Need some help with 86 Cherokee 2.5 TBI (Renix)

I have an '86 2.5 TBI Cherokee Pioneer with fuel delivery and wiring issues.

It will crank over from cold start, run fine for a few seconds then it will start getting rough and will die unless I burp the throttle rapidly, which occasionally makes it backfire through throttle body, but pouring gas down the throttle body makes it run smooth and fine.

I am getting stray voltage to injector, there is about 3.5 volts at the injector harness clip with ignition ON, and voltage does go up when I crank the engine. Injector also appears to be bad, it dribbles fuel if the fuel pump is activated, even with injector unplugged (so the stray voltage wouldn't be the only thing keeping it open)

The wiring diagrams I have say that the injector should be wired up to Orange and Light Blue, but mine has a different wire in place of the Light Blue, it looks Green, and diagram says egr solenoid should have Orange and Green, but it has Orange and Light Blue ???

Is there an easy way to find this wiring issue, by volt or continuity testing either the diagnostic D1/D2 terminals or the ECU harness? I have a digital multimeter.

I was thinking of continuity testing the ECU, one terminal at a time compared to engine ground, would that give me accurate diagnosis of the affected circuit? Should I look for stray voltage in the other circuits, to narrow down the possibilities?

*edit* The tachometer reads between 300-500 rpm with ignition switch OFF, and then goes down to zero with ignition switch ON.

I'd be looking at a ground issue. If a shared ground is bad no telling how the current will flow. I'm sure you have seen somebody turn on their left blinker and seen another bulb in the opposite side taillight light dimly, same thing, only in your engine management electrical system.

The fuel regulator is a known soft spot in the TBI and no biggy to replace. Autozone may have a writeup on how to do it.

I have a Chiltons someplace for the 86, I'll see if I can find it and see what they have to say.

I was thinking FPR as well, I am a competent enough mechanic to be able to replace it, it's just the electrical issues that I haven't had enough experience in.

I found some flow chart diagnostics, will start with those I guess.

SavageOne said:

I was thinking FPR as well, I am a competent enough mechanic to be able to replace it, it's just the electrical issues that I haven't had enough experience in.

I found some flow chart diagnostics, will start with those I guess.

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I'm not real familiar with the 86 specifically, but am competent in general auto electric. If the flow chart doesn't work out for you try Cruisers ground refreshing methods. They may not help, but in all likelihood need to be done on a vehicle that old anyway and may just cure your standing voltage issue and may help avoid future issues.

You might also try pulling fuses one at a time and seeing when the voltage disappears, this may help steer you in a productive direction. Often enough sub systems share a common ground and if the ground is flaky the voltage is going to find any ground path it can and may cross over at the high resistance ground and travel backwards through a completely unrelated system (wire).

I've generally had better results starting with the basic and then working on into the exotic. Better than half the time a sensor or component problem is in a connector or the wiring, swapping out components doesn't help any when the wiring isn't right.

When I remove the fusible link that feeds the fuel pump relay from the starter relay, voltage dissapears from the injector harness. The only ground I haven't "refreshed" yet is the braided cable from engine to body ground, about to do that now.

Flowchart says to replace ecu, no voltage between 1 and 7 with ignition ON.

SavageOne said:

When I remove the fusible link that feeds the fuel pump relay from the starter relay, voltage dissapears from the injector harness. The only ground I haven't "refreshed" yet is the braided cable from engine to body ground, about to do that now.

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That's super important on the 2.5s for some reason. More so than the 4.0s.

My multimeter had glitched, i have voltage between 1 & 7 with ignition ON. Found the stray voltage, my 5 volt supply from ECU to Map/Tps sensors is getting split into the injector harness, either externally or in the ecu if that's even possible. The 5 volts is divided, about 3.68 to injector and 1.32 on the MAP.

Question: How can I rule out the ECU? Should I check for voltage across the ECU 5v and body ground when ignition is on/off, or check for continuity between ECU 5v and ECU injector ground while the ECU is disconnected?

Just a couple of guesses. pull the fuel pump relay and see if the stray voltage disappears. Could be you have a leaky internally carbon flashed fuel pump relay. Might as well check the fuel pump ground while you are at it.

Not sure about your 86 but the 87 MJ i was working on (4 banger) had two grounds coming out of the ECM (ECU), believe it was the first two pins, pins 1&2, black wires. OHM test from the ECM connector(black wires) to battery ground. Best way to do it is to leave it connected and just strip back a little of the insulation. If it tests out OK, then pull the connector and double check the pins in the connector, pull on the wire in the back of the connector and see if the pins come out. The little locking tabs on the pins do flatten and the pins may just be touching and not making good contact. The pins do sometimes back out of the connector and make flaky contact.

The voltage remains when all three relays are disconnected. (fuel pump, B+ latch, and what I'm guessing is the a/c relay).

As for the ECM Ohm test, I do have both #1 & #2 as ECM grounds at the connecter. What OHM or continuity should I be looking for? Infinite resistance or full continuity, or somewhere in between?

SavageOne said:

The voltage remains when all three relays are disconnected. (fuel pump, B+ latch, and what I'm guessing is the a/c relay).

As for the ECM Ohm test, I do have both #1 & #2 as ECM grounds at the connecter. What OHM or continuity should I be looking for? Infinite resistance or full continuity, or somewhere in between?

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Ideally you are looking for "0" ohms, practically anything over 1 ohm is a cause for concern. If I do everything right (cleaning my grounds and connectors) I usually end up with .2 ohms or less and am happy.

I have a complete Jeep Factory Manual on that year. It is like 500 pages. If you are interested, I will be back home in about a week and I could pack it up and mail it to your for $30.

Found the electrical leak, was NOT the 5v reference signal from ECU, it was a 12v+ positive harness connector (2 orange wires) that feeds the A/C relay and possibly the trailer hook-up (it splits off and goes into the wiring loom behind the grill, but the lights work fine with it disconnected and I don't care enough to track it further)

I have ordered a TBI rebuild kit, if new o-rings don't stop the fuel dribble a new injector will be on the list as well, but I am going to clean and check the injector while throttle body is off.

Thanks for the help! P.S. Old_man, I don't think I need a paperback manual, I have plenty of digital resources available, it was just a pain in my ass to trace the wiring issue, especially with so many items powered or grounded off the same circuit.

I've got a new throttle body with injector.