ive got a challenge for you guys ( troubleshoot)

Okay boys and girls, ive got a 99 xj, 4.0.


Im getting the occasional slow return to idle problem. occasionally it will just hang up about 1200 rpm for a few seconds after i let off the pedal, and shes not very consistent about it either. sometimes it just hangs there for a moment other times it seems to go up a hundred rpm before returning to normal.
otherwise she idles fine, with a slight amount of needle bob. ( i cant tell if its normal or bouncing between open and closed loop)


here's where im at:
Fuel pressure is good.
TB is so clean you could eat off it.
New IAC
New TPS (have two that test and work the same)
New Coolant temp sensor
New Up & newish downstream o2
New cap rotor and spark plugs
butterfly plate isint sticky
No vaccum leaks, i even went as far as to cap all the lines off and try taking it for a drive.
im relatively sure its not the throttle cables, it will rpm hang when i move the Throttle by hand and can see its not hanging up on the cables.
Brand new Cat last week.
its not leaking at the TB-to manifold surface. gasket is good.
after posting elsewhere i tried seafoaming. its cleaner (i guess) but still the same problem.
ECU reset.
took all the injectors out, cleaned and tested.
no Codes
TV cable set
oil and air filters up to date.
fuel filter is in the tank , and i really really dont want to drop the tank unless i have to.

tailpipe is a little black at the end, looks like its running rich, question is why perhaps?

i feel like im chasing a ghost. i just spent the last couple hours checking the resistance for the grounds for the upstream O2, wondering if maybe it was poorly grounded. less then an ohm, and 14V supply to the heater.


help?

Engine off, open the hood, and blip the throttle lever by hand. Does it snap back like it should? Engine on, repeat, any difference? That should give you a clue if its mechanical or electronical.

Lots of things can cause the throttle to drag, like rusty throttle line or malfunctioning cruise control, usually its an IAC problem though. Resetting the computer might fix that.

My 96 has recently had that issue. Next time it happens, try putting your foot under the throttle pedal and lift it from behind and see if the rpms drop. I removed the cable and lubricated it.

cables were removed and lubed with graphite spray.
can repeat problem by hand with cables removed.:banghead:

Are you able to test the MAP sensor and verify it's working correctly. Did you check manifold bolts? They can work loose.

spring could be bad

blade shaft might be rusty

Could be some crud built up in the shaft bore. This typically requires removing the shaft, and cleaning it out with a pipe cleaner and solvent (a small tube brush may also be used.)

If you're going to do this, dismount the throttle body and do it on the bench - no sense in pushing crud down the intake.

(This is uncommon, but I've seen it a couple of times over the years...)

If there's room in the shaft for crud to build up, it will leak badly when its cleaned. Might be a good time to replace the whole TB

it DOES close properly, but lets say it is full of crud... if the plate is closing properly ( visually tested, snaps shut) what effect is crud going to have?

IH392 said:

Are you able to test the MAP sensor and verify it's working correctly. Did you check manifold bolts? They can work loose.

Click to expand...

ill see if i can google how to do that now.

map tests good, was also swapped out with a known working one.

Cracked exhaust manifold near the up stream O2 sensor?

Bad wiring harness wire-connector at the IAC?

I would get a high impedance volt meter and check the O2 signal while it miss behaves!!! Or use an OBD-II data system to watch it live.

From post#1:

"tailpipe is a little black at the end, looks like its running rich, question is why perhaps?"

How to test O2 sensors:

http://www.naxja.org/forum/showthread.php?t=1012701&highlight=oxygen+sensor+testing