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Renix Coolant Temp Sensor question..

stephenspann27

NAXJA Forum User
I'm in the process of troubleshooting an erratic idle...my '87 XJ AW4 will idle normal, then low, and then back to normal. If its hot, and sits a few minutes, when I crank it it will misfire for the first few minutes at idle ( I know it sounds like hot soak). However, it will have the erratic idle even when the engine is cold. I'm getting 20mpg... so nothing is terrible out of wack.

So far I've replace the MAP sensor, swapped throttle bodies, TPS, IAC, and tonight I replaced all the injectors with mustang #19's. Fuel Pressure is spot on.

While ago I crawled under it and unpluged the CST with the engine running, it didn't seem to make a difference pluged in vs, unplugged. However, i check the sensor with a multi meter and it read .547 with my multi meter set on "2k" According to this site: http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

it should be around 547 not.. .547.. The engine was probably around 80 degrees.. I had driven it an hour earlier.. and it had idled a few minutes.

Someone please let me know if that reading means its good.. it doesn't read "open" at least....

I noticed in the wiring that goes to the knock sensor, there is random bar wire hanging out before it goes into the sensor. The knock sensor only has two wires.. and it has two good wires going to it.. there just this 3rd bare wire hanging out of the harnes near it.. I also noticed I cut in the '02 sensor wiring.. and taht the O2 sensor plug is a little melted.. so I'll check that out soon.
 
Fix your oxygen sensor wiring/connector. May not be related but gotta fix this anyways to ensure proper O2 sensor operation.

Verifying proper resistance "swings" on the coolant temperature sensor between a cold and a hot engine is useful. If resistance is approx the same hot or cold, replace it.

You have addressed some of the usual suspects. How about EGR operation? Your vintage has an EGR valve and they are known to carbon up and can fubar your idle if it gets carboned up and allows some EGR operation (should be none at idle).

Another thing to consider is get a scan tool so you can see engine management data real time as there is no check engine light on your RENIX system.
 
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No they are not that hard to find. Look on evil bay(ebay) for a Snap-On MT2500 with the right cartridge for the Jeep. I have a snap on brick (MT2500) that I got to specifically read my Renix stuff. You are more then welcome to use it but the drive might hurt. I am in Flatonia, Tx a ways away. But the offer is there.
 
check your sensors by onnecting the meter to them and heating them up with a lighter, look for a resistace swing as the temp changes. don't forget the manifold air temp sensor,
 
My '87 had an erratic idle problem for years until last week. I decided to check the ground for the MAP sensor and found that I had 4 ohms to ground. It's best to check with an old school analog ohm meter instead of a digital meter. A lot of times the DMM can fool you when checking grounds through a wire harness. Anyway; the MAP, TPS, and MAT all have their grounds spliced together in the wiring harness that runs across the firewall to the C101 connector. All three of these sensors had a poor ground. I first found a splice for the fuel injectors and it was crimped and soldered. The splice for the sensors was only crimped. I soldered the crimp and then the resistance was down to 0 ohms and the engine now idles better than it ever has in the last 100K miles. I no longer have erratic idle speed or fast idle problems.
 
My '87 had an erratic idle problem for years until last week. I decided to check the ground for the MAP sensor and found that I had 4 ohms to ground. It's best to check with an old school analog ohm meter instead of a digital meter. A lot of times the DMM can fool you when checking grounds through a wire harness. Anyway; the MAP, TPS, and MAT all have their grounds spliced together in the wiring harness that runs across the firewall to the C101 connector. All three of these sensors had a poor ground. I first found a splice for the fuel injectors and it was crimped and soldered. The splice for the sensors was only crimped. I soldered the crimp and then the resistance was down to 0 ohms and the engine now idles better than it ever has in the last 100K miles. I no longer have erratic idle speed or fast idle problems.

That sounds encouraging is C101 the one that's up high on the firewall next to the brake booster? Or is it the one below the brake booster? I checked the Manifold Temperature Sensor, at lunch and it read .727, and the number when down as the motor got hotter, just like it should, I read the Coolant temp sensor and it read .247, which is a lower number (hotter reading) than when I checked it last night with a coolish engine, so they booth seem to be functioning ok.

I unplugged the following, and it caused no change in idle: Manifold Air Temp sensor, Coolant Temp Sensor, and Knock sensor..

Maybe its running in open loop? If it wasn't I don't think I'd be getting good mpg.. Also, the motor NEVER hesitate or miss fires when driving, its smooth as silk.. this is only an issue with idle.. it also.. never idles low enough to die.. it idles low.. then back up..

How exactly did you use your probes to ohm the harness grounds for the MAP? Stick one probe in the connector then the other probe where? To the pin on the C101 connector? Sorry I'm learning when it comes to using a multi meter..
 
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My '87 had an erratic idle problem for years until last week. I decided to check the ground for the MAP sensor and found that I had 4 ohms to ground. It's best to check with an old school analog ohm meter instead of a digital meter. A lot of times the DMM can fool you when checking grounds through a wire harness. Anyway; the MAP, TPS, and MAT all have their grounds spliced together in the wiring harness that runs across the firewall to the C101 connector. All three of these sensors had a poor ground. I first found a splice for the fuel injectors and it was crimped and soldered. The splice for the sensors was only crimped. I soldered the crimp and then the resistance was down to 0 ohms and the engine now idles better than it ever has in the last 100K miles. I no longer have erratic idle speed or fast idle problems.

Where in the wiring between the sensors and the C101 connector did you find the crimp that you soldered?
 
Not sure what they did but I cut my C101 out and soldered the wires one by one. Most of the splices in the harness are just crimped with a clip and a silver like duct tape over them.
 
I observed a strange phenomenon. I unplugged the connector from my new installed IAC valve and plugged it into a spare I have ( have two good used IAC's) with the engine running, the spare IAC that I was holding in my hand was pulsing and I could see it slowing (at first it seemed to not be moving at all just thumping in my hand) move in and out.. it finally moved out too far.. and seems too be trashed now.. anyways.. when the IAC was unplugged I actually had the best Idle I've ever seen. I then plugged the connector back into the IAC that was actually installed in the throttle body and the idle got a little less ideal but still good.. I killed the jeep.. and cranked it a few minutes later and the idle returned to crap.. IAC plugged in or not..
 
have you cheaked your spark? i know it kinda sounds unrelated but mine had poor idling, hard starting problems when hot or after it sat for long periods. i replaced the coil and it fired right up and has been running good since. might test the coil.
 
From the test my POS haynes manual says to conduct, it looks like even a marginal coil would pass that test.. I have some FSM documents.. is there a test that is more sensitive to a marginally good coil? I can take the coil off my '89 parts jeep but its a 4 hr drive away..
 
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