fuel pump problem?

I have a '96 Grand Cherokee w/5.2 V8. When I accelerate quickly from a dead start, the vehicle will hesitate for a second or two before it gets going. After that, it accelerates smoothly and runs strong at high speed. I took it to the dealer and he said the fuel pump was bad because the pressure was low. I have read about Jeeps being notorious for bad fuel pumps, but it seems it usually shows up as hard starting/no starting, rough idle, and/or stalling out. In fact, I replaced the fuel pump about 4 years ago because it wouldn't start. It seems to me that if the pressure were low that I would have other problems like poor performance at high speed. Aside from the hesitation from a start, it runs fine. I've changed the spark plugs, wires, fuel filter, air filter, PCV valve, cleaned throttle body, cleaned IAC passages, and replaced O2 sensor. I've heard there was a TSB about updating the software in the PCM for a similar problem. The dealer said that it wasn't necessary, but the PCM has never been updated. Any thoughts?

Reply to
Martin & Debbie Ghioto
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A fuel pump won't cause problems taking off from a dead start. the pump runs at the same speed whether your vehicle is idling or running at WOT. The excess fuel is returned to the tank via a return line. I would look into the throttle position sensor either being defective or out of adjustment. I'm not sure if your vehicle has an adjustment for this or not, but most do. The sensor also has resistance readings that can be checked with an ohm meter. Consult a service manual for the readings. The sensor tells the computer how far the throttle is open and adjusts the injector duty cycle accordingly. Also, make sure the injector nozzles are spraying a nice even "fan" pattern. If you see dribbling or an uneven pattern, then there is a problem with the injectors. the TPS would be my first place to look.

Chris

Reply to
c

It not only *can* cause this problem, it's fairly common.

the pump runs

Reply to
bllsht

Martin & Debbie Ghioto did pass the time by typing:

Clear some things up for me.

- By acccelerate quickly do you mean WOT (wide open throttle) i.e. mashing the gass, pedal to the metal, etc. or open throttle, but not mashed all the way down.

- Does this happen more with the vehicle cold or at operating temperature or both.

Not all sensors are used in all conditions. If your O2 sensor hasn't been replaced you probably should. They tend to fail slowly and as they do it causes your engine to run richer and richer. Gas mileage and power goes in the crapper.

The MAP (Manifold Air Pressure) sensor will also cause WOT problems.

The sensor diagnostics section on my webpage may help. It's for the I6 but the basic electronics are identical.

Low fuel pressure can be caused by a faulty pressure regulator or vac leak.

Before replacing any sensor it is best to clean the connectors. Lots of times a little corrosion is the real problem.

Reply to
DougW

It happens basically anytime I punch it, not necesarilly petal to the metal. I'm a little scared to jump into traffic because if I hit it too hard, it will stumble. I did replace the O2 sensor about 3 months ago and that cleared up some other problems, but the stumbling continues. I checked the MAP sensor and the TPS sensor according to the Haynes manual and everything checked OK. It seems to do it with a cold or warm engine.

Reply to
Martin & Debbie Ghioto

Martin & Debbie Ghioto did pass the time by typing:

How's your distributor? Take the cap off and see if there is any oil in there and if the rotor will move side to side. If it does you found another possible cause. Only solution in that case is to replace the distributor. (It's an easy job, don't fork over major bucks to the dealer)

Any engine codes being thrown other than 12?

Last but not always least is the CPS (Crankshaft Position Sensor) but usually when it goes your jeep simply dies. A flaky one can cause stumble though.

Still sounds like the MAP sensor to me. Mine tested ok but replacing it did improve acceleration. My solution was to buy new sensors and swap them out till the problems went away. Then just keep the rest as spares for the future.

Reply to
DougW

Reply to
L.W.(ßill)

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