Hesitating motion

[patrick]

I'm not sure which forum to put this under. Here's a little background first. 2002 trailblazer EXT. Within the last six months I've had a new pcm(?) installed. It's the one behind the passenger seat behind the driver. A new throttlebody was installed. And two days ago I had new brake pads and rotors installed all the way around.

Now with the problem. While driving there is a rhythmic hesitation in motion. I'm not sure if it's the engine because the RPMs stay the same. But it's very noticeable. Acceleration seems fine, and there's no vibration In the idle. Traveling about 45 to 55 mph when I let off the gas the car slows down incredibly fast in the RPMs drop down to idle. This morning I did a test, most cars while in gear from a stop will slowly roll forward under their own power. The trailblazer without any gas applied will roll forward about 10 feet and then it's like something is grabbing it and then it will continue rolling forward under power for about 10 feet and then it's like something is grabbing it again almost stopping it . It is very rhythmic. If anybody could give me any ideas of what to check for I would greatly appreciate it

 

[Matt]

Sounds like the PCM was not replaced. The PCM is in the engine bay, right hand side of the engine.

 

[patrick]

I'll check with the shop who did the work. I know the put a sticker on top of the fuse box under the hood, but I thought the part replaced was the computer under the rear seat.

 

[Mooseman]

How's the maintenance? Have you replaced the transfer case fluid in the last 50k miles? What about the diffs and tranny? What mode do you drive it in, 2HI or A4WD? Sounds like something binding in the driveline.

 

[Blckshdw]

Under the rear seat is the Body Control Module, so you were only off by 1 letter

What issues were you having before that required them to replace that computer? with Mooseman, sounds more like a driveline issue, rather than something that would be related to the BCM.

 

[ek02]

If it started doing it right after the pads and rotors were replaced, I might think it is a caliper sticking or a warped rotor. The fact that it rolls and then almost stops in gear sounds like a brake problem.

 

[patrick]

When I bought it had 142,000 miles on it. It now has 163,000. I have not changed any of the transmission fluids. It is not a four-wheel-drive model. Before they changed the PCM I got the dreaded reduce power mode, and it would only go about 15 mph. I took it into the shop and they said the throttlebody electronics were fried and the pcm needed replaced. I'm not getting any check engine light or anything but could there be a code that could be read?

 

[patrick]

If it started doing it right after the pads and rotors were replaced, I might think it is a caliper sticking or a warped rotor. The fact that it rolls and then almost stops in gear sounds like a brake problem.

It did start right after the pads and rotors were replaced. Which led me to think it was a brake problem also. I'm getting no indication from the engine that there's anything wrong with it.

 

[HARDTRAILZ]

Brakes are sticking....

 

[patrick]

If the brakes are sticking is it most likely a mechanical issue? Nothing like the ABS system ? I'm hoping its just something they did wrong when they put the pads and rotors on.

 

[HARDTRAILZ]

Probably. I would be taking it back to them. Might be a simple sticking pin or maybe a new caliper is needed.

 

[808_LS_EXT]

Brakes are sticking....

^^^ Ditto.

Try pulling over (into a safe lot) when you feel the hesitation at 45-55...
Then put your hand over each wheel... you will feel the extreme heat from the wheel that is dragging.

 

[patrick]

I'm here at the brake shop now. They are going to check it out. I'll post if they find anything.

Since you mentioned it. I just checked both front wheels are too hot to touch. The back ones are cool

 

[ek02]

With that many miles I would bet a caliper, or both, are sticking. When the calipers are compressed, if there is any build up of gook in the caliper, the pistons start to stick. Especially if they are phenolic pistons. I replaced mine with steel piston calipers when I replaced the pads. If your brake shop is nice they will replace the calipers for the price of the parts, and the labor for the calipers after the pads have been removed. I hope that is your problem.

 

[808_LS_EXT]

I'm here at the brake shop now. They are going to check it out. I'll post if they find anything.

Since you mentioned it. I just checked both front wheels are too hot to touch. The back ones are cool

I hope they're honest with you.
Good on you for noticing the "hesitating motion"...
You'd be amazed how many drivers don't pay attention to any of those red flags.

With that many miles I would bet a caliper, or both, are sticking. When the calipers are compressed, if there is any build up of gook in the caliper, the pistons start to stick. Especially if they are phenolic pistons. I replaced mine with steel piston calipers when I replaced the pads. If your brake shop is nice they will replace the calipers for the price of the parts, and the labor for the calipers after the pads have been removed. I hope that is your problem.

Phenolic > Metal

Q. Are steel pistons better than phenolic, which are plastic?
A. Phenolic pistons have an advantage over steel pistons;

• Phenolic pistons do not rust
• Phenolic pistons maintain original size regardless of age. They are not affected by heat or moisture in the brake system
• Phenolic pistons do not transfer heat to the brake fluid
• Phenolic pistons are lighter than steel
• Phenolic pistons are fuel efficient and noise dampening

Quoted from here... good read

Edit: Corrected link target.

 

[ek02]

Phenolic pistons may have improved in the last 30 years. I had bad experiences with them sticking when hot on Chrysler products many years ago. Technology has probably gotten a lot better. I tend to stick with what I know works well.

 

[patrick]

Damn I forgot about this. Yes the brakes were sticking. They fixed it for no charge.