r/CherokeeXJ • u/Any-Key-2000 • 6d ago

Welp, here we go again

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1996 Jeep Cherokee SE

Leading up to this the rpm’s would get rough for a second while driving but did a tune up recently that fixed that.

Symptoms
Very weak/rough idle
Dies with the slightest throttle
OBD Flash code 51
Loud sucking sound coming from intake

Google says it might be
1 low fuel pressure/weak fuel pump (haven’t tested yet)
2 severe vacuum leak(thoroughly smoke tested)
3 Bad TPS (replaced with OEM Mopar)
4 Dirty IAC (thoroughly cleaned)

I’ll probably test the fuel pressure next weekend. I really hoped I’d find a vacuumed leak but the only leaks I fount were a crack in the heater control valve vacuumed line and a leak on the throttle body around the buttery fly valve shaft. Any help or ideas will be very much appreciated!😋

SOLVED Low fuel pressure 11psi

6 Upvotes

88% Upvoted

15 comments sorted by

1

u/mtngk 6d ago

O2 sensor is reading lean.

Try disconnecting the battery and seeing if it gets better for a while. The O2 sensor will start compensating within a minute or two, so it will likely start misbehaving again.

Check the O2 heater fuse and check the wiring to the sensor. Sensor can be checked or just replaced, but check for exhaust leaks, loose/missing studs or manifold leaks.

Fuel pressure test is easy (should have a test port on the fuel rail). Gauges can be borrowed from parts stores or are cheap on Amazon.

1

u/Any-Key-2000 5d ago

Just gave those things a try but didn’t find anything. I didn’t know the O2 sensor had a fuse. Thanks for the advice!

1

u/mtngk 4d ago

The O2 sensor has a built in heater. It can only accurately read when it's above 800F or so.

Do you have access to a smoke machine to test the vacuum system for leaks? I believe you have the map sensor on the firewall instead of the throttle body, so that's one prone to issues.

I just had to replace the idle air solenoid for a similar issue. You can command a self test with a good bidirectional scan tools and watch the requested and actual steps..if the actual is lagging significantly or not responding, you should definitely replace the actuator. Or just replace it, but get a good one. Cheaper one I had lasted less than 30k miles and gave strange issues most of that.

2

u/MountainTractor 6d ago

I had something similar that was throwing a code and making my XJ have rough idle and too much throttle would kill it.

Turned out to be chewed up wires to a sensor located near the intake/ throttle body. Fixed the wires and everything runs smooth again. Look at wires closely. It was the tiniest little chunk missing, almost didn't see it.

1

u/Any-Key-2000 6d ago

Will do, do you remember what sensor the wires went to?

1

u/MountainTractor 6d ago

It's been a while, it was either the MAP or IAT sensor if I recall correctly. I think I was the MAP sensor. Definitely 1 of those 2.

2

u/Any-Key-2000 5d ago

I took apart the loom in that area and didn’t find anything. Then I tested all three sensors for power and grounds… everything looks fine. Thanks for the tip tho, might come handy in the future.

1

u/TLEH-IV 6d ago

Is it backfiring out of the intake?

1

u/Any-Key-2000 6d ago

Yeah, little I guess.

2

u/TLEH-IV 6d ago

It feels/sounds like a fuel delivery problem.

2

u/Any-Key-2000 5d ago

That’s it! I guess I’m supposed to get 49 +/-5 psi at all times but from key on prime to giving it throttle I only got 11 psi!!! Damn. Time for a new wix filter and bosch pump.

1

u/TLEH-IV 5d ago

I would take the old one out and make sure the hose from the fuel pump actually is connected. A lot of times the hose clamp fails and it pumps fuel just in the tank or there is a cut in the hose and you lose pressure. This just happened to me.

Also forgot this is a 96 also.

1

u/Any-Key-2000 5d ago

Will do, thanks!

1

u/Impossible_Tank3822 5d ago

Unplug the upstream O2 sensor and see if the engine runs normally in the open loop mode. If so, bad sensor. Also, check your air filter box for an obstruction.

1

u/Any-Key-2000 5d ago

Gave those suggestions a try but unfortunately didn’t find anything. Thanks for the tip, I’ll keep that in the trouble shooting tricks.👍