bad fuel pump? buzzing sound

90 xj laredo, 4.0l, manual trans .. 136k miles

so .. I thought the hesitations Im getting had to do with cold engine. seems that its doing it randomly now. and occasionally when I hit a bump

I also noticed the RPMs drop here & there (below idle) .. it doesnt die out though, just bucks like a horse

current fuel pump has about 25k miles on it

I changed the fuel filter to see if it was clogging up (only had about 12k miles on it though)

when checking for leaks on new filter .. I noticed the fuel pump can be heard.. kind of a buzzing sound .. it is pumping .. but I dont remember ever hearing a good one buzz like that (never intentionally tried to listen to one though) .. its not real loud but can be heard over my loud exhaust

still fighting this gremlin .. dont have a fuel pressure tester or code reader, so that test is out of the question .. thinkin this buzz could be a defective pump??

test drive w/ new filter .. still has the hesitations

and where in the heck do I get a new Manifold Air Temp sensor?? (MAT) .. cant seem to find anything local (dealer says theyre discontinued)

Every one of my 4.0's (4) has had the fuel pump buzz. It may be that the newer pumps are audible while the older ones are not?

For hesitation, my laundry list looks like this:

Spark Plugs
Rotor
Distributor Cap
Plug Wires
Air Filter

If all of those haven't been done in the last 30000 miles, post back after they're done and let us know how it's are doing. (Oh yeah, and if you haven't changed the coolant in the last 5 years, it's time for that also....either that, or have a look at the "overheating" threads)

Every one of my 4.0's (4) has had the fuel pump buzz. It may be that the newer pumps are audible while the older ones are not?

For hesitation, my laundry list looks like this:

Spark Plugs
Rotor
Distributor Cap
Plug Wires
Air Filter

If all of those haven't been done in the last 30000 miles, post back after they're done and let us know how it's are doing. (Oh yeah, and if you haven't changed the coolant in the last 5 years, it's time for that also....either that, or have a look at the "overheating" threads)

Spark Plugs ... brand new as of last saturday (champion copper, OEM) .. old ones looked great for having about 25k on em .. no oil on em, just the light oxidation on tips, all looked the same .. good too see

Rotor button = 10k miles
Distributor Cap = 10k miles
Plug Wires = newer .. beldens
Air Filter = 2 weeks old
CPS = 10k (shop installed)
O2 sensor = about 10k
cat/muffler = 10k

as far as engine .. ran great until the last 2-3 weeks

seems when in 1st & 2nd gear (especially hitting a bump) .. there will be a pause in power .. sometimes a bump cruisin in 4th or 5th will do it too, but very uncommon. sometimes revving in 3rd gear will do it. saturday it was 45 degrees, jeep sat for about 5 hours in that temp .. 2nd gear about 2k rpms was nasty .. then warmed up & it went away

as far as today ... in AM. the curvy crappy side street gave me the normal pauses here & there ... then drove fine doing 45 mph to highway .. then felt the loss in power once on highway, but just barely

when leaving work: same thing .. a few hesitations until I got on highway 5 blocks away .. then drove fine until on the curvy bumpy side street near home

as far as overheating .. this engine hasnt yet .. water pump replaced about 15k miles ago .. bearing was making noise .. so I guess I replaced a good portion of the coolant on that job .. the plastic overflow bottle replaced due to a crack about a year ago

IS THE CTS DOING THIS ??? coolant temp sensor ... Ive seen diagrams of it ,, but cant seem to locate it .. under the EGR valve ??

boggles me why in low gears it likes to act up .. cruisin is fine

when it is about zero degrees I notice the fuel pump float level is incorrect on dash gauge .. I filled it up completly last friday .. seems to be reading accordingly to mileage .. but its been a bit warmer this week

A thought: on my 2000, I had power surges and odd shifts, and it was due to the Crankshaft Position Sensor, CPS. The slight stumbles and such went on for a couple of months, at very odd intervals, in all temperatures. Then one day it died on me on the highway, but started right back up after I'd pulled over.

It's possible it is the temperature sensor. I don't think the 90 models had OBDII, but you could try to check for any codes and if the vehicle won't do it, several auto parts chains (Advance Auto and Autozone come to mind) will usually hook up an engine analyzer and give you the results.

yea. . maybe the CPS ... heard even a fraction of space can make it screw up

will check its tightness for sure .. or take it in for codes

hopin its not some ground wire, that will cost $80/hour to find = ouch

I would just replace the TPS .. but dont think thats the problem .. especially with hitting bumps .. somethin is loose !

they way this problem just "began to happen .. and got worse quick" reminds me of when i had the CPS changed last year ... drove it until it died, while trouble shooting the whole time & spending money on other parts

Yeah, well if you're on your original CPS, that's a decent place to start. Post back with the results......

Yeah, well if you're on your original CPS, that's a decent place to start. Post back with the results......

no .. CPS was changed a year ago november .. so has approx 12k miles on it .. maybe came a bit loose ?

Hmmm...dunno. Possible but not real likely imo.

Have you checked all the electrical connections under the hood, and leading to and from the computer? I've read a thread or two where those were the culprit.