TPS changes with engine temp?

[TONY1]

hoping for some help from any tech savvy people out there..........This may be a little [lot?] long winded but its complicated....My 2005 matrix 1.8 L ZZE132L with ‘fly by wire’ throttle. 5 speed tranny
does something very strange...I bought this car used in 09 , at 30,000 km.

During steady speed highway driving, it always seemed to have occasional changes in speed [pick up or drop a few MPH] without any change of gas pedal position, making it difficult to maintain a constant speed up and down slight grades. Most often, it would pick up power a while after adding just a bit of throttle for a slight uphill. Then, after cresting the hill, it seemed I could back of the throttle more than expected to keep from speeding up too much, when it would suddenly drop some power, and I’d have to give it more throttle or it would slow down too much.

Consistently, after a cold start, it would pick up power just before reaching full coolant temp on then gauge. You would be cruising along at 60 MPH on level ground, when it would suddenly pick up enough power to accelerate to 65 mph. I could hear the intake and exhaust note change slightly as well. It felt like someone opened the throttle a touch.
Recently I had a check engine light on, took it back to the dealer, they had codes p0441 and p0455 –they reflashed the ECM under warranty and the MIL problem was solved.
However, it did not change the power up/down symptoms.

Since there are no codes or MIL , and the symptom is very difficult to notice unless you are really paying attention for it, I figured I’d better try to eliminate as many things as I could before going back to my dealer and getting soaked for who knows how much to be told ‘no problem found’

I thought it might just be running cold, so I replaced the thermostat, and tried some fuel injector cleaner just in case –no better.

I have since been driving it with a scanner hooked up , and found that it was very hard to maintain a constant throttle position [ in degrees] Initially I thought that it was just my twitchy foot combined with a very sensitive gas pedal sensor, so I rigged up a pedal stopper to limit throttle to about 27 degrees, which translates to 60 MPH.

What I found is this; Consistently, after a cold start, when coolant temp reaches 80 degrees C [reading the scanner] the throttle position will jump 3 degrees [from 27 to 30] and speed increases 5 MPH. The power surge is instantaneous, feels like I flipped a switch on. This is without any movement of the foot pedal since it is jammed against a solid stopper.
The max temp I see is about 86 C.
If I stop and idle for a few minutes, or even just drive at 40 mph or so until temp drops a few degrees, , my max throttle will once again drop to 27 degrees. Drive a mile or so and the max jumps to 30 again, with corresponding increase in power. Once it’s hot the power up and down seems to happen at about 84 degrees. Drop below 84 = power down. Get above 84 = power up.

So , now I know what is happening, but I have no idea why! TPS? ECM? Another reflash ? maybe that’s just how these things are?

If anyone has any insight, I could sure use some….

[fishexpo101]

Hmm, interesting observations.

This would be a good question for someone that is intimate with the internal programming of the ECM. Which last I heard, the ECM code is still closed.

But you do bring up a good point about the throttle stop and a "feature" of some hall effect sensors. That in the absence of motion, the driving circuit can sometimes fluctuate - many times, it is engineered in that manner to make sure the sensor is reading correctly.

Sort of similar to how the O2 sensor and the ECM, drive the air/fuel mix slightly rich to slightly lean - part of the feedback control algorithm. As anything that is steadystate or in a static state - it is really hard for a computer to differentiate if the sensor is reading correctly or if the sensor is dead.

Being seemly keyed to a specific temperature window is pretty interesting. Why there seems to be a relationship between temperature and throttle position in this manner - I cannot say. Until we can look at the feedback control algorithm - don't even have enough information to speculate.

[TONY1]

thanks for the info.
I did some more testing today, and it actually does not seem to be temp related, other than it does not happen until at least 80c.
I can make it happen consistently by simply driving at 27 degrees t.p. / 65 mph until it jumps to 30 [a mile or two at highway speeds] then back throttle off down to 25 to maintain 65 mph. Within another mile or two it suddenly drops to 22 degrees and slows, , push it back to 27 to maintain speed, then it jumps to 30 , and the cycle repeats seemingly indefinitely.
It seems that when its in 'power down' mode it will take 27 degrees to go 65 mph, and when it's in 'power up' mode it only takes 25.

It may turn out to normal but it sure is annoying. If you had cruise on you'd never feel it.
I'm goin to run it by a couple of local techs as well, then take it in to the dealer and see what they come up with.
thanks again