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2000/2200 rpm dead spot

04-silver

By 04-silver
in Ask A GM Technician

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04-silver Enthusiast

04-silver

Posted
Posted

I started a post about this some time ago.....(apparently two now that I've back searched)

first post

second post

 

Seeing that camping season is over up here(Canada) and the truck will be due for its' bi-annual check-up soon I want/have to get this problem fix.

 

This summer I've been pulling our 5th wheel and this dead spot and lunge is a lot more prominent.

 

The problem, Readers Digest version somewhat:

 

-the easiest way I can repeat this is to drop the truck down to 80 kph

-accelerate to 95 kph(in a normal manner). RPMs will be approx. 2500/2800

-within 30 seconds of maintaining 95 kph the rpms will drop to 2200 there is a slight but noticable exhaust noise change, deeper/thicker??

-now that I am 95 kph and 2200 rpms I try to accelerate

-I will depress (gradually not hard)the accelerater to what feels like 3/4" to 1" and nothing, no rpm increase, no kph increase, nothing, nothing.... dead.

- then the pedal gets to a certain spot, bang! the truck lunges forward so hard I look in the review to see if I still have a trailer back there! This is a good jolt, this will clean out your cup of McDonalds hot coffee!

-the rpms have just jumped from 2200 to 3500 in a split second! and doing maybe 100 kph

 

-to avoid this sudden whiplash, when cruising at 95 kph/2200 rpm I must completely remove my foot from the accelerater for a nano second and redepress the pedal normally and all is well, rpms increase 22-23-24-........2800,.....kphs increase relative with the rpms.

 

This is about the best I can explain this, hope it makes sense.

 

 

I've had some by help/input

It's the torque converter clutch locking when you see the rpm's drop to 2000. Why it doesn't  go up from there is a good question. Sounds like a VSS (vehicle speed sensor) problem to me, or some corruption in the PCM program. The dealer only sees that the program is there and functioning. It cannot see if the program is corrupted if no codes are being set.

 

 

 

 

 

:chevrolet:

John

 

PS-the dealer mechanic told me already has latest program-Jan/05

GM-Tech Senior Enthusiast

GM-Tech

Posted
Posted
-accelerate to 95 kph(in a normal manner). RPMs will be approx. 2500/2800

-within 30 seconds of maintaining 95 kph the rpms will drop to 2200 there is a slight but noticable exhaust noise change, deeper/thicker??

 

Sounds like normal TCC engagement.

 

As for the second part of your description... also sounds like TCC, though disengagement obviously. Why it's so rough or sudden is hard to say. Is TPS percentage increase/decrease smooth throughout it's range?

04-silver Enthusiast

04-silver

Posted
  • Author
Posted

:chevrolet:

Is TPS percentage increase/decrease smooth throughout it's range?

 

If this is what our asking the TPA (accelerater) feels fine, no jurks, feels like squishing a bag of marshmellows :confused:

 

 

:flag:

John

GM-Tech Senior Enthusiast

GM-Tech

Posted
Posted
If this is what our asking the TPA (accelerater) feels fine, no jurks, feels like squishing a bag of marshmellows :chevrolet:

 

TPS is throttle position sensor. Good or bad, it's not something you can feel. Basically, it's like a potentiometer and it sends back a voltage signal based on the position of the throttle.

04-silver Enthusiast

04-silver

Posted
  • Author
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TPS is throttle position sensor. Good or bad, it's not something you can feel. Basically, it's like a potentiometer and it sends back a voltage signal based on the position of the throttle.

 

 

 

I can't answer that as I don't have a clue and I wouldn't even know where to start to check this out.

I will mention this to the dealer when the truck is in for its check up.

 

For the sake of an agrument lets say the TPS is the problem then this>

-I will depress (gradually not hard)the accelerater to what feels like 3/4" to 1" and nothing, no rpm increase, no kph increase, nothing, nothing.... dead.

would be where the voltage signal is not being delivered and then>

- then the pedal gets to a certain spot, bang! the truck lunges forward so hard I look in the review to see if I still have a trailer back there! This is a good jolt, this will clean out your cup of McDonalds hot coffee!

this would be when the voltage signal suddenly gets delivered again and says hey man I way up here get moving........ :chevrolet: so speak.

 

This seems to make sense, could be based more on where the pedal is then say the rpms, but on the otherhand, unless I don't get the drift of this TPS, this jolt or rapid rpm take off happens when in D also but different rpm & kph

 

when 3rd the rpms are 2200 and the kph is 95 (more promient in this gear)

when in "D" the rpms are 1800 and the kph is 80

 

I would think when in 3rd, the pedal is being depressed a lot farther down then when in D. Make sense maybe or am I going way out in left field?

 

Anyway the TPS is a start, I can ask the dealer to check that out when I take the truck in.

 

:flag:

John

04-silver Enthusiast

04-silver

Posted
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I'm kinda thinking not on the TPS. Seems to me, for it to be the TPS, the pedal would always have to be in the exact same spot when the truck acts up.

 

My educational guess(cause I don't know mechanics all that well... :flag: ) is that there is something causing the tranny to stick in 2nd or 3rd, whatever gear it is in at the time, anyway like I was saying, there is something causing the tranny to stick between shifts, hence when I take my foot off the accelerator for a nano second that might be just enough for the rpm's to drop a bit, which they do and thus unstick/loosen up or grip even for that matter whatever it is from present gear and be to up shift. BTW you can hear that the motor working more during all this.

 

Does this make sense, you think this could happen or maybe something else?

 

 

:chevrolet:

John

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