• NAXJA is having its 18th annual March Membership Drive!!!
    Everyone who joins or renews during March will be entered into a drawing!
    More Information - Join/Renew
  • 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.

Coolant temp sensor renix idle problems?

Precious88xj

NAXJA Forum User
Location
Los angeles
1988 Jeep xj 4.0 4x4 aw4

So I recently swapped in a high output motor and forgot to install the coolant sensor that mounts to the motor on the renix. I kept my electrical system from the renix. Would that cause me a rough idle I don’t have it connected?
Also I won’t be able to install it to it’s original location can I adapt it to the water outlet where the temp sensor goes located for the H.O
 
order a thermostat housing from a 93 4.0 and put your temp sensor in there. your going to need to make a longer harness.

it would change timing and mixtures a bit when its missing.
 
1988 Jeep xj 4.0 4x4 aw4

So I recently swapped in a high output motor and forgot to install the coolant sensor that mounts to the motor on the renix. I kept my electrical system from the renix. Would that cause me a rough idle I don’t have it connected?
Also I won’t be able to install it to it’s original location can I adapt it to the water outlet where the temp sensor goes located for the H.O

It needs that sensor for sure.

Too hard to get the plug out of the block now.
 
1988 Jeep xj 4.0 4x4 aw4

So I recently swapped in a high output motor and forgot to install the coolant sensor that mounts to the motor on the renix. I kept my electrical system from the renix. Would that cause me a rough idle I don’t have it connected?
Also I won’t be able to install it to it’s original location can I adapt it to the water outlet where the temp sensor goes located for the H.O

Yes, it will cause idle problems. The PCM needs to know if the engine is hot or cold (among other things) to adjust fuel accordingly.

If your factory temp sensor will fit into an HO thermostat housing, I would simply cut the wires a few inches away from the connector and splice in some new wire. If you can solder, I would recommend that and some heat shrink tubing. If not, most parts stores sell butt connectors with a heat shrink plastic "jacket" that also work very well.
 
I am not sure, not convinced a missing CTS sensor will cause idle problems. Simple test is to connect one to the wiring, just do not install it in the block. Cool the sensor with cold water, then heat it with boiling water. That will tell you if it is affecting idle or not.

If the O2 sensor and IAT temp sensor are working I would think it could it idle fine (closed loop).

Not having a CTS can affect starting the jeep (probably hard to start, easy to flood sometimes)!!!!

And it can waste gas!!!

Old-Man had a bad one, it was telling the ECU the temp was 30F or something all the time, and his problem was the ECU would not switch to closed loop, and it stayed open loop and drank gas big time, but he did not mention an idle issue. I think it was Renix. I think the ECU will assume an acoolant temp, guess, and then use the O2 sensor to control fuel feed.

IF the O2 sensor is having issues, fuel injectors have issues, or a dozen other things, then that is the probable cause of the idle issue.
 
I am not sure, not convinced a missing CTS sensor will cause idle problems. Simple test is to connect one to the wiring, just do not install it in the block. Cool the sensor with cold water, then heat it with boiling water. That will tell you if it is affecting idle or not.

If the O2 sensor and IAT temp sensor are working I would think it could it idle fine (closed loop).

Not having a CTS can affect starting the jeep (probably hard to start, easy to flood sometimes)!!!!

And it can waste gas!!!

Old-Man had a bad one, it was telling the ECU the temp was 30F or something all the time, and his problem was the ECU would not switch to closed loop, and it stayed open loop and drank gas big time, but he did not mention an idle issue. I think it was Renix. I think the ECU will assume an acoolant temp, guess, and then use the O2 sensor to control fuel feed.

IF the O2 sensor is having issues, fuel injectors have issues, or a dozen other things, then that is the probable cause of the idle issue.

I'll guarantee a missing CTS will cause problems.
 
Flores helped me diagnose a bad CTS: It idled great, ran great, never went into closed loop, could not pass emission testing.
 
I'll guarantee a missing CTS will cause problems.

I never said it would not cause problems, but I still say it will not cause a bad idle/problem if the other sensors and hardware are working!!!

"Coolant temp sensor renix idle problems?" is the title of the OP post.

But I agree with you, it will cause many other issues. For one it controls the initial fuel feed rate when cranking on start up!!! Because at that time it has no O2 sensor data!!! It is critical for start up fuel rates!!!
 
Flores helped me diagnose a bad CTS: It idled great, ran great, never went into closed loop, could not pass emission testing.

Well that confirms it. I am pretty sure old_man had the exact same story, his was sucking up fuel like a mad dog as the CTS was passing out bad coolant temp data to the ECU, claiming the block was always super cold, or something like that.

Depending on the bad data, random change in the reading, or always reading cold, warm or hot, would likely create different issues. One might be hard starting when cold, the other starting when hot. IMHO. But in Old_mans case it would not go to closed loop either, due to bad CTS, thus it wasted gas big time.

So a missing CTS needs to be fixed!!! But doing so fixes other problems, not a wandering idle!!!
 
order a thermostat housing from a 93 4.0 and put your temp sensor in there. your going to need to make a longer harness.

it would change timing and mixtures a bit when its missing.


93' thermostat housing will fit the 90 renix?
Also isn't it better to have the CTS in the block where the water has not had a chance to cool before going into the radiator?
 
In the head is better, you can see the coolant temp before the T-stat opens, if it opens. In the T-Stat housing, you do not see the coolant temp tell the T-stat opens if it opens. I Think the HOs had it in the water pump outlet feed to the heater, which bypasses the T-stat. That would see the hottest water in the entire system, whether the T-stat is open or not
 
Mine is in the side of the block, miserable to get to. Though it would be good to have it in the thermostat housing, that way it could be changed without loosing too much coolant. Do you think the later years housing would fit the renix?
 
You have three coolant temp sensors. One in the radiator drivers side, on on the top rear of the head, and the one you mentioned. One is a temp-switch to cycle the E-fan, one for the dash gauge (on the rear of the head drives side) last goes to the ECU for fuel and air trim
 
You have three coolant temp sensors. One in the radiator drivers side, on on the top rear of the head, and the one you mentioned. One is a temp-switch to cycle the E-fan, one for the dash gauge (on the rear of the head drives side) last goes to the ECU for fuel and air trim

The one in the radiator is a fan SWITCH.
The one in the block is a coolant temp SENSOR.
The one in the back of the head is a temp gauge SENDER.
 
Also note the Temp gauge sender comes in two flavors, and idiot light switch, and a temp gauge variable resistor/sensor
 
Back
Top