XJ Bogs with Throttle Wide Open

C

CpnJack

NAXJA Forum User
Location
Rio Rancho, NM
I've been chasing a problem for a while. Here's the details.

-94 xj 4.0 5spd 4.56 gears 210k miles
-symptoms: Runs fine until operating temp. At op temp consistently bogs at apprx 3-3.5k rpm (25 mph) in second gear with throttle wide open. Jeep pushes through bogging and eventual acclerates normally. Problem also occurs when turning in either direction even with light throttle. Occasionally problem gets very bad and vehicle comes to a stop but does not die. Goes nowhere with pedal to the floor but if I turn it off and restart everything is 'fine' again for a few minutes.

Attempted fixes and diagnostic
-No engine codes.
-Fuel pump (car had set for long time and I know the gas was bad), Fuel Filter, drop and clean gas tank, another fuel pump. Fuel treatment. No Luck
-Plugs & wires for the heck of it. No Luck
-Tested TPS. Showed 3.7v at open throttle so replaced. No Luck. Guessing bad test reading was operator error.
-Replaced fuel regulator. No Luck
-Pulled injectors. Cleaned tested w/12v and all clicked open each sounded the same.
-Installed new Crank Position Sensor (CPS)
-Waited for problem to occur. Disconnected O2 senser and became a little worse. Was hoping that since problem only happens at op temp that unplugging O2 may trigger open loop reading. Didn't run w/out o2 sensor long. Just enough to see that it only made minimal difference.
-Inspected hoses to MAP


Any suggestions will be much appreciated.
 
Operating temp would seem to indicate a problem when it enters Closed Loop, at WOT you are back to Open Loop--both of those pointing to o2 sensor.

Now, when you are corning things are flexing--I say it would be a good idea to inspect the o2 harness for bare/melted spots.
 
Try and replace the temp sender at the thermostat housing. I had almost the same exact problem, that was the fix.
 
I agree, sounds like temp sensor or upstream O2. My bet is that it bogs when you turn due to the extra load on the engine from the P/S pump, not a wiring harness issue, though.

Had the same exact issue on my 98 when I got it. It would bog and lurch and choke like it was running out of gas. I never had the balls to floor it though, it had no rear driveshaft and was in 4x4 driving on the front axle so I didn't want it bogging and lurching that hard and possibly stretching or breaking the t-case chain.
 
Thanks for the ideas. I'll check out the O2, temp sensor, and look for bare wires. Question though, didn't I test the O2 sensor already when I disconnected it and the problem remained roughly the same?
 
Not really - basically all you did was make the ECU run blind. It's about like cutting off your arm to find out if your thumb was broken... all you get is more pain, can't determine much.

You should be able to get more info from it if you put a code reader on it - though it's more difficult to find OBD-I code readers these days. Try putting an analog voltmeter on the signal line from the O2 sensor to the ECU, if you see a voltage that varies anywhere from a few times a second to every few seconds, it's at least semi-operational.

Unplugging it and having it operate the same actually makes me think it may be dead - mine was throwing two codes, O2 heater malfunction and O2 signal stuck high. Unplugging the sensor would result in the O2 signal either being stuck high or stuck low.
 
If there is a problem with the o2 sensor, and you are running Open Loop, and you unplug the o2 sensor that will put you into Open Loop--lack of change in that instance wouldn't mean anything.
 
Checked the CTS. It tested fine but reminded me of a sketchy splice I did in the wiring from the CTS a few years ago. There was some scale on the wire kind of like on a battery terminal. I'm thinking the splice may be bouncing loose and causing inaccurate readings. Going to mend the wires, test the connection, and take it for a drive.
If that doesn't work then I'll test the o2 sensor.
 
dont know if this will help but i had the same problem with my 90 and it was the a pluged cat. It would idle fine and when i would drive a lil it would start bogging down. Cut it out and she runs like a bat out of hell.
 
LLS at Kastein. Disco'ing the CTS will keep it in open loop. I would think a clogged cat would progressively get worse as the rpms increase--wouldn't think you rev through it. It sounds like it's starving for fuel or getting the wrong amount. Those rpms are where it transitions from CL to OL below WOT. . . maybe vacuum leak/dirty/clogged MAP line?
 
Tested o2 per
http://www.naxja.org/forum/showthread.php?t=1012701#post244627859. Only I used a digi VM because that's what I have but post calls for analog. The voltage ranged from .1 to .3 volts with engine at varying RPMs. Don't know if the digi VM nulifies the test but figure I've thrown enough sensors at the problem so far that running to the parts store for an o2 only makes sense. :doh:

I might have to throw in the towel if that doesn't work.
 
Analog is specified because it allows you to see a varying signal better, while digital updates the display too slow for you to see the variation.

You've got 210k miles on it, if you hadn't replaced the upstream O2 sensor yet, you were probably overdue for one anyways, so don't sweat it. O2 sensors are one of those things you should replace at 80-100k whether they need it or not, consider it the same type of thinking as a timing belt. I just replaced one at ~215k and saw an immediate 2-3mpg boost to my MJ's gas mileage, and another at ~150k that made my XJ less like a rodeo and more like driving a car again.
 
Well, traded the xj in on a 4 door JK....yeah right. Any how, installed the o2 sensor and at first it ran perfect, but then it always ran fine when the engine was cold. At op temp still had bogging and lunging at same RPMs as before but seemed to be less exagerated. Parked it and disconnected the battery for the night to reset the computer :dunno:.

Figure I'll check vacuum hoses one more time this weekend and then take it to a mechanic next week. Think I'm about $400 in on what is prob a $60 fix once it is diagnosed.

Thanks for all of the suggestions. I'll let ya'll know what it was when the mech figures it out for me.
 
Plugged cat.
 
hadn't been ignoring the suggestions to check the Cat just wasn't sure how to do it. Did a little research and looks like easiest most accurate way to test would be with a pressure gauge screwed into the 'upstream' o2 sensor (no downstream sensor on 94 xj anyway). Going to see what AutoZone has to loan.

Any preferences b/w testing manifold vacuum pressure vs back pressure? Better or easier ways to diagnose the cat?

If I can't get a pressure gauge that doesn't require tapping the exhaust I'm thinking I might just cut the exhaust off after the o2 sensor. Bad idea? I don't weld so would have to take to a shop anyway to reinstal whether or not the cat is bad.
 
You could do that. With a 94 I think you may be lucky, check if there is a 4-bolt flange in the exhaust pipe right before the cat. If there is, remove or cut those bolts (depending on how badly seized they are) and drop the cat+muffler+tailpipe, see how it does, then bolt it back up.
 
Checked it, no bolts. The exhaust is all welded. Guess I'll just cut it then use those muffler brackets to hold it back together till I get someone to weld it.
 
Tested the MAP before cutting the exhaust. MAP tested at 3.96v but spec is 5v at op temp. Swapped in a new MAP drove it and problem is still there. tested the new MAP and it also showed 3.9v. Was able to exchange the MAP for a MAT (or IAC). The orig MAT was testing at 1890 ohms but spec is "less than 4000 ohm" at op temp. Still no improvement with new MAT.

After everything that had been tried, figured all that is left is PCM and Cat.

I cut a 1 inch by 3 inch hole in the pipe immediately before the cat. There is significant exh flowing through the hole and probably about the same amount making it out the end of the tailpipe. Problem still exists. Does this rule out a clogged cat?

list of things that have been checked:
TPS-replaced
CPS (crank)-replaced
MAP-replaced but old reinstalled. MAP hose checked
CTS-Tested fine
IAC/MAT-replaced
o2-replaced
Fuel pump-replaced
Fuel filter-replaced
Plugs/wires-replaced
Fuel Injectors-checked
Air Filter-replaced
fuel pressure regulator-replaced
Cat-1x3" Hole cut in front to bypass

Runs fine under Op Temp. Runs fine at op temp for a few moments if I cut the engine and restart. Any other ideas?
 
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