1998 Wrangler 4.0L loses Power while Accelerating

Tiny
TISSUEMAN
  • MEMBER
  • 1998 JEEP WRANGLER
  • 97,000 MILES
I have a 1998 Jeep Wrangler Sport. I bought it three months ago. It rans rough and lost power all under acceleration. No check engine warnings appeared. I changed the plugs, plug wires, distributor cap and rotor. It ran better, but still low power under acceleration. I check the fuel pump pressure: it held 45-48 psi. The vehicle has always idled perfectly. It does not stall or knock. I put at least three bottles of fuel injector cleaner in the system: SeaFoam.

While idleing, I disconnected each fuel injector just to look for a difference: all made the idle the same amount of roughness. I disconnected the throttle position sensor: the engine stalled and the check engine came on as expected.

The check engine light turned off the next day. But, I had the computer checked and no warning codes appeared, accept for the throttle positioning sensor which I purposely triggered to make sure the check engine light worked.

The engine still loses power while accelerating. If I back off the gas, the engine smoothes out. At 65-70 mph, any hill is an issue. I have to floor the gas just to keep moving and I have to forget using fifth gear.

What should I check next? Thank you for your assistance in advance.
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Thursday, June 16th, 2011 AT 10:47 PM

19 Replies

Tiny
TISSUEMAN
  • MEMBER
I have considered most of those, but wouldn't they trigger a check engine light?
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Thursday, June 16th, 2011 AT 11:12 PM
Tiny
RASMATAZ
  • MEMBER
Don't always rely on the MIL/CEL/SES to give you a headstart-try scanning it and see if something pops-up for a start
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Thursday, June 16th, 2011 AT 11:16 PM
Tiny
DANLESABRE
  • MEMBER
Remove the gas filter and use your mouth to try blowing through it. If it is hard to blow through. There's your problem. Can you connect a fuel pressure gauge and position it so you can read it while driving?
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Friday, June 17th, 2011 AT 5:11 AM
Tiny
RIVERMIKERAT
  • MEMBER
Check fuel pressure at the pressure regulator.
Loss of power under acceleration is usually due to a plugged catalytic converter though. Try disconnecting the catalytic TEMPORARILY and installing a bypass. Then test drive it. Problem gone? Replace the catalytic converter.
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Saturday, June 25th, 2011 AT 9:39 PM
Tiny
TISSUEMAN
  • MEMBER
I have checked the fuel pressure: it was good. The fuel filter is in the fuel tank as part of the pump. I am in process of changing the cat convertor this weekend.
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Sunday, June 26th, 2011 AT 3:28 PM
Tiny
RASMATAZ
  • MEMBER
Prior to replacement, best check if it has an excessive backpressure-by disconnecting the exhaust pipe or use a vacuum gauge/gas analyzer/pressure gauge-
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Sunday, June 26th, 2011 AT 4:24 PM
Tiny
RIVERMIKERAT
  • MEMBER
Yeah, a cat is pretty expensive. Best to install a bypass and test drive first.
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Sunday, June 26th, 2011 AT 5:20 PM
Tiny
RASMATAZ
  • MEMBER
That's what I'm talking about that bad boy is not cheap. Now if you're not affected by an yearly emission certification install a test pipe and save your money
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Sunday, June 26th, 2011 AT 5:38 PM
Tiny
RIVERMIKERAT
  • MEMBER
Are there any states left that don't have at least bi-yearly testing requirements?
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Sunday, June 26th, 2011 AT 5:43 PM
Tiny
CJ MEDEVAC
  • EXPERT
Yeah down here

For the longest time, I thought emmission, was what you paid to get into the picture show.

The Medic
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Sunday, June 26th, 2011 AT 5:49 PM
Tiny
RASMATAZ
  • MEMBER
Then again you're polluting the enviroment which is not nice for health purposes I take my suggestion back on the test pipe-have it replaced if its the culprit-Good Luck
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Sunday, June 26th, 2011 AT 5:55 PM
Tiny
RIVERMIKERAT
  • MEMBER
Agreed razmataz
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Sunday, June 26th, 2011 AT 6:33 PM
Tiny
TISSUEMAN
  • MEMBER
Well, I changed the rear O2 sensor: seemed to help but no absolute cure. It worked for one day, then I notice a terrible vibration when I started in first gear. Further snooping allowed me to hear the noise while stopped and idling. The noise came from the Catalytic Converter. Ordered a direct fit Cat ($143) on-line and just completed installation. Appears to be the root cause. No more vibration and plenty (?) Of power. THanks for all your help.
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Tuesday, June 28th, 2011 AT 7:57 PM
Tiny
TISSUEMAN
  • MEMBER
P.S. I don't know of any state in the Southeast that requires an emission test.
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Tuesday, June 28th, 2011 AT 8:03 PM
Tiny
RIVERMIKERAT
  • MEMBER
Great! Glad to hear it. Thanks for letting us know.
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Tuesday, June 28th, 2011 AT 9:50 PM
Tiny
TISSUEMAN
  • MEMBER
Well, it's been close to five months since catalytic converter repair. Mind you, the performance is much better, but something is still not right. I still believe there is a hesitation: always during acceleration in any gear (expect 5th, there is no accelerating in 5th). Never feels as though I am getting the smooth power I should get. I will review the check list provided earlier. I have changed the O2 sensors, cleaned the MAP (knock) sensor, and changed the wires, plugs, cap rotor and wires. Still no engine fault codes. I have used several over the counter injector cleaners: Seafoam, etc: no difference.
Seems light the engine is running lean. Mileage is a constant 14mpg. The vehicle is lifted 3 with 33x 12.5 tires, so I don t expect better.
I have not played with TPS or the CSPS because that doesn t make sense: no starting or idle issues at all. Engine revs in neutral fine. Only under load does the engine seem anemic. I hate throwing money after money, but it is a Jeep!
Well, back to the garage. I am thinking coil as potential issue: weak spark? Any feedback is appreciated.
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Sunday, October 30th, 2011 AT 7:14 PM
Tiny
RIVERMIKERAT
  • MEMBER
Weak spark COULD be an issue. If you could put it on a diagnostic scope with a raster display, that could tell you if the spark is weak. You might also check whether the coil is receiving the voltage it needs (13.8 to 14.2).
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Sunday, October 30th, 2011 AT 8:20 PM
Tiny
JEEP-GUY101
  • MEMBER
I had that problem with my 98 base with the 2.5. Take the vacuum line off the fuel regulator and plug it up with a small bolt or screw, make sure its tight and wont come out. Then go try to take it up a hill, if it does fine you're looking at a new fuel regulator which is about 30 bucks and its held on by 2 bolts to your fuel rail. If that doesnt help, like it didnt for mine, you might need a fuel pump. About $110 and 3-5 hours to put on depending on what kind of aftermarket stuff you have on.
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Wednesday, October 7th, 2015 AT 11:01 AM
Tiny
KEN
  • ADMIN
Nice additional information here :-) thanks!

Best, Ken
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Wednesday, October 7th, 2015 AT 1:36 PM

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