Sudden RPM jump while accelerating

dpappyp

New Member
I just bought a "new" 2004 Trailblazer EXT with 105k miles. If I want to accelerate quickly from a stop for merging onto the interstate, for example, I am gradually pushing the pedal down more and more to try to get the rpm's up, but it seems like it just accelerates at the slow pace it wants to until I get to certain point (maybe pedal halfway down) and then all of a sudden the rpm's shoot way up and accelerates faster than I was intending. Is this normal for these TB's? Or could it be a faulty pedal sensor? A faulty throttle position sensor? A dirty throttle body? The engine otherwise idles perfectly fine and I have no other noticeable symptoms. I searched around the forums, but couldn't spot an identical scenario from someone else. Thanks for any tips.
 

TequilaWarrior

Well-Known Member
A few things come to mind. First, it's never a bad idea to clean the throttle body. There are several walkthroughs available on this forum. Second, without knowing the maintenance history, it would be a good idea to install new spark plugs, air filter, fuel filter, and change all fluids. If you have maintenance records that are accurate you may be able to skip some of that. The spark plugs in these are supposed to be changed at 100k miles and should only be replaced with AC-Delco 41-103 plugs. Do not gap them. If you buy genuine plugs, they are already gapped. This basic maintenance will help improve overall driving performance and gas mileage.

As for the sudden jump in RPM - what you're describing is very subjective (slow pace, maybe pedal halfway down). It may be that at the point the RPM jumps the transmission is downshifting a gear if the RPM jump is over 700 RPM. If it's around 700 RPM, it may be the torque converter clutch unlocking. Either way, this is by design.

Another thing to keep in mind is that these vehicles are "drive by wire" - meaning there is no cable that runs from the gas pedal to the throttle body. It's all electronically controlled and electro-mechanically actuated. The computer manages ~everything~ in that engine/transmission. The throttle position is used as an input and computer makes calculations based on current RPM, current throttle plate position, current engine load, current gear, etc... and decides what to do with that throttle position input. With a stock tune, the computer is very conservative and errs on the side of providing a balance between torque and fuel economy, opening the throttle plate slowly until there's a good match between the throttle position input and the engine RPM. A good PCM tune can improve its behavior dramatically (opening the throttle plate faster, changing shift points, increasing fuel amounts, etc...).

Much of this is subjective, but cleaning your throttle body and checking maintenance records is essentially free. You'll need throttle body cleaner and some rags and a toothbrush to clean your throttle body. Fluid changes don't have to be crazy expensive. Personally, I'd start with the throttle body, air filter, and fuel filter. Depending on how it runs afterward, maybe spark plugs.
 
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dpappyp

New Member
Thanks for the input. Spark plugs, fuel filter and fluids are all relatively new. I'm changing the air filter this weekend.

I guess what I was trying to say is that there doesn't seem to be a sensible relationship between acceleration and pedal position. It's as if the throttle either opens a little or all the way, going from gentle acceleration to trying to get the best 1/4 mile time. There isn't a steady progression in rates of acceleration between the two. I am aware it is fly by wire, so maybe that's just how the computer is programmed. I am leaning towards a faulty pedal position sensor that is sending the same signal to the computer whether it's at 10%, 20%, etc until it gets beyond a certain point, say 50%, then it starts sending the correct signal for that position again. So, it sends a signal for 8%, 9%, 10%, 10%, 10%, 50%, which no gradual variability in between. Just a theory. Can the pedal position sensor be logged via a scanner? I might have to buy the Chevy specific upgrade for mine to read it though.
 

MRRSM

Lifetime VIP Supporter
Welcome to GMT Nation...

This is an excellent, focused overview of "How the ACC Sensor (Gas Pedal) Works in Modern Computer Controlled Engines":

 

Chickenhawk

Well-Known Member
The other common cause of slow acceleration on the highway is a cat converter starting to get plugged. It bogs down when you try to accelerate until it forces a downshift. If the truck is new to you, it might have had a history of misfires or a low coolant temp sensor or bad thermostat. These are very common on our platforms, and as they age, we will see more and more plugged cats.

It won't throw a code because it is still cleaning the exhaust, just only letting 10% of it through at a time. This is why we notice the early symptoms the most when accelerating on the highway or climbing hills.

If it has ever been driven in the past with a flashing check engine light - indicating a severe misfire - the cat can become cooked in minutes. Or if the coolant temp is below specs for a few months, the same thing happens.

You may want to get an exhaust backpressure test done if you don't find any other causes. (Backpressure should be 0 at idle and no more than 1 or 2 PSI at 2500 RPM.)

Just to help us confirm this possibility, either photo your coolant temp gauge when hot, or let us know if the needle is slightly to the left, straight up and down or slightly to the right when hot. Interestingly enough, the gauge itself is not 100% accurate, nor is it linear, but the POSITION of the needle from center actually tells us a lot.
 

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