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Old 11-25-2016, 12:47 PM
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Chevy 350 TBI: Idle Air Control Valve

Hi All,


What should happen to a Chevy 350 TBI engine if the Idle Air Control Valve is disconnected? Engine is running, warmed up, and the vehicle is parked.

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Old 11-25-2016, 12:52 PM
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Quote:
Originally Posted by lt1silverhawk View Post
Hi All,


What should happen to a Chevy 350 TBI engine if the Idle Air Control Valve is disconnected? Engine is running, warmed up, and the vehicle is parked.
Most likely nothing. The iac will move in against the seat then back out so many turns when the key is switched on. After that as the engine warms up it closes to lower down from fast cold idle. Once the engine is warmed up it only moves a fraction of a turn in or out to try to maintain a stable idle speed, unless there's a load placed on the engine such as dropping into drive from park, or switching the AC on. In those cases it'll move back out to compensate for the lost rpm then settle back down. Once it's idling stably and smoothly iac will barely move.

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Old 11-25-2016, 12:56 PM
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Originally Posted by Tim C View Post
Most likely nothing. The iac will move in against the seat then back out so many turns when the key is switched on. After that as the engine warms up it closes to lower down from fast cold idle. Once the engine is warmed up it only moves a fraction of a turn in or out to try to maintain a stable idle speed, unless there's a load placed on the engine such as dropping into drive from park, or switching the AC on. In those cases it'll move back out to compensate for the lost rpm then settle back down. Once it's idling stably and smoothly iac will barely move.

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Thank you!

I am trying to diagnose a transmission issue, which has led me to the IAC. I have noticed the engine has a weird, uneven thumpy idle when at a red light. Could the IAC be a possible culprit?
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Old 11-25-2016, 12:59 PM
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Thank you!

I am trying to diagnose a transmission issue, which has led me to the IAC. I have noticed the engine has a weird, uneven thumpy idle when at a red light. Could the IAC be a possible culprit?
In my experience an iac either works or it doesn't, an uneven idle surge is usually caused by something else such as a vacuum leak, or a bad map or tps sensor. If it's capable of idling higher cold and kicking the idle up when put into gear it's usually OK

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Old 11-25-2016, 04:59 PM
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Does the idle change if you put it in park or neutral? If yes, then most likely a trans/TC issue. If not, then check the basics - vacuum, FP and spark (especially spark wander if this a high mileage motor. Spark wander is usually caused by a worn distributor or distributor gear, which will not act 'so' badly when driving under load.
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Old 11-25-2016, 05:51 PM
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Quote:
Originally Posted by Tim C View Post
In my experience an iac either works or it doesn't, an uneven idle surge is usually caused by something else such as a vacuum leak, or a bad map or tps sensor. If it's capable of idling higher cold and kicking the idle up when put into gear it's usually OK

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It does idle higher when cold. But once in gear and at a stop is when I feel the odd idle. Like a thumb every 5-7 seconds or so.




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Does the idle change if you put it in park or neutral? If yes, then most likely a trans/TC issue. If not, then check the basics - vacuum, FP and spark (especially spark wander if this a high mileage motor. Spark wander is usually caused by a worn distributor or distributor gear, which will not act 'so' badly when driving under load.
I have not paid attention to the idle after driving it around and putting it in park, will do that the next time I drive it.

The odometer shows 319k, but the motor appears to be newer and quite healthy. But I wouldn't be surprised if the other components including the distributor are original.

On a related note, the real "acting badly" that started this mission is that the transmission doesn't seem to want to shift into over drive. Tried replacing the TPS with an aftermarket one, and the transmission would constantly bounce between over drive and third. Just non-stop bogging and surging. But it drives as smooth as can be on the street. Started a thread about it here (click).
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Old 11-26-2016, 04:45 PM
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Yesterday, I decided to go ahead and see if there were any trouble codes in the computer. Video of the procedure posted below.


I got the codes "15" and "22".


Based on this website (Auto Repair and Maintenance Estimates | Auto Shop and Mechanic Ratings), code "15" is "low coolant temp error" due to the engine being cold.


Code "22" is "Throttle Position Sensor error (signal low)".


A few days earlier, I tested the original TPS, as well as an Autozone Duralast version. According to a Youtube video, the original TPS was putting out the right volts but the Duralast version was putting out too much. Video gave the acceptable range as between 0.4 volts and 0.6 volts. I also checked the original TPS by moving the throttle back and forth, and there were no dead spots from what I could see. Video posted below.


The original TPS keeps the transmission in 3rd for the most part. The Duralast TPS causes serious back and forth shifting between third and fourth.


If the error code "22" is correct, then could it be bad wiring that is causing a weak signal? I did have to "fix" the wiring on two other sensors already. Or should I try an new AC-Delco TPS?













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Old 11-26-2016, 05:01 PM
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I'd disconnect the battery for a few minutes to clear the codes, then run it again to try to see if any come back.

Code 15 means the coolant temp sensor is reporting the temp is stuck at -40*. This usually causes an extremely rich mixture or flooding and no starts. This could explain your surging idle. If this truck has the electronic transmission like a 4l60e or 4l80e the faulty coolant or tps reading will definitely affect the transmission.

Coolant temp sensors are pretty common failures on tbi Chevies. The wiring pigtail also fails, usually when you buy a sensor it comes with a wire pigtail. There are spec charts online that will give the resistance needed at different temps, you can check the CTS by reading resistance on a cold engine and comparing it to outside air temp, then running the engine til the top radiator hose is hot and comparing the resistance to the thermostat open temp which is around 195*f stock.

As far as tps goes I've seen .4 all the way up to 1 volt closed throttle. Most early (87-92) trucks ran .4-.6v. The later years ran closer to .6-.9v.

What wiring problems have you had to fix?

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Old 11-26-2016, 05:11 PM
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Quote:
Originally Posted by Tim C View Post
I'd disconnect the battery for a few minutes to clear the codes, then run it again to try to see if any come back.
I will try that next.




Quote:
Originally Posted by Tim C View Post
Code 15 means the coolant temp sensor is reporting the temp is stuck at -40*. This usually causes an extremely rich mixture or flooding and no starts. This could explain your surging idle. If this truck has the electronic transmission like a 4l60e or 4l80e the faulty coolant or tps reading will definitely affect the transmission.
In one of the videos I saw on Youtube, a person mentioned that that code 15 maybe showing up because the engine is cold. I wonder if this may be true in my situation.



Quote:
Originally Posted by Tim C View Post
Coolant temp sensors are pretty common failures on tbi Chevies. The wiring pigtail also fails, usually when you buy a sensor it comes with a wire pigtail. There are spec charts online that will give the resistance needed at different temps, you can check the CTS by reading resistance on a cold engine and comparing it to outside air temp, then running the engine til the top radiator hose is hot and comparing the resistance to the thermostat open temp which is around 195*f stock.

...

What wiring problems have you had to fix?
I actually just replaced both the CTS as well as the Temperature Switch because the gauge was reporting temperature closer to 100*. I thought that it was the issue that was keeping the transmission from shifting into 4th/OD.

When I replaced the sensors, the temperature was still reporting quite cold. I noticed that the wiring for the CTS was exposed and corroded at one point, as was the one for the alternator. So, for now, I cut out the bad parts in both wires and twisted and taped them. The temperature started reporting at 195* after that. However, the shifting issue remains.




Quote:
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As far as tps goes I've seen .4 all the way up to 1 volt closed throttle. Most early (87-92) trucks ran .4-.6v. The later years ran closer to .6-.9v.
Thank you for this info.
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Old 11-26-2016, 07:54 PM
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The IAC, can fill full of soot. Pull it off and shoot it with some carb cleaner pushing the plunger in a few times. That will "refresh" it. The sensor itself is cheap as well as the TPS sensor.

If you are getting a over rich condition you should have a o2 sensor yelling at you somewhere. Or a Egr silionoid/egr sticking.


Between your TBI and distributor you have a vacume line leading to a TVS switch. If this is clogged with oil it will tell the distributor your engine is cold. If your TPS is working properly it will adjust the fuel and the IAC will limit the fuel. But the distributor is only receiving vacuum signal that tells it the engine is cold and to keep it timed to warm up the engine.
Pull that TVS and check to see if it is full or oil/gunk.



Run your hand along the harness going to the transmission checking for any burns or cuts.
Check the pig tail connections and wiring going into the connections for any loose or damaged wiring. If they are full of oil and gunk hit them with your ex girlfriends toothbrush.

Ok check your transmission fluid color(google it). I am guessing your engine is getting hot and that gauge with 300k on it is no longer reading correctly. You could also have a air bubble in the sensor area giving the pcm false readings. Pull the temperature sensor and check it electrical wise(google). Then if it reads good reinstall it, after filling the port to the brim, inserting the sensor till fluid comes out, installing some liquid silicone, then inserting the sensor in place you know you will not have any bubbles.
I would run the engine with a aftermarket gauge with the aftermarket sending unit installed in a non factory position(so the computer reads the stock sending unit correctly) checking the cluster temperature vs the aftermarket temperature to let you know if your gauge is working correctly.


Does the engine rev higher in any gear like it is in neutral for a bit when driving?
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Old 11-26-2016, 09:16 PM
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Quote:
Originally Posted by cerial View Post
The IAC, can fill full of soot. Pull it off and shoot it with some carb cleaner pushing the plunger in a few times. That will "refresh" it. The sensor itself is cheap as well as the TP
S sensor.

If you are getting a over rich condition you should have a o2 sensor yelling at you somewhere. Or a Egr silionoid/egr sticking.
I removed and cleaned the IAC valve about three days ago with carb cleaner. It was rather grimey-looking. I think the plug connector from the harness to the IAC might be damaged. The plug has four prongs, and one them is missing a plastic cover but the other three have it.

I replaced the O2 sensor back when I did the tune up a few months ago.






Quote:
Originally Posted by cerial View Post
Between your TBI and distributor you have a vacume line leading to a TVS switch. If this is clogged with oil it will tell the distributor your engine is cold. If your TPS is working properly it will adjust the fuel and the IAC will limit the fuel. But the distributor is only receiving vacuum signal that tells it the engine is cold and to keep it timed to warm up the engine.
Pull that TVS and check to see if it is full or oil/gunk.
I checked but did not see a TVS, or a vacuum hose on the distributor. I also did not find any info on my truck having one. Its a 1994. I'll try to dig in tomorrow.







Quote:
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Run your hand along the harness going to the transmission checking for any burns or cuts.
Check the pig tail connections and wiring going into the connections for any loose or damaged wiring. If they are full of oil and gunk hit them with your ex girlfriends toothbrush.
Got it. Was thinking of checking the speed sensor while down there.




Quote:
Originally Posted by cerial View Post
Ok check your transmission fluid color(google it). I am guessing your engine is getting hot and that gauge with 300k on it is no longer reading correctly. You could also have a air bubble in the sensor area giving the pcm false readings. Pull the temperature sensor and check it electrical wise(google). Then if it reads good reinstall it, after filling the port to the brim, inserting the sensor till fluid comes out, installing some liquid silicone, then inserting the sensor in place you know you will not have any bubbles.
I would run the engine with a aftermarket gauge with the aftermarket sending unit installed in a non factory position(so the computer reads the stock sending unit correctly) checking the cluster temperature vs the aftermarket temperature to let you know if your gauge is working correctly.
I checked the transmission fluid when I got the truck a few months and again three days ago. It is pretty clean. I have feeling both the engine and transmission were replaced at some point.

I replaced both the Coolant Temperature Sensor (AC-Delco) and the Temperature Switch (AutoZone's Duralast brand).







Quote:
Originally Posted by cerial View Post
Does the engine rev higher in any gear like it is in neutral for a bit when driving?
No higher revving when driving or warming. It drives along just fine, especially on the street. Its when I start accelerating past 60 mph that it either the transmission stay in third, or bounces back and forth between third and fourth. It only a has a bit of a "thump" every few seconds or so when stopped at a light.
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Old 11-30-2016, 04:58 PM
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Just wanted to post some updates...


I left the battery disconnected overnight to clear the codes. The next day, I reconnected it, and warmed up the truck. I did not drive it. After warming it up, i check for codes and no codes were flashed.


Today, I warmed up the truck, and checked the transmission fluid. The amount of fluid is much higher than the level indicated for "Hot". Could this be the issue? See picture below.


I then took it for a spin. Still continues to stay in third gear on the highway. Afterwards, I checked for codes, and nothing flashed, except for the "diagnostic mode" code 12. No code "15" (coolant temp low) or "22" (TPS signal low) like before.


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Old 12-01-2016, 09:30 AM
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Quote:
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Hi All,


What should happen to a Chevy 350 TBI engine if the Idle Air Control Valve is disconnected? Engine is running, warmed up, and the vehicle is parked.


Read through this the first place I would go is the EGR they wear from the exhaust gasses and get leaky at idle where they a normally not functional. This will upset the idle while not throwing a code as the code only comes on if the EGR fails off when it should be on. The other potential failures are not monitored. This will cause an unstable idle but not always all the time.


Bogie
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Old 12-17-2016, 07:14 PM
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Read through this the first place I would go is the EGR they wear from the exhaust gasses and get leaky at idle where they a normally not functional. This will upset the idle while not throwing a code as the code only comes on if the EGR fails off when it should be on. The other potential failures are not monitored. This will cause an unstable idle but not always all the time.


Bogie
Thank you Bogie and others. Turns out the idle problem was cased by a damaged spark plug wire. Also, the original ground straps look very old, so it may help to replace those too, I think. I replaced the spark plug wire with a used one I had previously removed. Also, one of the ground straps was hanging loose by the passenger side exhaust flange. I couldn't locate a place to attach it to the back of the head, so I attached it to the exhaust manifold. Overall it seems much smoother.
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Old 12-17-2016, 09:53 PM
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Bad grounds and plug wires will definitely cause issues. Your IAC connector can be bought or grab one from a local pick and pull.

I can't remember if a 94 TPS needs to be adjusted, but you can tell via the mounting holes - slotted holes means it's adjustable. Read the first post in this : Initial Setup of TBI, TPI and MPFI systems. Min Air Adjust, TPS set and Fuel Pressure.

And again, if memory serves me correctly, the coolant temp sensor in the head is for the gauge, but the sensor that throws the code is in the intake (it's a two wire, not one wire sensor.) The joy of the TBI is the simplicity of it. Once you have an understanding of how a few sensor work and how the fuel system works you'll find it is really basic and easy to maintain.

I have to say - where's Wayne and LATech? These two will rattle off this info like it's there job - oh, wait a minute, I think it is Maybe send them a PM.
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