• Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

Low Idle??

CaliXJ

CaliXJ

NAXJA Forum User
Location
Oceanside, CA
Ok before i start... I have searched and searched. Now i have come up with other idle issues, but not LOW idle issues.
Info: 91 cherokee H.O. 4.0, AW-4, NP231 (92,000 mi.) Now the problem i'm having is that the idle is about 450-475 RPM. Now things i have done so far is plugs, wires, cap, rotor, ect. Now i've read about AIC and replaced. Checked TPS. It doesn't seem to idle rough But i did notice what seems like a surge in the idle. Meaning idle from 475 to 450 rpms. Now it's not much but usually after trips on highway. Any thoughts would help. Noob here for sure! Thanks in advance!
 
have you ever replaced your fuel filter? it is near the back along the drivers side of the rim of the jeep. i ran into low and rough idle issues and replaced mine (original with 110K miles on it!) and it poured out brown rusty gas.

after that, i ran bottle of lucas oil in a half tank of gas to clean out the injectors.

may not be the cause of your problem, but for $10 and 10min of your time to change, it is well worth it.

good luck.
 
Yep! Got that one covered when i first bought it about 3,000 mi ago. No change. Was hoping to get better that 17 mpg highway was the initial reasoning for this.
 
You say you replaced the IAC, but did you clean the throttle body with particular attention to the IAC passageway?

If you are running rich, that will reduce your idle speed. What thermostat are you running--195 degree? Is the engine warming to about 200-210 degrees indicated on the gauge? Any cooling system mods? Have you tested the coolant temperature sensor for the ECU? How about the o2 sensor?
 
joe_peters said:
You say you replaced the IAC, but did you clean the throttle body with particular attention to the IAC passageway?

If you are running rich, that will reduce your idle speed. What thermostat are you running--195 degree? Is the engine warming to about 200-210 degrees indicated on the gauge? Any cooling system mods? Have you tested the coolant temperature sensor for the ECU? How about the o2 sensor?
Click to expand...

During replacing the IAC i did clean passageway. (as best i could without removing TB) I am running a 195 thermo (small holes drilled as 6 and 12 to help burping) but did upgrade the fan to a zj fan as well as new OEM water pump. After warm up gauge reads about 195/200. IS the CTS for the ECU the one in the thermo housing or is there another one? How do i check for proper reading on this and the o2?
 
Renix the CTS for the ECU was in the block, driver's side, under the manifolds.

Yes, for the HO, the CTS was moved to the thermostat housing. Fully warmed up the sensor should read 200-300 ohms.

195/200 degrees is normal when running the ZJ fan.
 
joe_peters said:
Renix the CTS for the ECU was in the block, driver's side, under the manifolds.

Yes, for the HO, the CTS was moved to the thermostat housing. Fully warmed up the sensor should read 200-300 ohms.

195/200 degrees is normal when running the ZJ fan.
Click to expand...

So outside of just throwing parts at this... How do i check if my o2 is good? Could a bad o2 be the cause?
 
heyhar said:
Joe- on my '92, the T-stat housing sensor is the elec. fan switch. I think that location became the gauge sensor with OBDll, and then the fan became controlled by the ECU. I think?
Click to expand...

You may be right.

Renix the fan switch is in the radiator, the coolant temperature sensor is in the driver's side of the block, and the temperature gauge sender is in the rear of the head.

So, with the fan switch in the thermostat housing, are the CTS and the gauge sender in the same places as on the Renix?
 
Never had a Renix, don't know. But, I do know if I pull the plug on the T-stat housing on my '92, the electric fan starts. Put it back, it stops. Pull that plug on my '96, engine starts missing, stalls, throws a code, although I forget the number. Tried Junior's '95, fan runs, so must be OBDll thing.
 
heyhar said:
Never had a Renix, don't know. But, I do know if I pull the plug on the T-stat housing on my '92, the electric fan starts. Put it back, it stops. Pull that plug on my '96, engine starts missing, stalls, throws a code, although I forget the number. Tried Junior's '95, fan runs, so must be OBDll thing.
Click to expand...

Does the OBDII, 96+, have all three in one? CTS, temp sender, efan?
 
Now i have a few questions about the MAP sensor. I unplugged the vac line to it and the idle runs up to 1500 rpms or so. Plug it back and back down to 475. Now if i unplug the the connection plug it goes up to 750 at idle and put it in drive and it really wants to die but stays at idle about 400 or less. Does this point to a bad MAP? Also shoud there be any change in idle when the ICT sensor (aka MAT sensor) is unplugged? I have pulled the plug to this as well with no change in idle only CEL.
 
Vac line is in great shape. I went to JY today and picked up 2 MAP sensors and the vac line. I also grabbed a ICT sensor. Idle didn't change at all. Still idles at 450-500. While i was checking all other vac lines (engine running) I pulled the one that goes from the manifold to fuel pressure reg. and idle goes up to 750. Any thoughts on this would greatly help. This is really starting to drive me nuts!!!!:mad:
 
CaliXJ said:
Vac line is in great shape. I went to JY today and picked up 2 MAP sensors and the vac line. I also grabbed a ICT sensor. Idle didn't change at all. Still idles at 450-500. While i was checking all other vac lines (engine running) I pulled the one that goes from the manifold to fuel pressure reg. and idle goes up to 750. Any thoughts on this would greatly help. This is really starting to drive me nuts!!!!:mad:
Click to expand...

When you pulled the vacuum line did you plug the end with your thumb or did the idle go up due to a vacuum leak?

Removing the vacuum line would cause the 30/31 psi pressure to raise to 39 psi, if everything is working within specs.

Throw a fuel pressure gauge on it.
 
What is the normal idle speed of the cherokees? I have an 87 and it seems that it is ideling really low. My tachometer is mal functioning also. It goes below the 0, when it is ideling.
 
Normal idle is 700-750 rpm on the 4.0; if it is an auto the check is in DRIVE after the engine and transmission are fully warmed up, after about 20 minutes of operation.
 
You must log in or register to reply here.

Similar threads

B
High Idle Woes
Replies
7
Views
2K
cruiser54
C
I
High idle, smells like gas, running out of things to replace
Replies
2
Views
2K
Inverhyt
I
9
Renix lowish idle
Replies
10
Views
1K
90xj06
9
M
Oil pressure question
2
Replies
26
Views
3K
Money_Pit_Incoming
M
G
Low vacuum
Replies
6
Views
2K
GravyDavey
G
Back
Top