idle problem - The 1947 - Present Chevrolet & GMC Truck Message Board Network

bshank88 · 08-06-2004, 11:52 AM

I have a 1988 1/2t 4x4 with the 350 & 5 speed. 150,000 miles.
All stock except: Flowmaster single 3" exhaust, converter removed. K&N Gen II intake with Helix TBI Spacer. These were both done at the same time- about 2 years agos. I also changed the Injectors at the same time. My problem is a very low idle when I start the truck. Bought the truck 4 years ago & it always had the fast idle which slowed down when the engine warmed up but as of 4 days ago, it is gone. The idle is so low that if you turn on the A/C, the engine dies. If I keep it running a minute or so then it will idle, just very low. Runs great down the highway. I changed the fuel filter & MAP Sensor but no help. Checked all vacuum lines & pulled the Throttle Position Sensor out & cleaned it, no help. I did get a code 34 yesterday which is why I changed the MAP Sensor then reset the codes. Is there any way I can adjust the idle or is it all controlled electronically? What besides the TPS & MAP Sensor should I look at?

ChevyTech · 08-06-2004, 04:48 PM

My first guess would be you have a bad IAC (idle air control) valve.

The computer and IAC valve control the idle speed, fast idle when cold and the idle speed increase during A/C operation.

The IAC screws into the TBI Assembly. It's rearward of the TPS.

IAC valves do not tolerate carb clearner or other harsh cleaners. Try not to let any liquid get on the IAC plunger.

Is your code 34 returning. If so, you should check it out. code 34 is for low MAP voltage - high vacuum. You could test to see if you are getting close to 5 volts to terminal C of the MAP sensor.

bshank88 · 08-07-2004, 03:30 AM

The IAC is what I cleaned. Not the TPS, I had it wrong. I used Carb Cleaner to clean it & worked the plunger in & out by hand also. Should it be replaced now because of the Carb Cleaner? So, what about the TPS. Is there any way I can check it? Does it control idle speeds? No, the code has not came back.....yet.

ChevyTech · 08-07-2004, 11:25 AM

Yes, the carb cleaner most likely damaged your IAC. My bet is, it's junk.

The TPS can be tested with a digital Volt/ohm meter.

The ECM does use the TPS reading to determine when to control the idle, but if the TPS has failed it should set a trouble code.

Being your code 34 has not returned, a IAC may be all you need.

bshank88 · 08-07-2004, 11:29 AM

OK I'll try that. Thanks

08-06-2004, 11:52 AM	#1
bshank88 Registered User Join Date: Aug 2004 Posts: 6	idle problem I have a 1988 1/2t 4x4 with the 350 & 5 speed. 150,000 miles. All stock except: Flowmaster single 3" exhaust, converter removed. K&N Gen II intake with Helix TBI Spacer. These were both done at the same time- about 2 years agos. I also changed the Injectors at the same time. My problem is a very low idle when I start the truck. Bought the truck 4 years ago & it always had the fast idle which slowed down when the engine warmed up but as of 4 days ago, it is gone. The idle is so low that if you turn on the A/C, the engine dies. If I keep it running a minute or so then it will idle, just very low. Runs great down the highway. I changed the fuel filter & MAP Sensor but no help. Checked all vacuum lines & pulled the Throttle Position Sensor out & cleaned it, no help. I did get a code 34 yesterday which is why I changed the MAP Sensor then reset the codes. Is there any way I can adjust the idle or is it all controlled electronically? What besides the TPS & MAP Sensor should I look at?

08-06-2004, 04:48 PM	#2
ChevyTech Registered User Join Date: May 2004 Location: Minnesota Posts: 4,621	My first guess would be you have a bad IAC (idle air control) valve. The computer and IAC valve control the idle speed, fast idle when cold and the idle speed increase during A/C operation. The IAC screws into the TBI Assembly. It's rearward of the TPS. IAC valves do not tolerate carb clearner or other harsh cleaners. Try not to let any liquid get on the IAC plunger. Is your code 34 returning. If so, you should check it out. code 34 is for low MAP voltage - high vacuum. You could test to see if you are getting close to 5 volts to terminal C of the MAP sensor.

08-07-2004, 03:30 AM	#3
bshank88 Registered User Join Date: Aug 2004 Posts: 6	The IAC is what I cleaned. Not the TPS, I had it wrong. I used Carb Cleaner to clean it & worked the plunger in & out by hand also. Should it be replaced now because of the Carb Cleaner? So, what about the TPS. Is there any way I can check it? Does it control idle speeds? No, the code has not came back.....yet.

08-07-2004, 11:25 AM	#4
ChevyTech Registered User Join Date: May 2004 Location: Minnesota Posts: 4,621	Yes, the carb cleaner most likely damaged your IAC. My bet is, it's junk. The TPS can be tested with a digital Volt/ohm meter. The ECM does use the TPS reading to determine when to control the idle, but if the TPS has failed it should set a trouble code. Being your code 34 has not returned, a IAC may be all you need.

08-07-2004, 11:29 AM	#5
bshank88 Registered User Join Date: Aug 2004 Posts: 6	OK I'll try that. Thanks



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