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Stumble, Hesitation, Stalling At Low Rpm


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#1 nastyZ71

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Posted 06 May 2009 - 07:27 AM

first the basics.
2000 GM sierra, 5.3 4x4 etc

I have a problem that is driving me nuts trying to figure out.
at low rpm (truck in gear and moving) the truck hesitates, sometimes almost stalling out.
I have started with the fuel system doing a pressure test and found the pressure regulator was bad
so that was changed out. now the fuel pressure is spot on according to the manual.
then figured I would do a complete tune up, new plugs and wires all amking no difference at all.
have remaoved the IAC and cleaned it, the throttle opening was also cleaned at the same time.

I did a slow speed drive around the yard with my pressure gauge hooked up to see if I was loosing fuel
pressure when the stalling occours but the pressure does not change at all.
I am at a bit of a loss here as it only happens at lower rpm with the truck moving, it idles perfect with
no DTC's showing and out on the road when you put your foot into it is takes off just like it should.
at WOT there is not stumble or lack of power.

any suggestions where I should be looking next?
Y2K GMC Sierra 1500 4x4 SLT
5.3L with wait4me tune, magnaflow
exhaust, airaid power tube,
Ford T-bar keys for 2" lift and 285's
310,000 kilometers and counting.

#2 Stevens11

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Posted 06 May 2009 - 08:15 AM

Clean the MAF sensor and run some seafoam thru the brake booster to clean the engine. :nonod:

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#3 nastyZ71

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Posted 06 May 2009 - 09:24 AM

Clean the MAF sensor and run some seafoam thru the brake booster to clean the engine. :nonod:


I'll give that a try tonight. already have 2 cans of seafoam waiting to go.
what is the best to clean it with? will the throttle body spray cleaners be fine?
I would imagine with the MAF there will have to be no residue.
Y2K GMC Sierra 1500 4x4 SLT
5.3L with wait4me tune, magnaflow
exhaust, airaid power tube,
Ford T-bar keys for 2" lift and 285's
310,000 kilometers and counting.

#4 66nova

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Posted 06 May 2009 - 09:27 AM

Clean the MAF sensor and run some seafoam thru the brake booster to clean the engine. :nonod:


I'll give that a try tonight. already have 2 cans of seafoam waiting to go.
what is the best to clean it with? will the throttle body spray cleaners be fine?
I would imagine with the MAF there will have to be no residue.



They make a MAF cleaner. Go to you local parts store. From everything I have read and heard, you do not want to use Throttle body or carb cleaner on the MAF.

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#5 Stevens11

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Posted 06 May 2009 - 09:34 AM

What he said, but any good electrical contact cleaner should work just fine. Just don't touch the diode with your hands. The spray cleaner is all you need to clean it.

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2011 GMC Sierra SLT Crewcab 6.2L with NHT and 3.73 gears


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Current truck:
Denali Grill, 2" RC Leveling kit, Boss MT100 20x8.5 wheels w/ 305/50r20 Cooper Discoverer H/T Plus tires, Blackbear tuned, Extang Full Tilt Bed cover, GMC Bedrug, 15% Tint, and Custom Dynamics light bar.

Previous trucks:
2007 Sierra 6.0 Vmax crewcab 4x4, 275/55r20's Cooper Zeon LTZ tires, Corsa Sport Exhaust, Denali Grill, Bedrug, Extang Full Tilt Tonneau Cover, Blackbear performance Custom tune
2003 Silverado 5.3L extended cab 4x4, 6" Skyjacker lift, 315/75r16 BFG tires, Goodmark Cowl hood, Billet grill, K&N 77-Series intake, Magnaflow exhaust, Wait4me PCM tune
2001 S10 Crewcab 4.3L 4x4; 3" body lift, 31" BFG K/O tires, Flowmaster 50-series exhaust, K&N 63-series intake, relocated IAT sensor, E-fans, TB Spacer, Jet stage 2 chip with B&M shift improver

#6 bill190

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Posted 06 May 2009 - 09:49 AM

At idle, the throttle is completely closed. The engine gets its air from the IAC valve and the PCV system.

Can you slightly press on the accelerator and this makes the engine run just fine? This is allowing a little air opening in the throttle. So perhaps air problem?

Might want to check that the PCV valve is clean as well as the tubing. Then there is another tube going to the valve cover with is part of the PCV system. Make sure that is not blocked either.

Then for the IAC valve, peek inside the opening where the "pintle" moves in/out and be sure there is no gook blocking the pintle's ability to move back and forth in the opening. I use a mirror and flashlight to see inside the opening.

So far as testing the IAC valve, there should be 4 electrical connections. Each set of two should measure around 40-80 ohms on my truck (yours may be different). This is an electric "stepper motor" and it has two "windings". These windings connect to two electrical pins each. So test two connections and if nothing, move one test lead to the next close connection and you should get a reading. So if you have connections A, B, C, D. (4 connections). Try A and B. If nothing try A and C. If nothing try A and D. Then when you get a reading for two, the other two should get a similar ohms reading (use a multimeter).

So far as testing an IAC valve to see that it moves back and forth OK inside the opening and makes the idle speed increase or decrease as it moves in/out, this is impossible to test without buying expensive test equipment (because it is a "stepper motor"). So best to replace it if you suspect it is having a problem. But you should get a diagnostic code if it is not able to adjust the idle.

Then what makes the IAC operate? (You may have a perfectly fine working IAC valve, but the computer is not telling it to adjust the idle!)...

My factory fuel and emissions service manual says to check/replace the oxygen sensor (I've read this should be replaced every 60,000 miles anyway - regular service item.).

Then the throttle position sensor (or a "switch" on the throttle which should activate when the throttle is at idle). My book says if these are not "telling" the computer that the throttle is at idle, then it will not adjust the idle! (The computer adjusts the idle by sending signals to the IAC valve to open/close a little and thereby adjust the speed.) You would check for voltage on the throttle position sensor with a multimeter and it should give certain readings based on how far open it is. A chart for various positions and voltage readings should be in the fuel and emissions service manual.

Then the book says to check the coolant temperature sensor. On my vehicle, there are TWO temperature sensors! One for the gauge on the dash and the other is for the computer. The service manual should tell you what ohms (or voltage) based on temperature this sensor should be reading. (Use multimeter and thermometer.)

Then MAP sensor. You would test this using a voltage reading per the service manual specs for your vehicle.

Edited by bill190, 06 May 2009 - 12:57 PM.


#7 nastyZ71

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Posted 06 May 2009 - 01:47 PM

thanks guys. will look into this tonight
Bill, the truck idles perfect. when you are on the gas it pulls clean with no stumbles or hesitation.
it only acts up when you are cruising with little load on the throttle.
then it can't seem to decide to declerate, hold steady or accererate.
if you take your foot off the gas at that point then it really stumbles and sometimes stalles.
as soon as you put your foot into it there are no problems.
I had the IAC out and cleaned the valve and the port. the valev is not siezed and at idle there
is no changes unless the ac pump cycles but is otherwise smooth and even.
PCV seems to be fine with good vacuum at the valve and free motion of the valve.
will look at the TPS tonight.

was thinking of checking the resistance of the injectors but it starts without issue and is fine
on acceleration so I can't really see there ebing a problem.

could this be something ignition? but a misfire would trigger DTC would it not?

when it first started this I did change the fuel filter as well. forgot to add that.

Edited by nastyZ71, 06 May 2009 - 01:49 PM.

Y2K GMC Sierra 1500 4x4 SLT
5.3L with wait4me tune, magnaflow
exhaust, airaid power tube,
Ford T-bar keys for 2" lift and 285's
310,000 kilometers and counting.

#8 bill190

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Posted 06 May 2009 - 04:21 PM

So if you start it up in the driveway, it idles just fine?

This problem only happens when the vehicle is moving and foot off the gas?

How about after you have been driving and you stop somewhere, put it in park, and let it idle (warm engine and warm sensors [heat can cause things to not work as they should])?

How about if the engine is warmed up, and you put it in drive, but are not moving (foot on brake)?

Or sitting in the driveway in park, A/C on, headlights on, and you turn the steering wheel a little (placing a load on the engine other than via transmission)?

#9 nastyZ71

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Posted 06 May 2009 - 09:02 PM

ok, cleaned the MAF, no difference.
pulled the IAC and checked resistance, got a reading of 46 and 47 within limits.
TPS within limits and still no DTC's

I did find that when light throttle is applied (truck sitting in park) once it has leveled off it will surge then slow down.
the engine sounds different as well, not like a misfire but not as smooth.
tried loading the engine with electrical and steering and idle adjusyed itself just like it should.
the idle does flucuate slightly as the ac pump cycles but only about 100 rpm, nothing like it is going to stall.
it pretty much acts the same way hot or cold. temp seems to make no difference.

the way it stumbles when it is about to stall then catches itself it is quite abrupt.
being so abrupt I am almost thinking it is an ignition issue but I have no misfire problems.
Y2K GMC Sierra 1500 4x4 SLT
5.3L with wait4me tune, magnaflow
exhaust, airaid power tube,
Ford T-bar keys for 2" lift and 285's
310,000 kilometers and counting.

#10 daddy

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Posted 06 May 2009 - 09:30 PM

Check your ground.

This sounds very similar to a situation I had with my '91. I threw hundreds of dollars and many frustrating hours at a problem that eventually took $6 and 10 minutes.

It's a cheap possibility.

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#11 bill190

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Posted 06 May 2009 - 10:33 PM

The IAC sounds good. And if it is responding to a loaded engine, then it sounds like it is working. It's job is to adjust the idle speed.

I would check all those sensors with a multimeter to be sure they are giving the computer the correct information. (Per your factory fuel and emissions manual specifications for each sensor.)

If the computer is not getting the correct information from a sensor, then it can't adjust the engine to run properly. (Like if it thinks there is little or no oxygen in the exhaust when in fact there is, or if it thinks the engine is cold when it is warm, or if it thinks there is lots of air pressure and there is little, etc.)

If you test each sensor, then there is no guessing. Either it is working like it should or it is not. Then you don't waste money buying parts you don't need.

For example based on the information provide by the oxygen sensor, the computer would adjust the engine more rich or lean.

If you have an IAT sensor, this would retard the timing when it was hot.

The MAP sensor would cause the injectors to send more or less fuel.

And the coolant temperature sensor changes everything! (Computer adjusts everything based on this information.)

#12 green 788

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Posted 07 May 2009 - 08:18 AM

A totally different animal, but I'll share an experience I had with my 1994 Ford Escort...

It was doing EXACTLY what you're describing. I would have to rev the engine up at a red light or stop sign just to pull out. Once I got up to speed, things seemed fine. The car would idle just fine. But when I would go to pull out from a stop, or cruise along at 5 to 10 mph between red lights, etc., the car would falter. It's a 5 speed manual shift, so I'd just press the clutch in and rev up the engine then get her going like that.

I did pretty much what you've done to your Chevy truck. I changed the mass airflow sensor... I cleaned the intake plenum with the proper solvent. I cleaned the injectors... we checked various other specs and components.

In my case, it ended up being a crankshaft position sensing issue. The 1.9 Ford engine has notches on the outside of the harmonic balancer, and the crankshaft position sensor is mounted to monitor those notches. The computer, based on the input from the CPS adjusts the timing advance. My harmonic balancer was loose and wobbling, and this was throwing off the crankshaft sensor input readings to the computer. Fixing the balancer in my case solved the problem.

The Chevy engine of course doesn't use the harmonic balancer for crankshaft position info (at least I don't think your engine does)... but if you did have an issue with your CPS, it could be failing to keep the computer apprised of the exact position of the crankshaft, thereby not allowing it to set the proper ignition advance.

I would try removing the CPS and cleaning it. (It might have metal fuzzies on it). :thumbs: Make sure it's got good magnetism. Check its wiring, etc.

Good luck. :lol:

Dan

#13 nastyZ71

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Posted 07 May 2009 - 11:09 AM

that sounds like a good place to have a look at.
under load or full throttle life is good.
it's just when you get to a level load when it acts up.
accelerating it would likely by watching other sensors
only thing is would it not throw a code if there was an issue with the cps?

maybe a crank relearn may be in order?

Edited by nastyZ71, 07 May 2009 - 11:10 AM.

Y2K GMC Sierra 1500 4x4 SLT
5.3L with wait4me tune, magnaflow
exhaust, airaid power tube,
Ford T-bar keys for 2" lift and 285's
310,000 kilometers and counting.

#14 green 788

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Posted 07 May 2009 - 06:24 PM

I don't know if it would code or not... I know my little 1.9 Ford motor never coded, but of course it's quite a bit older than your truck.

You might take it by an auto parts store and see if their free diagnostic would pull anything up. Sometimes there are fault codes that don't actually light the engine warning light--at least that's been the case with other vehicles I've owned.

Post back and let us know what the eventual solution is. :pimp:

Dan

#15 nastyZ71

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Posted 18 July 2009 - 09:30 PM

it's been a while since I have had the time to look at my truck but was back at it this weekend.
after checking injectors, crank position and can't remember what all else I got to the EGR.

now when it's unplugged it runs perfect!! as soon as I reconnect it I start having issues again.
I pulled it and cleaned it with no difference. the pintle seems to move very easily.
I thought there would be more spring tension on the pintle.
is there any way to test the EGR?
a replacment will likely be pretty pricey so not looking to throw money without knowing it is bad first.

some 5.3's have no EGR on the engine. could I just block it off while leaving it connected to the EGR
so the computer thinks everything is there and not throw a code?
Y2K GMC Sierra 1500 4x4 SLT
5.3L with wait4me tune, magnaflow
exhaust, airaid power tube,
Ford T-bar keys for 2" lift and 285's
310,000 kilometers and counting.




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