Brian Day

My 1999 Cherokee sport, 4.0/auto is randomly losing power for a split second then comes right back it gets worse when I run the a/c. I've replaced everything I can think of including crankshaft position sensor (what most people have told me the issue is) the main computer was replaced less than a year ago, distributor also newer, iac replaced... the list goes on. I've scanned the ob2 several times no pending codes and no stored codes. Any info would be greatly appreciated

jordan96xj

Do you have a multimeter?

First quick test is to have the vehicle running, with the AC on, and test the system voltage at the battery terminals. Look for any strange variance in the voltage, like going up and down, or transient spikes.

When was the last time you cleaned up all of your electrical connections? (battery posts, chassis to battery ground, other ground points).

Brian Day

Yes, voltage is fine when it's at idle the problem only occurs when driving, as I've replaced pretty much every senor on the jeep I've checked all connections I can see or know about, even ran a few new grounds thinking that might be an issue.

jordan96xj

Ok, at least the most obvious stuff is already checked. Good thinking.

I would probably want to pull the steering column plastic and check the condition of the wires and connector at the ignition switch. I would also do some continuity tests on the ignition switch connector socket pins while slightly moving the key in the "on" position (with connector unplugged). To see if there were any strange interruptions to the continuity that could be leading to something transiently knocking out the ignition. This is relatively quick to do.

If that didn't show anything obvious/quickly, I would want to then determine if it is spark or fuel that is being interrupted. Checking for spark interruptions while driving might not be feasible. But you may be able to run a fuel pressure gauge so that you can see it while driving (depending on how long the hose for it is). Also, if it is the injector pulses that are dropping out (due to an electrical problem) the fuel pressure may look fine, but still be a fuel delivery problem.

Have you tried jostling the harnesses around while idling with the AC on in the drive way to see if you can get it to reproduce? Particularly the harnesses that run around the back/top of the fuel rail (also visually inspect for any harness chafing back there).

Other than the power dropping out for a brief moment, does it run well otherwise?

s346k

define "randomly loses power". like the engine completely cuts off or the engine simply bogs down? for starters - if it's cutting out altogether i'd be looking at ignition sources. if it's bogging down i'd be looking at fuel.

Brian Day

Thank you for the great information, yes I've tried wiggling the harness and other key areas to see if I can get it to do it at an idle, no luck, yes it runs perfectly fine just randomly for a brief second it will cut out almost like it lost spark for a quick sec. I'll pull the under dash today and see if anything is out of the ordinary there. I took a video of the gages but not sure how to post it on here to get a better idea of what's going as it's hard to explain. After reading your replies I'm willing to bet it's something to do with the ignition. Thank you again hopefully I'll figure it out today.

Brian Day

Definitely cutting out, as if I turned of a switch super quick and back on, only one time it did die completely but fired right back up as soon as I turned the key

Brian Day

So I've checked the whole harness as far as I can go all looks good, I checked voltage at the ignition everything is fine there, no fluctuation. I went and bought a obd2 scan tool that does live data... this is the first time I've used one and not exactly sure what I'm looking at but one thing that I did notice that looks a bit odd is the spark advance is fluctuating up and down from 10 deg to 20 deg at idle approximately 800 rpm. Is this normal?

Brian Day

Bugout4x4

These guys in here know more than I do about the eccentricities of this particular engine, but I wonder if a sloppy timing chain could be causing this fluctuation?

67 GMC

I had this on a much older vehicle. It was a bad O2 sensor. It would basically cycle every so many minutes. Would run fine, then lose power for 2-3 seconds, then back to normal. The O2 sensor was "slow" so it was sending bad information to the ECU which was trying to adjust the fuel / air mixture to keep it all running. It would go back to a default mode when the O2 sensor was sending garbage to it. Maybe watch it on your scan tool and see if the fuel trim changes just for a few seconds before it "dies". Not sure if FUEL SYS CL means closed loop. If it goes intermittently to OL (Open loop) that might be your clue.

Brian Day

I'm starting to lean more towards it being a fuel issue, I finally got my hands on a fuel pressure tester when I first turned the key on it did not build pressure right away as I cranked it the pressure came up and started it holds at 49lbs while running and drops off immediately to zero when I shut off the jeep. I realize this could just be a bad check valve at the pump but could this be part of the issue with the engine cutting out?