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Thread: carb idle issues caused by carb temp sensor discuss

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    3Geez Veteran lostforawhile's Avatar
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    carb idle issues caused by carb temp sensor discuss

    I wanted to start a discussion, I hear all the time that the carbs have idle issues,that seem to be unfixable, how many people know that the carb has a separate temp sender on the block just like the EFI? I was looking through the service manual, just because I'm glad I don't have all that crap anymore, and I had forgotten all about it. Everyone knows with EFI the gauge has one sender, and the computer another, well the carb computer also sets parameters such as idle,mixture etc. based on coolant temp. where is it? it's almost impossible to see, but it's screwed into the block under the intake manifold, with everything in the way, most people wouldn't even know it's there. I wonder how many have failed and if this is the root cause of all the idle issues, The carb computer is a lot more sophisticated then most people think, it really is a computer, so give it a bad critical input, and you are going to get bad outputs, the difference is the carb computer has no check engine light, so a failed sensor gives no indication. These sensors are a common failure on FI engines, so I would assume the same on carb engines. It would be interesting to see if it's the same sensor as the FI engine.



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    Re: carb idle issues caused by carb temp sensor discuss

    Interesting idea. So few people have successfully rebuilt these carbs, fewer still really understand them. It would be hard to know for sure what's up. Maybe we can talk one of these newbs into being a guinea pig. LOL
    Dr_Snooz

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    3Geez Veteran lostforawhile's Avatar
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    Re: carb idle issues caused by carb temp sensor discuss

    Quote Originally Posted by Dr_Snooz View Post
    Interesting idea. So few people have successfully rebuilt these carbs, fewer still really understand them. It would be hard to know for sure what's up. Maybe we can talk one of these newbs into being a guinea pig. LOL
    well I had forgotten about the sensor back there, most people don't even know it has one at all, it causes the same issues that a bad one on a FI car has, but there is no open loop so the car just runs bad, the computer won' drop the idle until that sensor says the coolant has warmed up, so if the sensor goes bad, the idle never drops. They last about 15 years on average on a FI car so the majority of them have probably failed. Even with most of the junk gone on my car, I have to get up under there with a flashlight to even see it, it even has the same connector as the FI cars, the green one. The carb computer actually has a map,just like a FI car, it uses data from the O2 sensor,the temp sensor, and other sensors to allow air into the intake to keep the mixture right, if the sensors are bad the mixture and idle are off
    Last edited by lostforawhile; 07-09-2012 at 09:43 PM.

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    Re: carb idle issues caused by carb temp sensor discuss

    Quote Originally Posted by lostforawhile View Post
    well I had forgotten about the sensor back there, most people don't even know it has one at all, it causes the same issues that a bad one on a FI car has, but there is no open loop so the car just runs bad, the computer won' drop the idle until that sensor says the coolant has warmed up, so if the sensor goes bad, the idle never drops. They last about 15 years on average on a FI car so the majority of them have probably failed. Even with most of the junk gone on my car, I have to get up under there with a flashlight to even see it, it even has the same connector as the FI cars, the green one. The carb computer actually has a map,just like a FI car, it uses data from the O2 sensor,the temp sensor, and other sensors to allow air into the intake to keep the mixture right, if the sensors are bad the mixture and idle are off
    my car runs alright until it gets warmed up. when the engine warms up and the fan comes on at least once, the car dies if you don't give it gas.. ill have to check that sensor

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    LXi User Buzo's Avatar
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    Re: carb idle issues caused by carb temp sensor discuss

    Also the intake air temperature sensor is used by the ECU to make decisions, so its important to have the little door (and its vacuum actuator) in the carburetor's cover and the hose that goes down to the exhaust manifold in good conditions. If the air temp sensor don't get to the expected temperature, some of the fuel efficiency features will never engage.

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    Re: carb idle issues caused by carb temp sensor discuss

    Quote Originally Posted by Buzo View Post
    Also the intake air temperature sensor is used by the ECU to make decisions, so its important to have the little door (and its vacuum actuator) in the carburetor's cover and the hose that goes down to the exhaust manifold in good conditions. If the air temp sensor don't get to the expected temperature, some of the fuel efficiency features will never engage.
    the resistor in the air cleaner actually only affects how fast the choke opens, if it's above a certain temp its switched out, the only one that directly affects the computer is the one screwed into the block,it's a crazy system, there are several systems that don't interact with each other, there is also a coarse and fine mixture control system which also don't interact with each other

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    Re: carb idle issues caused by carb temp sensor discuss

    Quote Originally Posted by g0dspeak View Post
    my car runs alright until it gets warmed up. when the engine warms up and the fan comes on at least once, the car dies if you don't give it gas.. ill have to check that sensor
    hi i have the same problem how do you fix it?? you have some photo??

    my car is carb
    Last edited by panthere0rose; 07-15-2012 at 02:25 AM.

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    Re: carb idle issues caused by carb temp sensor discuss

    Quote Originally Posted by panthere0rose View Post
    hi i have the same problem how do you fix it?? you have some photo??

    my car is carb
    I'm not saying for sure thats the issue, but it's one more item to check when you have idle problems since the computer for the carb uses it to change the idle on a cold engine, if you put the car up on jackstands,you can't get to it from above, it's screwed into the BLOCK, not the one screwed into the intake, directly below the intake manifold, it will have one of those connectors where you have to push the tab to release it, you would test it just like you would the one on the FI car, since most sensors that are subject to hot coolant eventually fail, I would say you have a good chance it's bad

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    Re: carb idle issues caused by carb temp sensor discuss

    Quote Originally Posted by lostforawhile View Post
    I'm not saying for sure thats the issue, but it's one more item to check when you have idle problems since the computer for the carb uses it to change the idle on a cold engine, if you put the car up on jackstands,you can't get to it from above, it's screwed into the BLOCK, not the one screwed into the intake, directly below the intake manifold, it will have one of those connectors where you have to push the tab to release it, you would test it just like you would the one on the FI car, since most sensors that are subject to hot coolant eventually fail, I would say you have a good chance it's bad

    thank you have you some picture of the sensor i wanna buy him
    Last edited by panthere0rose; 07-16-2012 at 12:03 AM.

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    Re: carb idle issues caused by carb temp sensor discuss

    Sounds like it would easier to test that sensor under the drivers seat at the control board. Just ohm it out there. Thermisters in general are quite reliable, but maybe an issue in use here. I have not had any problems with them, but the Thermowax vacuum valves can sure get flaky; partly open, not quite closed, slow to respond, kinda works well most of the time. Themovalve A being on the high idle unloader to step down in good time when warming up. ...It's summer now, so works perfectly every time.

    The mystery problem I have is when 'puter decides it wants to close the primary slow mix cutoff valve when it's supposed to be at idle. Extremely random and rarely happens, but this has been an issue since the car was new! (so not something wearing out) 'Jimmy' just has a temperament! Best I can guess is the RPM sense gets confused....maybe. I think Buzo's excellent diagnostic board is about the only thing to track that issue down.
    Sorry to drift OT with that one....
    Last edited by g.frost; 07-16-2012 at 03:11 PM. Reason: spelling

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    Re: carb idle issues caused by carb temp sensor discuss

    Quote Originally Posted by g.frost View Post
    Sounds like it would easier to test that sensor under the drivers seat at the control board. Just ohm it out there. Thermisters in general are quite reliable, but maybe an issue in use here. I have not had any problems with them, but the Thermowax vacuum valves can sure get flaky; partly open, not quite closed, slow to respond, kinda works well most of the time. Themovalve A being on the high idle unloader to step down in good time when warming up. ...It's summer now, so works perfectly every time.

    The mystery problem I have is when 'puter decides it wants to close the primary slow mix cutoff valve when it's supposed to be at idle. Extremely random and rarely happens, but this has been an issue since the car was new! (so not something wearing out) 'Jimmy' just has a temperament! Best I can guess is the RPM sense gets confused....maybe. I think Buzo's excellent diagnostic board is about the only thing to track that issue down.
    Sorry to drift OT with that one....
    the primary slow mixture cutoff valve sounds like an electrical issue, it is energized when the ignition key is at ignition 1 I believe, the computer doesn't control it, they are either on or off depending if the car is running or not

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    Re: carb idle issues caused by carb temp sensor discuss

    It also closes when decelerating and above 1500RPM or so.( &other vac. sensor inputs to determine conditions)
    Look at the data Buzo collected on that signal using his diagnostic board; it's active.
    ... If it ever gets bad enough I may just wire it to 12V ignition switch and be done with it. Suffer whatever the gas mileage loss.

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    Re: carb idle issues caused by carb temp sensor discuss

    Quote Originally Posted by g.frost View Post
    It also closes when decelerating and above 1500RPM or so.( &other vac. sensor inputs to determine conditions)
    Look at the data Buzo collected on that signal using his diagnostic board; it's active.
    ... If it ever gets bad enough I may just wire it to 12V ignition switch and be done with it. Suffer whatever the gas mileage loss.
    I'm thinking of the solonoids that close the float bowls off to prevent fuel vapor escape

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    Re: carb idle issues caused by carb temp sensor discuss

    Quote Originally Posted by g.frost View Post
    It also closes when decelerating and above 1500RPM or so.( &other vac. sensor inputs to determine conditions)
    Look at the data Buzo collected on that signal using his diagnostic board; it's active.
    ... If it ever gets bad enough I may just wire it to 12V ignition switch and be done with it. Suffer whatever the gas mileage loss.
    you want a carb computer to try swapping it? you can have it for shipping I have no use for it, I can't give it away

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    Re: carb idle issues caused by carb temp sensor discuss

    Thanks for the offer, but this problem occurs so rarely(like maybe once a year for a day or so..., I would not risk the potential of other trouble in swapping the control board.
    It happened quite a few times this summer as the car was used (and abused) much more.
    Last time I was teaching my daughter to drive a clutch/manual. One to many jump stalls and Jimmy had enough. He quit for a day or 2. (hence the diagnosis of 'temperament problem') I dropped her off at camp, & on the way back Jimmy thought he was safe, so started running so perfectly as usual.

    Earlier in the spring I was driving all over the county & put about 3k miles in a month on.
    Up one bumpy dirt road, Jimmy had the same 'I've had enough' reaction & quit idling.
    Talk nice, give him a bath, and rest; he's good to go again....

    I had the car to the dealer before 12K mile new car warranty was up for this problem.... no one has ever come close to a 'solution'. Well, it is very hard to reproduce, but I did get a bit farther than I ever have before with it this summer: enough to know the 'puter is telling the cutoff valve to close anyway, the problem is not the valve itself sticking.

    ...Running great now as it has for the last few weeks without a hint of the problem.

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    Re: carb idle issues caused by carb temp sensor discuss

    Quote Originally Posted by g.frost View Post
    Thanks for the offer, but this problem occurs so rarely(like maybe once a year for a day or so..., I would not risk the potential of other trouble in swapping the control board.
    It happened quite a few times this summer as the car was used (and abused) much more.
    Last time I was teaching my daughter to drive a clutch/manual. One to many jump stalls and Jimmy had enough. He quit for a day or 2. (hence the diagnosis of 'temperament problem') I dropped her off at camp, & on the way back Jimmy thought he was safe, so started running so perfectly as usual.

    Earlier in the spring I was driving all over the county & put about 3k miles in a month on.
    Up one bumpy dirt road, Jimmy had the same 'I've had enough' reaction & quit idling.
    Talk nice, give him a bath, and rest; he's good to go again....

    I had the car to the dealer before 12K mile new car warranty was up for this problem.... no one has ever come close to a 'solution'. Well, it is very hard to reproduce, but I did get a bit farther than I ever have before with it this summer: enough to know the 'puter is telling the cutoff valve to close anyway, the problem is not the valve itself sticking.

    ...Running great now as it has for the last few weeks without a hint of the problem.
    ok, I thought you might want to swap the entire computer in and see if it cured the issue, pretty much plug and play, it was working fine, I just don't have a computer carb anymore,if you ever need it let me know

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    Re: carb idle issues caused by carb temp sensor discuss

    Quote Originally Posted by g0dspeak View Post
    my car runs alright until it gets warmed up. when the engine warms up and the fan comes on at least once, the car dies if you don't give it gas.. ill have to check that sensor
    have you fix the problem i have the same problem

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    Re: carb idle issues caused by carb temp sensor discuss

    Quote Originally Posted by lostforawhile View Post
    ok, I thought you might want to swap the entire computer in and see if it cured the issue, pretty much plug and play, it was working fine, I just don't have a computer carb anymore,if you ever need it let me know
    You are so generous! I'm not sure how 'Jimmy' (my daughter named it, and it stuck) would handle a brain transplant. (Dr. Jekyl or Mr. Hyde?) It is from an '86? and if there were any Rev. changes from 86 to 88. CA emissions, etc.. Also, I don't know enough about the problem to be sure it is the control board at fault. According to the 'part swapping' test procedures in the manual, that's what they say to replace. But the problem occurs so rarely, I'm inclined to leave well enough alone. If it ever fails completely and consistently, I may finally have the chance to diagnose the 24 year old mystery.

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    SEi User import racer's Avatar
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    Re: carb idle issues caused by carb temp sensor discuss

    Well I called a parts store about a temp gauge sensor and they said It was the same one for carbed or fi.I tried three different ones I had in my car and every one made my gauge go to hot as soon as I plugged it in.

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    Re: carb idle issues caused by carb temp sensor discuss

    All the temp sensors likely use the same thermistor. An Ohm meter is required to check it: 2.5K Ohm is 68F, 1K ohm is 104F, 0.5K is 140F.... (graph is in the manual) It sounds like there was a short in the signal path if your gauge always pegged high.

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    Re: carb idle issues caused by carb temp sensor discuss

    Yeah I thought it might be the gauge itself.

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    Re: carb idle issues caused by carb temp sensor discuss

    the gauge uses a different sender, the gauge sender if the gold looking single wire sender screwed into the intake manifold on carb cars, the other sensor I'm talking about is screwed into the back of the block and tells the carb computer what the coolant temp is

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    Re: carb idle issues caused by carb temp sensor discuss

    Oh ok sorry I misundrestood.

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    Re: carb idle issues caused by carb temp sensor discuss

    Quote Originally Posted by import racer View Post
    Oh ok sorry I misundrestood.
    Its ok that's the point of the thread everyone thinks the carb only has one

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    Re: carb idle issues caused by carb temp sensor discuss

    I'm just wondering if the sensor location would be a good place to put a knock sensor on EFI cars. In case you were looking to go with some kind of electronic spark control. Electronic spark control is about the only thing I'd change on the EFI A20.
    Dr_Snooz

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