I am trying to help my neighbor figure out what is going on with his '94 Jeep GC, with the MPFI fuel injected V8 engine installed in it. I thought he had simple problem but it isn’t turning out that way in trying to figure it out.
When I first got involved with it he told me that the last two cylinders ( 5,7 & 6,8) on each side were not showing signs of firing. We verified this by pulling the connections to the individual injectors. Power to all of the injectors has been verified with a noid light. Pulling the front 4 connections makes a big difference in the way the engine runs and the back four make no difference. I decided to check how the new plugs were running. All of the plugs were solid black. I have never seen plugs that black before. After we cleaned them up the engine ran better but the rear four cylinders still showed no signs of firing. He also stated that firing pulses were getting to all the plugs but I haven’t verified that yet myself though I trust his judgement.
The very rich mixture made me think the fuel pressure regulator was bad or the CTS sensor was faulty. The Chiltons manual stated the regulated pressure should be 31 pounds at the fuel rail. My new fuel pressure gauge showed 36 pounds there. The CTS sensor seems to a little out of tolerence also. The resistance seems to be a little too high for the actual engine temperature so he is going to replace it. I’m not sure about the fuel pressure regulator reading. Could 5 extra pounds make a big difference in fuel mixture?
The compression is good in all cylinders. Though the disty doesn’t control the timing I wonder if the position of it could have something to do with the issue though I have to ask why only four cylinders are effected. I am now wondering if something is going on with the PCM that could cause this to happen. One other possible clue, the owner stated the fuel filter was pretty bad when he changed it.
What do you all think about this? I though I was going to solve this issue real quickly. Not! We need some good help.