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What sensors come online when...

98XJSport

Destiny is the rising sun
Location
Western Maine
... the idle switches from the higher warm up idle to the lower normal idle? I have a 98 4.0 auto. Just trying to track down an acting up sensor that only happens after it switches idle mode.

Thanks
 
The O2 sensor is only active after appropriate warm up. Generally this means engine temp in excess of ~150*. Additionally, the O2 sensor must be at operating temp (~600*) before it can generate the rapidly changing signal the ECM needs to control the fuel ratio. With the correct engine temp, and an O2 signal to the ECM, the system goes into closed loop; meaning the ECM is using the O2 signal to constantly vary the fuel mixture between rich and lean. Before going into closed loop, the ECM uses predetermined fuel tables to control the fuel mixture, not the O2 sensor.
 
Is the above true if you start it up and the engine is already at operating temp?
 
Isn't the input from the MAP ignored until closed loop mode?
 
Saudade said:
Isn't the input from the MAP ignored until closed loop mode?

heck no. try this: pull the vacuum line of the MAP sensor and plug it. then start your jeep when its cold. it will run like balls. actually, dont try it just trust me

-Tim
 
So just thinking out loud, if the O2 sensor heater is bad, it won't be able to accurately control the air/fuel ratio when it first switches over, but it may start working properly once the sensor is fully heated?
 
The MAP is used during warm-up, as well as post warm-up. I think it's used at all times; but not sure if it's used during wide open throttle--the ECM usually has a special "program" for wide open throttle conditions. Generally, the ECM has an internal timer that holds off for ~1 minutte before allowing closed loop operation. So on a warm engine restart, with the coolant already above 150*f, two things must occur before going closed loop: The timer must elapse, and the O2 sensor must start sending a varying signal to the ECM. If the ECM doesn't see the O2 signal rapidly varying between approximately .2 and .7volts, it won't go into closed loop, but will continue to adjust fuel based on engine temp and MAP input. For the O2 sensor to operate, it must reach its operating temp of arround 600*f. I beleive all XJ's have O2's with a heater circuit to insure operating temp is maintained, but it works in conjunction with exhaust gas heating (some systems don't use an O2 heater circuit--my engine Chevy doesn't).
Why don't you share with us what your experiencing?
Basically, a cold start will have a richer fuel mixture than a warm, restart; this is a direct result of the engine and MAT sensors. Don't no about non-Renix ECMs, but early XJ's always had an idle "flare-up", even on a warm restart--the ECM always opens the AIC at engine shut down, in anticipation of the restart. This increased idle should be very short lived after a warm start, as the engine temp interacts with the ECM to control idle speed; warm engine=low idle--just like in the olden days of carbs with automatic chokes.
 
The long story short of what Ive got going on is low/rough idle for a min or so after startup and going into closed loop mode. It kicks, sputters, bucks, then just starts working perfectly normal until you shut if off again for any length of time over like 15 mins. No CEL, and it won't do it if I go WOT, telling me it's a sensor. Ive seen someone fix something similar with a new upstream sensor, and I really don't want to drop the cash unless Im sure that's it.
 
So, you're saying cold idle is too low? Runs OK after it warms up? If so, check out the engine temp sensor on the front of the engine--the one that feeds the ECM. The one on the back of the head feeds the temp sensor--at least that's where the are on the early 4.0's.
The ECM varies fuel mixture proportionately to the resistance of the temp sensor--it's the modern-day equivalent of a choke--cold engine=more fuel. As the engine warms up, the extra fuel is reduced. The temp sensor is also instrumental in setting cold idle speed. Cold engine=high idle.
Without cold enrichment and higher idle speed, you could experience stumble and hesitation until the engine warms up.
A bad heater in the O2 may result in intermittent operation of open loop, but generally only causes bad gas mileage, and eventually a bad catalytic converter; the same for a bad O2 sensor. And again, the O2 sensor is not in play for at least a minutte, or so after starting the engine, even for a warm start.
I'd check the temp sensor and its associated wiring. I'd also check the AIC for operation. Most systems, these days, have a shutdown routine that sets the AIC (idle control) to the correct position for the next start-up.
 
Its not the cold idle, its the warm idle. It starts to stumble and bog after running perfectly for about a minute after startup. Runs fine for a min or 2, then goes to hell. Will do that for a few minutes, then everything if normal again. It does this regardless of engine temperature.

I already went down the CTS route and replaced the front one, as well as a new TPS and MAP and cleaned the IAC housing.
 
98XJSport said:
Its not the cold idle, its the warm idle. It starts to stumble and bog after running perfectly for about a minute after startup. Runs fine for a min or 2, then goes to hell. Will do that for a few minutes, then everything if normal again. It does this regardless of engine temperature.

I already went down the CTS route and replaced the front one, as well as a new TPS and MAP and cleaned the IAC housing.

IF you were to say it ran like crap immediately after a hot restart, and then went away after a minute, I would be looking for a leaking fuel injector by doing a fuel pressure leakdown test. But you're not. I still would hook up a fuel pressure guage and see whats going on that way. If nothing jumped out at me, than I might tap with a screwdriver on the base of the distributor while it's running, crappy or otherwise, to see if I could induce a hesitation or stumble. This would indicate a problem with your primary ignition pickup-
No codes huh?
I would still be looking at your data stream on the scan tool to see if there was anything irregular.

Does the crappy running come and go during a drive cycle? Or is it only once after a restart and just for a minute?

Just came to me: Check your IAC system. Check the valve and use the special noid light for IAC systems and verify your pcm is sending both extend and retrieve signals consistently.
 
Last edited:
Nothing happens when messing with plugs, injectors, etc. Got new wires, plugs, cap last fall, then started doing this last winter. Haven't checked fuel pressure, only because if it was a problem there I would think that it would be happening all the time.

The best way to fix it I know is to hook it up to a scan tool, but I dislike dealerships.... It would easily be cheaper to throw a new O2 sensor on. Its got close to 170,000 on it. Plus with my little obd2 scanner I get a O2 heater circuit pending code.


Now my next problem is getting the sensor out...
 
hey,

when i first got my jeep, it would run like hell until it got warm, the problem that i finally found was the wires for the CPS was kinda long, and it had dangled onto the exhaust manifold and melted, causing it to short and run funky. crankshaft position sensors get dirty, too so maybe you could try cleaning that.

-Tim
 
The MAP is used during warm-up, as well as post warm-up. I think it's used at all times; but not sure if it's used during wide open throttle--the ECM usually has a special "program" for wide open throttle conditions. Generally, the ECM has an internal timer that holds off for ~1 minutte before allowing closed loop operation. So on a warm engine restart, with the coolant already above 150*f, two things must occur before going closed loop: The timer must elapse, and the O2 sensor must start sending a varying signal to the ECM. If the ECM doesn't see the O2 signal rapidly varying between approximately .2 and .7volts, it won't go into closed loop, but will continue to adjust fuel based on engine temp and MAP input. For the O2 sensor to operate, it must reach its operating temp of arround 600*f. I beleive all XJ's have O2's with a heater circuit to insure operating temp is maintained, but it works in conjunction with exhaust gas heating (some systems don't use an O2 heater circuit--my engine Chevy doesn't).
Why don't you share with us what your experiencing?
Basically, a cold start will have a richer fuel mixture than a warm, restart; this is a direct result of the engine and MAT sensors. Don't no about non-Renix ECMs, but early XJ's always had an idle "flare-up", even on a warm restart--the ECM always opens the AIC at engine shut down, in anticipation of the restart. This increased idle should be very short lived after a warm start, as the engine temp interacts with the ECM to control idle speed; warm engine=low idle--just like in the olden days of carbs with automatic chokes.
X2, though I don't think the O2 signal will vary rapidly till it actually enters closed loop, the reason it varies is because of the mix going from rich to lean to rich to lean quickly which only happens when it's using the O2 sensor instead of just the MAP/CTS and fuel trim tables.

Also, a bit of info on the way the O2 sensor heaters are powered - at least on a 96 (and I believe on all OBD-I and OBD-II XJs) the heaters for both the upstream and downstream O2 sensors are powered directly from the ASD relay, which also powers the ECU, alternator field coil, injectors, and a few other things I forget. If your downstream O2 sensor cable comes disconnected from the body and drags on the driveshaft, then shorts out, that fuse blows and you are dead in the water... ask me how I know! It's a 30 amp minifuse in the PDC.

Its not the cold idle, its the warm idle. It starts to stumble and bog after running perfectly for about a minute after startup. Runs fine for a min or 2, then goes to hell. Will do that for a few minutes, then everything if normal again. It does this regardless of engine temperature.

I already went down the CTS route and replaced the front one, as well as a new TPS and MAP and cleaned the IAC housing.
Mine did exactly this. My bet is that your upstream sensor is at the end of its life. Buy a new one on rockauto (cheaper than most parts chains), toss it in, call it a day. Especially because you're getting a code for it!

It's a 7/8 inch wrench, use a fully boxed wrench (or strong 7/8 flare/line wrench if you have one, or a specialized O2 sensor socket with a slot down the side) to crack it loose, then toss some pbblaster into the threads and get it out the rest of the way. Daub a decent amount of antiseize on the threads of the new one, you'll be glad you did when you change it next.
 
It was a malfunction of the upstream O2 sensor, in the heater cicuit. When the heater was on it would cause a backfeed, causing the mix to go way lean and the idle to go to hell. New sensor, and it hasnt acted up since been almost 4 years exactly.

And thanks for searching :cheers:
 
It's a 7/8 inch wrench, use a fully boxed wrench (or strong 7/8 flare/line wrench if you have one, or a specialized O2 sensor socket with a slot down the side) to crack it loose, then toss some pbblaster into the threads and get it out the rest of the way. Daub a decent amount of antiseize on the threads of the new one, you'll be glad you did when you change it next.

for future searchers:

I'd stay away from the O2 socket to break the old one free-- they tend to split at the slot with any real force-- use the 7/8th box end wrench and save the specialty socket for installation.

Also, if you decide to add anti-seize to an O2 sensor, try to source the correct stuff. The standard grade stuff will melt and run off, you want an anti-seize that contains itty-bitty glass beads that won't deteriorate with exhaust temps.
 
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