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1993 Jeep grand cherokee running on three cylinders

I have a 1993 Jeep Grand Cherokee Laredo that was running well, but I stopped at a restaurant for lunch, and when I came out and restarted the car, it ran like garbage. I babied it 50 miles home, where I replaced the spark plug wires, cap and rotor. (I figured it hadn’t been done since I bought the car, and wouldn’t hurt, might help) No change. unplugging the injectors on cylinders 4,5,6 makes no change. Fuel pressure is a little weird, as it reads between 20-37 lbs, but whatever the pressure is reading, it runs poorly. The fuel pressure leaks down almost immediately after shutting off the truck.

  1. Why do my 4-5-6 injectors appear not to work?
  2. Could the apparently bad check valve in my fuel pump cause my symptoms?
  3. Is an inline check valve an option to repair this? (money is an issue.)

The sudden nature of the malady suggests to me that it is not plugged injectors, though I am open to suggestions.

Since the problem was sudden and is not confined to one bank of cylinders, I wonder if you have a wiring problem or a problem with the driver circuits in the PCM. If you pull the electrical connectors for the injectors on the affected cylinders, then start and shut off, does the fuel pressure still leak down immediately?

Have you pulled codes yet?

Extracting Fault Codes - 1993 Jeep:
Turn the ignition switch ON, OFF, ON, OFF, ON and observe the CHECK ENGINE LIGHT. The CHECK ENGINE LIGHT should begin to flash a series of codes. The flashes will be separated by short an long pauses. The long pauses separate the “ones” and “tens” positions, and each code from the next. A code 55 will be displayed at the end of the test.

Diagnostic Trouble Codes - 1993 Jeep:
11 IGN reference signal.
13 Fault in the map sensor pneumatic circuit.
14 Fault in the map sensor electrical circuit.
15 Fault in the vehicle speed sensor circuit.
17 Engine running too cool.
21 Problem in the oxygen feedback circuit.
22 Fault in the coolant temperature sensor circuit.
23 Air temperature sensor.
24 Fault in the throttle position sensor circuit.
25 Fault in the IAC motor drive circuit.
27 Fault in injector driver interface circuit.
31 Purge solenoid circuit.
33 Fault in air conditioning cutout relay circuit.
34 S/C solenoid circuits.
35 Fault in fan relay circuit.
41 Fault in the charging system or no field current.
42 Fault in the ASD relay driver circuit.
43 Fault in ignition coil control circuit.
44 Battery Temp Voltage.
46 Battery voltage is too high.
47 Battery voltage is too low.
51 Lean condition is indicated.
52 Rich condition is indicated.
53 Internal problem in module.
54 Fault in the distributor high data rate pickup circuit.
55 Completion of fault code display.
62 Unsuccessful attempt to update EMR mileage.
63 Unsuccessful attempt to write to an EEPROM location by the controller.
76 Fuel pump resistor by-pass relay circuit

I just pulled codes, and got a 12, 13, 55

I did unplug the map sensor at some point while looking for a vacuum leak, so that is not surprising. Not sure what the 12 is though.

@oblivion, When you say the problem is not confined to one bank of cylinders, what constitutes a bank of cylinders? I thought that cylinders 4-5-6 could be a bank, but that info is not easily found.

I did find that one of the pickup wires from the distributor looked suspicious, but I replaced the pigtail with another from the junkyard with no improvement. I forgot to mention earlier, I changed the fuel filter also, to eliminate that.

Also, I changed the fuel pressure regulator with a junkyard part, which produced no change.

Uh, inline 6 or V8 motor?

Sorry, 4.0 straight six

Doesn’t the vacuum for the MAP sensor come from the rear of the intake? If so I would check that vacuum line and the MAP sensor for leaks. Also chech for anything else that draws vacuum from the intake.

I will check that this afternoon.

Changed plugs, 2,3,4 had a lot of carbon. No change though.

I did find this little sensor plug broken though. Not sure what it is for?

Btw, scratch the idea about the check valve, I am getting 30 psi steady. I rechecked with a better fuel gauge after I found the o ring in mine was missing.

Code 12 is low battery voltage or a battery that has been disconnected in the last X number of starts, 55 is end of test.

A broken camshaft could disable the last 3 cylinders, but I have NEVER heard of that happening.

I think you are looking for something wrong with the wiring to the back 3 injectors since you say the plugs were dry.

This guy found rust plugging an injector:

Do a leak-down test between those three adjacent cylinders to see if a head gasket failed.

I had a Wrangler come in with the 4.0 liter engine with non- firing between two adjacent cylinders because the head gasket failed.


It seems that the coolant temperature sensor was broken internally. Not sure how that would affect my injectors?

The CTS tells the ECM the engine’s temperature. If cool, it enriches the fuel mix, like a choke on ancient carbs. If broken, the ECU sees a really cold temp, like -40 degrees. It then floods the cylinders and causes the plugs to carbon up and foul. Like what you saw on the plugs, all that black carbon crap.

Well, I replaced fuel temp sensor. But it still runs like garbage. The fuel pressure dropped off to 15 lbs for a while though, so now I am back to the fuel pump being the main suspect. I can’t figure out why only the last three injectors are not firing, unless fuel pressure at the last three injectors is lower than in the first three, and not enough to go through.

Have you replaced the fuel filter? The pressure is too low to run the engine. Maybe the regulator is bad. That would seem cheapr and easier than the pump. One test I remember is to pull the vacuum line on yhe regulator and see if the fuel pressure changes. No vacuum should boost the pressure.

I haven’t checked a wiring schematic and this is just food for thought, but any chance the injectors have power provided in sets of 3?

Some vehicles (say a 6 cylinder for example) will have power through Fuse 1 for 3 injectors and Fuse 2 for the second set.

I’ll dredge up a schematic for a look-see later as I’m on the way out and if no one clarifies the issue before then. Later… :slight_smile:

I have replaced the fuel filter, and the fuel pressure regulator. I did find two blown maxi fuses, for “Alt power 1” and “Alt power 2” The guy who owned this vehicle before me did some weird stuff.

The Jeep just got a head gasket replaced a month ago, but I will check to see if that is a problem. I may pull the fuel injectors at the same time to swap them around to see if the problem cylinders follow them. The abrupt nature of the change leads me to think not injectors, as that would be a more gradual issue, no?