My 95 jimmy runs fine in Open loop. (Actron scanner). Problem is it starts running rough as soon as it goes into Closed. Fuel pressure readings are normal. Entire fuel system was replaced including the fuel.

Here's the readings I'm getting:

Cold/Off

MAP - 28.9
coolant - 37 (air temp 35)
IAT - 41
O2 1 - .460
O2 2 - .155

Open Loop at start up

MAP - 10.6
O2 1 - .480 and very slowly rising
O2 2 - .140 and very slowly falling
TPS - .4
Calc Load - 10.6
IGN ADV - -20.5
vacuum - 17-19

Closed Loop initially

MAP - 13-14
coolant - 178
IAT - 48
O2 1 - .725
O2 2 - .125
Short Term Fuel Trim - -9.4 (max but often less than throughout test)
Long Term Fuel Trim - -16.4 (throughout test)
TPS - 0-.4 (throughout test)
Calc Load - 4.7
IGN ADV - -26
vacuum - 14-15

Closed Loop when warmed up

MAP - 13-14
coolant - 189
IAT - 48
O2 1 - .900
O2 2 - .130
Calc Load - 4.7
IGN ADV - -26
vacuum - 14-15

Not I'm not too familiar with these readings but do know a little about them. Those O2 readings sure look funky to me.

Thanks for your help.

 

she looks rich, check egr valve for stuck open or sticky lazy. rreplaced fuel...? spider? reg? feed tubes is intake? those tubes dont like to be touched may have cracked one very common. that front 02 sensor could be poop too. i wish i was there to see more alot of things can cause this. if it really does run well cold, i mean totally normal and faults when warm id lean towards front 02 sensor myself but again im not there to expierience

 

Yeah. Check EGR first. Stuck open {or sticky} EGR causes false rich reading {due to low oxygen} so PCM leans it out too much. Also possible contaminated O2 sensor on 1.

on second thought, I would start with replacing bank 1 sensor. Check EGR valve too but it cant hurt to check that engine off disparity and have a spare O2 on hand when you might need it

 

Like I said, the entire fuel system was replaced. The fuel lines were leaking last spring so I parked it. In the fall I pulled the tank off and found the sending unit had rotted and the tank had a hole in it. So, I replaced the tank, fuel pump, sending unit, fuel lines, filter, and regulator and it still wouldn't start. I then replaced the 2 year old (!) spider and the feeder lines and she fired right up, but, runs rough when in closed.

The question I have is the rear O2 sensor never really moves. Could that indicate a plugged catalytic converter? Would a plugged converter make the front O2 sensor slowly rise like it does because the gases are backing up?

I'm no mechanic but I'm too cheap to pay someone.

And one other thing. Don't believe the dealers and web sites that say certain pigtails don't fit a 1995 Jimmy. I replaced 2 that fit perfectly for a 1994 Jimmy. I'm gussing they put a 95 body on a 94 frame, or something like that. The 95 pigtails they wanted to sell me wouldn't fit if I tried. And the dealer told me I had to buy the wiring harness for $170. I found the correct pigtail at Oreilly's for $23.

Thanks again.

 

no and no. rear sensor sounds normal, cat cant tell by these figures. front sensor will be slow to rise until it is at full operationg temp. should fluctuate from high to low floating around .5 as a median but never!!!! resting. your front sensor indicates rich.


i just looked at ur readings again. man thats rich, check that egr valve again, if looks good put a front sensor in it a oem type not the universal tell the parts man that. said its been sitting... this thing doesnt have 2 year old fuel in it does it??? long shot but humor me

 

alright I'll check those. The fuel is new. The front sensor never fluctuates, just slowly rises. One other thing it did do once was revert back to Open Loop, after being warm, and then it ran fine... till it went back to Closed.

 

Just checked my 2002 Trailblazer just out of curiousity. When off/cold the front sensor reads around .450. When in Open Loop it rises as does the rear one. The rear one follows the front one; rises into the .600's. When in Closed the front sensor goes up and down like it should, correct? And the rear one follows the front one going up and down as it should. Do you think both sensors are bad on the Jimmy?

Thanks.

 

Buy one. Put it in front.. or swap the two. And check your wiring while you are at it.

If the front one doesnt work, the back one wont act right, either.

Here's an idea. Disconnect and block EGR and see what happens. Remember.. too much EGR gives a false rich reading. And IIRC egr doesnt operate until closed loop.

 

yeah hes right front one only one that affects performance, rear is to test cataylist function.your 02 sounds proper

 

ok. I went to take off the front one which wouldn't budge. After some time it finally did but the threaded ring came with it. The weld broke. I then had to beat on the sensor with a chisel to separate the ring from the snesor. (I think it had failed anyway since I couldn't get an ohms reading from it.) Needless to say I need a new sensor anyway, and a new crossover pipe. I'll write back once that is finished.