Low idle problems

After doing a search and not coming up with anything that really helped, I would like to ask about low idle. My 1990 has always had a bit of a rough idle/low idle when hot, but it has never dropped this low. In the past, the lowest I have seen it drop is around 450RPM at hot idle.. On cold start up, idle is as it is supposed to be, but lately since the weather has gotten a little colder and wetter, the hot idle will drop down to between 250-300RPM when I stop at a light or when I park. I'm not sure what's causing this. The jeep runs well otherwise, and has decent power and pickup for a 17, going on 18 year old engine.
The idle has never gotten so low as to die out, but now I'm wondering if it's just a matter of time now.

clean throttle body and iac. also check/adjust the tps with voltmeter.

Sorry to hijack!!!
I found this thread because i have the exact same problem... Its been like this for awhile now in my case. 90 RENIX 4.0 Automatic.
Everything is new etc.. TPS adjusted perfectly .. no other problems.
Jeep runs just fine with super strong acceleration. Ive tried three different IAC's (one of them is brand new) and about 3-4 different TPS's. Ive even had this problem with 3 different throttle body's.
After start up / while warming up the idle is perfect .. around ~750 rpm's (third line after the 0 on the tach)
When warmed up (im assuming after reaching closed loop) the idle is low... ~330 RPM's (first line after the 0 on the tach). I usually dont notice it untill i start driving / aka in gear. It runs fine when idling this low. Its very quiet \ i barely notice its running which i hear is very unusual for a 4.0.
The only reason why i want to fix it is because at night when running the lights, heater, and my secondary electric fan (taurus fan on low) im at the limits of what the alternator can put out at this RPM range. I have verified this visually (dimming lights) and with my digital multimeter measuring both amps and volts.
The problem does not exist when i give it a little gas to bring it up to ~750 RPM's. (where i feel it should be).
Yes the TB is clean ... all three of them i have tried are.
EGR valve is "newer" and also removed it and cleaned it (although it shouldnt be part of the problem without thier also being some rough running involved)
As i said above i have a new IAC and two other "known good" ones that i have tried.
4 different TPS's adjusted correctly exhibit the same issues.
New CTS (changed when i had the exhaust/intake manifolds removed... had the same problem before replacing it)
New Head Gasket (dont think it makes a difference)
Intake / exhaust manifold bolts are tight to spec.
No Vacuum Leaks.
All new RELAYS including the +b Latch relay.
Tried a "known good" MAP sensor .. same problem.
Tried a couple "known good" IAT sensors .. same problem.
CPS is about 6 months old now. (Shouldnt contribute to the problem)


As you can see im at the end of my rope.. everything has been replaced or swapped out or verified to be within spec with a multimeter.. but the problem is still here.
Does anyone have any advise? Is it possible that i should accept that my Jeep is unique, and fork over the cash for a HO alternator? Is it possible that my alternator is bad causing it to not put out enough amps at that low of RPM's (seems unlikely most are designed to run at higher rpms then 330).

Help and advise is appreciated.

Well, as far as I know, the temp sensors work in conjuction with the ECU to control Idle speed as the motor warms up...I know you said you changed the Coolant Temp Sensor, and the Intake Air Temp Sensors, as well as the IAC, but that doesn't mean the circuits are pronounced 100% functional. I can only suggest measuring the temp sensors right at the ECU input pins to see exactly what the ECU is seeing from these sensors...(from the 88 docs I have...I don't have any 1990 documentation)

CTS & MAT SENSOR TEMPERATURE-TO-RESISTANCE VALUE​

​

​

-40 (-40) .............................. 100,700

Probably the fastest check would be to warm it up to operating temp, shut off engine, and then quickly measure resistance between, sensor ground, (D3 at ECU) and CTS sensor input (C10 at ECU) and then the IAT sensor input (C8 at ECU) and compare your readings to the chart...( again this chart and pinout info from 1988 docs, but I think they are the same for a 1990)...

just my 2 cents...

Thanks for the information. I haven't had a chance to pull the ecu down to check the values at the ECU connections... although i have verified them at the sensors themselves, and they are all operating correctly according to the 90 FSM (basically the chart above).

However i was sitting at a 24 hour Mexican food drive in tonight waiting for my food, and i started doing some testing.

Please remember that these tests where done when stopped in park with a warm engine unless otherwise specified.
1.) As always, at idle I'm around ~330 RPM's
2.) If i give it a quick jolt of gas that makes it go above 2500 RPM's the idle goes down to exactly where it needs to be. ~750 RPM's
3.) If i put the Jeep into drive it drops down to a low idle again, even if i immediately put it back into park. Wont idle well again until i go back to step 2.
4.) If the idle is good, and i turn on my electric fan (either low or high doesn't matter) it slowly starts to drop down to the low idle. Also wont idle well again until i go back to step 2.
5.) In step 4, i turn on my electric fan the volt gauge show a small drop in voltage (going into low speed probably around 12v according the the gauge) larger drop into high speed (maybe as low as 10.5v). Either way it will climb back up to anywhere between 12v and 14v. While its climbing back up is when the idle goes down. (remember this happens when its idling well after step 2)

I know the gauge isn't very accurate, but it seems odd that the idle starts to drop as the voltage climbs back up. (the purpose of explaining 5) .. and also how it also drops just by putting it into gear.

What do you guys think of all that?
I hope it all makes since also...

BTT

Bump again .. anyone?

I'm experiencing the exact same symptoms, but unlike you, mine also dies randomly during deceleration and turning. This weekend is going to be an electrical replacement extravaganza, for me. I'll let you know how it goes for me.
Good luck,
Jon

I can tell you what fixed mine, though yours may be different.
Multiple IAC cleanings and piston oiling, new battery, tightening the intake manifold bolts and finding a loose sensor ground splice about 4-6 inches under the wrapping on the harness on the firewall drivers side. It was the TPS ground wire, it tested good, but wasn't.
The idle got better incrementally, the most bang for the buck was the intake bolt tightening and the TPS ground, with the new battery being a close second place.
I also cleaned out and slightly adjusted the throttle plate bypass passage. This was a trial and error type adjustment, 1/8 turn at a time, to split the difference between high idle cold and low idle warm.
Somewhere in the process I hit the magic combo and multiple problems disappeared, no more high rpm hot starts. Idles to 1200 momentarily at start up, idles around 650-700 cold and finally settles into a steady 500-550 idle (by the XJ tach). My XJ tach seems a bit stingy, I'm going to have to compare it to my electronic tach one of these days.

8Mud said:

I can tell you what fixed mine, though yours may be different.
Multiple IAC cleanings and piston oiling, new battery, tightening the intake manifold bolts and finding a loose sensor ground splice about 4-6 inches under the wrapping on the harness on the firewall drivers side. It was the TPS ground wire, it tested good, but wasn't.
The idle got better incrementally, the most bang for the buck was the intake bolt tightening and the TPS ground, with the new battery being a close second place.
I also cleaned out and slightly adjusted the throttle plate bypass passage. This was a trial and error type adjustment, 1/8 turn at a time, to split the difference between high idle cold and low idle warm.
Somewhere in the process I hit the magic combo and multiple problems disappeared, no more high rpm hot starts. Idles to 1200 momentarily at start up, idles around 650-700 cold and finally settles into a steady 500-550 idle (by the XJ tach). My XJ tach seems a bit stingy, I'm going to have to compare it to my electronic tach one of these days.

Click to expand...

Ive done everything here except for the bypass screw on the drivers side of the throttle body ( i believe we have had this discussion before lol). I guess thats the cure. All the throttle bodys i have ever ran into (for the RENIX) have that screw as tight as it can go.. so i guess i keep getting leary of touching it.

Muad'Dib said:

Ive done everything here except for the bypass screw on the drivers side of the throttle body ( i believe we have had this discussion before lol). I guess thats the cure. All the throttle bodys i have ever ran into (for the RENIX) have that screw as tight as it can go.. so i guess i keep getting leary of touching it.

Click to expand...

Well i officially tried the bypass screw and it did absoloutly nothing. All that it did is make a slight whistling noise.
For shits and giggles i went ahead and unscrewed it all the way, and well it still did nothing. the vacuum from the intake kept it closed and if you wiggled it you could make the whistling noise a little louder, but other then that it did nothing.

Does anyone else have any ideas? This is driving me nuts...

I adjusted the throttle stop on the throttle body on mine when I was trying to keep my high idle managable during trouble shooting for months, then when I finally solved the high idle problems I had suddenly had a low idle, caused by the prior throttle stop adjustment. Readjusting the throttle stop fixed the low idle on mine.

As I recall I also had a low idle caused by fixing the high idle TPS ground problem that was causing the high idle. That low idle was fixed by recalibrating the TPS which changed its idle voltage due to the ground wire harness fix on the TPS ground.

Only other low idle I ever had was cyclic running up and down every 6 seconds from 200 to 1,400 rpm, and that was a TPS connector that was installed backwards on the ECU side. Anyway, it was cyclic, not just a steady low idle.

Muad'Dib said:

Well i officially tried the bypass screw and it did absoloutly nothing. All that it did is make a slight whistling noise.
For shits and giggles i went ahead and unscrewed it all the way, and well it still did nothing. the vacuum from the intake kept it closed and if you wiggled it you could make the whistling noise a little louder, but other then that it did nothing.

Does anyone else have any ideas? This is driving me nuts...

Click to expand...

I have gotten about 200 RPM difference with the bypass on two different XJ's. Did you check the passage to make sure it was open and not plugged with gunk? Did you pull the screw adjuster all the way out and look at the end?

Adjusting the throttle stop can do the same thing, but don't forget to re adjust your TPS. If you have a hard time getting the right RPM's the bypass is a finer adjustment and a lot easier to trial and error adjust.

8Mud said:

I have gotten about 200 RPM difference with the bypass on two different XJ's. Did you check the passage to make sure it was open and not plugged with gunk? Did you pull the screw adjuster all the way out and look at the end?

Adjusting the throttle stop can do the same thing, but don't forget to re adjust your TPS. If you have a hard time getting the right RPM's the bypass is a finer adjustment and a lot easier to trial and error adjust.

Click to expand...

Ive tried both of these things, and yes the passage is clean... I really dont think its related to the TB at all... The idle is great when in open loop (when cold) Once it gets warm is when it starts going low. And then i go through all the steps that i listed in post #5.
The only things that i havent replaced (but have tested good via a multimeter) are the TPS, IAT, and MAP sensors... ive also tried swapping those parts out with other "known good" parts that also test good with a multimeter. Everything else is new or newer (not older then a year) and have tested good via multimeter.
Maybe i should just start throwing new parts at it again... $$$$

As an update, i think i have it fixed. I had the TB off again to put in a TB spacer that i had made for me:
http://www.jeepstrokers.com/forum/viewtopic.php?f=25&t=62

Anyway, while i had it breifly off, i went ahead and moved the throttle stop screw to open the butterfly a bit more then it should (or at least i thought it should). Anyway, i put everything back together and made sure the TPS was adjusted to spec. Now idle is great both cold and warm. Just a tad bit higher then normal, but satisfactory in my book.
Thanks for the help