CHT sensor question

[Shun]

I know that the Cylinder Head Temperature sensor takes a dry temperature of the cylinder heads, and that the sensor then sends information to the ecu that is used to send information to the temperature gauge in the instrument cluster and turn on the radiator fans. now I believe my thermostat is stuck closed so I am going to check it this weekend, but I am still trying to figure out why my temperature gauge and radiator fans aren't working.

I have replaced the bypass tube that is mounted to the chassis over the radiator. I have replaced the fan assembly resistor pack and I have replaced the Cylinder Head Temperature sensor. Is there anything else along the way, before from the CHT to the ecu or from the ecu to fan/resistor? this really hasme more than a bit baffled, trying to find out what is allowing my car to run abnormally hot

Edit: before anyone asks, I had to replace the bypass tube because it cracked from either too much pressure or too much heat.

[Broke_Two_Foci]

did you check your fan relays??? easiest way to do this is turn on the a/c if the fans do not turn on then you have a blown fuse or bad relay.

[Shun]

they come on randomly with the A/C on. I perhaps the relay is bad. I did the fans always on mod from FF but it kept the both the low and high speed fans on, which kept engine temperatures too low and reduced gas milage considerably. I meant to replace the relay but completely forgot about it, so thanks for reminding me. other than that, I wonder if my engine/cylinder head temp just isn't high enough to make any noticeable changes with the temp gauge. I will still be open to any other suggestions/comments though

[Liquid_02_ZX3]

Possible cooling fan resistor pack, mine has went bad and the fans wouldn't kick on. Put the new one in and back to normal.

[Shun]

already replaced it, as stated in the first post of the thread

[Broke_Two_Foci]

I know the temp gauge in the focus is about as usless as an idioit light. I ended up installing an autometer water temp gauge to find out what was really going on with my car.

[Shun]

I plan on doing water and oil temp gauges asap. but I'm still just trying to figure out this over heating problem

[Camel04]

Did you ever figure out what the problem was with your temp gauge? My friend's focus is having what sounds like the exact same problem. His car overheats, but the fans won't kick on with the AC off and his temp gauge always reads normal. I've replaced the resistor pack, and I was suspecting a bad CHT sensor, but it sounds like that didn't fix it for you. Did you ever get it working?

Thanks,
-Mike

[Shun]

I haven't really figured this out yet. though i'm not entirely sure that there was ever anything wrong to begin with. most engine bays, especially the focus, will give the impression of over heating since it's a small engine bay stuffed with everything and not much ventillation. You could go and buy the CHT sensor and replace it. you must remove the alternator to get to it or - I haven't even bothered doing it yet, but you may want to have your friend invest in an auto meter water temp gauge and adapter part #ATM-2281 then run the adapter in line with the hose coming off the back of the thermostat housing as that is the best place to get an accurate reading.
it will read hot until the car is up to operating temperature and the thermostat opens. If you do this and the water temp gauge never goes down then you would pull the thermostat and check that it's operating properly. if he is smelling anything coming from the vent's then that would be his valve cover gasket, which was the biggest thing that gave me the impression of over heating.
I replaced the gasket and the smell went away, untill I made a dumb mistake during a tune up and got oil on my exhaust manifold and all that good stuff and have yet to wash it off with a degreaser so I get that damn smell again in my vents when I run the air. so to sum things up:

If he sparadically smell's something like burning, then replace the valve cover gasket.
If he doesn't then either pull and test the thermostat, or install a water temp gauge first and see if the car is actually over heating. as Broke_Two_Foci said, the stock instrument gauges are so inaccurate and unreliable that they can't be trusted

[skateman1990]

my car is doing the same thing but i don't know what the problem is...i think the best way to check is get a infrared thermometer and point it at engine, then at the thermostat so you know you get the right temp. i need to do that but i don't have the tool to do it with

[Shun]

Skateman, Why do you think your car over heats or runs abnormally hot? what kind of symptoms are you having?

[skateman1990]

Well naturally the temp gauge starts going up..i haven't been loosing coolant taht i can notice.. i have lost a little but that happened with my spi motor also, but the thing is i don't want to let it go to red and it actually be right., i have checked out all my fans, changed resistor pack, the thing is with my car when idling it starts to overheat and the fans are not kicking in also...so i am thinking that it is a bad ground or the car is playing tricks on me, either way its retarded

[Camel04]

Thanks for the reply. The car was definitely overheating when he brought it to me. It was bad enough that coolant was venting from the reservoir. The reason I suspected the CHT sensor was that the gauge never registered it overheating, even when it was spewing steam, and the fans also don't come on unless you turn the AC on. I think I'm going to test the thermostat tomorrow, and go from there.

[Shun]

skateman - your thermostat is probably sticking, pull and check it via these directions: http://www.focushacks.com/index.php?modid=60

camel - replace the radiator cap. it sounds like the cap isn't pressurizing the system, which is why the fluid is boiling. if after replacing the cap this persists, have the system tested for exhaust gases.

[Shun]

oh, skateman, also before you check it pick up a thermostat and a thermostat housing as well as gasket. if it is sticking then definitely replace it. also check the housing for cracks or chips as that would explain a loss of fluid. if you don't find any cracks or chips then return the housing but definitely use the new gasket

[BUR_ZX3]

Originally Posted by Shun

camel - replace the radiator cap. it sounds like the cap isn't pressurizing the system, which is why the fluid is boiling. if after replacing the cap this persists, have the system tested for exhaust gases.

More than that. Replace the reservoir tank and cap. They were redesigned after they discovered that the threads would warp and pressure be lost.

Happened to me. Lost an engine because of it.

[skateman1990]

well i know i can smell a little coolant everyone once and a while, but i don't think that it is leaking i know that the fans are not kicking in at idle but when driving they do. i guess i will change the thermostat since it is only 10 bucks

[Shun]

Originally Posted by BUR_ZX3

More than that. Replace the reservoir tank and cap. They were redesigned after they discovered that the threads would warp and pressure be lost.

Happened to me. Lost an engine because of it.

Hey man, thanks for the input.

[FrdFocus01]

My car is having some same problems. If I have been driving it for a while without the a/c on or I let it idle it gets too hot. Then if I turn my car of to go in some place quick and come back out the car will start and then die right away. Do you know what could cause that? The thermostat is already replaced and I replaced the CHT sensor today but Im wondering if that will fix the problem. Its idling lower now, it used to idle at about 3/4 of the way to 1000 now it idles around 1/2 and it seems like it wants to die. U think thats because of the sensor and my car is supposed to idle that low or what? Let me know what you think thanks.

[skateman1990]

changed thermostat and the fans are still not kicking in ...i thinking it is my relay....or a ground..not sure yet..goign to look into it

[Shun]

Well there are some instances where the whole thermostatic relay system needs to be re-wired. it's about $50 parts at an auto parts store. but you should pull the relay and see if you can find someone to test it as i'm not sure how many ohms or amperages it's supposed to put out. the relay is RL-15 and it's gray, in the fuse box under the hood

[skateman1990]

nahh my pcm harness is messed up so i am looking for a new one

[Shun]

Interesting! perhaps I should look into that.. even though my car doesn't really show signs of over heating I never really hear my fans kick on except for when I run the a/c XD

how did you come to this conclusion?

[skateman1990]

yeh my fans just won't kick in at idle changed relays thermo and resistor pack and they are still not kicking in for me still no conclusion yet until i get a new harness