Hard to Start - Stumbles/revs at part throttle

OK
The Patient:
1993 Jeep Cherokee Country
4.0/AW4/231

History :
Had an exhaust leak, cracked header at down tube join and leak at back of head (missing nut/washer on last stud, missing bottom/rear manifold bolt).
Replaced exhaust manifold, replaced all hardware, torqued to spec/in pattern. All bolts seem tight at this time/no obvious exhuast leak noises. I _know_ it has some wiring problems, because occasionally the starter will not engage until I pop the hood and touch the wiring bundle near where the starter relay wires come out. As soon as I touch that though, the starter cranks over willingly and quickly.

Symptoms :
Was fine for a week after manifold replacement.
Last night on the way home it started to stutter/buck at mid-throttle.
Tried to die at idle unless I revved it.
Hard to start.
Will only start with liberal application of pedal, long cranking does nothing, waiting with key in run for a long time before cranking does nothing.
Got it home OK and it's obviously running rich (real stink of gas).
No obvious leaks around fuel rail, injectors, fuel pressure regulator.
No CEL, no codes other than 12 (I did disconnect the battery to reset the adaptive memory after replacing the exhaust).
I can drive it around but it's obviously down on power.
If I start it up in the driveway and rev it, the revs are VERY slow to come back down.

Current plans:
Spray some TB clean around the fuel rail/injectors/Intake manifold seating area to see if there's a vacuum leak.
Check ignition stuff, cap, rotor, plugs.
Test resistance of coolant temperature sensor.
Check readings on O2 sensor wiring.
Clean IAC, check TPS resistance.

Anything else I should check? That revs-are-slow-to-return-to-idle thing screams vacuum leak to me but IDK.

Disconnecting TPS and Coolant sensor results in expected codes - no perceptible change in behavior.

No change in idle quality/ease of starting or idle speed when spraying TB cleaner around the manifold or the injector bases.
Pulled the plugs, 1-3,5,6 all look tan, little bit old but nothing crazy. 4 is dark/wet. Gonna try swapping out injector on 4 to see if it's stuck open possibly.

Coolant temp sensor resistance looks in spec.

TPS idle pwm voltage is .68V and seems to sweep cleanly up to 4.56v at WOT.

I appear to have 0V on the Oxygen Sensor pin 3, which if my reading is right shoudl have full battery voltage in run. Tracing that wire back now (already backtapped it at the end of the fuel rail, still 0v)

I'm wrong, looks like. It is titled as a 93 and was sold to me as a 93 but it's a 94. I cannot find a ballast resistor or ballast resistor bypass relay. Can anybody tell me where the O2 sensor heater circuit power comes from on a 94? Bottom of one of the relays in the PDC? All diagrams I've found are either for 91-93 or 97 up.

Well I found some 1995 doco and the colors seem to match up. I have conductivity between the O2 sensor heater circuit source wire and pin 9 in the PDC (fuel pump relay).

Pretty sure you won't get power there unless the motor is cranking or running. Just turning the key to run won't do it. The oxygen sensor heater and the fuel pump are the same circuit. You may get a voltage blip to the O2 heater when you first turn the key to run and the fuel rail primes for a couple of seconds.

Thx. I'll check it when cranking tomorrow.

OK, I have voltage on the O2 sensor heater when cranking/running.
Went out and got a fuel pressure tester.
When placed into run, the pressure blips to 20psi briefly then drops back down again.
When cranking, it goes to 40psi, but as soon as you let off the key it drops to 4-5 psi, then 0. Like in the time it takes to pull the key out.
When running, 31 psi, tops.

Bad fuel injector. Pulled the rail and replaced with a known good set and we are back to purring like a (tractor) kitten.

jarvitron said:

Bad fuel injector. Pulled the rail and replaced with a known good set and we are back to purring like a (tractor) kitten.

Click to expand...

Success! :smoker:

One thing to note - the manifold was wet with standing gasoline, that's how much was leaking through that injector. Incredible that it still ran!