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ronkeller8 08-31-2011 10:01 AM

Loss of Power Problem
Hi folks. I've been trying to solve this riddle for some time now, but I'm not getting any closer to a solution so I thought I'd throw it out there....
Background: 1997 TJ, 4.0 litre. 5 Speed. My o2 sensor went bad some time ago (upstream) and I couldn't get it out, so disconnected it. Jeep threw CEL, but otherwise ran fine. Then, about a month ago, it started running rougher and rougher. Got the o2 sensor replaced and it ran okay after that. However, it seems now that after driving for about 40 minutes, the Jeep will start to have intermittent problems loss of power under acceleration, hesitation, backfire once in a while and rough idle if left idling too long. Problem is exacerbated when using a/c. Engine does NOT throw a CEL, however.

I've replaced the plugs, wires, flushed the radiator (engine doesn't run hot), replaced the IAC, the MAP sensor, TPS, cleaned the throttle body assembly, tried fuel additives, tried using higher octane fuel. Also, checked the rotor cap, plugs and wires using a grounded screw driver at night for sparks none. Tightened down the valve cover and the exhaust manifold. Looked for vacuum leaks, sprayed engine with brake cleaner no change in RPM.
One possible other clue: When I start the Jeep after sitting for a while, it won't catch quickly. If I turn ignition switch off and then restart, it fires right up. My thought was that the fuel was siphoning back into the tank. Also, the fuel gauge has been acting wonky fluctuates from idle to throttle. Could these symptoms be caused by a fuel pump going bad and if so, could they be related to my other problem? Why would it work worse after 40 minutes of driving fine??
Appreciate any thoughts / comments....I'm out of ideas!

OIIIIIO1997 08-31-2011 10:38 PM

I have a 97 4 popper doing the same thing
I hooked up my scan tool, on live view it showed erratic TPS values. I did pick up a TPS sensor on the way to work I sure hope that is it.
I only had one code no check engine light the code was a #4 miss fire.
I will update if it worked.

OIIIIIO1997 09-01-2011 02:16 AM

well the TPS made zero improvement. to go further into it, the thing has verl little power and will not rev above 4k with no load on the motor.
so any ideas where to look or go now would be great.

jeepsr4grls 09-01-2011 02:34 AM

Recomendations will be to check your fuel pressure. This will help to determine whether it is your pump or not. SEarch the threads that relate to catalytic converters. Not sure if that may relate or not. Maybe compare symptoms to see if that may be the issue.

Shelby427 09-01-2011 11:12 AM

Fuel pump going south is a good possibility. They just don't die, they will do all kinds of odd ball things instead. And heat, from longer running will multiply the problems.
Also the fuel pressure regulator going bad is a classic, and the multiple key starts to get started is the #1 symptom.

Also running with no O2 sensor might have added to your problems. No signal and the ecm goes full rich on the mixture to try and get one. Makes the pump work harder, and it's also dumping an excessively rich mixture into the cat convertor.

Taking a shot but I'd say you are looking at a fuel pump & regulator, which both require dropping the tank. Hopefully you won't need to replace the cat too because of running with no O2 sensor.

Buddha 09-01-2011 11:21 AM

This sounds like classic fuel pump going bad- just to screw with you it will run good for a little while to let you think it's fixed -then it will start cutting out again - usually when it heats up. Get a fuel pressure gauge and check your fuel pressure.

If the fuel pressure is good (consistently) one other thing it might be is the coil.

ronkeller8 09-01-2011 01:54 PM

Yeah, I'm leaning towards the fuel pump/regulator being bad as well. My other thought was a plugged cat, but wouldn't the downstream o2 sensor throw a CEL if that were the case?
I just had to do a bunch of brake work, to the tune of $400, so with any hope, the fuel pump won't take a dump on me before I can afford to replace it. It sure *looks* simple to do, but we all know how that usually turns out, right? Funny story: I had a client in my office the other day who's a mechanic. His box truck died, he was 100 miles from home, and was trying to replace the in-tank fuel pump on the side of the road....right after he filled his gas tank. With 55 gallons. Yikes. He was able to find a guy locally with a fork lift to lower the tank for him. Hopefully...mine will be a bit simpler (and better planned out).....
One other question: To check the fuel pressure, can I just get a meter of sorts and hook it up to the Schrader Valve on the fuel rail? Is it that easy?

erickpl 09-01-2011 02:06 PM

Did somebody steal your 4.0 and put a 2.5L in its place?

Buddha 09-01-2011 02:09 PM

Yeah it's that easy - fuel pressure gauge to Schrader valve...

ronkeller8 09-02-2011 09:47 AM

1 Attachment(s)
So, on the way home last night, Jeep decides to start having problems right in the middle of the road. Stalled, couldn't get it restarted. Was finally able to chug off the side of the road where it died. Called AAA - police officer wanders by, who happened to be a mechanic in a previous life. We start talking, tried starting it up (which of course it did, then fluctuated on the RPM's, then choked out and died). He seemed to think that the fuel pump would either work or it wouldn't (not so sure about that one), but he said the engine was running very rich (could smell the exhaust gases). He thought there was more to it. Soooo- before I take the time this holiday weekend to drop my tank and spend the money on a replacement fuel pump, two questions: 1) Will the replacement fuel pump unit include everything I might need, such a fuel pressure regulator and filter? And 2) If I hook up a fuel pressure gauge when the Jeep is running (i.e. at home, not stuck out on the road somewhere), what will that prove, since the Jeep will be running at that time - other than perhaps a weaker than ideal flow, perhaps?
I don't want to go through this to find out it's something else entirely....

Buddha 09-02-2011 12:22 PM

I can't tell you what you will get when you get the fuel pump because I have a YJ and not a TJ and they are different. I want to say that I've read on here that the regulator and filter are part of the pump assembly on TJ's but don't hold me to it.

I can tell you that when my fuel pump was going out some days it would run like a top for a while then quit. As best I can tell from my personal experience, they don't just go out all at once unless there's an electrical problem with them or something like that. When I finally wised-up and decided to check the fuel pressure what I found was that it was reading about 20 PSI instead of 31 (again the TJ pressures may be different). At that point it was idling fine and not cutting out, but had I driven it with that low pressure it would have sputtered and chugged, and I suspect that as it heated up it would have gotten worse.

As for the gas smell and running rich - when mine was going out it would do that even to the point of backfiring at times. I thought it was odd because the pressure was low which should have meant less gas - not more. I finally decided it must have something to do with the computer trying to adjust the mixture to make up for the lack of fuel - but that's all way over my head - so that was just a guess.

Good luck this weekend if you decide to drop the tank - get as much gas out of it as possible before dropping it - it will be much easier to handle.

ronkeller8 09-02-2011 07:49 PM

Thanks for the thoughts. I think everything you say makes sense, TJ or YJ. I'm leaning towards doing it, but may check the fuel pressure readings first, although unless its hot and under load, i don't see how that will help tell me much. Id like to hook up a permanent fuel pressure gauge in the cabin, not sure how to do it though.

Buddha 09-05-2011 03:04 PM

Like I said above - my fuel pressure showed low even when it was just idling so I knew I had a problem. It just got worse sporadically when under a load. New fuel pump fixed it.

ronkeller8 09-10-2011 07:33 PM

UPDATE: So, it seems like the general consensus here and elsewhere was the fuel pump. So, I replaced it earlier today. Twice, actually. It seems that, like a bad surgeon in the O.R., I left a tool in the patient. I was also doing the 15 to 19 gallon mod on the tank, and left my PVC cutter IN the tank. Of course, I didn't realize this until I had EVERYTHING back together and working. Could have been worse, I could have driven it around and had a metal tool sloshing around, slamming into my new fuel pump. Cest la vie. Lets just say that the second time was MUCH quicker than the first.
So, it seems to be running smoother through the power band, the vehicle is starting right up now, and the vehicle with the a/c running doesn't seem as sluggish. All good things. The real test will be Monday, when I drive to/from work for 1 hr each way. Let's see how it holds up then - will keep everyone posted.

Buddha 09-12-2011 12:31 PM


Originally Posted by ronkeller8 (Post 1558272)
I was also doing the 15 to 19 gallon mod on the tank, and left my PVC cutter IN the tank.

Sounds like you did the tank mod the hard way if you had your cutters IN the tank :D But hey - at least you got 'er done.

Looking forward to hearing how the drive to/from work went today.

Tyewilly 09-12-2011 02:06 PM


Originally Posted by Buddha

Sounds like you did the tank mod the hard way if you had your cutters IN the tank :D But hey - at least you got 'er done.

He did it right, the vent tube doesn't pull out on a tj like it does on our yjs. The hard way is the only way.

ronkeller8 09-12-2011 09:13 PM

Well, so far, so good. I was able to get to/from work without a problem. Let's see if it continues all week. I filled the tank up on the way home and noticed that the gauge now reads well below empty, even though there was 2 gallons in the tank, and at full it reads well above full. Any thoughts on that? Do I need to recalibrate the gauge somehow? I used the fuel pump for the 19 gallon tank, so I presumed it was set for empty/full based on 19gals. Plus, for some reason, the 19 gallon fuel pump was $50 cheaper than the 15 gal fuel pump...weird. Ideas anyone?

Buddha 09-15-2011 09:46 AM

After doing the 15-19 mod on mine and replacing the fuel sending unit it now reads way past full for about 75 miles then starts to fall off and read correctly. When I get to a quarter tank it has about 4 or 5 gallons in it - I couldn't tell you which sending unit I installed (15 or 19) so I know this info isnt much help.

Most Jeep owners will tell you that all Jeep fuel gauges are squirrelly and you just have to figure out how yours is working. Many just use the trip odometer and go by how many miles it's been since the last fill up. I just try and make sure I start looking to get mine filled when it reaches a quarter tank on the gauge.

ronkeller8 09-15-2011 03:01 PM

Yeah, that's probably just gonna be the way it is. From what I read on alldata, it seems that the unit sends a certain amount of voltage to the PCM depending on the position of the float. Perhaps the new unit is sending different voltages than what the PCM is expecting, hence the problems. Guess I have bigger fish to fry than worry about this, but it would be nice for it to work accurately.

TJWenatchee 09-15-2011 05:46 PM

My 97 TJ has a 19 gallon tank. Fuel light came on and still drove it another ten miles or so. Put 15.10 gallons in it. When full mine always pegs the needle way above the F mark, yet low fuel light comes on about an 1/8" before the red and the E. Stays at full for along time and then drops to 3/4 of a tank. Jeep Gas gages are odd :whistling:

zimi117 09-15-2011 06:30 PM

I would also reset ur computer
Just disconnect the battery and leave it for a couple of hours. Ur ECM is using old mixtures at least that's how it was when I replace my o2's in my 98 4.0l
Just a thought

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