98 XJ - Mysterious loss of power and MPG

[blistovmhz]

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.

I don't know what to believe regarding the dent. Lots of guys say it just hits the axle, others say it was factory dented for axle clearance, and others still say it's there to increase backpressure. I've talked to two guys who actually worked on the assembly line who say they're sure the dent was not there from factory, but then I've talked to another guy who also claims to have worked on assembly, who says it was there for axle clearance. Either way, I've had another jeep with a new, non-dented downpipe, and never noticed a difference.

RE: vacuum leaks. Yup. And if I could find it I'd be happy happy, but nothing doing, I can't find it. MAP and vacuum gauge both say I'm getting about 17in vacuum which is better than a lot of other Jeeps I've driven, which get 20mpg just fine. I still half suspect vac leak, but can't find it.

 

[blistovmhz]

Checked fuel pressure. Stable at all RPM at 47.5psi. So that was a $100 tool I'll never use again

Got sidetracked for a few days with my rear disc swap (KJ 8.25 rear disk to my XJ D44), but waiting for a spacer now, so I'll pull the intake and exhaust off again tomorrow and try to find a vacuum leak.

 

[old_man]

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.

On the renix vehicles, that dent is for clearance on the pinion at full stuff. I built a pipe and it ended up with the dent from hitting the pinion.

 

[blistovmhz]

I'm thinking the problem is something mechanical at this point.
This Jeep has never given me the same power of my other Jeeps, despite everything I've tested, seeming to be in better condition. My 90' XJ had 700,000km, had been run dry outta oil several times, overheated often, would see sudden drop (to 0) in oil pressure while driving, and still made more power than my current Jeep (and got better mileage).

Wouldn't a bad timing chain/cam/lifters/pushrods result in loss of power, slow rev up and down, and decreased FE?

How do you test this all in the 4.0L? Can't find any reference to diagnosing cam issues in the Jeep. I figure you must measure valve height and stuff, but can't find any specs.

 

[Dragonslayer]

I'm with Holysirsalad. Check for a vacuum leak. 17 in HG vacuum at idle seems a bit
low. My own '93 XJ consistently draws 20-22 in HG vacuum at idle. A vacuum leak will
cause a lean condition and O2 sensor will send that signal to the computer that will then
try to compensate for that by opening the fuel injectors a bit more (= less MPG, hard
starts, fouled plugs).

My hearing is goofed and so I got a doctor's stethoscope (~$15), disconnected the head,
and installed a 1/8" x 12" brass tube. Use that to probe around all the little vacuum
hoses, the base of the throttle body, and around the ex-in manifold gasket.

 

[yossarian19]

For grins, perform a running compression test.
It's exactly what it sounds like.
Remove only one plug at a time, put your compression tester in, start her up and see the number. Expect roughly half your regular compression test number. Don't worry about the number itself unless it is more than 10 psi under 1/2 your regular compression test number. Look for one cylinder not like the others - your camshaft is likely damaged on that cylinder.
Also - is your 17 hg of vacuum a totally steady needle, or does it wiggle? I don't recall if you said.

 

[blistovmhz]

For grins, perform a running compression test.
It's exactly what it sounds like.
Remove only one plug at a time, put your compression tester in, start her up and see the number. Expect roughly half your regular compression test number. Don't worry about the number itself unless it is more than 10 psi under 1/2 your regular compression test number. Look for one cylinder not like the others - your camshaft is likely damaged on that cylinder.
Also - is your 17 hg of vacuum a totally steady needle, or does it wiggle? I don't recall if you said.

Vacuum reading definitely shakes a bit, but not by more than about 0.5psi.
I just finished the rear disc conversion today, so will be back to troubleshooting engine again tomorrow. Probably pull the header and intake for inspection, and make sure the header flange face is the same thickness as the intake.

 

[blistovmhz]

Pulled intake and header, ground the header flange down 0.033" to match the thickness of the intake flanges. Re-assembled, no change. Took it for another test drive and averaged 16mpg (down from an average on this drive of 21mpg). However, the idle seemed less wonky after the drive. After installing the header, any time I drove over 100km/h for more than a few minutes, the idle would increase to around 900-1100RPM until I restarted the engine. Didn't do it this time.

Vacuum reading didn't change at all. Still a slightly shaky 17in. Disconnected all un-necessary vacuum stuff and plugged at the intake to rule stuff out. No change.

Will try a running comp. test tomorrow after oil change. Other than the cam, or a wonky piston, I can't think of anything else.

 

[moparmansfield]

How about something transmission related? I've had 2 very similar jeeps that felt miles apart when talking about performance and mpg. difference was a rebuilt aw4 and one with 160k(well used) on it. you said you wheel this thing pretty hard, sounds like the tranny has probably gotten pretty hot. Id say drop the pan and check out that magnet to see how much metal is on it. I'm no tranny expert but maybe the torque converter could be on its way out or your clutches a little burnt up, or one of the bands.

Also, some people may not agree with this, but I felt a difference when switching from dextron3 to Atf+4. Atf+4 felt like it lost the tightness it had before, like it was foaming or something. just something to think about.

 

[blistovmhz]

How about something transmission related? I've had 2 very similar jeeps that felt miles apart when talking about performance and mpg. difference was a rebuilt aw4 and one with 160k(well used) on it. you said you wheel this thing pretty hard, sounds like the tranny has probably gotten pretty hot. Id say drop the pan and check out that magnet to see how much metal is on it. I'm no tranny expert but maybe the torque converter could be on its way out or your clutches a little burnt up, or one of the bands.

Also, some people may not agree with this, but I felt a difference when switching from dextron3 to Atf+4. Atf+4 felt like it lost the tightness it had before, like it was foaming or something. just something to think about.

AX15, and yes, my clutch is slowly dying, but doesn't feel like it's slipping. Actually the opposite. It often wants to grab when it shouldn't. I'll be changing oils tomorrow (hopefully) as well and will take a look at the tranny while I"m at it.

 

[old_man]

AX15, and yes, my clutch is slowly dying, but doesn't feel like it's slipping. Actually the opposite. It often wants to grab when it shouldn't. I'll be changing oils tomorrow (hopefully) as well and will take a look at the tranny while I"m at it.

Grabbing when it shouldn't is a sign of a leaking rear main and getting fluid on the clutch disk, or a leaking slave cylinder.

 

[blistovmhz]

Grabbing when it shouldn't is a sign of a leaking rear main and getting fluid on the clutch disk, or a leaking slave cylinder.

I've got no leaks whatsoever. I suspect the TOB or something is binding on some mud because the problem comes and goes, usually gets worse after drying out after a day of wheeling, and cleans up on it's own after a few days (most cases).

Yes, I know... My Jeep's falling apart and it makes troubleshooting any single issue more difficult. I've just had nothing but bad luck with this one. Had sticky valves when I got it 5 years ago, replaced the head. CPS died out in the middle of nowhere but managed to rig it to limp home. Engine harness is probably corroded all over, but every time I test stuff, everything checks out fine. It may just be cursed.

That said, the slightly rough idle and fuel leakdown (back to tank) are the only actual symptoms I can point to in the engine. I recently added 33"x12" shoes, but I clearly recall pulling 19-20mpg highway with them for the first week installed. I'm almost positive the power/mileage loss came after working on the exhaust which was a week after new shoes. Just to be sure I'm not crazy, I'm going to throw some smaller rubber on and give it a test again.

 

[yossarian19]

Power and mileage loss after exhaust work... have it smoke tested. A leak could do it, no?

 

[yaromb]

Interesting. I have kind of the same issue. Once in a while, my idle will go to 1100 and stay there until I turn the car off. I also have really bad mpg issues. No leaks were found but I have a spectre intake, bored out throttle, 703s and 2.5" catback...

No loss of power.

 

[blistovmhz]

Interesting. I have kind of the same issue. Once in a while, my idle will go to 1100 and stay there until I turn the car off. I also have really bad mpg issues. No leaks were found but I have a spectre intake, bored out throttle, 703s and 2.5" catback...

No loss of power.

Starting to think the idle issue may be IACV. I thought I'd swapped it out with a known good unit, but maybe I forgot. Will try today. I seem to have several donors in the yard right now. Friend left her Jeep (was mine years ago) in my yard while she's in Hawaii.
I'm starting to seriously consider swapping her entire engine over to make sure my issue has anything to do with the motor, and isn't something stupid like fuel delivery.

 

[blistovmhz]

So I think the high idle was just a sticky throttle plate. Stopped on the highway when it happened, popped the hood and pulled the plate down and problem solved.

On the loss of power and intermittent misfire, still no idea. Replaced cap/rotor/wires/plugs, fuel injectors, ECM, and all sensors except the CPS.

A sticky valve or bad distributor is all I can think of that would cause an intermittent misfire, aside from all the stuff I've already replaced.

Any more idea's? Any way to test and confirm valve clearances?

 

[Sidewinder CC]

Here's a site that shows various vacuum diagnostics that you may want to try.

http://www.secondchancegarage.com/public/186.cfm

Attention should be given to a bad/clogged CAT.

17 in/hg may be a tad low if it stays there continuously. The FSM says 13 in/hg is min. I see 17 when the engine is warming up then it ends up at 21 in/hg when at 210*F coolant temp.

There's other vacuum diagnostics websites if you Google for them. Youtube has a few as well.

 

[yossarian19]

Try a new factory crank sensor. I wouldn't shop for sensors based on price - get the Mopar, is my advice, unless you are real savvy on which brands build what for who.

 

[MAY POP]

Distributor advance is controlled by the PCM. Put a chalk line on the vibration damper 1 inch in front and behind and on TDC. Check the timing at idle,1000 and 2000 rpms with a timing light It should be the same on both engines. Timing plays a large part on how an engine accelerates (Revs). Perhaps the timing is different between the 2 engines. This will prove both are similar.
I also suggest swapping HWs rubber onto yours and I bet it will seem like a raped ape.

Ron

 

[blistovmhz]

Distributor advance is controlled by the PCM. Put a chalk line on the vibration damper 1 inch in front and behind and on TDC. Check the timing at idle,1000 and 2000 rpms with a timing light It should be the same on both engines. Timing plays a large part on how an engine accelerates (Revs). Perhaps the timing is different between the 2 engines. This will prove both are similar.
I also suggest swapping HWs rubber onto yours and I bet it will seem like a raped ape.

Ron

What is "HWs rubber" ?