CHT test, jumpy reading Options

[Java2570]

Hi - I've got a 74 2.0L with stock EFI and it's developed a lean cold idle condition that I've been trying to diagnose. I've procured a backup AAR to try out as mine "seems" to work but I need to test it further. My other thought is the CHT sensor is bad...in taking a reading at the plug jack, I'm getting a jumpy reading of anywhere between 1.5 and 7 ohms. This is at ambient temp of about 85 F in my garage. I have not taken a warm temp yet to see how it goes. I'm wondering if there is a short or bad spot in the wires somewhere that is causing the fluctuation or if this is a normal scenario?
Basically, the car has a low idle at cold start and sometimes tries to die in the process. I also can't adjust the idle with the air bleed screw and ECU mix has not much effect. Could a fluctuation in the CHT cause an issue such as this or should I look elsewhere?
All vacuum lines are new, including intake boots. I've also replaced the throttle body gasket and intake gaskets. Decel valve has been unhooked/hooked up with discernable difference. I have not tested vacuum on MPS but doesn't this typically cause a richness problem? TPS has not been touched but would that cause an issue?
I know it could cause the cold idle problem but what about the lack of idle adjustment?
It's possible the AAR is not functioning correctly and that's on the list to unhook and test for power, etc. I did check for suction at cold idle and it was sucking.... not sure about after warmup. I have not looked at the oil filler gasket yet or the PCV valve.
I've been spending a lot of time on B. Anders site looking at possible causes, etc.
Am I missing anything in this equation? I'm trying to not get frustrated and give up and when I do get too annoyed I just take a break. Let me know what you all think.

[r_towle]

time for a new CHT

[Java2570]

Also, when it states "unplug and test resistance to ground" what exactly does that mean? It's been awhile since I've done any electrical stuff and I want to make sure I'm doing what I'm supposed to! Hey, there's no shame in asking the dumb questions once in a while right? (IMG:style_emoticons/default/blink.gif)

[r_towle]

Set meter to ohms.
Put red lead on the lead coming from the CHT.
Put black lead on a solid ground.
Lead from CHT needs to be unplugged from the ECU or you will get false readings.

Take reading when cold.
Plug it back in...warm up the car...turn off car.
Umplug the CHT.
Take reading again when warm.


Rich

[Java2570]

Set meter to ohms.
Put red lead on the lead coming from the CHT.
Put black lead on a solid ground.
Lead from CHT needs to be unplugged from the ECU or you will get false readings.

Take reading when cold.
Plug it back in...warm up the car...turn off car.
Umplug the CHT.
Take reading again when warm.


Rich

Ok, gotcha; I just did the cold reading to ground and it's all over the place, especially when I moved the wire around. Will do a warm test in a while but I think it looks like I need to order a new CHT! Thanks Rich

[r_towle]

Dont bother...you need a new CHT.
It should be steady no matter what you do to the wire.

Rich

[Java2570]

Dont bother...you need a new CHT.
It should be steady no matter what you do to the wire.

Rich

Yep, my thought as well! At least I feel like I'm making progress and I'm getting pretty good at taking things apart on it!

[Java2570]

Dont bother...you need a new CHT.
It should be steady no matter what you do to the wire.

Rich

Yep, my thought as well! At least I feel like I'm making progress and I'm getting pretty good at taking things apart on it!

I do have an interesting thing to report though: I have the black lead coming out of the CHT but also a pair of red/black wires coming out of the CHT recess that hooks up to a white plastic connector and proceeds to go...well somewhere! What the heck?
Is this normal? Or what could it be? Puzzled....
At any rate, the black lead is either giving me zero or once in a while a bizarre reading that's too high. I will replace it but I want to know what the other pair of wires does.

[jcd914]

I do have an interesting thing to report though: I have the black lead coming out of the CHT but also a pair of red/black wires coming out of the CHT recess that hooks up to a white plastic connector and proceeds to go...well somewhere! What the heck?
Is this normal? Or what could it be? Puzzled....
At any rate, the black lead is either giving me zero or once in a while a bizarre reading that's too high. I will replace it but I want to know what the other pair of wires does.

Cylinder head temperature sensor (thermocouple) for an after market cylinder head temperature gauge. Sensor is a large ring connector under the #3 sparkplug.

Jim

[SLITS]

Two wire connector is for a Cylinder Head Temperature Gauge.

It has a ring terminal at the end that is usually placed under the spark plug. If they placed it under the CHT sensor, it might cause problems as it is a thermocouple. Based on temperature, it changes resistance.

[Java2570]

Two wire connector is for a Cylinder Head Temperature Gauge.

It has a ring terminal at the end that is usually placed under the spark plug. If they placed it under the CHT sensor, it might cause problems as it is a thermocouple. Based on temperature, it changes resistance.

D'oh...I just realized that it's the cylinder head temp gauge when I was out walking the dog! I'll have to remove it from the sensor location; how difficult is it to mount it correctly under the 3rd plug? I'll do some searching and find out more info about that.
I wonder if I'll ever stop being surprised by what I find on this car!! Thanks guys!

[Java2570]

Oh, so you just attach the ring probe to the head of sparkplug before threading in the head? Awww, I can do that! My question is: why would someone put it on the CHT
instead of on the plug?

[Tom]

They were uninformed as to the correct place to put it.
Tom