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Old July 29th, 2021, 15:43
Cvldwell Cvldwell is offline
NAXJA Forum User
Join Date: Aug 2015
Location: 805
Posts: 7
92 High Fuel Pressure/Stinks/Runs Rough

Hey gents. 1992 XJ 4.0 with an interesting and long one for you. I have done research on all forums, etc until my eyes bled and I can't figure it out. Any ideas would be much appreciated.

TL;DR - Jeep stinks and won't smog, abnormally high fuel pressure for a 92, lots of replaced parts previously, runs pretty well at all ranges with a slight misfire at idle probably vacuum/exhaust leak related. Ready to throw a new head, new sensors again, and the new fuel pump in.

Facts and Issues:
- Recently started running with a strong typical "smog" smell, can't pinpoint the exact timeframe of when it started. This mostly happens when the e-fan kicks on and brings the temp down, but it the smell is becoming worse as the days go by, now stinking all the time. Before it would only happen after idling for 20 minutes and once the fan brought the temp down from 210-185.
- Fires up immediately when cold, like second rotation.
- Long cranks if firing once warm, but doesn't exhibit symptoms of heat soak as it doesn't stumble or run poorly, it just cranks 6-7 rotations before firing like its trying to bring fuel back up (but the fuel is already there per the gauge).
- It has had a myriad of parts replaced recently as I'm a "throw everything at it" kind of guy. I've replaced injectors with Ksuspension 4holes, walker muffler, IACV, air charge sensor on the TB, coolant temp sensor in the thermostat housing, fuel filter, exhaust manifold as it was cracked, o2 sensor, MAP sensor, plugs/wires/cap/rotor, throttle body gasket, probably forgetting some things. I have reset it every time after replacing parts.

Here's where things get interesting.
- I checked the fuel pressure at the rail and its 40psi+ with key on/not running and the same while running, and 50psi+ when running with FPR vacuum line disconnected.
- I checked the return line from rail to tank and don't see any kinks that may cause a high pressure.
- From what I understand, this year should be around 31psi at the rail.
- I can't tell if its rich or lean, but it stinks like all hell and runs a little rough at idle. No indication of either on the plugs, they're completely clean and were replaced before this all started.
- It does leak down from 40psi to 25-30psi after about 20 minutes after shut down, though I haven't waited longer to see if it drops further.

I thought maybe tank check valve leaking down or injector bleeding, but the results of the rail pressure test show the exact opposite? Spark plugs are neither black nor white.

Is there anything in the sending unit/pump that can cause high pressure? I have a new unit, just don't want to replace it if I don't need to. Can anything in the head cause this like a stuck valve? Can it throw some kind of loop when the temperature drops back down from 210 that causes it to run rich or lean? I think it's on the OEM catalytic converter, but unsure how it would cause high fuel pressure. I haven't compression tested or vacuum tested for exhaust restriction.

I'm at a loss. Thanks in advance!
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