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'94 Jeep Grand Cherokee Is Running Way Too Rich

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.

The compression is said to be good. What are the numbers? I ask this because many post on this forum with allegedly good numbers and nothing could be further from the truth.

Will ponder this some more anyway but let me also ask this. You state the noid light shows power at those injectors. Does the noid light flash when the engine is cranked over?

Thanks for the reply OK4450. We sure need some help with this problem.

From what I was told, all of the compression numbers were in the area of 133psi. As far as the noid light test goes, from what I understand the test was made while the engine was being cranked or possibly while it was running. I would have to verify that but he did say it was flashing. I compared the resistance of a couple of injectors, one each in a working and non-working cylinder and both injectors read about 15 ohms with my meter.

The engine had just been through an IM test before this problem happened and passed with flying colors. My neighbor let someone work on it after the testing was done and it came back running like it is now. He was supposed to do a ‘tune up’ on it. The only thing he worked on was the distributor as far as I know but there must be some others things he touched I am not aware of right now. I have a suspicion about the disty since that was worked on. The cam position sensor is inside it and I wonder if something has happened with it. The ignition timing doesn’t come from the disty but what about the injector timing?

I have been trying to make logical sense of why the back 4 cylinders are not working. What could cause this to happen? The firing order is 1,8,4,3,6,5,7,2 and 6,5,7, an 8 are dead. At first I thought some of the plug wires were in the wrong order but after checking that they proved out to be ok. The only other thing I can think of is there may be a problem with the fuel rails. The fuel filter was pretty bad I heard and the rails tilt towards the back. I doubt there is debris built up in there but we haven’t removed the rails to check that.

I would like to know what you think about the fuel pressure test readings. The book stated 31lbs is the regulated pressure and my gauge showed 36lbs… Do you think that is the cause of the problem? I don’t have enough experience in that area to be sure. Off hand I would dimiss it as the problem but maybe that is a mistake. The regulator costs about 70 dollars so the owner didn’t want to repace it if there wasn’t a real need to do that.

Please let us know what you think.

While responding to another Jeep problem I just realized we haven’t checked anything to do with the air flow metering for the engine. In fact the ducting to the throttle body is not currently in place and I don’t know where the sensor is supposed to be located. Maybe the injectors are running wide open. We also haven’t checked for any codes. Just some more thoughts here.

The 133 is low and that may or may not be a problem. Normally, you would look at that and say the engine is worn. However, since it’s running excessively rich it could be that it’s simply a matter of the cylinder walls being washed down with raw gasoline.
I would think that if it were this bad the engine oil would have a faint gasoline smell though.

The ohms on the injectors are about right and I don’t think the 36 PSI fuel pressure is the problem. After looking over a cylinder layout and dist. cap layout there doesn’t appear to be much of a pattern there other than the 2 rear cylinders on each bank.

I’m by no means even a mild Jeep expert but if the ducting is not in place between the throttle body and air cleaner I would suspect a MAF problem as they’re located in the intake tract between those 2 things. Without air being pulled through the MAF it will run roughly.

If this gets sorted out, runs well, etc, and IF the engine oil appears to not be diluted with gasoline I would revisit the compression issue with a wet test. If those 133 readings take a jump up to 160, 180, etc. then I’d say it has a piston ring issue developing on all cylinders.

Have you checked the fuel pressure regulator for a leak?

If the regulator is leaking and fuel is being pulled into the intake manifold, the excess fuel is going to run towards the back of the intake flooding the last four cylinders.


Thanks for the info guys.

I thought the compression numbers were low also and though the vehicle is fairly old I wonder if the timing chain has slipped and caused those numbers to be lower than they really should be. The trouble happened while the fella that was supposed to tune it up had it. Whatever is causing the trouble it happened suddenly because it was running pretty well before he worked on it.

I will check out the MAF or MAP issue and see what is up there. We’ll try and check to see if the injectors are leaking also.

The pick-up in the distributor effects the injector timing but moving the distributor housing doesn’t cause a noticeable change in perfomance. The DRB III has a utility to synchronize the distributor (set sync signal).
It is possible to turn the dist. housing enough that the rotor is aligned with the wrong cylinder on the cap. Set the crankshaft at TDC and see if the rotor is aligned with cylinder #1.

Thanks very much for the very good info Nevada_545. I was wondering about the wiring that tied to the disty and what was going on with the three leads to it. While the firing order checked out to be ok I have been thinking that the disty position has to effect the firing somehow. We will definately look at that tomorrow. Since the disty is the only thing I really know the person worked on, it is prime suspect in my book.