quick question

My 89 4.0l has been idling rough and stumbing right off idle and all the way through the rpm range. stutters so bad that sometimes it takes more than a mile or so to get up to 50-55 mph. Happens more often than not, but some times i'll get in start it (cold motor) and it will run great for the first few minuts but then starts stumbling and being cranky. tps checks out fine (4.9v in .7v out), but i just ran a fuel pressure test and something seems funky.

Start-up fuel pressure is 30psi

drops to 20-23 psi (no change with disconnecting p. regulator)

turn off engine and pressure drops to 0 in less than 10 secs

Does this sound like a faulty pump to you guys. I've replaced the pump twice in the two years I've had this jeep but it never acted like this. Usually when the pump went south the engine would just start and die, or not even run.

Ive been told it could be the egr valve, but without a vaccum pump and guage i dont know.

Should i just throw yet another pump at it and hope for the best or continue searching and testing things? (ok, maybe this isn't such a quick question but still any help would be much apreciated)

I could be wrong, but dont the 89's have the fuel pressure regulator on the fuel rail? I have heard of people haveing problems with that. You may check into that instead of thinking it is the pump.

Jump the ballast resistor, then squeeze the return line after the regulator and see if the pressure goes up.
It may be the pump (or regulator), it may also be something in the wiring. I had a similar thing going on, I replaced the regulator with no improvement and later found that the harness connector near the rear door frame right inside the left rear panel was fried. It tested 10+ volts engine running at the pump, through the ballast resistor (which is near normal) but wasn't passing enough amps to get the job done. During start the ballast resistor is bypassed and you get whatever voltage is left over from the starter running, during run (engine running) the current flows through the ballast resistor and is typically 2-3 volts lower than battery voltage.
On another occasion (years later) I had low fuel flow, turned out to be the ballast resistor. I took it off and looked at the back side, it was fried.
It's always a good idea to replace the fuel filter. Even if it is fairly new, I've emptied them of fuel and tapped the end more than a few times to have a small pile of gunk come out of the filter.

ok, so with the return line pinched off, and the ballast resistor bypassed there is no rise in fuel pressure. The pump is getting ~13v while cranking and then ~14v with the engine running, and the resistor bypassed. so i dont think its an electrical problem. Haynes manual says if the pressure doesn't rise with return line pinched look for fuel supply issues. im confident that the fuel filter is not the issue as it is new. Is there anything else to look at or does this mainly point to the pump?

The other thing is that it has an inconsistent idle symptom that has emerged fairly recently. Idles normal then drops to barely running for a few sec and either dies out or comes back to normal idle, then it will repeat for the entire time im sitting there letting it idle. some mornings on my way to school i drive down street and when i go to put the clutch in it sputters and dies. it will start back up, do the same thing at the next stop sign, and then not die for the rest of the day but does the high/low idle scenario. I've been searching and it seems many renix 4.0 have the same issue and the cause could be a host of things. I'm just hoping that it is related to the low f.p. instead of something else. any more ideas?

Two other possibilities besides the pump itself, are one remove your gas tank cap and see if the pressure increases (a long shot but what the heck why not). And second is the rubber hose between the pump and the tank outlet has been known to deteriorate and spew most of the fuel back into the tank.
Low pressure and your check valve failing at the same time may indicate something other than the pump itself.

thanks 8mud, i will take out the pump today and see if anything looks odd. does any one know where the check valve is, because while I was under there yesterday i could hear the gas leaking back into the tank as soon as the engine was turned off. by replacing this Check vlave do you think my pressure would increase.

IIRC, the check valve is internal to the pump. It's usually just a ball bearing or needle valve that will seat with the absence of pressure from the pump and residual pressure in the system. I'd try a concentrated fuel additive and see if you can loosen up any varnish built up on the seat before I pulled everything apart for the third time.

shorty do you mean something like fuel injector cleaner or the like? And if the valve is stuck open or malfunctioned could that be the cause of low pressure? (i'm visualizing a scenario where when the pump is runing its just pumping it back into the tank and not getting the chance to build the right pressure at the fuel rail. I could be way off though?)

"Yes" to the injector cleaner

"I don't think so" to the low pressure during opperation. It will bleed down rapidly when you shut it down though if it's stuck.

ok so i found the problem. it was the in-tank fuel line going from the pump to the assembly itsleft. funny thing is I thought i replaced it when I put the new pump in last time.:doh: Fuel pressure back to 31 and it drives 100% better:clap: Cheers to you guys for your help with this one. Thanks 8mud.