302 erratic idle question.
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i ain't rite
Joined: Mar 2005
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From: Marlboro Mental Hospital.
302 erratic idle question.
one of the neighbors stopped by a few minutes ago with a strange problem/question i have never seen before.
truck is a 88-91 F150 2 wheel drive with a 5.0 engine and 5 speed trans. with climate control turned off it purrs.
but turn the heat or AC on, and the idle will waver from 400 to 1100 rpms.
the air and heat work, and air comes out the vents selected, so i don't think it is a vacuum leak..
any ideas??
truck is a 88-91 F150 2 wheel drive with a 5.0 engine and 5 speed trans. with climate control turned off it purrs.
but turn the heat or AC on, and the idle will waver from 400 to 1100 rpms.
the air and heat work, and air comes out the vents selected, so i don't think it is a vacuum leak..
any ideas??
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From: Denver Metro Area, CO
I can understand the different behavior with the AC turned on, the engine idle gets bumped up at that time to draw more air across the radiator but I don't know of any relationships with the heater.
Junior User
Joined: Jan 2012
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Clean the IAC valve located right hand side of the throttle body. It controls the idling. When you turn the a/c on it is supposed to when adjust the idle to allow for extra load. Take it off and give it a good cleaning carb cleaner or throttle body cleaner will do.
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i ain't rite
Joined: Mar 2005
Posts: 65,455
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From: Marlboro Mental Hospital.
that is what has me baffled too Chris. i could understand if it was only on AC.
Bucks, it idles perfectly without the heat or AC on, and also does it with the heater on too. that is why i don't thing it is the IAC or TPS
Bucks, it idles perfectly without the heat or AC on, and also does it with the heater on too. that is why i don't thing it is the IAC or TPS
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From: Denver Metro Area, CO
The thing to come to immediate mind is perhaps an excessive power draw by the fan motor, although I'd expect it to pop the circuit breaker if that was the case.
Nevertheless, I guess I would take some voltage readings at the ignition module source, do it first without the fan out to get a baseline, then switch the fan on to see if anything significantly changes.
I'm assuming factory AC which uses the same fan motor as the heater.
Another thing that comes to mind is wiring behind the dash having been mucked with and is tangled up in the HVAC controls and (partially) shorts out in with levers in certain positions.
When the idle goes up and down, can look at the IAC motor and see it causing that? Raising & lowering the throttle? Could the computer perhaps be trying to raise the running speed because it senses the engine is gonna die?
Nevertheless, I guess I would take some voltage readings at the ignition module source, do it first without the fan out to get a baseline, then switch the fan on to see if anything significantly changes.
I'm assuming factory AC which uses the same fan motor as the heater.
Another thing that comes to mind is wiring behind the dash having been mucked with and is tangled up in the HVAC controls and (partially) shorts out in with levers in certain positions.
When the idle goes up and down, can look at the IAC motor and see it causing that? Raising & lowering the throttle? Could the computer perhaps be trying to raise the running speed because it senses the engine is gonna die?
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Thread Starter
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i ain't rite
Joined: Mar 2005
Posts: 65,455
Likes: 5,523
From: Marlboro Mental Hospital.
i have nothing to pull codes with, and it does not throw a CEL.
i doubt very much anyone messed with the wiring, he is the original owner. this just started happening a short time ago.
and it is not fan related. it will do it with the fan on low or high.
yes, it is factory AC in a XLT lariat truck.
i was thinking a sensor in the AC sending a signal to the computer, but eliminated that thought when i realized it was doing it in heater mode too.
i doubt very much anyone messed with the wiring, he is the original owner. this just started happening a short time ago.
and it is not fan related. it will do it with the fan on low or high.
yes, it is factory AC in a XLT lariat truck.
i was thinking a sensor in the AC sending a signal to the computer, but eliminated that thought when i realized it was doing it in heater mode too.
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Joined: Nov 2007
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From: Denver Metro Area, CO
Well, even if it's not throwing the light, I would pull the codes anyway. You can do it with a paper clip, here's how:
Fuel Injection Technical Library » How To Run a Self-Test
Reps to 88dually, pulling codes is usually one of the first things to come to mind on any computer-controlled engine and I forgot for some reason.
Fuel Injection Technical Library » How To Run a Self-Test
Reps to 88dually, pulling codes is usually one of the first things to come to mind on any computer-controlled engine and I forgot for some reason.
Fleet Owner
Joined: Oct 2005
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From: Kentucky
Turn on the A/C or heat then drive the truck. Give it some throttle, I bet the air diffuser will drop to the default mode of defrost. I bet you have a vacuum leak in the HVAC system. That funky white line that frequently goes bad leads back to the control **** inside the cab. There are several vacuum motors that can also cause issues, but I would start with the lines leading to and from the HVAC control panel.
Some great info in this thread from another forum: A/C and/or heater stuck on defrost - FSB Forums
Some great info in this thread from another forum: A/C and/or heater stuck on defrost - FSB Forums
Senior User
Joined: Aug 2001
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From: Lexington Park, MD
I'm curious, when you say heater, is that Vents, Floor or Defrost? IIRC the A/C compressor cycles when Defrost is selected. When checking it, did you use Defrost primarily? Do the RPM fluxes occur in time with the compressor cycling possibly? Just a thought.
Chris
Chris