LeakyForLife

A few weeks ago my 91 cherokee started rough idling on start up but would suddenly smooth out while driving after a mile or two. I even made a 120 mile trip with it two weeks ago and it ran 70mph smooth as could be. Since then it has rough idled and doesn't smooth out. I have KSuspension 4 hole injectors. I figured it was an injector since a code 27 was thrown. I went to the pullapart and pulled several 4 hole injector from another chrystler and cleaned it with carb cleaner and threw it in port 2 after finding disconnecting that one didn't change the idle. Didn't help. I checked the ohms on all the injectors and they were 15 omhs with continuity, including the one I pulled out and put in. They all click with 12v across the coil. I tested all grounds for continuity and ohms from injector harness to ecm plug. They all had continuity and <0.4 ohms. The injectors positive lead with the key on but not running had 6.4 volts to all the injectors 1-6 and was the exact same. I swapped to another ecm and I got all injectors showing 0.44volts on the positive harness leads. Checked the coil input voltage and it was 12.3v and the ground in that harness had continuity. Going through and unplugging the injectors with the engine running showed no change in rough idle with injector 2 disconnected. All of the other shuddered. I swapped injector 2 with another JY injector and it didn't help.

Why do I only have 6 volts two ALL of the injector harness positive leads with one ECM and only 0.44 volts with another ECM. I have read They should read 12v with key on. With both ecms the engine cranks and runs the exact same just a rough idle and both show no change with injector 2 unplugged. Could I have a short on the positive feed? Bad coil? I'm lost at this point and wanted input before I tear the harness apart looking for abraded wires.

lawsoncl

A scope or noid light would be really helpful to see if that injector is getting voltage to fire. Sounds like you've confirmed good grounds and continuity back the the ECU. When you were testing, did you check that the positive wires were not shorted to ground or each other? I agree the next step is looking at the wiring harness. Try wiggling and shaking the harness all the way back to the firewall and see if it affects the idle. You you tried swapping the injector connections between 1/2 or 2/3 and see if the dead cylinder moves? Connectors inside the plugs not damaged?

LeakyForLife

So I swapped 1 and 2. Injector 1 is definitely firing and 3 is definitely firing. Didn't change much of the idle swapping 1/2 or 2/3. I did take a spare injector and put it on 1 and 3 with engine running and can feel the injector firing so the 1/3 harness is working. When I put the injector on 2 it's not firing. I'm going to start going through the harness today and I'll let you know what I find. Thanks for the reply.

LeakyForLife

Well I found what looks like a short across injector 2 harness where I spliced EV6 connectors in. It was a very small hole that developed in the heat shrink between the soldered wire ends. I re insulated with self sealing silicone tape and put it back together. Still have the same problem. Will a short across a single driver cause the ecm drivers to burn up? I checked continuity across the other pos/negative on each injector connector and none had continuity. Wish I had checked that before I started taking apart the harness to make sure that was the cause. Still not firing on injector 2 though...

lawsoncl

Ouch. Unfortunately, shorting the wires can damage the ECU and blow the driver for that injector. I'd try another junkyard ECM and see what happens.

LeakyForLife

Yep it's blown. Carefully removed the rubber coating and tested the mosfet and it got continuity across two pins on the number 2 driver (red arrows)where the other mosfets do not have continuity. Looks like I'm ordering another mosfet and resoldering it. I'll head to the JY and get a spare as well they have a 93 there. Hope it still has the ecm in it.

shout out to wcjeeper's write up on replacing the drivers. I knew just where to look and he had the part hyperlinked for purchase!

lawsoncl

Cool. Glad you have ability to replace the mosfet. Most people get cross-eyed just trying to use a voltmeter, much soldering on a board. I had actually saved a bookmark to his writeup in case I ever needed the part number. https://www.cherokeeforum.com/f2/how...m-95-a-248325/

When you seal it up, don't use the silicone that smells like vinegar when it cures. The acids aren't too friendly to the copper traces. I typically spray a conformal coating (clear polyurethane spray in a pinch) and then repot using either cheap Silicone II kitchen/bath caulk which doesn't out-gas acetic acid as it cures or a polyurethane caulk.