Engine Hesitation and Stumble TPS? Paging 5-90

This problem started yesterday. Last week I installed a new fuel pump and filter, and everything worked fine after a test drive.

Yesterday, I was driving the Jeep to go and meet a member of my local 4wd club to sell some parts. On the way home, I turned into my neighborhood, and when I went to accelerate, the engine lost power, I released the gas and pressed it again, and the engine responded. I parked the jeep for about 10 minutes, and went back out to see if I could get the problem to happen again. Well this time, it happened while turning from a stop sign at low speed, I continued home and it happened again while entering my neighborhood.

I suspected that it might have something to do with the Fuel Pressure Regulator, because I found out that it has not been hooked up for a long time. (maybe even since I have had the Jeep in 2005, While checking fuel pressure, I found that the vac lines were routed wrong, probably by the PO, and the FPR did not have vacuum to it.)

I have a little over 1/4 of a tank of gas, and I have driven before with less gas and have not had this problem.

I checked the TPS at the square plug, since this is an automatic, and I get from 4.64 to 4.7 votls comming to the TPS, last night when I checked it had 4.9 volts though. I have also adjusted the TPS so that I get .8 volts at the other pin in the square connector.

Still after doing this, If I blip the throttle by hand, the engine will stumble, but if I gradually increase the throttle nothing happenes. The same thing happened on a test drive, where if I took a turn and quickly accelerated the engine would stumble, but if I slowly accelerated, I could hear the engine almost stumble, but it didn't.

What could I try? A couple years ago, my brother replaced the battery cables, and there is one that runs from the negative terminal to the wires next to the dipstick tube on the block. I also checked the sweep of the TPS with an analog volt meter, and the sweep looked smooth with no dead spots.

you must be talking about an 87-90, so the tps should be adjusted so the output voltage is 83% of the input volage.

for example, if your input is 5volts, then the output should be 4.15 volts.
4.15 divided by 5 = .83 or 83%

i would check fuel pressure with a gauge and even drive with the gauge attached so you can see what the pressure is under load.

might want to pull the fuel pump assembly and check for anything that may have come loose and that the sock is on tight.

also check the polarity of the pump wiring. when i changed the pump on my 89, the replacement pump had the + and - terminals on the opposite sides of the oem pump.

all sensor info can be found at www.lunghd.com

x2. your TPS is not adjusted correctly. output needs to be around 83% of your input.

This is on an 87, fuel pressure is 31psi at idle, 39psi with vac line removed, when I rev the engine up and let off, fuel pressure does drop a little, and then comes back up where it should be.

Is the output checked on the square plug or the other flat plug, does anyone have a pic of where I should be checking at?

O2 sensor good? cat clogged? Air filter dirty?

I'm not convinced the TPS is adjusted properly.

I readjusted the TPS to get 3.8 volts at the transmission plug, which gives .91 volts at the square plug. I went for a test drive, and just like the last time, once the engine got up to temperature, and on the way home, it happened again. I rechecked for flat spots on the tps and nothing showed up on the meter.

Filter is clean, cat is a magnaflow unit that was installed a couple years ago. The cat does have a couple of dents in it from rocks but when I hit it I don't hear any rattle. I will check the O2 sensor when the engine cools off.

Throw a vacuum gauge on the engine and slowly raise the RPMs. If the exhaust is restricted you will lose vacuum as the exhaust backs up.

Are you certain it is fuel related? Could be early signs of thermal failure of the crank sensor or coil.

The thing that makes me think that it is not the exhaust clogging up, is that it runs fine after the stumble.

I adjusted the TPS back to .8 volts instead of the 3.xx from the other method. I removed the neg battery cable and will see if that lets things reset and fixes anything.

I also tested the o2 sensor, and voltage bounces around like it is supposed to, but my resistanace is 10 instead of the 5-7 it is supposed to be, but it is not infinate.

The crank sensor was replaced a few years ago, when I thought it might be causing starting problems.

From searching, I found that terminal B is the ground. I tried to ground B of the square plug to the firewall ground, and the transmission would not come out of 1st gear, so I tried to ground B of the flat 3 wire plug and whenever I start the engine, it revs to about 3000 rpms and then comes back down (I didn't try to drive it this way). I put everything back to normal and drove it.

The real odd thing is that this only happens when I turn right. I would try and make a left turn after making a righ turn, and I couldn't get it to stumble, but when I made another right turn, it would happen. I am taking the turns slow, and from a stop.

Carefully check the crank sensor CPS/CKP harness for burned spots that could be grounding out the sensor when you turn.

Use a bright spotlight, run your fingers along the wires.

EDIT: do the same for the o2 harness.

One other thing--did you leave the fuel gauge hooked up and make any right turns? Possible bad hoses in the tank and its dropping pressure in the right turns--will it cut out in a right turn with a full tank?

I will try and check for shorts tomorrow, I will also recheck the pump and add some more fuel to the tank. I can't leave the fuel pressure gauge on and drive at the same time, because it isn't long enough to reach outside of the hood.

I checked for shorts and wire chafing, and I didn't find anything. I did however, get the engine to die when I stayed on the gas in a turn (still just a right turn). I noticed that if I accelerated and coasted through the turn, the engine would slow but not fully stumble. So, I filled the tank up with a little less than a half a tank, so that I can still take the pump out and check it, and I cannot get the problem to happen again. So, later on once everything cools back down, I will check for proper direction of flow, and if something came loose in the tank.

I just don't get it. The fuel is flowing in the correct direction, and the pump did not come appart in the tank. I added 4 gallons of gas and the problem went away. I just don't understand how I have previously ran the tank down to right at the E where the fuel light came on and never had this problem.

Are the hoses from the pump to the flange deteriorated? Is the pickup tube that is covered by the filter "sock" dislodged?

I just replaced the pump with a new Bosch unit last week. The "sock" mounts directly to the pump on the new style. It also came with a new hose that goes from the pump to the flange on the pump mount assembly. Everything in it is new except for the tank and the pump mount assembly.

I'm at a loss then. Only one other thing I can think of is if the tank has been replaced with an aftermarket unit and it doesn't have the "bowl" in it that keeps the fuel from sloshing away from the pump when cornering. There are some pictures/drawings around here of the inside of the tank showing the "bowl" and where the foot of the pump locks in.

Me too, the tank has the bowl and the foot piece still in there. I guess I will just drive until the fuel gets back below 1/4 tank and see if the problem comes back.

TMXONR said:

Me too, the tank has the bowl and the foot piece still in there. I guess I will just drive until the fuel gets back below 1/4 tank and see if the problem comes back.

Click to expand...

One of the most important things my Father taught me was to always keep at least 1/2 of a tank of fuel--just in case of a Zombie Attack. Well, he didn't actually say the zombie part, but he did say you never know when you might have to sh*t and git, and you don't want to be sitting in a line at a service station when you should be gitting.

Well, the problem came back when it dropped below 1/4 tank, and again only in a right turn.