I have been suffering the last couple months with an insidious problem with my '91 4.0 Sport: Every time I take it up in the mountains, far from home it starts stalling. After about 1 hr of uphill freeway, it stalls. Repeatedly, for 2-5 seconds at a time, requiring a roll start or use of the starter if I was going slow enough. Issue will persist until I leave the jeep to sit for 6 hours or so. The problem does not happen with regular around-town driving. Only on long-sustained uphill drives.

At first I thought it was heat related, so I installed an e-fan (since my 91 Sport didn't have one stock) and insulated some things. That didn't help. Yesterday, I thought to check the fuel pressure. While the fuel pump makes the right pressure at idle (32 psi) and with vacuum disconnected (39 psi), it won't hold fuel pressure. (pressure totally goes away in 15 or 20 seconds). I verified that it's not leaking through the injectors or back through the fuel regulator. So the fuel pump has a bad check valve.

So my question is: could a bad fuel-pump check value cause an an engine to stall when it's at medium-heavy load?

Sigh. Thanks so much for any input you have. This problem has thrown a monkey wrench into my last 4 wheeling trips and has made me really sad.

 

Don't be sad, we're here to help

A bad checkvalve/drain back valve is usually giving symptoms of slow start/increased crank times. It only stops flow in one direction and has no other function.
Once the engine is running, its pretty much not having anything to do as such. So from that angle, it should not given you the stalling whilst driving. (my drain back valve is virtually non existent, have to prime the fuel pump afew times before starting and pressure drops immediately to nothing after shutting the engine off. However, no issues during driving)

Maybe a somewhat marginal pump would cause it, or a faulty pressure regulator. I'd be more inclined to look for the cause there.
It would be great to be able to know the fuel pressure whilst you'd be doing those long uphill drives when it starts to do that but I'm not sure how that could be achieved. Keep in mind there might be other causes for the stalling.

Good thing though: IF its your checkvalve/pump: the pre-update fuel pump is easy accessible, compared to the 97+ where we need to drop the tank.
IIRC correctly, the 97+ have two drainback valves, at least one in the pump assembly; but maybe I don't recall this correctly.

 

Have a tach?
What does it do when the Jeep hesitates?

 

Thanks for the tips/clarifications.

Jeep does have a tach. It doesn't hesitate or stumble. It just dies. So if I put the clutch in when it's dead, RPM drops to zero. (then if I release the clutch after a few seconds, it will roll-start back to life for another minute or so, then repeat the drama).

Oh, the first thing I changes was the crank position sensor, since that sounds like it could have cause this problem. Forgot to mention that in my post.

I was suspicious that the check valve was mostly about priming. I checked the fuel regulator and it seems to be functioning correctly. (correct pressure with and without vacuum, pinched return fuel like line and saw pressure spike to ~100psi). Is a fuel regulator something that can work intermittently? I was planning to bring the fuel pressure gauge with me on a test trip in the next couple days. This way, I can see if the pressure if fluctuating wildly during these stalling sessions.

Thanks again.

 

So you have a manual and a pre-update, right?

I dont have much experience with those, especially the separate fuel regulator, so others need to advice on that.
I think it would only be helpful to see the fuel pressure as you are driving, but I reckon that would need the gauge to be hooked up while driving and I'm not sure how one would go about that.
Maybe your fuel pump has a poor connection or otherwise starts to play up while you are driving. Not sure if ASD or other components are in play either, including CAM and CPS. The CPS can give heat related issues.
Challenging to diagnose this.

 

Well, based on your symptoms, it still sounds heat related. A long steady climb with a good load generates a lot more heat in the engine and transmission than driving around town. Yours, being a manual, likely removes it from the "suspect list". Problem is getting to troubleshoot while it's in the no-run condition. It's important to try and determine if this is a fuel or ignition issue.

Meanwhile, check your grounds, verify condition of your battery cables, disconnect//inspect/clean/reconnect the connectors for your sensors and the ignition coil.

Depending on the source of your CPS, I wouldn't rule it out yet. There are plenty of posts about cheap sensors failing quickly.

Good luck. Hope to see your FP results soon.