Intermittent fuel delivery issues after sitting for a few months

[SoCalManche]

So I broke the clutch pedal on a test run up to LA from San Diego on Christmas Eve last year and the ol' girl sat the first half the year until I finally grabbed another pedal. Up until that point, she was running like she was brand new, no issues. After I got a replacement clutch pedal, she was running like $#!& for a bit, but now pretty much back to (almost) perfect. I get an intermittent issue where while driving, she cuts out completely as my foot is on the throttle, and as soon as I let up she goes back to normal.

 

With my REM, it looks like this:

Driving --- ST value is bouncing around normally

Fuel randomly cuts off while foot on throttle, ST value hits zero and remains at zero until I pull off the throttle and then fuel returns. 

 

I thought I pinpointed when the fuel cut off happened, once it got up to operating temp, but there is no rhyme or reason, or at least from what I can tell...She also dies sometimes when I come to a stop, but immediately restarts and can always hear the fuel lines pressurize by the pump. I've swapped out multiple injectors, issue remains. All grounds refreshed, pretty stumped on this one. If this issue has been discussed on here before, I'd love the link to the thread.

[Gojira94]

How did you arrive at the conclusion that fuel is cutting off? You say short term fuel trims look normal- from slightly negative to slightly positive? How much both ways, min/max ST trims? You say that while driving the ST goes to/ stays at zero? Not many things turn off ST fuel trims, but one of those is when PE mode (WOT) is commanded/ detected. If the WOT switch is glitching it may fool the ECU into thinking it's in WOT, zero the ST trims, add an arbitrary amount of extra fuel otherwise suitable for PE mode at whatever RPM/MAP it's at, and bog until you lift out of the throttle. You might want to test the WOT switch to make sure it's not inappropriately entering PE mode.

 

Procedure here: https://www.jeepforum.com/threads/how-to-test-wot-wide-open-throttle-switch.368246/

Edit: test the TPS, too 

[SoCalManche]

18 hours ago, Gojira94 said:

How did you arrive at the conclusion that fuel is cutting off? You say short term fuel trims look normal- from slightly negative to slightly positive? How much both ways, min/max ST trims? You say that while driving the ST goes to/ stays at zero? Not many things turn off ST fuel trims, but one of those is when PE mode (WOT) is commanded/ detected. If the WOT switch is glitching it may fool the ECU into thinking it's in WOT, zero the ST trims, add an arbitrary amount of extra fuel otherwise suitable for PE mode at whatever RPM/MAP it's at, and bog until you lift out of the throttle. You might want to test the WOT switch to make sure it's not inappropriately entering PE mode.

 

Procedure here: https://www.jeepforum.com/threads/how-to-test-wot-wide-open-throttle-switch.368246/

Edit: test the TPS, too 

Because the fuel IS cutting off? Let's say I'm cruising down the highway at 65mph (give or take), and I go to increase my speed to 70mph to pass someone. Randomly, as I'm trying to speed up and essentially pushing down on the throttle more, the ST value will drop to zero, I'll get the "check engine light" on the REM, and my engine cuts out. She doesn't die perse, but if I'm on the pedal, I'm losing speed because no fuel is being supplied to the cylinders until I fully let of the pedal, ST trim values return to normal, I feel the engine kick back in, and then I can continue on. 

 

ST Trim values in normal operation hover around just shy of 100 and no higher than 152. All though, I do remember before having this issue that my ST values went up to 180 once in a blue moon thanks to the southern California hills and valleys that do a number on these 2.5L. When I stomp on it and go into WOT, it only hits 152, so I don't know if that's anything to notate.

 

Sounds like a good test to do; I will definitely jump on that ASAP. Quick question, what is PE mode?

[cruiser54]

Fuel pump ground behind the left tail lamp?

 

CRUISER'S MOSTLY RENIX TIPS

IMPROVING THE FUEL PUMP GROUND

JANUARY 3, 2016 CRUISER54 59 COMMENTS

The fuel pump and fuel tank sending unit ground at a sheet metal screw up behind the spare tire on an XJ, and behind the driver’s taillamp on an MJ.

Not only is a sheet metal screw a lousy way to ground things, this ground path is long and travels through some connectors that are prone to corrosion and moisture.

Locate the black wire on  the HARNESS side of the fuel pump/sender 3 wire connector. Remove a 6″ length of the split loom covering. Strip back about 1/2″ of insulation from the BLACK wire.

Take your new ground wire, preferably at least 14 gauge and 12 to 18 inches long as needed, strip it about 3/4″, and wrap it around the exposed part of the harness plug wire.Solder the connection. Tape it up and reinstall the split loom covering.

At the other end of your new ground wire, add a crimp on eyelet. Attach the eyelet under a bolt that goes directly into the chassis. Be sure to clean the attaching point til shiny and apply OxGard to the contact surfaces.

[MiNi Beast]

Man, never thought of this. I think this is very well my issue. Thanks for the details cruiser54. Gonna do jusst this tip tonight. 

[cruiser54]

can't hurt and can only help. 

[Gojira94]

15 hours ago, SoCalManche said:

what is PE mode?

Sorry, GM terminology. “Power Enrichment” mode. Like when TPS positive delta (change) of 40% or whatever is detected from TPS, ECU enters a mode of adding fuel, much more than say for Acceleration Enrichment for mild/ moderate acceleration. A WOT or PE mode usually also ignores O2 input, stops doing ST fuel trim and LT fuel trim learning and commands a fuel amount from a table lookup of RPMs vs manifold pressure in kPa. Why I asked about ST fuel trims gong to zero. Indicated to me that the ECU purposely stopped doing the math, possibly for a false WOT mode or maybe a false closed throttle indication from the TPS.

 

We used to hook up a fuel pressure gauge and tape it to the windshield to confirm fuel pressure when issues were suspected. Since these trucks don’t have a pressure sensor at the rail, the REM can’t tell you if you suddenly lose fuel pressure. Loading up too much extra fuel feels almost the same- bogs/ dies and then clears up when you lift.

[SoCalManche]

-UPDATE-

 

Got caught up with work crankin' 12-14 hour days for the better half of the last month or two. Hadn't been able to tackle any testing just due to lack of time, but as my days were going back to normal, the she-beast decided to spring a small leak in the t-stat housing, and once I replaced that, it gave way to a much larger issue, the water pump failing and dumping massive amounts of coolant on the ground; gotta love green street paint! Due to coolant being spat throughout the engine bay, once she was up and running again, she was bucking like the wildest horse you ever rode. Essentially my initial intermittent fuel delivery problem x100. She got all cleaned up and is now back to her regular fuel delivery issues. 

 

So away I go to test and see what I can come up with. Also, I added the splice for the fuel pump in when I did the original RENIX Tips 1-5 ages ago.