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Detailed fuel pump problem on my 93 - humming n worse

Nick_XJ

NAXJA Forum User
Location
Michigan
First off I bought this 93 Jeep Cherokee Sport w/ a 4.0 Liter for $400 so I am not looking to dump a bunch of money into it. The jeep was sitting with the fuel tank out (because of a bad fuel pump) for about a year before I bought it. When I got it home I installed a new Airtek (yes cheap AutoZone junk) Fuel Pump in the tank after I did a thorough cleaning. I replaced all of the hoses going from the tank to the metal fuel lines and installed a new fuel filter.

The Problem:
When you start the Jeep it runs perfectly. It has no lack of power, no hesitation, no smoking, no ticks, knocks, or any other noises. After 5 minutes the fuel pump starts "humming". The humming is the normal fuel pump noise only 100x louder.

After 5 mins of the humming the Jeep will start bogging down and missing under heavy acceleration.

After 10 mins of humming the Jeep will start bogging down under even normal acceleration. You can get it up to 55mph at this point but only if you barely push on the gas pedal and very gradually accelerate.

After 15 mins of humming the truck will barely run and eventually shut off completely; however it will start back up, but shuts off after a couple seconds.

If I turn off the ignition for 1-2 minutes, I can start the Jeep back up w/out humming and the 5 min before humming / dying cycle will start all over again.

The times listed above are under normal acceleration. Under heavy acceleration, the truck will start humming and shut off completely within 5 minutes.

When the pump starts humming, if you put your hand on the hoses by the tank the biggest hose only feels like its vibrating really bad.

Also it does not matter if the gas tank is full or empty, the problem never changes.

The things I have tried:
I am not a mechanic but as soon as I heard the humming I immediately thought it was a bad fuel pump. I took the pump out and exchanged it for a brand new one. I also bought another new fuel filter and reinstalled the new pump and filter. The problem is still the same. No changes at all, the times didn't even change.

*When I pulled the first new fuel pump out, the rubber spacers on the top and bottom of the cylinder pump were slid into the middle of the pump. I will post the picture of the pump after I took it out of the tank when I find it.

At this point I took it to a mechanic and paid him to track down the problem. A day later he tells me that he has no idea what is wrong with it. He put a fuel pressure guage and let it idle for over an hour. The fuel pressure was a steady 31psi (exactly where it should be). The voltage was also exactly where it should be and never swaggered. However he only tested both at idle and never under a load.

When the mechanic couldn't figure it out I called Airtek tech support and they said it's either a bad fuel pump or another problem, but they were not mechanics so they could not help me. The only advice they gave me was to exchange the pump yet again.

Hesitant to pull the pump again since both fuel pumps had identical problems and the origional went out for a reason, I decided to talk to a Chrysler mechanic. The mechanic told me it sounds like my catalatic converter is bad. I replaced my whole exhaust system and the problem didn't change.

I then turned to Yahoo! Answers and the only good response I got was to replace the Ballast on the drivers fender under my hood. I replaced it and still no change in the problem.

At this point I started trial and error. I have replaced all rubber hoses under my hood to make sure there were no vacuum leaks. I have also took off my gas cap to see if it is a ventilation problem. The problem still exists and has not changed at all!

Other ideas I have that haven't been tried yet:
- Check the Fuel Pressure Regulator (doing this tomarrow, I will post results)
- Check the voltage under a load (if its high or low, what would cause this? possibly the computer? any info on this would be great)
- Replace the fuel pump relay in the fuse box under the hood (just thought of this while writing this, will do it tomarrow)
- Exchange my fuel pump yet again, maybe it really is another bad pump?


If anyone has any suggestions, ideas, or simliar problems that they have solved please let me know! I am quickly running out of ideas and I would love to get this Jeep running correctly.


Help Please!!! :rof:
 
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Hallo, Maybe useful, there is also a ballast resistor bypass relay.
You can renew this relay? It involves the voltage to the pump ,when starting. After that, the ballast resistor takes it over. :spin1:



'92 XJ
 
Hi,
I do not have any new ideas, you seem to have tried most everything. I just wanted to say that I have fitted a new Airtex fuel pump complete with the frame and fuel level sender to my 93 XJ and it is very quiet, so maybe you do have another dud pump. The chances of getting 2 dud pumps in a row must be millions to 1. The ballast resistor is there to keep the pump quiet but you say you have fitted a new one. If the rubber foot was not seated correctly in the tank that would make the pump noisier. Other than that I cannot think of what could be the problem. Sorry could not be of more use.
 
I would start by checking votage on the pump, both at idle and again under load. Verify that all of you grounds in the engine bay are in good shape (replace as necessary). If you have to swap the pump again, i personally would upgrade to a better quality unit (Bosch).
 
you need to test fuel pressure when the pump is humming.

swapping relays may give you piece of mind but i doubt its the problem. the relay will either not send current through it or it will click repeatedly open and closed.

for the ballast resistor, you could just jumper the 2 wires leading to it together and eliminate it from the curcuit for diagnostic purposes. basically taking it out of the equation.

also make sure your not reversing the polarity of the fuel pump. when i installed an airteck pump on my 89, the + and - terminals were on the opposite sides than the oem pump i replaced.

did you check for a clogged fuel filter or obstruction in the fuel line from being disconnected for a year?

if fuel pressure and fuel pressure regulator check out, i would look into:

thermal failure of the coil or possibly the cps.
sync signal(i believe your year has one) in the distributor
test the map sensor

did you try and flash/retreive any codes stored in the computer?
 
I have diagnosed a simular problem before. Pressure was OK at an idle but I found it fell when driving and under load. As it turns out the fuel line had been hit by a rock and was pinched. It limited the flow to enough for an idle but not for running under load.

Install a pressure gauge and route it to where you can see it when driving.
 
Inside the tank is a tab where the rubber stopper on the end of the pump assembly must correctly fit or you will have NOISE and VIBRATION.

Is the rubber stopper there? Did you get the rubber stopper properly located in the tab?
 
Thank you all for your suggestions and keep them coming.

I did the fuel pressure test. When the vacuum is hooked to the Fuel Pressure Regulator it idles at 33 psi, when I unhook the vacuum it goes up to 41 psi. The pressure regulator is working correctly.

Hallo, Maybe useful, there is also a ballast resistor bypass relay.
You can renew this relay? It involves the voltage to the pump ,when starting. After that, the ballast resistor takes it over.

I replaced this..its in my description.

I have diagnosed a simular problem before. Pressure was OK at an idle but I found it fell when driving and under load. As it turns out the fuel line had been hit by a rock and was pinched. It limited the flow to enough for an idle but not for running under load.

Install a pressure gauge and route it to where you can see it when driving.

pinched line, I never thought of that. I'll definately take a look.
I know the hoses are good but I never inspected the metal line.
 
Inside the tank is a tab where the rubber stopper on the end of the pump assembly must correctly fit or you will have NOISE and VIBRATION.

Is the rubber stopper there? Did you get the rubber stopper properly located in the tab?


Yes the rubber stopper is there and yes it fit like a glove.
 
Buy a Bosch pump, I believe the number is 69302, many here swear they are the best.
 
Buy a Bosch pump, I believe the number is 69302, many here swear they are the best.

My fuel pump is still under warranty, I am not going to buy another pump for it.

here is a picture of the 1st new pump I installed after i pulled it out of the tank:

b51yqh.jpg
 
Take the pump back where you bought it.

Tell them you are tired of dealing with their POS junk.

Upgrade, i.e., pay the difference, and get a decent pump.
 
They might have a bad lot of pumps too. You've already talked to Airtek support so call them again and tell them that you want a new one direct from the factory. And get a prepaid FED-EX number from them to send yours back for analysis. If they won't do this, get a refund and another brand of pump. How much work are you going to go thru chasing this?
 
i would verify the voltage at the pump and make sure your not reversing the polarity.

if it worked for 5 minutes and then starts humming, than something else is wrong other than the pump. things that will kill a pump is the incorrect voltage at the pump and reversing the polarity(spinning the pump backwards). other possibilities could be an obstruction in the fuel line and improper installation, which from the picture it looks ok, and overheating from the lack of lubrication(low on fuel).

maybe you got thtwo bad pumps, but i think you need to make sure there is another issue causing the pumps to go bad.

why was the tank missing in the first place? because the pump was bad, so did the previous owner try to replace it and the new pump failed or did they just not put in another? and why did they remove the tank to get the pump out? its a 93 and the pump is installed on the back side of the tank. did you check the fuel filter for blockage?
 
i would verify the voltage at the pump and make sure your not reversing the polarity.

if it worked for 5 minutes and then starts humming, than something else is wrong other than the pump. things that will kill a pump is the incorrect voltage at the pump and reversing the polarity(spinning the pump backwards). other possibilities could be an obstruction in the fuel line and improper installation, which from the picture it looks ok, and overheating from the lack of lubrication(low on fuel).

maybe you got thtwo bad pumps, but i think you need to make sure there is another issue causing the pumps to go bad.

why was the tank missing in the first place? because the pump was bad, so did the previous owner try to replace it and the new pump failed or did they just not put in another? and why did they remove the tank to get the pump out? its a 93 and the pump is installed on the back side of the tank. did you check the fuel filter for blockage?

The problem still occurs when the tank is full of gas. Yes maybe I got 2 bad pumps in a row but that wouldn't cause the origional oem pump to go out. The tank was out of the truck because the previous owner tried to replace the fuel pump but ended up getting a new truck instead and never went out and purchased or installed a new pump. (the fuel pump is on the front side of the tank w/ not enough room to pull the pump w/out dropping the tank) When I asked what the problem was the previous owner simply said that the fuel pump went out and the jeep shut off when he was driving home from work one day. He had it towed home and thats where it sat.

My thought is, if the origional pump went out and now both of the new pumps are acting the same way, then there has to be another problem other than just buying a bad pump.

As for the wiring, the plug from the new pump only connects to the plug in one way, so there is no way I could hook it up backwards.


I thank you for your suggestions on upgrading the pump but I am unemployed and do not have any money to pay the difference for another pump. So please stop suggesting this.
 
OK, understand the money thing.

Have you measured the voltage at the pump--12V?

Have you measured the ground resistance at the pump--0 ohms?
 
-Might take a Real Good Look at this pump + and - marking on the pump itself and what the + and _ of the 12VDC actually supplied. As mentioned above it can be Wrong.

-While meter attached and observing reading -cycle key on-off and be sure full 12 volts.

- Junk yard- pumps different brand bosch or oem. (if the front end is crunched, it WAS running!) (Hopefully cheap)

-flow test- loop test hose pump out to tank filler, let it run a while. (trying to narrow it to pump itself)

We are ALL glad to help if we can.

Good Luck,

orange
 
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-if the in and out hoses were open while parked, could be crud or bugs in lines. might try blowing thru hoses from front.

Good Luck, all around, Job too. (BTDT- real ups and downs)

orange
 
Consider doing a capacity test. It should pump at least 1/4 liter in 7 seconds. Failure of test would indicate kinked line or fuel pressure regulator failure. Hope you get it fixed easily. There always is SOME answer.
 
Install a pressure gauge and route it to where you can see it when driving.
this is excellent advise and it will not cost you anything! try autozones loan a tool program and tape it to the windshield. then go driving. pressure at idle is useless info to you since the problem isn't happening at idle! you need to know the pressure when the problem is happening!

it is also possible that their is enough pressure but not enough volume being supplied.
 
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