Fuel pressure for 91 4.0L, engine misfires

[Tyler_Smith_28]

Hello,

Does anyone know the correct fuel pressure for the 91 Jeep Comanche 4.0L? I did a fuel pressure test while running and it’s steady at 20 psi. Is this low? Also, I manually bypassed the fuel pump relay to keep the fuel pump on for a test without the engine running. It builds to 20 psi, but as soon as I turn off the pump, the pressure drops to 0 psi very quickly. 

 

I get engine misfires when the engine is in any gear but park or neutral. Also, sometimes the engine will just die. Also, I get check engine code 33 (AC clutch) & 54 (cam position sensor inside distributor, crank position speed sensor). There’s a very slight vibration in the harmonic balancer, but I don’t think it’s enough to cause an issue with the knock sensor.

 

So far I have:

> checked major grounds

> sensor grounds

> ensured vacuum line(s) integrity

> cleaned throttle body

 

Replaced:

> Fuel injectors with brand new

> Fuel Filter

> Distributor with remanufactured

> Rotor & Cap

> Spark plugs wires & plugs

> TPS 

> Crank Position Sensor

> MAP Sensor

 

[schardein]

The fuel pressure is a little low.  It should be around 29psi.  The fuel pump has a check valve that prevents drain back.  It sounds like yours isn't working.  You mention a knock sensor- no knock sensor in the 91+ system.

[cruiser54]

It's supposed to be at least 39. 

You're on the right track. 

[schardein]

Read this for a full understanding of what the fuel pressure should be.

[gogmorgo]

Fuel pressure is definitely low, and your symptoms are consistent with low fuel pressure, not wanting to rev with load, backfiring, stalling. 
The pressure test is done with the engine running. Should be in the low 30’s at idle, and jump up to at least 39psi when you pull the vacuum line off the pressure regulator (at the end of the fuel rail).
 

Bypass the fuel pump ballast resistor, see if it changes. 
Also confirm you put the new filter in the correct direction, if there’s an arrow on it, it points in the direction of flow, from the tank to the engine. It should point forwards in the truck. If it was backwards you’ll want a new filter, just to be sure nothing caught on the wrong side gets pumped up to the injectors. 

 

Did you have the code 54 before or after changing the distributor? I’d be a little skeptical of a reman. If you pop the rotor off and can wiggle the distributor shaft around it can be enough to throw off the sensor, and cause erratic readings. I’ve had code 54 on a couple engines now and on at least one of them there was significant play in the shaft, and both were corrected with a replacement distributor. They also backfired and died on me a bunch.

The redesigned distributor for a ‘95 Cherokee is a plug and play replacement, and is a serviceability improvement in that you won’t need to pull the distributor out and apart to change the cam position sensor. But I wouldn’t just go out and change it without a reason, I doubt there’s any other advantage to it. 
 

Depending on the source of the rest of the sensors, I’d be inclined to put the originals back on, especially if you didn’t diagnose a problem with them. That said I’ve had to change the TPS on every early 90’s Jeep I’ve owned (after a code 24), and the parts store sensor’s always been fine. I’ve never personally seen a Crank Position Sensor go bad but that doesn’t mean they don’t, just that I’m skeptical of it being the internet’s go-to solution to everything. But I do know there’s definitely been lots of problems reported with an aftermarket CPS so the recommendation is to always go Mopar there. 

[Tyler_Smith_28]

Thank you @schardein@cruiser54@gogmorgo!!! You guys are amazing resources. It ended up being the fuel pump. Truck is running great again. It still has the check engine code 54, and not sure if there is a way to clear codes, or if it is an old code versus a re-occuring one. If it's a re-occuring code, I may go with a 95 cherokee distributor replacement and see if that fixes the issue.

Thanks again!

Tyler Smith

[gogmorgo]

Bookmark this thread:

The Check Engine Light only comes on while a code is active, so if the light is off it’s probably not a huge concern.
Inactive codes are stored for 50 key cycles or until they’re cleared.
I’ve always been able to clear codes just by disconnecting and reconnecting the battery, let it sit for a couple minutes for good measure. Hornbrod’s procedure there with the headlights and touching battery terminals together ensures all capacitors in the system get discharged and all data gets erased which might help with a stubborn code, but I’ve never needed to do it. If the code comes back it typically means the problem wasn’t corrected.

[schardein]

Glad you got it sorted out.  Like gogmorgo said, the 95+ distributor isn't an upgrade in itself, but if you ever need to replace the sync sensor (camshaft position sensor, distributor pickup coil), it's a much simpler procedure.  In fact, I think you can put the new style sensor in your old distributor.  You'll need to pull and disassemble your distributor to get the old style (original) sensor out, then reassemble and put in the new sensor.  But in the future, the new style sensor can then be replaced without pulling the distributor.