Topic: YJ idles ok but lacks power while driving

[Eucalypta]

Hello Guys,

I swapped the TPS with the ZJ (same partnumber) but no improvement.
Don't think there is a tube knicked; with my bodylift the top of the fuel tank has enough room for the fuel and vacuum lines.

Then I reconnected the fuel pressure gauge and again on idle 31PSI (=OK) and with vacuum disconnected 39PSI (=OK).
Reading here the sugestions to monitor the pressure while driving I was trying to find a way to move the gauge inside the cabin.
But this seemed impossible to do without getting real long fuellines.
So I connected the fuel pressure gauge and started driving in our street. After just 50m (the problem is getting worse) the engine starts hickupping again. I jumped out the car and looked under the hood, just in time to see the pressure jump from 20PSI to 31PSI.
So repeating this I got it to the point I could see it dropping even to 0PSI.
Good call guys !   

So I guess the problem lies with the fuelpump, which deliveres the right pressure but cannot deliver the flow needed to supply the engine with enough fuel.

Next weekend I am going to exchange the fuel pump and just to be sure, even the mechanic said they changed it, also the fuel filter. Ofcourse I will check the fuel lines and think I will clear them with air, just to be sure there is no debris inside.
Will keep you posted on what I find!

[sharpxmen]

Hello Guys,

I swapped the TPS with the ZJ (same partnumber) but no improvement.
Don't think there is a tube knicked; with my bodylift the top of the fuel tank has enough room for the fuel and vacuum lines.

Then I reconnected the fuel pressure gauge and again on idle 31PSI (=OK) and with vacuum disconnected 39PSI (=OK).
Reading here the sugestions to monitor the pressure while driving I was trying to find a way to move the gauge inside the cabin.
But this seemed impossible to do without getting real long fuellines.
So I connected the fuel pressure gauge and started driving in our street. After just 50m (the problem is getting worse) the engine starts hickupping again. I jumped out the car and looked under the hood, just in time to see the pressure jump from 20PSI to 31PSI.
So repeating this I got it to the point I could see it dropping even to 0PSI.
Good call guys !   

So I guess the problem lies with the fuelpump, which deliveres the right pressure but cannot deliver the flow needed to supply the engine with enough fuel.

Next weekend I am going to exchange the fuel pump and just to be sure, even the mechanic said they changed it, also the fuel filter. Ofcourse I will check the fuel lines and think I will clear them with air, just to be sure there is no debris inside.
Will keep you posted on what I find!

aw12345 had an interesting suggestion, you might want to give it a shot - what if there's a venting problem of the gastank - so just for a test, take the fuel cap off the filler neck and try it that way - on our Jeeps the fuel cap has a valve that lets air in only 1 way and it blocks vapors from coming out, if it's blocked  both ways then you could have that issue - it's worth a shot, dunno it's the case but might as well try it. Also look under the jeep to make sure the skid is not bent upwards (see some previous posts).

[aw12345]

He tried it without a gas cap, did not help, but then the pinp pong ball is the filler neck could seal it shut. Fuel pressure dropped, could be the pump, could be the venting problem or a bunch of gunk in the tank that gets sucked on the fuel pump sock and mostly clogs it. At one time or an other I have had all three happen or a combination of some, with a frame mounted pump it would be simpler to narrow it down.
Atleast you got it narrowed down to a fuel delivery problem, which is a good start, from here I would check for crap in the tank, if it's not there, I would check the venting system, John you have the factory manual, so use it, it has all the info you need, if all that checks out it's time to cough up some coin for a new fuel pump

[jfrabat]

He tried it without a gas cap, did not help, but then the pinp pong ball is the filler neck could seal it shut. Fuel pressure dropped, could be the pump, could be the venting problem or a bunch of gunk in the tank that gets sucked on the fuel pump sock and mostly clogs it. At one time or an other I have had all three happen or a combination of some, with a frame mounted pump it would be simpler to narrow it down.
Atleast you got it narrowed down to a fuel delivery problem, which is a good start, from here I would check for crap in the tank, if it's not there, I would check the venting system, John you have the factory manual, so use it, it has all the info you need, if all that checks out it's time to cough up some coin for a new fuel pump

When this was happening to me, I dropped the gas tank and cleaned it while I was at it.  I was replacing the pump, so why not do it anyway.  Dropping the tank is not that hard (getting it back in place is a bit trickier, but I did it with some help from the wife in my garage with only a floor jack, so nothing you cannot do).  I would start thinking about dropping the tank and cleaning it up, and mayble replace the pump while at it...

[Eucalypta]

Hej Guys,

Indeed, I tried to drive the YJ with the fuelcap off. No solution. => are there any checkvalves or air slots in the fuel filler??? If so, detaching the vent hose from the fuel filler might help.
A few weeks a go me and a friend of mine have dropped his fueltank to check his fuel pump and cleaned the tank; just 1 or 2 hours of work.
My own tank was the day before cleaned and inspected by a dealer; they claim they cleaned the tank, lines and injection rail and placed a new fuel filter. Just 100 miles after the problems ocurred.

Thinking about this problem some other issue a year a go sprung to my mind; when I just bought the car, I drove it for about 200 miles and the tank was half full. Then the car stopped and it looked like I ran out of gas (didnt crank it because I was sure I ran out of gas). After refueling it ran great and I kept the tank full.
Never tanked more than 35 liter of gas.
What if the pump has an internal breach; as long as it is submerged in fuel; the pump works ok. When the pump gets above fuel level; gas (air) can be sucked in and the pump fails.
If it is a double membrane pump and one of the membranes is punctured; this would mean it still can bring pressure but not deliver the fuel needed (less flow).
I have heard of other YJ owners who claim their cars run better with a full tank of gas.....

With the dropping of the fuel pressure the pump is my suspect no 1. The problem gets worse with every mile I drive. I cannot get it out our driveway now (or tow it).
I just got 2 types of fuel pumps from my parts suplier; I can always bring back the parts I have not used (great service!).
This weekend I will  drop the tank, inspect every line, replace the fuel filter and if needed the fuelpump. Will also try and check the venting valves.
Will keep you ppl posted 

[Eucalypta]

Problem seems to be solved:

The mechanic who claimed that they cleaned the tank didnt do a very good job; They haven't even opened the hatch with the pump. The sieve on the pump was filled with mud, also the fuel filter was clogged with mud.
Also noticed that one of the venting tubes of the rollover valve was squeezed and no air could get through. Must have been when I didnt have the body lift yet. When the tank/skid plate hit a boulder, it got stuck between body and tank.

We thoroughly cleaned the tank, replaced the pump with a new one (just to be safe), replaced the fuel filter, replaced the venting tubing and cleaned the fuel lines.

Car runs like new

 (at)  AW1245; When you hit the fuel tank on a boulder or something and the bottom got dented; the suction end of the fuelpump can get blocked; the rubber nozzle where the sieve is attached to can be pressed to the pump and obstructing the flow.
The return flow can also be blockedas it is next to the inlet nozzle of the fuel pump.

fellow Jeepers; Thank you all for your help!

[sharpxmen]

    

[dwtaylorpdx]

Well now I know what I will be doing tomorrow....

Dave