Odd fuel/throttle issue

[KMW-NWXJ]

Had an issue pop up today with the XJ. It's a '91 with 4.0 and 5 speed. It's pouring rain so hard I haven't been able to do anything with it yet. But I figured while I wait I might as well seek some suggestions on things to look for.

My son was driving it and it stalled taking off from a stop. He had to hold the pedal to the floor to get it to start again, and then it didn't want to rev up. He got it to a driveway and called me.

I found that it won't start unless the pedal is to the floor. Then it sputters and shudders and puts out white smoke and smells rich, before clearing up and revving. Then it will idle fine. It sounds smooth and doesn't shake like it's missing.

However, if you press the accelerator slowly, it will sputter and die right off idle. If you let off quick it will recover and continue idling.

If you press the accelerator quicker and a little further, it will sputter just a second and then rev.

It is as though there is a "spot" in the throttle opening where it will die. If you pass through that "spot" quickly enough it's fine again. Once I figured this out I was able to drive it home just fine. Going down the road it is normal. No loss in power. It's only in the just-off-idle throttle position that it has a problem.

Anybody had anything like this? I'm kind of thinking maybe TPS? At first I thought fuel filter, but once I found it ran fine once I got past the "spot", that doesn't fit.

Thanks for any ideas!

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[4.3LXJ]

You probably have a check engine light on. If you check the code it will probably be for the throttle position sensor

[KMW-NWXJ]

There was no light on, but I guess it's possible it doesn't work. I'll go by the parts store and check for codes.

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[slvmart]

Not sure about a '91, but my 2000 has similar symptoms if the battery discharges. No idle and rough running, with foot on throttle, until the battery charges enough. Then all is fine. If you've ever drained the battery down because you left the lights on. Or as in my case, the battery has a few years on it and it gets below 10 deg out.

[Carves]

Let us know how you get on after checking for codes.

[Cheromaniac]

A bad throttle position sensor will cause a fluctuating idle and some hesitation when changing throttle position, but the engine should still be able to start and drive reasonably well.
Your Jeep has all the classical symptoms of a bad MAP sensor. Luckily it's a cheap part to replace and it carries a GM part no. so a GM 1-bar MAP sensor of similar vintage will interchange.

[oderdene]

Mine was TPS, bad around 1000-1600 RPM. replaced with Standard brand 2 years ago, so far no issue

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[KMW-NWXJ]

Well, went out this morning to start trouble shooting it, and it fired right up and ran perfect. Went and drove it to town and no issues. So far it's driven 20 miles with starts and stops, and not a single hiccup. That's great, but now I don't trust it! I did pick up both a MAP and TPS so I'll have them on hand.

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[KMW-NWXJ]

Very strange. Been driving it a lot and nothing. Running like a top! Hard to diagnose when it won't do it again! If it comes back I'll update with what it turns out to be.

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[oderdene]

Very strange. Been driving it a lot and nothing. Running like a top! Hard to diagnose when it won't do it again! If it comes back I'll update with what it turns out to be.

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Maybe loose connector, if it run ok & you have a spare on hand I say HAKUNA MATATA [emoji1]

[KMW-NWXJ]

Well, after two months it's started again. We checked codes, and it's showing a code 76. The chart I found says: "Fuel pump bypass relay circuit. Need specialist or dealer diagnostics"

When it started I was quite a ways from home so I got to feel it out pretty good. Idle is fine. You have to rev it up quick to get going without stalling. And it absolutely won't start without holding the pedal to the floor.

Then once you're driving, if you are smooth with the throttle it does fine, but if you punch it quick you get a lurch as it completely falls on its face for a second and then takes off just fine. It kind of acts a lot like a carburetor with a bad accelerator pump.

Anybody know what the fuel pump bypass relay circuit is?

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[4.3LXJ]

What year is your Jeep?

[KMW-NWXJ]

It's a '91

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[4.3LXJ]

I believe you have a ballast resistor on the driver fender well. Check it and see if it is OK. Normally the fuel pump just quits, but maybe not in your case

[KMW-NWXJ]

Ok, thanks. I'll take a look for it as soon as I can get some time. I would think it would just quit as well.

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[KMW-NWXJ]

Ok, finally got a day off and I'm messing with the XJ. So still will not start without throttle to the floor, warm or cold. Once started runs really rough at first and seems very rich. Like it was flooded. Once it clears up it idles smooth. When you apply more throttle it stalls, then catches and revs. It doesn't matter where the throttle is. Off idle, or holding at 2500 RPM and then applying more throttle, it does the same thing.

I bypassed the resistor thing on the fender well, and that made no difference at all.

I did notice that if I push on the injector plugs on some of the cylinders it will slightly roughen the idle. Kind of odd.

Right now I'm off to grab a new TPS just to rule that out.

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[KMW-NWXJ]

Ok, neither the TPS nor MAP made any difference at all.

I was thinking maybe the idle air valve, but that shouldn't affect it after it's off idle though, right? And I unplugged it and it wouldn't idle at all that way.

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[KMW-NWXJ]

Well, I have bypassed the fuel pump relay and the resistor with no change.

I have verified that the system is working as designed, i.e. fuel pump runs about 2 seconds when key is turned on and then stops, shut down by the Auto Shutdown Relay, as I understand it. Then when you start cranking it over the fuel pump starts up again, powered through the starter circuit, according to my research. Then once it starts, the fuel pump is still running, now powered through the fuel pump relay circuit, per my research. The pump sounds smooth and normal at all times.

I went and rented a fuel pressure tester. Per my research pressure should be 30-40 psi, and should pressure up and hold from the initial 2 second priming when the key turns on. Idle should be about 31, and with the vacuum disconnected from the regulator it should jump to 39.

I hooked it up and on prime it jumps right up to 40 and then instantly settles at 31. Once started it idles at 31 and jumps to 39 with the vacuum off. Seems to be as it should be. However, it still fell flat when you try to give it throttle, and still needed full throttle to start and would sputter and cough several seconds before it could idle. Once shut off pressure held at 31.

Now here's where it gets weird. While it was running I pushed the pressure bleeder for the tester just to see the fuel flowing. No real thought or reason, I just did. I let fuel run out the bleeder line for several seconds. Then I hit the throttle and it revved smooth and normal! Shut it off and it started right back up normal with no throttle and again revs fine.

Now I'm really stumped! It's running just fine and I can't get it to sputter, and nothing has changed! Is it possible there was a vapor lock issue in the fuel rail? Or some kind of debris that was blocking an injector that got flushed out when I let it flow through the bleeder line? That's all I can think of at this point. I'm at a total loss, and I feel like I just can't trust it!

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[4.3LXJ]

Did you let some vapor out of the line?

[KMW-NWXJ]

I'm not sure. There were bubbles, but I'm not sure if that was from the fuel rail, or the tester line.

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[4.3LXJ]

I'm not sure. There were bubbles, but I'm not sure if that was from the fuel rail, or the tester line.

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The last symptom is what you would get if you had air or vapor in the fuel rail and then bled it. It may be that you are getting bubbles from heat or air from the pump or something

[KMW-NWXJ]

Maybe I should hook it up again and let it flow and see if the bubbles keep coming. At the time I wasn't paying super close attention as I wasn't trying to accomplish anything. It was more like it was a button, so I pushed it!

There really wasn't much heat at the time.

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[KMW-NWXJ]

Well, it drove around all day today just fine. [emoji53]

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[KMW-NWXJ]

Probably sick of this thread popping up by now! [emoji16]

It started doing the same thing again. I pulled all the plus and they all had black soot coating them indicating a rich condition. Consistent with how it sounds and acts. Then I unplugged the injector plugs one at a time while it was running, and you can hear it start missing with each one. Then, after messing with the injector plus it was suddenly running just fine again. Starts right up, and revs smoothly and normally. Drove it today about ro mules pulling a trailer and it ran perfect. [emoji35] I am really getting frustrated with this thing! The only thing consistent I have come up with odds it starts running fine after I mess with the fuel system. The first time it was after I tested the fuel pressure, and this time after unplugging/plugging all the injectors.

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[KMW-NWXJ]

I'm back! So, it's been running fine since August. Just started doing it again.

Takes full throttle to start it, and then it sputters and coughs for several seconds and pumps black smoke before it will settle down and idle. And smells strong of gas.

From idle, as soon as you crack the throttle it wants to choke and die. You have to sort of pump and feather it through this "dead zone". Once you're above the "dead zone" it smooths out and will run. But then when you abruptly add throttle there will be a cough before it goes.

I took a video showing the off idle "dead zone".

I unplugged all the injectors again, and bled fuel from the rail, but this time that did not stop it. Once veggie, after unplugging and plugging each injector it suddenly ran fine again, and did for several months. The next time I was checking the fuel pressure and bled some fuel out while it was running, sms it suddenly ran fine, and has since August.

I've changed the throttle position sensor multiple times, as well as the idle air valve controller, and neither has had any effect.

Here's the video. Maybe someone will have a fresh idea?

https://youtu.be/FQmK_ePWSqw

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[4.3LXJ]

How is your fuel pressure? Stock injectors?

[KMW-NWXJ]

Stock injectors as far as I know, (I've never changed them, and I know the previous owner wouldn't have) and when I checked the pressure I August it was all exactly what the specs say it should be.

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[4.3LXJ]

I think you need to know what the pressure is now. It sounds like it might be too high. Also it would be better if you filled out your vehicle info in the User CP so we could see exactly what you have

[KMW-NWXJ]

Sorry, I'll do that. It's a '91, 4.0 with 5 speed.

I'll rent a tester at the parts store later today and check it again.

And thanks for helping out! I am getting so frustrated with this issue! I think this is the 4th time since March of last year, and every time it just quits doing it while I'm trying to figure it out.

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[4.3LXJ]

One other thing, take your battery cables off and touch them together and reinstall. That might fix it too. The Jeep ECM is not to smart

[KMW-NWXJ]

One other thing, take your battery cables off and touch them together and reinstall. That might fix it too. The Jeep ECM is not to smart
Ok, just did that, and nothing changed.

Here's another video from a different perspective. If you watch my foot and the tach, you can see the cough and then rev. So it's not just of idle. It's initial throttle application from any RPM. It acts just like a carburetor with a bad accelerator pump.

https://youtu.be/VauqR29FNL4

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[KMW-NWXJ]

Ok, so wife just got home with the fuel pressure tester. I hook it up, turn the key to just crank it and see what the pressure is cranking, and it just starts right up without having to hold the pedal to the floor. No sputtering, no rough idle, revs smoothly, just perfect.

WHAT. THE. HELL?????????????




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[KMW-NWXJ]

Oh, and right now fuel pressure readings are all dead on what they should be.

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[4.3LXJ]

Not sure what to say. Sounds like you need to catch it screwing up to figure it out

[KMW-NWXJ]

At this point the only thing I know for certain is it quits doing it when I start messing with the fuel rail. Either by testing pressure or unplugging/replugging injectors.

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