Still hunting an intermittent low idle on my ‘89 Laredo and need some help! Everything is stock except 2” budget lift. Very clean and well maintained. 209K miles.

Symptoms:

-My idle will go down into the 500’s in gear at a stop and the REM unit will show my IAC signal closing (<) and go all the way down to 0… the only time I get an open signal (>) to the IAC reading on the REM is when my A/C is on or if I punch the throttle and let off and the Idle starts stumbling it will also send open signal for a few seconds.

-my idle tends to hunt slightly up and down kind of like an old man snoring when in gear and especially right after it goes from open loop into closed loop… The idle is more consistent with the a/c on (less hunting) and maintains a slightly higher idle of 600-650 in drive…

-It idles perfect in open loop but in closed loop it starts going down and if I punch the throttle and let off, it starts to stumble and then catches itself and idle comes back up to 500-600 range (note that the engine never actually stalls out though and the REM will open IAC for a couple seconds and then it goes back to sending closed signals even at low idle of 550 rpm range)

-When I’m at a stop my battery volts will go down to 12.5 on the REM and my headlights will dim with A/C on. If I lightly hold the throttle to idle around 750 the battery volts raise back up perfect between 13.5-14 volts. Cursing down road with all accessories on and blower motor on high my volts stay around 13.5-14 so I don’t think it’s my alternator.

-Idle is slightly better in park and neutral (650-700 rpm’s) REM still sending closed IAC signals in P/N

-Frustrating part is it happens intermittently, sometimes I’ll come up to a stop and my idle acts normal more so after engine is warm and I’ve been driving on highway, however not always the case, sometimes coming off highway at full operating temp I’ll get the low idle syndrome.. 🤨

Things I’ve done to address the issue:

-IAC is new, cleaned IAC inlet and throttle body

-tps is set correctly .83 and 17%

-new 02 sensor (Bosch) swings normal

-no noticeable vacuum leaks, I have tightened all manifold bolts, inspected vacuum lines

-cleaned / refreshed all grounds and check for resistance on sensor grounds

-Throttle plate had been tampered with by mistake however I have reset it per cruisers tips using a .003 feeler gauge to get it as close to being closed as possible without bulging into throttle body.

-I checked my egr valve today and it has movement when I rev the engine. However I tried to push in to see if idle lowers but couldn’t get it to move by pressing with a screw driver. May need to check this out more?

Could this be a fuel pump issue? Maybe fuel pump is not sending consistent pressure at idle? Original fuel pump

What I don’t understand is why the ECU keeps telling my IAC to close even when the idle is super low! Makes no sense…

I’m determined to figure this out. Thanks in advance for any help / advice

 

How quickly is the O2 sensor switching Rich to Lean?
Should be quite fast. Like less than a second one to the other.
Do you have voltage at the reddish orange wire on the O2 sensor when running?
Disconnect and plug the vacuum to the EGR and see if anything changes.
There's a possibility your EGR isn't fully closing, giving you an undetectable vacuum leak.

 

it seems to be switching, but sometimes it seems to be a little slow on the switching. I’ll revisit the rem readings. It will hang up on rich sometimes then switch to lean and back and forth. It’s a Bosch. Maybe I’ll switch in a new NTK to see if that gives me quicker switching

im showing 14 volts to the 02 heater relay consistently

Also wanted to note that I recently fixed my TCC switch under my brake pedal and now that the TC is locking up again my LTFT have gone from 130 to 122.. seems like my jeep is using less fuel now. Not sure if this helps but wanted to add that.

how much movement should I see on the EGR? When I rev my engine to 1500 rpm’s I see the pintle going in and out very quickly but only a tiny bit. How far is this pintle supposed move in and out?

If it does end up being the EGR valve it sounds like I can’t even get a replacement for this part anymore.

 

But is that 14 volts getting to the O2 sensor?
Not sure on the EGR movement/travel.
Do you have emissions testing where you live?

 

How do I check this again? I have a volt meter set it on 20V setting?

Do I backprobe the orange wire connector with the red lead? But where do I put the black lead from the multi meter?

no I don’t have to get inspection anymore. Live in NC


Do I put the red lead into “A” and black lead into “B” or “C”?

 

Red lead to red wire. Black lead to black wire with meter on DC volts.
Better yet, Red lead to red wire and black lead to known good ground.

 

looks like I’m getting close to 14 volts

@cruiser54 so my swing happens at idle from 1-4.92 volts back and forth quickly but it also swings that wide when cruising. Should I be seeing the same range of swing while cruising as when idling? I’ve read on some threads where it mentions at cruising speed the switch range wouldn’t be as wide as when it is at idle.

 

You have a good range of swing. You have great voltage at the oxygen sensor heater.
Might check the resistance on the sensor ground. If that's not good you can have issues. I shoulda suggested that earlier...

 

which setting on this volt meter would I use to check the sensor grounds? im still new to using this thing. Thank you for your help.

 

Ohms scale 200

 

If I cap off the vacuum source going into the EGR solenoid on driver side, will this prevent the EGR from opening? If so the next time my idle is normal around the 750 I’m going to cap it off and see if that prevents the low idle situation from coming back. This should tell me if my EGR could be causing this annoying intermittent low idle.

Also I’m going to be doing the OHMS test on the sensor ground today as well.

 

Great plan!!

 

So plugging the vacuum line for the egr yielded same results. Had no effect on my idle.

I did the grounds test and getting 4.7 Ohms. Can you check my settings? I had the black lead in negative battery post.

my value was steady though. It did not fluctuate when I wiggled the harness.

is this enough resistance to cause my issues?

 

Absolutely.

 

shouldn’t the ohms be moving when I’m wiggling the harness?

I’m going to recheck that the ground at the dipstick tube is really good. And then recheck the sensor ground readings.

I’m not sure how comfortable I feel getting into the wiring harness, cutting those splices out and soldering. It sounds like the wires don’t always match up the same for each jeep. I’ve been reading your tip 6.