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Anyone ever had a weak fuel pump? - SOLVED

Fuzzydog

Red Skull Member
Joined
May 20, 2020
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581
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159
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BC Interior
My 2000 grand Cherokee (4.7l) is kicking my ass with a running issue, mainly under moderate to heavy load.
No codes, - crank and cam sensors recently replaced with OEM Mopar sensors.
I am suspecting fuel delivery but all my life a fuel pump has either worked or not worked, making it easy to diagnose.
This kind of feels like a weak fuel pump issue and I don't know if that is possible. Fuel pumps either pump or they don't, right?
 
My last nissan Titan would randomly just lose power on the highway. I'd pull over, power cycle it and it would run like normal and usually wouldn't do it again for several weeks. After some time, I was on a road trip to AZ, stopping to pick up friends in TX on the way and it started doing it way more often on that first leg of the trip. Told my friend about it when I got to TX and he said "your fuel pump is about to go out." Apparently another friend had the same problem and they had just done his a week before that. So luckily Napa had one in stock so we picked it up on the way out of town. The first day out in the desert after we made it to AZ it got really bad. I had to stop every 15-20 minutes and shut it off and let it cool down for 5-10 minutes and it would start back up and run like normal. Swapped it out in camp that night and it was 100% for the rest of the time I owned it.


Cliffs: I had intermittent power problems that eventually got worse. Fuel pump was going bad and gave lots of warnings that I mostly ignored.
 
I put a mechanical fuel pressure gauge on the fuel rail schrader on my 4.7’s just to make diagnosing things like this easier. Though if it only ever happens while under load you wouldn’t exactly be able to watch it live unless you put a gopro under the hood with it haha.

Do you have a fancy scan tool where you can watch sensor outputs? I’d also be curious if your TPS has a bad spot somewhere at part throttle, would be nice to give it a full sweep and see what it’s doing.
 
Yes, repeatedly. Cause issues going up hill, causes hot starting issues.

How does it start hot?

Way back when Hp fuel pumps and fuel injection was a new thing fought a miss and after all the screwing around it was the fuel pump.

Tested ok. Within spec for pressure and flow but it was not enough under a hot load.
 
Tiha said:
...it was not enough under a hot load.
Click to expand...
cover10.jpg
 
Fuzzydog said:
My 2000 grand Cherokee (4.7l) is kicking my ass with a running issue, mainly under moderate to heavy load.
No codes, - crank and cam sensors recently replaced with OEM Mopar sensors.
I am suspecting fuel delivery but all my life a fuel pump has either worked or not worked, making it easy to diagnose.
This kind of feels like a weak fuel pump issue and I don't know if that is possible. Fuel pumps either pump or they don't, right?
Click to expand...
Had same vehicle basically. Messed around with it for quite awhile. Did a couple of warranteed cheap fuel pumps. Always kinda the same after a couple of months. Similar conditions to yours. I bought a good one from maybe NAPA or something similar and never had another problem. No codes but it made owning the rig not fun for awhile. If that happened to me now I would buy the best pump I could find. Life is too short.
 
I have the auto enginuity scan tool with enhanced chrysler pack - it does have live data, etc but most of the time I don't even know what I am looking at. I do know that green numbers are good, red are bad and I do have lots of green.

I did check the schrader on the rail and there was strong pressure affter engine shut down but it did seem to me the fuel came out much more misty than I would have thought. more like it was already vaporized in the rail , than full of fluid. I am digging into it again tonight and will get the fuel pressure gauge on it.
glad to hear confirmation that fuel pumps sometimes do give notice that they are failing. I have changed a lot of fuel pumps over the years and always was because they flat out quit working. sometimes got them to get me home by hitting the gas tank with a crowbar :lmao:
that is why I was skeptical of being a FP issue - I guess if it quacks like a duck.....
will order a new pump - $257 CAD for Bosch OEM :eek:
 
AgitatedPancake said:
I put a mechanical fuel pressure gauge on the fuel rail schrader on my 4.7’s just to make diagnosing things like this easier. Though if it only ever happens while under load you wouldn’t exactly be able to watch it live unless you put a gopro under the hood with it haha.

Do you have a fancy scan tool where you can watch sensor outputs? I’d also be curious if your TPS has a bad spot somewhere at part throttle, would be nice to give it a full sweep and see what it’s doing.
Click to expand...
hmm good call - I should monitor the TPS through full range. I know the tool will do that. Will check out tonight, thanks for the tip.
 
Yep, current crawler has a fuel pressure sensor, last one had a gauge, and I've got a couple other gauges in my toolbox for chasing the same.
 
I usually start with replacing the filter on the frame. Maintenance item anyway.
Fixed a couple of “bad” pumps this way.
 
Ditchrunner said:
I usually start with replacing the filter on the frame. Maintenance item anyway.
Fixed a couple of “bad” pumps this way.
Click to expand...

Fair point, and with it being the regulator it wouldn't be a bad first step before fuel pump because it's cheaper and easy. But it would be showing low fuel pressure at the rail too so that's still a good first diagnosis step.
 
AgitatedPancake said:
Fair point, and with it being the regulator it wouldn't be a bad first step before fuel pump because it's cheaper and easy. But it would be showing low fuel pressure at the rail too so that's still a good first diagnosis step.
Click to expand...
I’ve had filters allow pressure at idle but as soon as demand ramped up they couldn’t handle the flow requirements.
Had bad fuel pumps do the same too.
 
xr-nut said:
"Anyone ever had a weak fuel pump?"

Everyone who has ever owned a gm product......





















:flipoff2::laughing:
Click to expand...
As my 1500 can't go above 3000 because of a weak pump, I'd like to tell you to politely go fuck off :flipoff2::laughing:
 
Fuzzydog I know there are fuel pressure tester kits out there, but if you want to actually add a permanent fixed gauge, I believe this is the parts list I used (not the exact brand, but same parts AFAIK). Also double check that you actually have a schrader, I've heard rumored that there are a few WJs out there that have no fuel rail schrader which is wild to me.

But it should be a -4AN fitting, so you can use a 90 degree swivel like this one at the rail itself (note, remove the schrader core first):
Amazon.com

Then a -4 male to 1/8" pipe female adapter:
Amazon.com

and a small liquid filled gauge with 1/8" pipe threads:
Amazon.com

Then just clock everything how you want, and you're good to go, besides being able to watch it under load. Doing an actual sensor with a remote gauge like ScottRS would be sweet
 
Picked up a filter/regulator yesterday. When the pump arrives next Tuesday from Rockauto I will do both.

I really appreciate all the feedback, makes me feel not so bad about throwing a $250 part at it
 
It definitely has a Schrader, I cracked it open yesterday. I really should consider a perm fp gauge. If I get this sorted and don't just torch it, I'll put it next on the to-do list
 
Fuzzydog said:
Picked up a filter/regulator yesterday. When the pump arrives next Tuesday from Rockauto I will do both.

I really appreciate all the feedback, makes me feel not so bad about throwing a $250 part at it
Click to expand...


I had a fuel filter plug up and do weird stuff at random times.


I've also found that often times the $200 pump "assembly" can be avoided. Just pull the pump and Google the part number on the side. I've fixed a few Denso pumps for $25-40 searching their part number vs looking up the application and only funding the entire bucket.
 
I know for me (this was years ago) not having quality data other than a cheap OBD code reader and a FP checking tool it was frustrating. If I could have seen real time FP I could likely have solved it much easier.

My buddy is a real tech savvy independent mechanic. When he brings out his 20K -or something close- scanner he gets to the root of the problem immediately. Much nicer but I try and not bother him.
 
KarlVP said:
Fuel pressure regulator. On the top of the tank. It's basically a filter.

Drop the tank and change it out.

Profit.

Thank me later.
Click to expand...
But no need to drop the tank. I did one on my daughter's same vintage grand Cherokee and it is at the front of the tank.
She was out in the middle of nowhere on a gravel road and a rock kicked up and cracked hers, leaking all their fuel out. Daddy to the rescue - 2 hour drive each way.
 
so swap out the filter/reg on yours and see if that's it before you put a chinkanese replacement pump in
 
I've been slow to address this due to other concerns, but did dig in last night.
Basically just spit balling my diagnosis here, helps me work through things, but I welcome any BTDT or pointing out things I overlook.

After sitting about 5 days, jeep started right up and I moved it around in the yard to turn it around, which required me to get the right front high on a bank (long arm flex baby! :smokin:) and it died out, would not restart (cranks fine, just won't fire). Pushed it down to flat again, dicked around and tried to start...and it did!.
Made me think back and the cutting out happened a lot when taking a corner hard, or going uphill. Has about 1/4 tank of fuel now, and issue on hwy was at just about half tank for sure.
Put a fp gauge on it and get 50psi, rock solid when running, at idle or revving up, and very slowly bled down to about 45 psi after being shut down about an hour.
This morning it is at zero and I don't see any evidence of leakage at the Schrader or tester, I think it bled down internally.
Also hooked up Auto enginuity and tested tps and map
Tps voltage is smooth through the whole throttle range
Map voltage also changes as expected, but is at 4.12 engine off. Autoenginuity displays the 4.12 in yellow which I think is their way if indicating a poss problem. As I understand it map voltage should be 4.0v with key on engine off. Is that .12v a big deal :confused:
Seems a map issue would throw a code, maybe not obd but I have the Chrysler enhanced package with autoenginuity and it better have that.
New tps should be here in a couple days, will swap out anyway.
I have a Bosch premium fp assembly and fuel filter/regulator on hand ready to go in, but don't want to do it until I see some funkiness on the fp gauge.
First thing after work today, going to run it up on that bank again and see if I can reproduce the dieing out, with the fp gauge attached.
 
I put an extended hose on my fuel pressure gauges to tuck them under the wiper to check under load. I've followed shitty mechanics who've replaced heads and I've fixed it with a fuel pump...


I also will absolutely put a $300 oem fuel pump assy in a vehicle I plan on keeping around, before it fails, as a maintenance item

Did my burb at 165k when I bought it. The sequoia has 270k and I'll probably put a pump in it at my convenience. I don't fuck around with my traveling suvs

No start project cars I'm flipping get Chinese pumps
 
Just a thought but, could it be as simple as a malfunctioning fuel gauge?maybe I am way lower on fuel than the gauge shows and it starves for fuel going up hills or on hard cornering?:eek:

Nah, can't be. I filled right up before the last outing and still had half tank on the way home when prob started, which is right about where it should have been
 
Fuzzydog said:
Map voltage also changes as expected, but is at 4.12 engine off. Autoenginuity displays the 4.12 in yellow which I think is their way if indicating a poss problem. As I understand it map voltage should be 4.0v with key on engine off. Is that .12v a big deal :confused:
Click to expand...

Map with the engine off is the same as the ambient barometric pressure
... this will vary depending on your altitude and the actual local barometer reading
... you can check you local weather forecast and get today's readings (that said, it doesn't matter that much)

As long as the reading changes when you fire up the engine, and varies with the load on the engine, it's probably okay


... just make sure it doesn;t think you're below sea level or above 17,000 feet with the engine off

I think the answer will be found when you pull the pump / sending unit

... whoever said you can buy just the pump is a genius, and the suggestion to check the fuel pressure regulator in the tank is also a very smart play

... I'd also be sure to check the all the electrical connectors for any evidence of corrosion/damage

... if there's any rubber line anywhere in the tank, I'd replace it with quality, pressure rated fuel hose and quality hose clamps (if used)
 
Pumps often gradually get worse with run time.

... if there's any rubber line anywhere in the tank, I'd replace it with quality, pressure rated fuel hose and quality hose clamps (if used)
Click to expand...

And make sure it submersible. Not all fuel line is safe to be.
 
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