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Unread 10-18-2008, 10:30 AM   #1
mondojeep
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89 Cherokee 4.0 severe power loss & backfire!

I am trying to help out my father in law with his 89 cherokee 4.0 auto. Last week it started running bad, idles fine but soon as I try to accelerate it backfires and has no power at all. I changed plugs, wires, cap, rotor, pick up coil, fuel & air filter to no avail. It always had the crank forever before starting issue but ran fine once fired up. I did resarch on this board and I am leaning towards the CPS now, called a-zone they have one in stock.

But one thing that is bothering me is I only have like 5lbs of fuel pressure at the rail even after changing the fuel filter. Could it be the fuel pump?

ALSO NO CHECK ENGINE LIGHT!

Any help will be greatly apreciated by me and my father-in-law.

RIck

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Unread 10-18-2008, 10:43 AM   #2
88XJSport
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1988 XJ Cherokee 
 
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Any 87-90 era Cherokee will have a long-ish crank time before starting (up to about 6 seconds.) There is no check engine light on an 89 Cherokee, so dont bother to try and pull any codes.

5 lbs of fuel pressure is not good. It sounds like you are starving for fuel, and when you said 5lbs, you confirmed it. The pressure should be atleast 31psi when you turn the key ON, and the pressure should hold there. Mine had the same issue back in 05, I had only a few psi, and couldnt keep the engine running. A new Bosch fuel pump, and my pressure was back up to 31.

I say change the pump. Remove the lock ring on the back of the fuel tank, and the pump and float comes out in an assembly. I got a new Bosch pump at autozone for about 80 dollars. It would also be a good idea to change the little rubber fuel hoses that connect the pump to the lines.

Good Luck.
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Unread 10-18-2008, 10:46 AM   #3
BrunoS
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hello:

some clues:
idle is ok (means compression, valves, plugs, computer is ok)

1) backfiring is related to spark timing that is off, or too much fuel in intake manifold (could be from long crank times to start)

2) lack of power while driving is a severe symptom of lack of fuel.


i would: check that fuel filter is installed correctly, doublecheck distributor cap/plug for misrouting.

also, try to disconnect battery for a minute or two, allow the computer to reset itself and try running it again.

5 psi of fuel pressure is way too low, should be near 39-49 psi on the 4.0 motor. the lack of flow could be a collapsed fuel filter or fuel pump going south.

try my suggestions first, let me know how it goes.
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Unread 10-18-2008, 10:47 AM   #4
jvilla112
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just had the same thing, no codes, it's your cps
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Unread 10-18-2008, 10:50 AM   #5
mondojeep
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Quote:
Originally Posted by 88XJSport View Post
Any 87-90 era Cherokee will have a long-ish crank time before starting (up to about 6 seconds.) There is no check engine light on an 89 Cherokee, so dont bother to try and pull any codes.

5 lbs of fuel pressure is not good. It sounds like you are starving for fuel, and when you said 5lbs, you confirmed it. The pressure should be atleast 31psi when you turn the key ON, and the pressure should hold there. Mine had the same issue back in 05, I had only a few psi, and couldnt keep the engine running. A new Bosch fuel pump, and my pressure was back up to 31.

I say change the pump. Remove the lock ring on the back of the fuel tank, and the pump and float comes out in an assembly. I got a new Bosch pump at autozone for about 80 dollars. It would also be a good idea to change the little rubber fuel hoses that connect the pulp to the lines.

Good Luck.


Thanks for the quick response and info! I guess the pump will be the next step, will call a-zone now. Its just strange cause I would think the pump would just quit not work and pump low.
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Unread 10-18-2008, 10:55 AM   #6
mondojeep
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Quote:
Originally Posted by BrunoS View Post
hello:

some clues:
idle is ok (means compression, valves, plugs, computer is ok)

1) backfiring is related to spark timing that is off, or too much fuel in intake manifold (could be from long crank times to start)

2) lack of power while driving is a severe symptom of lack of fuel.


i would: check that fuel filter is installed correctly, doublecheck distributor cap/plug for misrouting.

also, try to disconnect battery for a minute or two, allow the computer to reset itself and try running it again.

5 psi of fuel pressure is way too low, should be near 39-49 psi on the 4.0 motor. the lack of flow could be a collapsed fuel filter or fuel pump going south.

try my suggestions first, let me know how it goes.

Thanks, the fuel filter & wires / cap are definitly installed properly. I am going with the pump next for it is much easier to change than the CPS.
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Unread 10-18-2008, 11:02 AM   #7
BrunoS
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be aware that IF your CKP (CranKshaft Position) sensor is going belly up, the computer won't see the crank rotating, and will cut power to the fuel pump. so a bit of warning if you replace fuel pump first. since it's a 89, that's 19 years on the fuel pump...not bad...and you can replace it without moving the tank.

If it barely idles, and does rev without a load on motor...then yep, fuel pump is shot.
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Unread 10-18-2008, 11:03 AM   #8
88XJSport
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Dont forget to replace the little rubber fuel hoses in the pump assembly (they're just worm clamped on) as the old rubber hoses have been sitting in fuel for 20 years...mine were mush when I removed my pump...
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Unread 10-18-2008, 05:50 PM   #9
mondojeep
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ITS FIXED!!!
Yep it was the fuel pump, but that damn ring was so rusted on even after drowning it with lube it took a while to get it off and bent it in the process but nothing that the BFH couldn't fix. I did replace the rubber hose cause it was mush, installed new pump and shes purring like a kitten again.

However I am noticing that the ignition key needs to be held at crank postion much harder now to get it to crank, maybe starter is going from constantly cranking it with bad pump? Maybe ignition switch is taking a dump?

Well I guess I am not done after all, but I'll let that slide for a while and see what happens,

thanks for all your support! You know I have to say that I am very impressed with the quick intelligent responses I received from this forum, now if I could get that from the camaro z28 board life would be good.

Take care I'll be back again.
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Unread 10-18-2008, 06:24 PM   #10
88XJSport
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Quote:
Originally Posted by mondojeep View Post
ITS FIXED!!!
Yep it was the fuel pump, but that damn ring was so rusted on even after drowning it with lube it took a while to get it off and bent it in the process but nothing that the BFH couldn't fix. I did replace the rubber hose cause it was mush, installed new pump and shes purring like a kitten again.

However I am noticing that the ignition key needs to be held at crank postion much harder now to get it to crank, maybe starter is going from constantly cranking it with bad pump? Maybe ignition switch is taking a dump?

Well I guess I am not done after all, but I'll let that slide for a while and see what happens,

thanks for all your support! You know I have to say that I am very impressed with the quick intelligent responses I received from this forum, now if I could get that from the camaro z28 board life would be good.

Take care I'll be back again.
Thank you for the follow up post.

9 times out of 10 a new member will post one question and never come back.

So your problem was exactly the problem that I had back in 05. The old 87-90 cherokee's had long crank times when new, (anywhere from 5-8 seconds) so theres not much to start as fast as new cars on the road.

Yes any more questions, please come back.
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Unread 07-16-2009, 04:44 PM   #11
JarVan
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'89 Laredo, 4.0, Manual Trans, 153k

I just went through a nearly identical situation with the additional symptom of a poor idle that faded to zero after about 5 minutes of run time. Could only keep the beast running with a foot on the accelerator. Max RPM was 2500 with a lot of backfiring and minimal power.

In my situation the output hose from the fuel pump had partially slipped off the fuel line (inside the tank). Fuel pressure was extremely low at the rail. Five minutes after shut down I could hear air gurgling into the tank when the fuel line siphoned empty.

Easy fix once the pump was out. For some reason my '89 has a straight end on the fuel line, no bulge to keep the hose from slipping off when it degrades a little over time.
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Unread 04-24-2010, 07:29 PM   #12
bubbaxj
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1991 FSJ Cherokee 
 
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91 xj 4.0 with power loss and max of 3000rpm

hi, i have a 91 4.0 H.O. The problem seemed to happen overnight. Sounds completely different, wont shift(auto tranny aw4), when rev it will make it to 3000 rpms and then gets a death wobbile and wont go any higher. idles good, sounds bad but is at right rpm and is constant. Used to have a deep tone to it, now seems high pitched air almost whitsle, just bad. im running out of ideas of what could be wrong. check engine light not on. did the ignition thing and no fault codes. PLEASE HELP.
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Unread 04-24-2010, 10:59 PM   #13
srhcherokee24
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have you checked and seen if you have a bad fuel pump balaster restister
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Unread 04-25-2010, 09:21 PM   #14
bubbaxj
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not sure on that.. dont have a fuse box cover, which one is it? or is it some where else?
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Unread 04-26-2010, 05:13 PM   #15
bubbaxj
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1991 FSJ Cherokee 
 
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you will never believe what it was..it was the catalitic converter! raised the back pressure so much that the jeep wouldnt even shift. all the power is back and it shifts and runs like a top
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