SOLVED! RPM Drop On Acceleration.

Mektek

Original poster
Member
May 2, 2017
656
FL
I'm experiencing a random issue with accelerating from a stop. As often as half the time the engine RPM momentarily drops to 500 when the accelerator pedal is pressed, after which it accelerates normally. It seems to be more prevalent when the engine is hot. No SES light.
Any ideas?
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
Has the Throttle body cleaned lately?

I had that on my 02. Seemed to happen if I let it idle for a while. Never could figure it out.
 

mrrsm

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Oct 22, 2015
7,639
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Before attempting this action... Please be prepared to Fully Investigate and Repair problematic Wiring Harness connections prior to doing this Test which MAY make the situation WORSE:

Tools and Supplies at the Ready:

(1) Scope On A Rope - Lighted Continuity Probe.
(2) DMM or DVOM.
(3) Automotive Grade Wire Heat Shrink Insulators.
(4) Black Electrician's Tape.
(5) Soldering Kit with Gun & Rosin Core Silver Solder.
(6) A Bright Flashlight.
(7) Various Colored 6" Long Automotive Wires.
(8) ,... Better Yet, Obtain a Replacement Harness Connector. (...around $10.00 on Amazon):


TBTHROTTLEBODYHARNESS1.jpgTBTHROTTLEBODYHARNESS2.jpg

(9) Having this Inexpensive Broken Wire Probe Kit would help you to locate and isolate any Broken Wires and there are quite a few Training Youtube Videos to demonstrate just how handy it will be:


BROKENWIRETESTER1.jpg

With the 4L60e Shifter in PARK and the Emergency Brake SET, Start the Motor and allow the vehicle to idle for a few minutes and warm up. Then address the Wire Harness leading to the Throttle Body Connector and gradually move them around it slow, small semi-circular motions and observe and listen carefully for the slightest signs of any Engine Stall or sudden increase in RPM.

If your actions can cause this to occur, clear the Work Field, lay down a Wide Work Towel. Then carefully un-wrap the OEM protective coverings in a non-destructive manner from around the TB Harness Wire Bundle.

As the Engine Idles around 600-650 RPM... attempt to repeat the RPM Drop Event by Slightly, LIGHTLY pulling on One Wire at a Time right where each Wire enters the back side of the Weather-Pack Connector and gradually work your way along each wire ...very slowly. If the Engine Sputters and Stalls... focus on that Single Wire-Connector point. Be aware that while Only One Wire should be repaired at time, MORE than One Wire may be involved.

In some cases, the Light Pulling Action will be enough to completely separate the Sketchy Wire Hidden INSIDE of the Wire Insulation. Naturally, an On-The-Spot Repair would be called for and so having the necessary Gear prepared and on hand will move things along without delay.
 

Mektek

Original poster
Member
May 2, 2017
656
FL
The throttle body was cleaned a few thousand miles ago. It also happens after idling for a while - like at a stoplight.
I'll check the the wiring harness to the throttle body, I suppose it's possible to have a problem there, although I haven't had any other problems with the harnesses anywhere else.
 

gmcman

Member
Dec 12, 2011
4,656
@Mektek what year is your vehicle?

With a slight stumble off idle, here's a few suggestions.

If an early model, how old is your fuel filter?

How old are the plugs?

When you open the throttle body, there is a momentary increase of compression, if the plugs are weak, they may pose a problem igniting the higher fuel/air charge. At the same time, you may not be able to compensate with fuel at the moment of higher air flow if the filter is restricted but the filters take quite a bit to restrict flow. Could also be dirty injectors or a faulty FPR, but dirty injectors would be more likely.

Possible very lazy O2 sensor.

Do you have a bluetooth OBD adapter? You could monitor the upstream O2 sensor.

Check for a vacuum leak. Make sure the intake manifold bolts are tight, but not too tight, only 89 inch-pounds.

Honestly, my goto treatment before any wrenching in a situation like this would be a can of BG44K. I'll go out on a limb and say any other fuel treatment is a compromise.

After the 44K, if over 80K miles on the plugs and upstream O2 sensor, those would be the next to replace, replace both the plugs and sensor.
 
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Mektek

Original poster
Member
May 2, 2017
656
FL
It's a classic 02. New plugs, fuel filter and pump. The o2 sensor could be original, I don't know. But a bad 02 should cause other driveability issues, not just the acceleration off idle issue.
When I'm checking the harness to the throttle body I'll look for a vacuum leak too.
 

gmcman

Member
Dec 12, 2011
4,656
The vacuum leak idea is based on since they removed the master cylinder ensuring the booster diaphragm is good is worth a check.

I would think if the upstream O2 sensor is lazy it might throw a code. However not knowing the long-term fuel trim at idle it's a possibility.

I wish there was a fuel pressure port on the rail, hard to say what it actually is since the fuel pressure regulator is in the circuit.

This is where the Torque Pro app is very handy. Monitoring cylinder misfires, knock retard & fuel trims helps out.

Could be a weak coil pack or maybe one of the springs inside the boot is damaged.
 

Mektek

Original poster
Member
May 2, 2017
656
FL
I wiggled and twisted the throttle body and MAP sensor harness with the engine running and that did had no effect.
It got a bit cooler in the morning and I while I drove it without AC it did not do it. Later in the day I had the AC on again and the random RPM drop on acceleration returned again, although perhaps not as dramatic as before.
So I'll still try removing the throttle body and cleaning what ever there may be to clean. That will also trigger an idle relearn, which may do some good.
 

gmcman

Member
Dec 12, 2011
4,656
Hard to say without knowing how the upstream O2 sensor is performing. You could be idling a little rich then maybe it takes a few cycles to clear the cylinder when accelerating.

Anytime I'm presented with mild driveability issues, BG44K is my goto.

I add a can every fall and spring, matter of fact, going to do so this week.

Never know what the combustion chambers look like and this stuff clears it up pretty good.
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
Don't forget to disconnect the battery or pull the two PCM fuses while cleaning the throttle body.
 

mrrsm

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Oct 22, 2015
7,639
Tampa Bay Area
The VOP presents with the same problem on his 2003 Trailblazer and one of the respondents suggested Changing The "Throttle Pedal Switch" (ACC Pedal Module ?) Since doing so, this Fixed his Problem.

THROTTLEPEDALSWITCH.jpg

 
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Mektek

Original poster
Member
May 2, 2017
656
FL
I suppose after a quarter million miles there could be wear on the pedal sensor. Or the throttle position sensor. It should set a code if it's really bad. I'll pull them both and check with a DVM for smooth operation.
Or it might be prudent just to replace them as PM.
BTW - when I cleaned the throttle body before I did not disconnect the battery or fuse. The PCM did the recalibration automatically and found the correct idle after a few minutes of "hunting"
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
You MUST disconnect the battery or pull the fuses for 30 minutes. The PCM automatically adjusts to compensate for the dirty TB and that must be cleared when it's cleaned. Conversely, you must clean the TB if the battery is disconnected.
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
It is well documented here, the old site, the Internet, YouTube. Heck, I think there's even a TSB about it somewhere. That's why when a battery is replaced and the TB is dirty, it will idle poorly, especially when the A/C is on.
 
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mrrsm

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Oct 22, 2015
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...and not necessarily anything any Walmart Automotive Technician performing New Battery Sales R&Rs would include in his actions...
 

Mektek

Original poster
Member
May 2, 2017
656
FL
I pulled the throttle body and checked it. It had very few deposits - definitely not a cause of the problem.
However, when I shook it, I could hear something rattling inside the gearcase. I removed the cover and found two plastic teeth that fell out:badday:
So what happens is that the throttle plate is rotating to a particular position, moves to the area of the missing tooth, and the spring loaded plate closes by the angle that would maintained by the tooth - instant rpm drop!

There are reman TBs sold, so it might be repairable. I think you have to remove the plug on the side to be able to remove the shaft.
A new one is $125, used around $35. If I go for the used one I'll shake it and listen for teeth rattling around to avoid buying one with the same problem.

Stupid Engineers - making critical components exposed to high temperatures out of crappy plastic. If the gears were metal they would last forever.

Another oddity of an aging engine on the GMT to watch out for.
 

Mektek

Original poster
Member
May 2, 2017
656
FL
Something I noticed when shopping for a replacement. The listing shows 03-07 model years. I have an 02 that looks identical, but I wonder why it was excluded. I can see that the 08-09 model years went to a six pin connector. Any ideas why 02 would be different?
 

Mektek

Original poster
Member
May 2, 2017
656
FL
Installed the new throttle body - a fine Chinese reproduction of the GM original. For only five bucks more than a used one at the PYP it was a great deal :celebrate:
I removed the PCM 1,2, and B fuses and reinserted them half an hour later. That had no effect - when started the engine there was no special "relearn" - it started and ran normally.
The low cost unit seems to work well, and I would even speculate that the acceleration is smoother and quicker than before.
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
Could you post a link to the CCC throttle body?
 

Mektek

Original poster
Member
May 2, 2017
656
FL
Here's a dissection of the throttle body:
throttle body1.jpg
Look at the center gear and you can see the two missing teeth. The plastic is a special polyphthalamide (PPA) reinforced with 30% glass fiber and 15% polytetrafluoroethylene (PTFE). There was no lubrication anywhere so the plastic is self-lubricating. The spring is a very unusual braided steel.

Repairing this or pretty much any other problem (other than cleaning) with the throttle body is impractical. To disassemble to this stage is difficult and must be done in a particular sequence to avoid damage. If I did it again there is a small hack to the position sensor that would probably make it easier to remove.
But hopefully the replacement is as good as the original and it will go to the PYP when the TB's days on earth are done:diggrave:
 

Mektek

Original poster
Member
May 2, 2017
656
FL
I took it for a longer ride under varying conditions and the problem was gone. That also confirms that the low cost replacement throttle bodies can be well made and work properly, although only time will tell.
The old TB was clean and when I installed the new one no relearn procedure took place. I tried to trigger it by removing all the pcm fuses but that did not do it. Maybe it's different between the different PCMs - I have the first generation.
So I conclude that the new unit was made identical to the old and the pcm had no problem with it.
Mark this one as SOLVED!
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON

Mektek

Original poster
Member
May 2, 2017
656
FL
Appearance is similar, but I don't have the dot matrix serial number on the motor housing, so it's not the same.
 

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