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Engine ground Ohm's while running

M

markaboo929

NAXJA Forum User
Location
Mooresville,NC
OK I am chasing a rough idle,rich running issue.I can see my O2 Sensor reading on my snap on mt2500.It shows rich while driving around .75 or so and does not switch until at idle or low throttle.It bucks/surges while slight throttle as well.I have changed every sensor with test ones including the ecm...no change.Also swapped in a different set of injectors,same thing.I did an Ohms test on the grounds and came up with around 2.0 or less on the grounding points on the engine.The one on the dipstick bolt is 0.00.I did measure it while running and it was around 20.0 ohms.I am really at a brick wall with this issue.I have been a master ASE Tech for 28 years now and can't think of anything else that could be the cause.I even disconnected the Alt due to it has the green/orange wire that feeds the injectors,coil,O2 heaters.Replaced fuel pump and regulator as well.Strange thing is it happened after I had a front end collision...yes I checked the wiring to make sure it did not short anything,it was only the header panel/front light harness.Any ideas?
 
#1 You can not use an Ohm meter to measure resistance in an ACTIVE circuit.

Resistance is a static measurment, and the Ohm meter measures the voltage drop across the resistance by supplying a voltage and reading the drop.
Because the circuit you are reading is active, and there is a voltage drop on the wire, your reading when running is meaningless.

I would start with the ISAC motor.
 
markaboo929 said:
OK I am chasing a rough idle,rich running issue.I can see my O2 Sensor reading on my snap on mt2500.It shows rich while driving around .75 or so and does not switch until at idle or low throttle.It bucks/surges while slight throttle as well.I have changed every sensor with test ones including the ecm...no change.Also swapped in a different set of injectors,same thing.I did an Ohms test on the grounds and came up with around 2.0 or less on the grounding points on the engine.The one on the dipstick bolt is 0.00.I did measure it while running and it was around 20.0 ohms.I am really at a brick wall with this issue.I have been a master ASE Tech for 28 years now and can't think of anything else that could be the cause.I even disconnected the Alt due to it has the green/orange wire that feeds the injectors,coil,O2 heaters.Replaced fuel pump and regulator as well.Strange thing is it happened after I had a front end collision...yes I checked the wiring to make sure it did not short anything,it was only the header panel/front light harness.Any ideas?
Click to expand...


What year is it?
 
Note from DJ
Have you checked for a vacuum leak between your intake manifold and the vacuum storage canister (football or egg) located behind the front bumper?
See Post #3 by cruiser54 for location:
http://www.cherokeeforum.com/f2/grill-clips-egg-shaped-thing-inside-bumper-198179/
A vacuum leak will cause the Jeep to run rich and there have been past posts regarding the plastic line braking; especially under the battery box.

Another question did you get neat and wash down the engine compartment after the removal of your damaged front end parts; before the installation of the replacement parts?
 
I did replace the vac canister..it was cracked in the accident.I did not wash any thing away it did not have that much debris.It does however surge when slight cruising and the rpms bounce up and down about 100 rpm or so.When you are driving it on the highway it just falls on its face.Cat has been gutted due to it glowing a dull red...being overworked by rich running.Floor it and it takes of like nobodys buisness.Stock 3.55 gears on 235/75 15 tires...next step is to go 4.10 gears:) Should have NO problem cruising with those gears:cool:
 
T Body mounted...reads 14.1-15.2 ish at idle.I checked for vac leaks everywhere and just replaced the manifold header due to the usual cracking at weld joints.
 
what I have noticed while it bucks or surges while cruising,is that the sort term fuel trim bounces between say -15.0 to 0 Snap On MT2500 installe.Just stumped really.
 
More times than not, the vacuum leak isn't the bottle, it is in the line under the battery.

Is the ground from the head to the fire wall in good shape?
 
I tried driving it with the line to the vac/cruise control capped at the manifold...same thing,also checked it for leaks.Ground strap from back of head bolt/stud to firewall measured around 2.0 Ohms as well.Almost ready to go 4.2 HEI Dist. and Motorcraft 2100/2150 or Holley 500 2 barrel at this point.Of course I would use the manual controller for the AW4.I just hope I figure it out,it is really driving my nutz!If all end to go carb then so be it.

http://www.ebay.com/itm/231308778862?_trksid=p2055119.m1438.l2649&ssPageName=STRK:MEBIDX:IT

http://www.ebay.com/itm/Rebuilt-Hol...Parts_Accessories&hash=item339377140c&vxp=mtr

Except it would be an electric choke.
 
If a vacuum leak is small and constant the PCM adjusts for them. My bottle came disconnected after a trail run and I didn't notice any change until I tried using the HVAC controls.

I still think you should take a look at the IAC motor for grins.
It's a motor with a worm gear that drives a slug for trim on the TB.

When mine went out, I looked though the yard's collection of XJs. 50% of them were stripped out and the worm gear couldn't reliably set the slug where the PCM wanted it. The symptom mine showed was erratic idle and sometimes low speed cruise.. Some times so rich it stunk of eggs, and sometimes so thin it wouldn't idle at all. Pretty much what you describe. I just about went bald looking for it.
 
Last edited:
2 ohms seems like a lot to me, I usually shoot for zero ohms and manage to live with 0.5 ohms on occasion.

I'd run a hefty ground from the AC bracket to the battery negative and see what happens. Bypass the head to firewall strap and all the body panels in between.

If the engine temp. sensor is out of spec it will run rich.

If the battery temp. sensor is out of spec it will run rich.

Don't ask me how, why or what for, but when my battery temp. sensor went bad, my mileage went to chit and I had a chronic rich condition. I have no idea why the battery temp. sensor would affect fuel trim, but it does.

TPS is one of the primary inputs for timing, bad O2 sensors can cause a lean condition and cause buck as can the wrong timing.

You really have to test the wiring and connectors as much as the sensors.
 
Ok I have tried a different IAC motor.There is a factory ground strap besides the head to firewall one...actually 2 of them that go from the battery to the coil bracket.I will try to look at the battery temp sensor...it is around 130-ish running?
All sensors have been swapped I mean ALL 'cept for batt temp.
 
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