Been trying to diagnose my friends truck for him and Im pulling my hair out here.

If you start it up it runs great, revs smooth and what not, after about a minute it starts running rough as hell, wont rev, misses, barley even idles, then it jumps up to like 2k. CEL isnt on, cant figure it out, replaced cap, rotor, plugs, wires, dist. fuel filter and it still does it.

Pulled the codes but I really cant read them, I know ho to read them on the pre-OBD Hondas, one long blink is 10 and shorts are 1 but it seemed a little different on the XJ. Whats the deal here guys I would really appreciate the help.

 

Fuel delivery?

 

Seems to have good pressure at the rail, I dont have the fancy tool to actually see what its at psi wise but when its running it will shoot fuel to the opposite fender.

 

That's an iffy test, friend. Haha, but w/o the proper equipment... So either way, I'm still thinking fuel delivery issue. Check fuel filter. Or heat soak (doubtful) or a short (doubtful) or a computer issue, more likely a sensor. Check o2 sensors.

 

Btw- is this a jeep were talking about? You said friends truck... Making this off topic..... MOD!!!!!!! Haha... Just messin..

 

Fuel filter is brand new. Pump is still OE. It gets real frusterating replacing all kinds of stuff and it not changing anything. And yes we are working on a Jeep.

 

Huh... Unplug **** and see if stops doing it? Still thinkin o2's, TPS, IAC valve, improperly grounded or shorted signal... Idk.. Screens gettin blurry, so that's all I got to offer tonight... Good luck...

 

I agree with VTJeep that the oxygen sensor is a definite suspect here.

When the engine first starts, it is in "open loop" control and uses (mainly) default fuel/air mixtures. But in very short order as the oxygen sensor warms up, the engine goes from open loop to closed loop and then uses the input signal from the oxygen sensor (and other engine management sensor components) to determine fuel/air mixture.

As you describe, this engine runs fine upon COLD start, but quickly runs poorly when the engine drops into "closed loop", I'd either test or replace that oxygen sensor with a new NTK sensor. The oxygen sensor is often overlooked and has a finite lifespan of between 100k and 150k, depending on engine efficiency.

And yes, the only accurate way to determine proper fuel delivery is with a fuel pressure gauge. Observations tell you nothing. Many parts stores will rent you this gauge inexpensively. For a 94, you should have 31 psi at idle. Then disconnect the vacuum line leading to the fuel pressure gauge and the pressure should jump to approximately 39 psi. Be darn sure that vacuum line is pulling good vacuum and also is not wet with raw fuel (which would indicate a ruptured fuel pressure regulator).

 

Figured it out today. The simplest thing. Another friend had an extra fuel pump we were going to try. Well upon pulling the old pump out the short piece of rubber hose from the pump itself to the hard line that exit the tank was loose. The way I figure it was sucking air/not pushing fuel through. Tightened up the clamp, put the sending unit back in and all was well. Crazy.