98 Grand Cherokee 'bogging down' at half throttle, 3k rpms

While driving a few days ago, I started noticing a severe loss of power while going up hills. I got home, shut it off, checked all the wires and connectors to make sure nothing got unplugged, and started it back up again. The check engine light immediately came on, and it started idling rough. I ran the codes, and they showed a random misfire, misfires in cylinder 3, and misfires in cylinder 5.

It will run fine in park at about 1/3rd throttle, everything sounds as it should, but when increased past that, it starts bogging down terribly.

Things I have done.

1. New spark plugs

2. Replaced Dis. Cap and Rotor

3. New ignition coil

4. New Spark plug wires

5. Removed fuel injectors and cleaned them

6. Dry gas in the gas tank.

7. Fuel pressure test. Manual says 43-48psi, mine came in at 41psi. About 3psi increase in fuel pressure when throttle is applied.

8. Checked Air filter and made sure there was nothing clogging the hose.

Any help would be extremely appreciated. I really am stumped.

You need a new fuel pump. The fuel pressure is below the minimum spec of 43 PSI @ 41 PSI.


Thankyou for your timely reply. I am going to do another fuel pressure test to make sure it really is coming in low. A question though, is it normal for the test gauge to go back and forth rapidly with the car on and idling between like 40 and 42ish psi? I had a hard time getting a good reading when the car was on because of this.

No. The pressure qauge shouldn’t fluctuate rapidly. It should read steady. That just confirms the fuel pump requires replacement.


Tester is certainly correct. Here’s one other thing if you need a little more convincing - it is below spec sitting at idle. Now imagine that you’re tooling down the road trying to get the engine up to 3K rpms. This is increasing load on the pump and by then your pressure is probably much more below spec. If you like you could figure out how to rig the fuel gauge to the outside of the windshield and watch what happens when you try to throttle up.

Of course, your list didn’t include a fuel filter. A clogged filter will keep the pressure down - I don’t know the specifics on this car but if you have an external filter change it and then check the fuel pressure again.


Replaced the fuel pump and filter. Still same problem. Any other idea’s?

Pressure went up to required specs.

Check the engine compression and check for exhaust blockage. The latter is pretty easy with a vacuum gauge.

I’ve ran a compression test and all cylinders were within 7 of each other. How would I go about checking for exhaust blockage?

Have a good mechanic reset your onboard computer(s). I think the computer is in the limp mode because of previous engine problems including the bad fuel pump. The computer may need re-programmed or replaced.

Engine exhaust blockage may be due to a bad catalytic converter. Have yours checked to see if it is stopped up.

When it comes to car electronics, im fairly clueless, but when the computer is in the limp mode, doesn’t it restrict the transmission to just gears 1 and 2? Would disconnecting the battery and resetting the PCM do anything? I’ve tried after the fuel pump was put in, and it still did it.

Later today im going to be disconnecting the exhaust in front of the cat to see if there is any change in how it runs. I’ll keep you guys updated.

You can have someone with the equipment actually measure the pressure to see if it is within specs. This means taking it to a shop.

A quick check for exhaust blockage can also be done with a vacuum gauge. These are not very expensive and way handy. Check this out: http://www.secondchancegarage.com/public/186.cfm

“Scenario 14” describes blocked exhaust.

Yeah, easy enough with a vac gauge. The vacuum should hardly decay with no load regardless of the rpm. The engine just runs out the air:Fuel mixture to speed. If the vac starts dropping off at a given rpm, you probably have a clogged cat. You then tap it with a mallet and see if you here debris rattling inside. This is not always the case, but frequent enough.

“I’ve ran a compression test and all cylinders were within 7 of each other.”

I more or less spaced that out the first time through. Within seven what of each other? What are the actual readings?

Lowest was 126, highest 133. 7psi. I should have clarified that.

I just got a chance to get out there and put the jeep on ramps. Cut out the catalytic converter, turned it on, and bam she runs again. Sounds like she’s ready to go out on the strip and drag race, but will go to redline.

Thankyou all for your help. I just wish it hadn’t ended up being as costly as it was.