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High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

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Old 07-28-2011, 12:34 PM   #1
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High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

I have a High/Erratic Idle, Missing/Bucking situation on my 91 S10 2wd 5spd 2.5L. The check engine light comes on when running like this. Odd thing, sometimes the problem goes away a couple miles down the road. Checked timing (engine was running fine at during the test), within spec.

New Parts Replaced:

Ignition: cap/rotor/wires/plugs, Ignition Control Module, Coil

O2 sensor (Bosch), Throttle Position Sensor, Delphi EGR, Map Sensor

I have a GM factory shop manual, but can't seem to nail down this problem. The battery & charging system was checked, all passed the test.

Appreciate your input
Old 07-28-2011, 12:38 PM   #2
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Re: High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

Truck has 69,000 miles. Really like this truck, great fuel mileage (24/28) and has good power, handles well, and good leg room. Hope to get her back in good working order.

note: the ICM was installed with the silicone jelly underneath as detailed in the shop manual.
Old 07-28-2011, 02:08 PM   #3
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Re: High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

I replaced the rubber vacum lines with rubber vacum tubing. Tested the hard plastic line that feeds into the TPI with a vac gauge, held tight. Also, the EGR control Valve was changed when the Delphi EGR unit was replaced.
Old 07-28-2011, 09:12 PM   #4
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Re: High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

Check your timing and see if it is stable at idle...... If you used rubber line to replace the hard plastic lines you may have screwed yourself.... the ol' duke likes the very narrow passages of the hard plastic lines......
Old 07-28-2011, 09:31 PM   #5
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Re: High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

Regarding vac line, replaced rubber for rubber, plastic for plastic. VW's are the same. My guess is the plastic line doesn't collapse on small signal or small volume vac applications, probably responds quicker to changes.

Regarding the timing, I'll have to catch it when it starts acting up. Currently, the timing is dead on the money (with electronic advance disabled per shop manual).

If the timing is off when acting up, what would that point to?
Old 07-28-2011, 09:31 PM   #6
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Re: High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

Regarding vac line, replaced rubber for rubber, plastic for plastic. VW's are the same. My guess is the plastic line doesn't collapse on small signal or small volume vac applications, probably responds quicker to changes.

Regarding the timing, I'll have to catch it when it starts acting up. Currently, the timing is dead on the money (with electronic advance disabled per shop manual).

If the timing is off when acting up, what would that point to?
Old 07-28-2011, 09:46 PM   #7
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Re: High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

Could be a bad hall effect sensor I suppose, just kinda wanted to make shure it had been checked and it was @ 8* adv.
Old 07-28-2011, 10:27 PM   #8
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Re: High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

Fortunately I bought a AC Delco Hall Effect switch from a GM dealer who was liquidating stock last year. Is that the same as the sensor or is it a part separate from the switch?
Old 07-28-2011, 10:34 PM   #9
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Re: High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

http://en.wikipedia.org/wiki/Hall_effect_sensor

Looking at this wiki article on the Hall Effect Switch/sensor, "Hall sensors are used for proximity switching, positioning, speed detection, and current sensing applications." I noticed a 2-4 second delay for the idle speed to adjust after the truck is at full stop.
Old 07-29-2011, 11:19 PM   #10
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Re: High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

Nope, that is the one...... Is the reluctor clean and free of rust/ debris?
Old 07-30-2011, 04:08 AM   #11
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Re: High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

I'll have to check the reluctor. Is that the sprocket/multi-finger looking wheel that attaches to the distributor shaft?
Old 07-30-2011, 02:11 PM   #12
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Re: High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

Thinking back, the reluctor did have some surface rust on it. I'll have to take some naval jelly to it for removal.
Old 07-31-2011, 12:50 AM   #13
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Re: High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

"The check engine light comes on"
so check for fault codes
Old 07-31-2011, 07:48 AM   #14
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Re: High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

Quote: Originally Posted by max1million
"The check engine light comes on"
so check for fault codes
It doesn't have any codes stored.
Old 08-03-2011, 10:55 PM   #15
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Re: High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

IF the CEL did come on it SHOULD have stored a code ASSUMING the ECM is good. Jumper the A/B terminals (upper R two pins) of the ALDL under the column and turn ignition on, dont start, count flashes. First Code should be a 12 (each code is repeated 3x, then starts over with first code (12). 12 is no tach signal from distr......
Old 03-29-2012, 10:14 AM   #16
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Re: High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

Same problem. High erractic idle (only on cold start), and the engine bucks when I'm accelerating or easing off the accel (but it only does it sometimes, and more often when fuel is low). 2.5 Iron Duke 5 speed. I have a new EGR. Anybody think TPS?
Old 03-29-2012, 09:35 PM   #17
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Re: High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

Check fuel pressure if it acts up more so when low on fuel....... Especially if below 1/2 tank, you may have a split in the rubber line connecting the pump to the top of the tank, or a leaking pulsator (if it has one instead of the rubber hose), poor pump, etc......
Old 04-03-2012, 12:37 PM   #18
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Re: High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

i am having a similar issue, i've changed tps, o2 sensor, coolant temp sensor, icm, hall effect, cap and rotor button, went out and bought a new p/u coil, and ignition coil, and i am still having an issue. i'm tracking the problem down, been three weeks of constantly throwing something else in the fire, the problems lessening though. could have multiple issues. my next guess for me is a idle air control. or egr for me. something else that may show up is when you come to a sudden stop. rpm's shoot way down past idle speed then back to normal. (atleast mine does)

good luck finding the problem. i'll keep my problems out on front also!
Old 04-06-2012, 10:36 PM   #19
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Re: High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

My 93 2.5 m/t doing the same thing as yours. I cut the exhaust loose still got the problem. Im replacing the ECM (brain) tomorrow. I rebuilt the TB an the distributer,New fuel pump,filter an injector filters. When I set the timing dead on 8 it want restart...timing on the engine want hold at times moves around when the engine gose above idle on it own . Cant figure it out....
Old 04-06-2012, 10:44 PM   #20
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Re: High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

Timing will move around unless you have the ALDL jumpered, or your timing gears are wore out.

Check your MAP sensor, its wiring, and the vac supply to it. Also check over ALL your Vac lines for leaks.

Also common issue on these trucks, check under the exhaust manifold steel cover for cracks in the manifold to make shure the ECM isnt chasing the O2 sensor with an exhaust leak.

Might pull and clean the TBI and especially the IAC.
Old 04-06-2012, 11:03 PM   #21
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Re: High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

Thanks for the fast reply Lester ! Where is the MAP sensor located is it on the breather or fire wall next to the canister ? I replaced the EGR also. What is the TBI and IAC? I have replaced the exhaust manifold last year no cracks as I can tell. Checked all the Vac lines no leaks as I can tell....Thanks again....John
Old 04-06-2012, 11:24 PM   #22
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Re: High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

Map sensor is on the breather, has a C clip on the inside to hold it up, should have a green 3 wire connector and a vacume line. Make shure it has vacume to it. Make shure clip is holding connector tight. Make shure the wires are ok.

Why did you replace the EGR and what brand was the new one/ old one.

TBI is Throttle Body Injection (kinda slang for the carb looking piece on the intake manifold)

IAC is Idle Air Control Valve, it should sit on the passenger side of the TBI, if older style intake it will look like a roll of quarters screwed into the side of the TBI with a square plug and 4 wires. If newer style will be a short round mini cupcake looking device with a right angle on the jack for the plug and a rectangular 4 wire plug. The IAC controls Idle Air Bypassing the Throttle blade to set the idle speed of the engine based upon the ECMs definitions in the PROM for current sensor inputs.

I would DOUBLE CHECK all the vacume lines, especially the little black plastic ones as they are awful about either cracking or wearing thru, and being virtually impossible to detect. I had one that caused me grief for months, and actually found it trying to do another repair, the line had been worn thru on the bottom, when the line sat on the intake/whatever it was on it was fine, but would leak under certian circumstances...

Good luck.
Old 04-06-2012, 11:37 PM   #23
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Re: High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

Thanks Lester I'll check your suggestion out tomorrow an get back with yall....if I dont get back with yall it's because Im still beating my head on the bumper....LOL
Old 04-06-2012, 11:45 PM   #24
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Re: High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

:-)
Old 04-07-2012, 11:40 PM   #25
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Re: High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

It was the ECM !! Back running like she should ...Well I'm not sure it was ECM ...I change so many parts I dont know but changing the ECM was the last resort and it running fine now...then the water pump went out ..crap... banging head against the bumper again...0
Old 04-08-2012, 01:17 PM   #26
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Re: High/Erratic Idle, Missing/Bucking 2.5 Iron Duke

Little John--

Glad to read that you got it going again--

Buy/use the best water pump you can find. New Old Stock is best, but near impossible to find, NAPA's rebuilt
Gold line would be good also.

My 2 cents on the pump..
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