Wh does my truck idle too fast, then to slow

Tiny
JACOB844
  • 1987 GMC SIERRA
  • 188,000 MILES

I have replaced all vacuum lines, replaced the following: EGR Valve, TPS, IAC, o2 sensor, rebuilt throttle the body, replaced the water pump, radiator hoses, temp sender, checked vacuum booster and flushed the cooling system. Truck is not posting any engine codes. This truck will idle about 2000 rpm then as low as 200 and fluctuates back and forth. It has stalled on occasion. Although sometime it will idle normally but that is rare. I have also replaced the exhaust manifold on the passenger side as it was cracked. Last year during winter season, I had trouble with engine at operating temperature but no heat. I had flushed the system, replaced the water pump and that resolved the problem. There is no oil in the cooling system.

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Wednesday, December 28th, 2011 AT 8:09 PM

22 Replies

Tiny
RIVERMIKERAT
  • MEMBER

Check ignition timing and compression. Check fuel pressure. Have it scanned for soft codes and live data. Soft codes won't set a CEL, so some shops won't try anything if they don't see it, so you need to tell them soft codes.

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Wednesday, December 28th, 2011 AT 8:34 PM
Tiny
JACOB844
  • MEMBER

Timing is correct for this make and model year 5.7L. Fuel pressure is within specifications on either tank. I have taken this to a shop and the consensus was to "smoke the motor" as they could not readily determine the problem that is causing the erratic idling. I am told that I most likely have an intake vacuum leak. I am almost resigned to just removing both heads and intake and replacing the gaskets.

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Wednesday, December 28th, 2011 AT 9:39 PM
Tiny
RIVERMIKERAT
  • MEMBER

Smoking the motor checks for vacuum leaks. You can come close to the same results with a can of carb cleaner, usually. When the leak is found, the idle will smooth out. It may not be an intake or head gasket. It could be a hose, plastic line, or fitting. How well does the throttle shaft fit in the throttle body? Is there any play in it?

Did the shop just check for hard codes (setting the CEL) or did they check for soft codes, as well? What about polling live data?

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Wednesday, December 28th, 2011 AT 11:40 PM
Tiny
JACOB844
  • MEMBER

When I was looking for the problem, I replaced all the plastic lines, hoses etc. I sprayed starting fluid all around the intake, TB, vacuum lines etc. With no difference. I also removed the plug to the IAC with no difference with regards to idling issue. I determined that spraying the TB around the vacuum lines, base gasket, linkage it was not the problem area. I must digress. I had the truck at a 2nd garage for repair. They could not resolve the issue and suggested I replaced the water pump. Replacing the water pump did not solve the problem. I had introduced into the cooling system which was to seal an internal leak (if it had one). I did notice an improvement with regards to cab heat but it did not last long; however that issue resolved itself, so maybe that material that was introduced into the cooling system did work, and in fact I do have an internal vacuum leak. I would lose heat from the temp gauge got over 200. Flushed out the heater core several times and that resolved that issue. The idling issue did not resolve itself. As far as codes, the mechanic just stated it did not throw any codes, did not elaborate hard or soft or mention about polling data.

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Thursday, December 29th, 2011 AT 1:04 PM
Tiny
RIVERMIKERAT
  • MEMBER

Your statement about nothing happening when you unplugged the IAC made me sit up. I'm going to ask one of my colleagues to peek in here and give his opinion, but I believe that maybe the IAC is bad. Please have patience, as he may take an hour or two to receive the message and have the time to check in.

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Thursday, December 29th, 2011 AT 7:44 PM
Tiny
MERLIN2021
  • EXPERT
  • 17,549 POSTS

My opinion is a manifold gasket leak or fualty injector. But this is the way to set the IAC up correctly.
THROTTLE BODY INJECTION
1. Remove air cleaner and gasket. Plug TBI unit vacuum port for THERMAC (Thermostatic Air
Cleaner). With Idle Air Control (IAC) valve connected, ground terminal "A" to "B" at ALDL
connector.
2. Turn ignition switch to "ON" position, DO NOT start engine. Wait approximately 30 seconds for
IAC valve pintle to extend and seat in throttle body.
3. With ignition on, disconnect IAC valve electrical connector. Remove ground at ALDL connector.
Using an awl, remove plug for minimum idle speed adjustment by piercing, then applying
leverage.
4. Start engine. Adjust idle screw to specification. Turn ignition switch "OFF". Reconnect IAC valve
electrical connector. Install air cleaner and gasket.
SLOW IDLE SPEED (RPM)
NOTE: Adjustment is necessary only when throttle body assembly has been
replaced.

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Thursday, December 29th, 2011 AT 8:34 PM
Tiny
RIVERMIKERAT
  • MEMBER

Thanks Merlin.

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Thursday, December 29th, 2011 AT 9:54 PM
Tiny
RASMATAZ
  • MEMBER

Happy belated Merry Christmas and a Happy New Year 2012-Boston Mike and nice to see you again old timer-

My opinion sounds more like the IACV is defective or its wiring back to the computer-who knows could very well be the computer-also might be stuck partly open or operating on its default values

Unplugging the IACV should caused the idle speed to react its purpose is to target the idle speed at all times at curb idle speed or you can adjust for minimum air as suggested by merlin

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Thursday, December 29th, 2011 AT 11:24 PM
Tiny
MERLIN2021
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If its not set up right it may not work. That's why I included the instructions too set it up!

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Friday, December 30th, 2011 AT 12:35 AM
Tiny
JACOB844
  • MEMBER

Merlin. Thanks for that information. Is A and B the two top right hand openings that are side by side on the ALDL? I will have to look at the ALDL to see where it is grounded. Could you be more specific with regards to "Using an awl, remove plug for minimum idle speed adjustment by piercing, then applying
leverage." Remove what plug? From where? At what point should I reconnect the ground to the ALDL? Thanks

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Friday, December 30th, 2011 AT 3:05 AM
Tiny
RIVERMIKERAT
  • MEMBER

Thanks guys.

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Friday, December 30th, 2011 AT 3:12 AM
Tiny
JACOB844
  • MEMBER

Did as you suggested (1) jumped across A/B at ALDL. (2) with key turned on, you could view seated pintel in the TBI body and you could hear and feel the IAC running.(Type 1 with collar) When cold, truck would not idle without applying throttle. When warm, truck would idle between 800 and 400 rpm.

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Friday, December 30th, 2011 AT 6:41 PM
Tiny
MERLIN2021
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Get the smoke test. I still think injection or intake gasket. Injector should have fine spray, no drops of gas.

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Friday, December 30th, 2011 AT 7:11 PM
Tiny
RASMATAZ
  • MEMBER

Has the TPS closed and wide open throttle voltages been checked/tested if not check it

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Friday, December 30th, 2011 AT 9:42 PM
Tiny
RIVERMIKERAT
  • MEMBER

Jacob, look for a small screw on the throttle body that stops the throttle from closing. That screw is the idle speed screw, or idle air adjustment. It has a plug over it, covering the screw head. It works in the same way that the idle speed screw does for carbureted engines. I can't find any images of the throttle body to help.

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Friday, December 30th, 2011 AT 10:03 PM
Tiny
JACOB844
  • MEMBER

Merlin. I had removed the TBI and utilized a rebuilt kit, including two new injectors. I observed a fine spray when it idles. On another note and not sure if it is related, my fuel gauge (on both tanks) fluctuates back and forth as the fuel sloshes around.
Yesterday was the first time that I managed to hear as the engine idled up and down. On the up stroke I thought I heard a high pitched whistle, I have yet to find it. After 20 minutes or so with the motor idling. I could no longer hear that sound. I also did a thump test on the computer module (another mechanic suggested that idea).

Rasmataz. I had replaced the TPS and no, open throttle voltages have not been checked and or tested, at least not by me. I have had this at three garages and either it was done but results where normal or it was not done and I tossed my money away to a garage that did not do their job. I will have to refer to my tech manual on this and see what I need to do to check the voltage and what it should read.

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Saturday, December 31st, 2011 AT 2:58 PM
Tiny
MERLIN2021
  • EXPERT
  • 17,549 POSTS

Here's the complete list of causes for poor idle.
ROUGH, UNSTABLE, INCORRECT IDLE OR STALLING
1. Check ignition timing. Check Park/Neutral switch circuit. See PARK/NEUTRAL SWITCH
DIAGNOSIS chart in GENERAL MOTORS COMPUTER COMMAND CONTROL article in
COMPUTERIZED ENGINE CONTROLS section.
2. Check injector for leaks. See DIESELING, RUN-ON. Check fuel pressure. See FUEL SYSTEM
PRESSURE TEST. If idle is rough only when engine is hot, perform the following checks:
Check for vacuum leaks by installing Plug (J-330247) in idle air passage. If closed throttle
engine speed is above 650 RPM, locate and correct vacuum leak.
Check Park/Neutral switch circuit. See PARK/NEUTRAL SWITCH DIAGNOSIS chart in
GENERAL MOTORS COMPUTER COMMAND CONTROL article in
COMPUTERIZED ENGINE CONTROLS section.
Check for sticking throttle shaft or binding linkage that may cause a high TPS voltage (open
throttle indication). If binding occurs, the ECM will not control idle. Check TPS voltage.
Reading should be less than 1.2 volts with throttle closed.
Check EGR valve operation. See appropriate EGR SYSTEM CHECK chart in GENERAL
MOTORS COMPUTER COMMAND CONTROL article in COMPUTERIZED ENGINE
CONTROLS section.
Check battery terminals, cables, and grounds straps for loose or corroded connections.
Erratic voltage will cause IAC motor to change its position, resulting in poor idle quality.
IAC valve will not move if voltage is below 9 or greater than 17.8 volts.
On 2.5L engine, check power steering pressure switch. See POWER STEERING
PRESSURE SWITCH chart in GENERAL MOTORS COMPUTER COMMAND
CONTROL article in COMPUTERIZED ENGINE CONTROLS section. ECM should
compensate for power steering loads.
Check MAP sensor operation. Start and allow engine to idle. Disconnect MAP sensor
electrical connector. If idle improves, substitute a known good sensor and retest.
Check A/C for high refrigerant pressure. Check A/C compressor or relay. If inoperative, see
appropriate A/C CLUTCH CONTROL DIAGNOSIS chart in GENERAL MOTORS
COMPUTER COMMAND CONTROL article in COMPUTERIZED ENGINE
CONTROLS section.
Inspect oxygen sensor for silicon contamination from fuel or improper use of RTV sealant.
Sensor may have a White, powdery coating, and will result in a high but false signal voltage
(rich exhaust indication).
Check PCV valve for proper operation by placing finger over inlet hole in valve end several
times. Valve should snap back. If not, replace PCV valve. Perform compression test.
Tips Click a link to view tip
Tech1 Question: hard starting hot or cold check engine light stays on intermitant

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Saturday, December 31st, 2011 AT 5:18 PM
Tiny
RIVERMIKERAT
  • MEMBER

Fuel gauge fluctuation with fuel sloshing is normal. It means the sending units are reacting properly and immediately.

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Saturday, December 31st, 2011 AT 7:30 PM
Tiny
JACOB844
  • MEMBER

This is a 1987 Sierra Classic V2500, full power group, 4 spd muncy, 5.7 TBI. I added additional ground straps this past summer as I thought too it was a ground issue. Replaced both batteries in October. Replaced items when I did a tune up, ie: wires, cap, rotor, pvc, fuel and air filter, wire set. I am going to fetch a new set of gaskets for the TBI 220 after the New Year even though I don't see a difference when spraying carb cleaner. I did find that a slight alteration to the foot peddle was causing it to hang at over 2000 rpm. Removed that aluminum foot peddle and that solved that issue. At present, it will start out idling at 650, the start to go from 400 to 800. Did notice will start then stall unless fuel is applied.

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Sunday, January 1st, 2012 AT 8:57 PM
Tiny
RIVERMIKERAT
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"Will start, then stall until fuel is applied."
If the engine doesn't want to stay running at idle, and everything else is right, this is usually an idle speed setting. Perform that idle speed adjustment mentioned above and it should fix the problem.

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Sunday, January 1st, 2012 AT 9:24 PM

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