the vacuum thing doesn't pan out, have someone watch your brake lights while you step on the pedal. I think that the fuel pump grounds at the same location as the tail lights. You could have a dirty ground back there and a funky electrical gremlin messin with ya, Just a thought.

[mythreesons]

:agree: Sounds like a grounding issue.

[echdaviem]

ok so i spiced the two wires together and just like that problem fixed!

but now i think i do have a ground issue or something cuz i can step on the brake

with the engine off and the fuel pump will turn on with the brake.

where is it grounded at exactly?

[Pete M]

behind the drivers side taillight. there are 4 screws holding the taillight housing in place.

[kastein]

If the vacuum thing doesn't pan out, have someone watch your brake lights while you step on the pedal. I think that the fuel pump grounds at the same location as the tail lights. You could have a dirty ground back there and a funky electrical gremlin messin with ya, Just a thought.

GREAT point, and sounds like the actual problem. I forgot that there were two common points between the brake/fuel delivery systems on the MJ (grounds and vacuum) - most of the tech info I have learned is XJ specific :nuts:

[echdaviem]

well i finally got around to messing with the wiring. after cleaning out the fuse box, the problem went away. which kinda bugs me cuz i really don;t know what the issue was. so now i have a new issue, well not new but it's been getting worse. my fuel pumps makes noise and it louder in the afternoon than in the morning. is my pump going bad? should i replace it now before it leaves me on the road? if so, is there a certain pump thats better than others?

[HOrnbrod]

so now i have a new issue, well not new but it's been getting worse. my fuel pumps makes noise and it louder in the afternoon than in the morning. is my pump going bad? should i replace it now before it leaves me on the road? if so, is there a certain pump thats better than others?

 

The fuel pump is getting louder because you bypassed the ballast resistor. It's a load resistor, and it's purpose is reduce the voltage to the pump after starting to prolong the lifespan of the pump and to reduce noise by lowering the operating voltage. Sure you can bypass it; but it's not a good not a good idea. It may last or may not depending on corrosion at any power or ground splice/connector joint in the fuel pump circuit. Check all these points for corrosion, clean them up, and if the pump is still noisy, get a new 1.3 ohm ballast resistor, and replace it. Unless you enjoy replacing fuel pumps prematurely. :ack: