ok folks, I'm pretty new to jeeps... I have an 87 jeep cherokee 4.0 5 speed with a real mind boggling problem. It started with burnt cps wires, which I have fixed. It has spark but is randomly loosing voltage to 2,3,and number 6 injectors. I'm fairly mechanically inclined but this one is beating me. I was curious if it has a limp mode which would cut juice to some of the injectors or of other know problems that would create this issue. any help would be appreciated.

 

Could you describe how you determined you are loosing voltage to the injectors. Method used.

 

I just used a test light. It flashed on the injectors that were firing and not on the others... 2, 3, and 6

 

I also can start it sometimes and it will run smooth for a short time, then I shut it off and it runs rough again. rough more than smooth though.

 

I just used a light tester.

 

Here's a diagram showing the ECU and the fuel injector circuits. Note the ECU connector, fuel injector pin numbers, and wire colors.

To make the images larger, depress and hold the Ctrl key on your keyboard while you tap the "+" key.



Here's the ECU pinouts. Note the 24 pin connector.



Locate the ECU. Remove the 24 pin connector. Inspect the pins and cavities for corrosion. Remove the connectors from the subject injectors, and inspect them.

Using a digital Ohmmeter set on 200 Ohms, read the resistance of the subject injector wires from the ECU connector to the fuel injector connector. You should see continuity. Wiggle the wire harness to see if the resistance changes.

 

Ill give this a try. I did clean the tar like substance out of this connection like instructed and hoped that was it... ill see what happens with the test.

 

ok, I went and cold started it, and it was firing on all six cylinders and purring like a kitten. I took it for about a 5 mile test drive and it did well the whole time. Came back put it in the driveway, shut it off, and restarted it.... it is now missing again and running very rough.

 

also before I started it I opened up the connection to the c101 or what ever it is called and tweaked the pins to the injectors, then I started it and was running on all six with just the slightest little random miss. a very minor miss. I wiggled all injector connections and wire harness to them with no effects on how it was running. This is the same thing it did last weekend after shutting it off and restarting, it started missing horribly. I will also add when I got home last night I started it and it was running very rough also on a cold start.

 

sorry to keep rambling on but, I keep thinking of things that might help with diagnosis. Usually when I have a vehicle not firing on all cylinders I start with spark. I started pulling spark plug wires until I found the ones not firing, I then pulled the plugs and spark tested them, they were sparking and dry, and this is what led me to fuel. I then pulled the injectors and cleaned them by manually opening them and using air and cleaner blew them out. all opening and spraying fine. I then hooked them all to the fuel rail and had a friend crank over, a little more to this story.. lol a real no brainer, needless to say when I put the fire out and unhooked the overlooked coil wire nothing was hurt but I did discover that three of the injectors were not firing most of the time, but then sometimes very seldom they would fire/spray. so then I put it back together and started pulling wires from the said 2,3,and 6 injectors testing them with my light tester and found no juice there. so then I tested one that I knew was firing and it lit up my tester when it cycled that injector. I cleaned, tweaked, cursed, etc. and am still stumped as to what would change between shutting it off and an immediate restart.

 

the burnt CPS wires might have damaged the ECU

if the wiring is good and you have good connectivity AND good ground connections - replace the ECU with a known working one and test again

 

Check continuity of the wires I outlined above. Of course, wiggle check is always in order with Jeeps. Examine all the harness wires from the ECU to the injectors. It sound like there's some chafed/broken wires somewhere.

 

$$

 

In 1987 on the 4.0, it was common for the injector harness to chafe where it exited the firewall area and went to the back of the valve cover.

 

yeah - but if the wiring harness is good and he has spark, what other things can he check?
how expensive are scrapyard ECUs...if there are still any left? :P