Bouncing RPM's at WOT

[1fox2go]

Yes this is true. It could be multiple issues

[1fox2go]

I am referring to Crankshaft sensor as well.

I am feeling confident that it is the crank sensor only because the information i have from the computer tool we had hooked up to my jeep showing that it failed intermittenly under heavy throttle

Also, my jeep is an xj...if that matters

[1fox2go]

Meh, it seems as something else is causing my crank sensor to fail. Put a new one in and still have the same issue..

[Jeeptech01]

Cracked flexplate?

[1fox2go]

Possible. Really, really, really hate to have to pull the trans to fix it

Lets hope not. Going to trouble shoot some more this week / weekend

Did the TPS fix your issue DP96zj ?

[dp96zj]

Possible. Really, really, really hate to have to pull the trans to fix it

Lets hope not. Going to trouble shoot some more this week / weekend

Did the TPS fix your issue DP96zj ?

I'm still working on it, sadly. I swapped in jfowlzj95's TPS, and I didn't have ANY throttle issues anymore. I put the pedal to the floor, got around 4-5k rpm's, no bouncing. But then I put my TPS back in, and the same thing, no throttle issues . Granted, the heep wasn't warmed up 100% yet, so it still could be the fuel pump. The weird thing though, is that before the TPS switch, even if I gunned it with a cold engine, it would still (slightly, but noticeably) bounce at full throttle.

I went to 4 different auto stores around here, no one has a fuel pressure gauge for rent.

Today I have to go to town (2hrs there and back), and will report back on how the shitwagon behaves. I still have jfowlzj95's TPS in, so we'll see how it goes.

[1fox2go]

Thanks man let me know. It could be doing the swapping things around the sensor is cleaner, or a slightly broken wire is better connected now...

[dp96zj]

Just got back from the road... The heep drove fine for the first 45 miles or so, going 70mph the whole way. After that, I could slowly feel the power decreasing, until I was almost struggling to keep up with traffic going 65. I made it into town, parked it for about 30min to go pick my stuff up, and then hit the road to come back home.

At this point, I had roughly 1/2 a tank, and stopped about 10 miles outta town to fill it up. Topped off the tank, and noticed that the filler nozzle was pretty cold, so the gas going into the tank was obviously cold as well. I hit the highway to drive the remaining 45 miles home, and had no issues at all. My guess is that JeepTech's right, and that the 1/2 tank has significantly less cooling power than the full tank (of cold gas), and when I've got a clogged fuel pump, well you do the math... I'm sure if I would've drove another hour or so, the gas in the tank would've warmed and given me trouble again.

If I can get my hands on a pressure tester, I'll use it. If not, I have a new fuel filter that I just bought, I may throw that on to see if it helps any before I drop the tank (although the filter that's on there now is less than 3 months old).

Does anyone see any reason why this might NOT be the fuel pump?

EDIT: I've looked at the Swampy Bypass thread to get some more ideas on testing to see where exactly the problem lies. And since the repair probably won't happen until school's out, I have more time to look for a definite solution instead of going and dropping the tank immediately. I'm not looking into doing the bypass at the moment, since he said it's a bit harder for '96+ ZJ's, and a little excessive for my Jeep. But I'm most likely gonna cut a hole in the trunk in case it happens again, and probably do a killswitch while I'm at it.

[1fox2go]

Well here is an update on mine. I drove into work this morning. More aggressive than usual and the jeep ran like a champ, granted I didnt floor it and hit WOT but it was more than usual. Get to work notice that the engine is still pulling even after I let off the throttle, so i put it in neutral and the jeep revs to 1500-1800 and holds it, had to pat the gas to get it to settle down...


Sound like a TPS issue?

[dp96zj]

Well, I went to half a dozen auto stores, none of them rent out pressure testers, so I dropped $40 and got one.


The results: At idle and normal driving, the fuel pressure is around 40-50 psi. At WOT, it gets down to 10-15 psi. That's when the engine starts choking out. I'm going out to replace the fuel filter right now, we'll see if it affects it any...

[dp96zj]

replaced the fuel filter, same drop in psi.

On a different note, when in neutral or park, I can get the revs up to about 4k/redline, and the fuel pressure stays steady. Is there any difference in fuel delivery when the vehicle is in gear or not in gear? Or could this in fact point to a sensor?

Regardless, I'm back home for the next two days and would like to tackle whatever project I have ahead of me over the weekend, if possible.

[ajmorell]

I would think that fuel delivery varies with load as well as speed. When you are revving it in P or N there is no load/minimal load on the engine.

[dp96zj]

That's what I was thinking. I'll probably go ahead and drop the tank.

[ajmorell]

That's what I was thinking. I'll probably go ahead and drop the tank.

I'm nearly positive that it does, I know it isn't really the same but I just checked with some data that I have for an industrial diesel and there is a significant difference in the fuel flow for very low load and very high load (like 2-3x as much under high load).

[Jeeptech01]

I would think that fuel delivery varies with load as well as speed. When you are revving it in P or N there is no load/minimal load on the engine.

Exactly. Load requires more fuel plus it takes wAAAAAAY longer for the 4 slow to hit redline in gear than neutral

[dp96zj]

I'm nearly positive that it does, I know it isn't really the same but I just checked with some data that I have for an industrial diesel and there is a significant difference in the fuel flow for very low load and very high load (like 2-3x as much under high load).

ok that makes sense. Thanks for checking it out man. I'm sure some other guys will throw out some info as well.

[dp96zj]

Exactly. Load requires more fuel plus it takes wAAAAAAY longer for the 4 slow to hit redline in gear than neutral

heh good think I don't have to worry 'bout that pesky 4 liter

Thanks for your input guys, that's about as much reassurance I'll need.

[ATL ZJ]

heh good think I don't have to worry 'bout that pesky 4 liter

well, you could say the same thing about the 5.2

my tech contribution: it would be an easy time to do the swampy bypass and install a less expensive, higher flow pump, but in those hot texas summers you might be better off sticking with the stock pump and using the fuel as somewhat of a heat sink.

edit: I'm dating myself if no one in this thread even remembers swampy

[dp96zj]

well, you could say the same thing about the 5.2

damn you and your fancy engine

my tech contribution: it would be an easy time to do the swampy bypass and install a less expensive, higher flow pump, but in those hot texas summers you might be better off sticking with the stock pump and using the fuel as somewhat of a heat sink thing.

I was considering doing the swampy bypass, but I keep leaning more towards the stock one. You make a good point with the whole fuel=heat sink. Not to mention, I'm kinda short on time and want to finish it relatively quickly. Since the stock one would be plug and play (other than the kill switch), i'll probably go that route.



Thanks for helping me out guys!

[1fox2go]

I hope mine is just a TPS. Full tank or not it still bounces.

Also my pump is ~a year old so...bleh

[Jeeptech01]

Oops I just took 1.2 liters away from youe e-cawk LOL

[dp96zj]

Oops I just took 1.2 liters away from youe e-cawk LOL

Damn right you did!

[dp96zj]

ok guys, I know this is kinda newbish, but judging by the symptoms that my jeep has, should I get just the fuel pump or the whole assembly?

I walked in today to check prices, and it's $208 for just the pump (bosch) or $450 for the whole assembly (bosch). I'd prefer NOT to be bent over and pay $450 if I just need the pump. But if it'd be best to get the whole assembly, I'd put the project off for a few weeks and source it cheaper online.

[Jeeptech01]

Fawk thats expensive. My wj one was like $130 for the whole thing from Napa IIRC. If it was me, Id replace the whole deal since the level sensor is prolly getting tired too. You've got to be able to get a better price than that though.

[dp96zj]

Fawk thats expensive. My wj one was like $130 for the whole thing from Napa IIRC. If it was me, Id replace the whole deal since the level sensor is prolly getting tired too. You've got to be able to get a better price than that though.

I just bought the entire assembly at a different store. It was $280 including tax for an "Airtex" pump. I've never heard of it, but apparently it's an OEM supplier and the pump is made in America, so I'll try it. The price is a little steep compared to what my dad paid for his, but at least I've got the parts and can fix this damn thing.