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Bad Coolant temp sensor?

This is a discussion on Bad Coolant temp sensor? within the Technical / Maintenance forums, part of the General Discussion category; How can I tell if my coolant temperature sensor is bad? I am thinking it might be part of the ...

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    Bad Coolant temp sensor?


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    How can I tell if my coolant temperature sensor is bad? I am thinking it might be part of the problem with my truck running like crap once it warms up but runs beautiful when it's cold. Im not talking the sending unit to my gauge, but the one on top of the block at the front of the intake manifold. any ideas?

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    Yes, I have a FSC as well X-Runner's Avatar
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    Re: Bad Coolant temp sensor?

    Buy a new one, they are like $20 IIRC

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    Re: Bad Coolant temp sensor?

    Quote Originally Posted by burntfrog
    How can I tell if my coolant temperature sensor is bad? I am thinking it might be part of the problem with my truck running like crap once it warms up but runs beautiful when it's cold.
    burntfrog,

    I've seen a lot of posts on these forums that blamed CTS for their problems and a few that said it was the problem. If you want to test it, here is a link. It's usually easier to replace, a fairly cheap fix, and probably not a bad idea on an older model ride. Also, may be time to get a new O2 sensor.

    http://www.autozone.com/az/cds/en_us...rInfoPages.htm

    You didn't indicate any mods that you've done; tbi mods, timing, custon chip, cam, exhaut, etc., so assume a stock engine (but most likely not).

    On a cold engine, you are running in open loop where idleing and running are controlled by the ecm. After engine is up to op temp you're running in closed loop that runs according to spark and fueling specs set in the ecm. ECM has a control and learning feature that helps control fueling but other settings run as commanded.

    It's hard to give a definite answer without more info. About the best I can do is make an educated guess.

    If your probem started recently, replace the sensor and see if it helps.

    Good luck,
    Buddy
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    Exclamation Re: Bad Coolant temp sensor?

    Well, I just got the truck a couple months ago and it had sat rotting away in some field for 5 years, seriously. But surprisingly fired right up when I jumped it. I put new plugs, wires, cap and rotor on it. also, one of those old style round chrome air filters on it. But other than that it's stock. It's had the problem since I got it but now that I've fixed some of the more major issues like putting a new tranny and new intake manifold gasket, valve cover gaskets, little head work, she runs absolutely perfect for an old truck with 145k on it. Except for this and it's bugging me.
    It doesn't actually run bad when it's warmed up but develops this problem where at low speeds (0-5) if I just go real light on the gas, like in stop-n-go traffic or parking lot, it bogs down and kinda does the train impression; chuga-chuga-chuga. But then if I give it some more gas it feels like it's catching back up to itself and takes off like a shot, or if i let off the gas while it's doing the chugging it usually dies. seems like it's flooding out or something.
    It will do the same thing cruising in 4th at around 40-45, real light on the gas, probably 1200rpm, starts chugging and causes the truck to surge so I either have to speed up or downshift manually to third to get the engine into higher rpms...
    No 'check engine' light ever comes on when it does this either. My injectors looks like they are working good, gota a new fuel filter on it also.
    SO i'm kinda stumped, I have heard it might be the Throttle position sensor as well. But, as I am strapped for cash right now (went back to college, finally). I would like to pinpoint it instead of replacing things and seeing what happens.

    Sorry, this got kinda long winded, but too much info is better than not enough. Anyways, thanks for any input!

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    Re: Bad Coolant temp sensor?

    Sounds like the truck runs good in closed loop but not open loop. You need a multi-meter and start testing some of these sensors cold. and after the truck has warmed up and see if there is a change. Sometimes you can pull the sensor and test it with a hair dryer or a heat gun. Do not use a open flame torch.
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    Re: Bad Coolant temp sensor?

    Just remembered something, when mine messed up all I had to do was unplug it and the truck ran better. Might be worth a try

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    Re: Bad Coolant temp sensor?

    yeah well, the other day my truck wouldnt start and it turned out it was because the connecter to the plug had wiggled loose somehow and the ecm went nuts and flooded me out. plugged it back in and it started right up. but, I may try unplugging it after the truck has warmed up and see what happens

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    Re: Bad Coolant temp sensor?

    Yeah you have to unplug it with the truck running or it wont start.

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    Re: Bad Coolant temp sensor?

    ok, so i unplugged it today and a few things happened: first off, it idled a lot higher as soon as it was disconnected. Then, when I drove it, the lugging was worse at low rpms but once it 'worked through it' there was an enormous increase in power. what the hell? I had to plug it back in just cause the lugging was ridiculous but when it kicked up to full power I was very impressed cause I thought this old motor was trash but somewhere in it is a beast waiting to come out!
    Like I said before, I just went back to school and every dollar counts so I don't want to replace things I am not 100% about. ideas?

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    Exclamation Re: Bad Coolant temp sensor?

    Okay, so today I put a new sensor in and noticed a large increase in power/throttle response and the low end lugging was almost gone. I took er out for a spin and the acceleration was 'neck-snapping' for a 17 year old TBI 350. At least more so than it has ever been. But, the low end lugging is still there to a degree, and it actually stalled when I was backing into my parking spot after my test drive cause I gave it a little gas and let off, and like I said in earlier posts; it likes a lot of gas at low speeds and a little bit causes iut to bog down and stall.
    SO....the coolant sensor was obviously a big part of the problem, now any ideas what else might be causing the rest of the problem? All I really want to do is be able to ease forward in stop-n-go traffic w/o it dying. Starting it ever time I move is no good for the pride and/or the starter.

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    Re: Bad Coolant temp sensor?

    When was the last tune up done?

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    Re: Bad Coolant temp sensor?

    By tune-up do you mean having it hooked up to an actual good scanner at a shop? who knows, I've only had the truck a couple months but I am thinking of just biting the bullet and doing it so I can hopefully get a definitive answer instead of playing a wild goose chase with my voltmeter on any sensor I can get my hands on.

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    Re: Bad Coolant temp sensor?

    Plugs, wires, cap, rotor etc

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    Re: Bad Coolant temp sensor?

    He performed the basic tune up when he purchased the truck. I would look into the exhaust. is you O2 sensor in good shape? Is your cat working properly? Also would look into EGR vacuum leaks as well cuz that can cause a rough idle. DOes it idle strangely when you push down the brake pedal?

    I don't mean to be a dick: but you have your signature as a 91 c1500 4x4. the c and k designate 2 or 4wd chassis respectively. so a 4x4 would be a k1500. i don't mean to be an assh-ole i just figured i'd clear it up.


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    Talking Re: Bad Coolant temp sensor?

    hahaha, funny you should mention that, I just noticed that on the last post I made, not sure how I did that, musta been drunk when I made my profile

    I was thinking the o2 sensor but doesnt that set off the 'check engine' light when it goes bad? how can I test it?

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