ZJ Stuttering

[theonetruetom]

Hi all,

I think I'm running into a fuel pump issue with my ZJ, but I'd like to bounce some ideas off the brain trust and see if I can confirm this.

For the past couple months, when I'm on the freeway (97 ZJ 4.0 2wd) the truck will act like it's losing all engine power. There are no CELs, no codes, and no other indication of problems. It was a momentary stutter that straightened itself out within a half-second. Then it started getting worse. It went from once every 2 weeks or so, to once a week, to once a day, and now today on my way in to work (30 mile commute each way) it started doing it every few minutes. Stop lights are where it happens most, and it's acting like it isn't getting gas at all, or is getting it in surges. If I turn the truck off, and let it sit for a minute or two, the problem goes away (probably crap getting sucked up into the strainer?) but only for a mile or two. The issue hasn't been consistent enough currently to where the jeep won't start and drive, but it is definitely causing some driveability issues for me.

What do you guys think, am I on the right track with the pump?

 

[XJReaper]

I would say you are on the right track, but beforehand I would check the filter and see if it's full of gunk. If so replace, if the problem persists then move to the pump.

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[theonetruetom]

so according to RockAuto, the filter assembly by itself is $100 or so... and the pump/strainer/sender/filter/regulator module is running about $75-100. At that point I think it'd be foolish to replace one part when the entire module is less.

 

[JPNinPA]

If you have a scan tool that could read your O2 sensors I'd look at the upstream O2. there can start to go bad or become slow and cause what you are experiencing.

Also is this at a certain pedal position.? If so look at your TPS. Or check for vacuum leaks.
Lastly note if it is at a certain engine temp. Temp sending units can act up and end up sending wildly swinging temp data to the pcm that in turn swings the air fuel ratio.


JPNinPA
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[theonetruetom]

@jpinpa:
I don't have a scan tool that can read those. If I unplug it it should run in an open loop, though, right? Bad fuel mileage but if the issue goes away that should point to the issue. Also, wouldn't that trigger some sort of CEL?

It doesn't seem to happen at a specific temp or pedal position, and the couple times it happened on my way to work today pushing the pedal down seemed to worsen the issue and make it stall harder... if that makes sense?

 

[XJReaper]

so according to RockAuto, the filter assembly by itself is $100 or so... and the pump/strainer/sender/filter/regulator module is running about $75-100. At that point I think it'd be foolish to replace one part when the entire module is less.

I agree with you there. I'm pretty sure you can go to oreillys and they can hook a code reader up for you so you don't need to spend un needed $

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[JPNinPA]

The O2 doesn't always throw a cell. When it's starting to fail and becoming slow to respond, it hasn't thrown any codes.
You are correct unplugging it will run open loop and should hide the issue. You will get a code because it's unplugged.

Ok with pressing the pedal worsening it it's likely not the TPS.
I have to look if you have one or two temp sensors. The one to the PCM is what is in question.

On another thought how cold is it there? Could it be bad gas or ice in the tank?

You could check the fuel pressure at idle and at different RPM up to app 3300RPM. Should remain fairly constant.


JPNinPA
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[theonetruetom]

Based on the fact that this has happened on multiple tanks of gas from multiple different gas stations I'm pretty confident we can rule out bad gas. As for ice, I suppose it's possible since it does freeze at night, but during the day when this is happening it's between 40 and 50 degrees F. It's also pretty dry here so I'd be inclined to rule out condensation leading to water in the gas. Plus we use ethanol here between November and March, so that should absorb any condensation anyway. Guess I'm at a pressure test.

 

[JPNinPA]

Missed the "past couple of months" statement.

Fuel pressure
O2
Temp sensor.
Vacuum leak?


JPNinPA
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[theonetruetom]

So, based on my gut instinct (which is usually fairly accurate) I pulled the tank today and swapped fuel pumps. I'm 99.9% sure that was the problem, because the strainer was so dirty it wouldn't allow light through, and the fuel that came out of the filter output when I turned the old pump on its side was brown and extremely dirty. I won't know until I commute to work on Monday, but I'm fairly confident I can now trust it to make that commute without leaving me on the side of the freeway.

 

[theonetruetom]

So that seemed to fix the issue for about a week or so, and juuuuust as I was about to declare victory, it's back. This time it died on the way home from work twice, *but* it started throwing codes finally. I have P0108 and P0351, which would point to MAP high voltage, and... Crank sensor? Coil? I'm starting to get deja vu with my Wrangler, because those two codes look awfully familiar and it ended up needing head work.

 

[RestorationRides]

Based on the fact that this has happened on multiple tanks of gas from multiple different gas stations I'm pretty confident we can rule out bad gas. As for ice, I suppose it's possible since it does freeze at night, but during the day when this is happening it's between 40 and 50 degrees F. It's also pretty dry here so I'd be inclined to rule out condensation leading to water in the gas. Plus we use ethanol here between November and March, so that should absorb any condensation anyway. Guess I'm at a pressure test.

I think you could be on the right track with the fuel pump as the source of the problem. However, it's very easy to blame other problems as a fuel delivery problem.

Letting the engine idle for a bit as a way of letting it "catch up" as you say, doesn't really give you the kind of test you need. Instead, you would do better to get your Jeep out on the open highway at low speed, floor that throttle...and see if it truly 'runs out of fuel" at higher loads as you run through the higher gears. Under sustained higher RPM heavy loads, any fuel delivery issue should become obvious pretty quick...

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[theonetruetom]

I did a little poking around online at work the other day, and it led to checking the connections at the PCM. I drove home the slow way (which doesn't involve speed limits over 70) and the CEL actually went out and the problem didn't show up at all. I know better than to declare victory until I take some terminal cleaner to all of the primary connectors and the problem stays away at all speeds for a week, though.

 

[JPNinPA]

I'm going to throw out this to consider a very basic step and a maintenance item every 30k miles.

Check the battery and all connections including the wires.
Load test the battery and alternator. A weak battery or even a good battery with resistive connections will cause weird faults and poor running conditions.

The connection to the PCM is a good example. That can happen.

Second verify the pump connections are still good.


JPNinPA
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