98 XJ - Mysterious loss of power and MPG

My 98 XJ has recently lost of a tonne of power and fuel economy and I can't for the life of me figure it out. I'm going to leave out most of the back story as I suspect it'd just make things more confusing.

I have another 98 XJ for reference. We'll call reference Jeep "hippywagon" and my Jeep "crumbly".

Hippywagon gets around 550 to 600km per tank and when you kick the pedal in neutral, the RPM's come up and down real fast.
Crumbly was getting about 450 to a tank a few months ago, but now only gets about 350km, and it revs up and down about half the speed of Hippywagon.

Crumbly should be in much better condition than HW. List of new parts on crumbly:
- Catalytic converter
- 2.5" exhaust
- upstream O2
- TPS
- Exhaust header
- Head gasket
- Head is about 3 years old
- Wheel and unit bearings
- COI (cowel)

Crumbly get about 17in vacuum at idle while HW only gets 15in. At idle, Crumbly consumes about 0.76 gallon per hour, while HW consumes about 0.88GPH. Despite this, HW is faster and gets almost double the fuel economy. HW also has a blown exhaust and is missing the downstream O2, and CEL is on.

Crumbly isn't throwing any codes and the only issues I'm having with it are as follows:
1. Takes a few seconds for fuel to hit the rail on cold startup. Suspect checkvalve is defective.
2. After driving for a while, the idle will go high (around 1100RPM). Sometimes it'll kick back down, while sometimes I'll have to shut it down and restart it.

Crumbly has 160psi across all cylinders +/- 6psi. Plugs all look nice and clean, I recently had the head off and didn't see any issues. Leakdown test is solid, and I can't find any vacuum leaks (and I'm making 2in more vac than HW).

So that said, Crumbly is a bit taller and has 33" shoes, but it's had bigger shoes (and 4.10 gears) for years, and always managed at least 450-500km per tank.

WTF causes slow rev response, horrible mileage, and crap power?

Run a can of BG44k or Chevron Techron injector cleaner through before starting any major troubleshooting. It never hurts.

Most likely you have some clogged/gummed up injectors. The next bet would be a bad CAT or dented exhaust pipe.

x2 on what old man says.

whenever this happens to me I do plugs, wires, cap and rotor, clean TB, sea foam in intake and gas and crankcase. If that doesn't do it, then usually then do the TPS then the O2. Ive had newer TPS's go bad pretty quick.

I already changed wires, rotor, cap. Just installed a new cat and most of the exhaust has just been redone and just installed a new TPS to test. I have less exhaust pressure than HW jeep, and after going through both vehicles and comparing parts, I'm amazed HW even runs. I started swapping sensors across Jeeps and found my IAC, despite being cleaner and looking to be in better condition, seems maybe causing the intermittent high idle. Will have to test drive tonight.

My Jeep has been sea-foamed about a year ago, though it does see a lot of mud and abuse. Will throw another can through this weekend if I can get it outta town long enough to burn it through.

Also found that my throttle at idle is 20%, while HW's is only 17.3%. Seems my timing is slightly more advanced as well, though this would make sense due to the slightly higher throttle position. I always thought the TPS on the 98 wasn't adjustable? Am I wrong?

This is pissing me off as I normally get at least 21mpg city/highway, even after installing the lift and 33" shoes, and I take way better care of my Jeep than the girl does (owner of hippywagon), but she's getting nearly double the fuel economy, and after test driving her Jeep today, it's WAY faster. She'd do 0-60 easily 4 seconds faster than me, with stock 3.73's and 30" shoes.

blistovmhz said:

I already changed wires, rotor, cap. Just installed a new cat and most of the exhaust has just been redone and just installed a new TPS to test. I have less exhaust pressure than HW jeep, and after going through both vehicles and comparing parts, I'm amazed HW even runs. I started swapping sensors across Jeeps and found my IAC, despite being cleaner and looking to be in better condition, seems maybe causing the intermittent high idle. Will have to test drive tonight.

My Jeep has been sea-foamed about a year ago, though it does see a lot of mud and abuse. Will throw another can through this weekend if I can get it outta town long enough to burn it through.

Also found that my throttle at idle is 20%, while HW's is only 17.3%. Seems my timing is slightly more advanced as well, though this would make sense due to the slightly higher throttle position. I always thought the TPS on the 98 wasn't adjustable? Am I wrong?

This is pissing me off as I normally get at least 21mpg city/highway, even after installing the lift and 33" shoes, and I take way better care of my Jeep than the girl does (owner of hippywagon), but she's getting nearly double the fuel economy, and after test driving her Jeep today, it's WAY faster. She'd do 0-60 easily 4 seconds faster than me, with stock 3.73's and 30" shoes.

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TPS at 20% is way too high. It's probably a P.O.S. BWD TPS. 17.3% is excellent. If I'm talking the same TPS talk, replace that 20% TPS with a Mopar TPS. Extra bux but you can install it and forget it...it's that good.

The OBDII TPS's are not adjustable.

Seafoam is useless. Use Chrysler Combustion Chamber Cleaner if you want to de-carbon your cumbustion chambers or use BG44K if you want to clean out your injectors.

Sidewinder CC said:

TPS at 20% is way too high. It's probably a P.O.S. BWD TPS. 17.3% is excellent. If I'm talking the same TPS talk, replace that 20% TPS with a Mopar TPS. Extra bux but you can install it and forget it...it's that good.

The OBDII TPS's are not adjustable.

Seafoam is useless. Use Chrysler Combustion Chamber Cleaner if you want to de-carbon your cumbustion chambers or use BG44K if you want to clean out your injectors.

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Should've mentioned I just took off the head for inspection last week, and cleaned up the combustion chamber while I was in there. I also swapped out two known good TPS's and both also showed 20% throttle at idle. I noticed there was a little back-play in the throttle plate, so I took down the set-screw a half turn which brought idle throttle down to 18%. Dunno if that means anything.

I'll swap out a set of known good injectors and rail today to see if that's the problem.
Can the 98 rail be unplugged from the fuel line without having to replace that clip?

blistovmhz said:

Should've mentioned I just took off the head for inspection last week, and cleaned up the combustion chamber while I was in there. I also swapped out two known good TPS's and both also showed 20% throttle at idle. I noticed there was a little back-play in the throttle plate, so I took down the set-screw a half turn which brought idle throttle down to 18%. Dunno if that means anything.

I'll swap out a set of known good injectors and rail today to see if that's the problem.
Can the 98 rail be unplugged from the fuel line without having to replace that clip?

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What 'clip'? The safety clip on the outside of the connector or the lock clip inside of the connector? In either case, no you don't have to replace them. You'll need a 5/16 lock clip release tool to get it off the rail however.

Note concerning the TB setscrew: You shouldn't reposition the setscrew to obtain a TPS position. If the throttle plate is that buggered up, think about replacing the TB.

Welp, swapped throttle bodies, and still no change.

Took HW Jeep for a quick test drive using my TB and connected sensors, and it's getting 23MPG, and I even got in a quick race against a Civic in there. It's WAY faster than my heep, and getting 35% better fuel economy.

Going to try to swap the fuel rail and/or injectors now. Any other suggestions when this doesn't solve the problem?

Is your MAP competent?

What does Short Term Fuel Trim look like? I'm assuming you're using a scanner from your reference to throttle position %.

blistovmhz said:

Welp, swapped throttle bodies, and still no change.

Took HW Jeep for a quick test drive using my TB and connected sensors, and it's getting 23MPG, and I even got in a quick race against a Civic in there. It's WAY faster than my heep, and getting 35% better fuel economy.

Going to try to swap the fuel rail and/or injectors now. Any other suggestions when this doesn't solve the problem?

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You're kidding aren't you.

Sidewinder CC said:

You're kidding aren't you.

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? I've tried almost everything else that I can think of, short of just replacing the longblock, and just about done trying everything that's been suggested. The only stuff left (of the suggestions) to test is the BG44K, and swap that fuel rail and injectors.

It's killing me that my Jeep looks to be in better condition, and uses less fuel at idle than HW, but HW still has so much more power and fuel economy.

These "two known good TPS's" wouldn't happen to be BWD TPS's would they.

I went through two BWD's because they all ran about 20%.. Installed a MOPAR TPS and it cured the problem.

Wait...what's the 5 volt supply voltage at your TPS. If it's 4.5 volts it's within limits (5 +/- .5 volts) and 20% of that is .90 volts, which is within TPS idle voltage limits of less than .95 volts.

Ok. Start back with basics. Well timed fuel, spark and compression. Replacing parts in a mad frenzy will eventually fix it but only based on the law of averages. Put down the shotgun and lets diagnose. I agree with holysirsalad. What are short term and long term fuel trims at? I gather you have some sort of scan tool at your disposal. Look at those PIDs. If it is a positive number the computer is trying to add fuel to compensate for a lean mixture. If it is a negative number it is taking away fuel to compensate for a rich mixture. Small variations from zero are normal.

joejeep92 said:

Ok. Start back with basics. Well timed fuel, spark and compression. Replacing parts in a mad frenzy will eventually fix it but only based on the law of averages. Put down the shotgun and lets diagnose. I agree with holysirsalad. What are short term and long term fuel trims at? I gather you have some sort of scan tool at your disposal. Look at those PIDs. If it is a positive number the computer is trying to add fuel to compensate for a lean mixture. If it is a negative number it is taking away fuel to compensate for a rich mixture. Small variations from zero are normal.

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This is the productive line I try to get and stay on

Fuel trim's average between -3 and +3, swinging between each end depending on current drive cycle. If I let it idle, FT, STFT and LTFT all sit really close to 0 +/- 0.5 on average.
Upstream O2 swings between 0.1 and 0.9V, but I don't have an analog meter or oscilloscope to check the frequency (though I believe my meter does have a frequency function I just haven't figured out how to use.
Downstream O2 tends to hang out around 0.8 during idle, but afaik, downstream is only used to determine cat function, and plays no role in FT.

I really only have two issues I can point to.
1. The fuel check valve seems to be toast. When I start up in the morning, there is no fuel pressure in the rail and it takes 2-3 key turns before I can hear the pressure has built up. I can just crank it, but it'll take 3-4 seconds to start because no fuel. If I do the 2-3 key turns first, it fires right up immediately.
2. Engine stumbles veeeery slightly. I've never been able to figure out which cylinder is the cause. If you're not listening for it, you'd never know it was there, but you can feel it shaking the Jeep slightly. I've owned 6 XJ's so I know how smooth they're supposed to run.

I was getting 23mpg a few weeks ago, so I'll describe what's been changed.

First, I wanted to fix the "axle clearance" dent in the downpipe. It took a lot of anger to get the cat off the muffler so I could pull the downpipe out. Once it was out, I hammered out the dent and put it all back together. Then noticed that when starting up cold, I was blowing a lot more vapour than I'd ever noticed (though I never start my Jeep up and look at it from the outside as I'm usually driving). I figured I must've busted my cat or something and started troubleshooting. At this point I was still getting 23mpg, but failed emissions miserably with extremely high NOx. I stupidly didn't start at the cat, and decided to look for HG leaks. Replaced HG, threw on a new exhaust header, and THEN my mileage took a big dip.
Since then, I've verified my compression is 160psi across the board, leakdown is good, new TPS, Swapped TB's and all sensors with a donor Jeep and installed a new cat. That's when I discovered my high NOx was because the previous owner installed a cat shell (no actual cat). So at this point, I've checked for exhaust restriction again, checked all my sensors except CPS, and FT looks good. On paper, I should be getting 23mpg, but the reality is somewhere around 15mpg.

Decided to look at brakes last night and found my rear drums (D44) were toast, so I'm swapping those out now and hoping they were dragging real bad causing the loss of mileage and power, but I still can't explain why my engine revs up and down at about half the speed of my other Jeeps.

All these fancy numbers are way over my head but I'd sure like for my XJ to get 18mpg... I've never seen a single XJ get consistently 23mpg.

Lowrange2 said:

All these fancy numbers are way over my head but I'd sure like for my XJ to get 18mpg... I've never seen a single XJ get consistently 23mpg.

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I'm starting to wonder if you guys (in the US) just have really crap fuel. Every time I fill up in Point Roberts, I see a few MPG decrease in economy, which doesn't really make up for the lower price.

My ex (now the owner of HippyWagon) consistently gets a bit over 600km to a tank, combination city/highway/bush driving. Works out to around 21mpg I believe.
I took her Jeep out the other night for a test drive to confirm and saw 22.8mpg according to my OBD app (Torque) as well as at the pump.

Fuel trim numbers as stated are ok. Upstream 02 should oscilate above and below 450 mV which richens and leans out the mixture to assist in catalyst efficiency. Downstream should be pretty much flatline as the oxygen should be mostly used up in the oxidation process of the catalyst. As far as your tps is concerned check for voltage into it and voltage out. Voltage in should be 5 volts and voltage out varies between .5 v and 4.5 v.

Also if your computer is not responding quickly enough to engine vacuum changes that could cause your concern. Open throttle plates=low engine vacuum. If the computer is not seeing this even a split second too late it can sure slow down things

Sounds like you've been taking parts on and off a lot. Have you checked thoroughly for vacuum leaks?

I don't think I saw you write how the MAP sensor looks

Also I must say I'm impressed that you managed to hammer out the dent in the downpipe. For reference, that was put there by the factory to increase EGT by way of more backpressure. This was done to get the cat to heat up faster. With the temps up there I'm not surprised you failed an emissions test if you didn't let it warm up extra. By your description of a fairly even DS O2 output and increase in vapour it sounds like the old cat was junk, clogged, worn out high flow, or something.