Slight hesitation or bucking in 4th gear.

[bugeyed]

'04 Trailblazer @ 130k.
Not sure I am using the proper terminology, but I will try to explain. Out of the blue, my TB started to hesitate & slightly buck or miss while driving at between 50 & 60 MPH. I don't have to be accelerating, but it seems that the issue happens when in 4th gear & driving steadily. I feel a slight miss. If I accelerate, & it downshifts, I don't notice a problem. The fact that it is in 4th gear & under light load, like a slight incline, seems to show the problem. Not getting a code & have cleaned the throttle body & done the relearn. I suspected a fuel issue, but I have run that tank down to less than 1/4 tank & filled up & have driven maybe 50 miles with no change. I am now focused on a coil pack, but without a code don't know which to target. Maybe fuel filter? I don't think it's ever been changed, so I need to anyway. Is it likely to be the culprit? Any help is appreciated.
Thanks,
kev

 

[The_Roadie]

Are you certain it's not the behavior of your TCC (torque converter clutch) going into and out of lockup? What's your transmissions service history?

 

[bugeyed]

I don't think it's the tranny. Filter & fluid changer at 100k. I was considering the tranny, but after paying close attention, I don't think it's the problem. It may be, but I need to change the fuel filter anyway. In case I still have issue, what can I do to better identify the torque converter as the culprit?
Thanks,
kev

 

[The_Roadie]

I think pressing the brake pedal enough to get the light to turn on, but not to drag the brakes, will suppress TCC lockup. Otherwise you need to go for a drive with a well-equipped mechanic with a high end Tech II tool or equivalent. And catch it in the act.

 

[bugeyed]

I think pressing the brake pedal enough to get the light to turn on, but not to drag the brakes, will suppress TCC lockup. Otherwise you need to go for a drive with a well-equipped mechanic with a high end Tech II tool or equivalent. And catch it in the act.

Well, hopefully the fuel filter was the culprit. It was due a change anyway. I couldn't reproduce the problem on a short test drive, but I will know more when I get on the road & warm things up better. It's funny, I just had the motor mounts replaced, after tolerating the vibration for a year & the thing was running smoother than ever. Then this happened?!?! Oh well, if the fuel filter was the issue, I am happy 'cause I now have a sparkling clean throttle body & fuel filter, new rear brakes & motor mounts. I may be good to go!
Thanks,
kev
P.S. Roadie, care to elaborate on your procedure above? Are you saying that applying some brake pedal will cause some light to come on? And when you say suppress TCC lockup, do you mean that it will relearn or adjust to different conditions & not ba a problem any longer? Just curious, 'cause I may need to know!
Thanks,
kev

 

[The_Roadie]

No, it's just that when the PCM sees the brake switch closed, it does some things like suppress TCC lockup. Same thing happens when you press the brakes with the cruise control on. It drops out.

 

[IllogicTC]

No, it's just that when the PCM sees the brake switch closed, it does some things like suppress TCC lockup. Same thing happens when you press the brakes with the cruise control on. It drops out.

It seems to me that also when coasting it'll drop out. Slight application of the accelerator, and you'll see the tach come up then drop in while the TCC locks up.

 

[bmcutright]

I'm having a similar problem when at idle/near idle speeds, engine seems to bog down, RPM drops from 600-700 fown to 200-300, little gas picks it back up to keep it from stalling, hard to do at a red light or stuck in traffic. No difference between AC on or off. Mainly when the engine is hot. Had a problem with HDM a few weeks ago, thinking of replacing the HDM and fan solid state relays to see if that would do the trick.

 

[bugeyed]

OK, I'm back on this again. It appears that I am experiencing the TCC lockup issue as described above. Yesterday I was able to test by touching the brake when the car is surging/bucking & I immediately feel the RPM change & smooth operation. I am wondering what I should do next. i am due a fluid/filter change, so should I just drop the pan & change filter & add fluid or, since I am experiencing an issue, should I do more. I have been warned to NOT having the system flushed, so should I just drain what I can from the pan, change filter & go? BTW I don't have a drain plug. Should I include some product like Lubegard Red ATF fluid protectant? Vehicle has 150K miles & fluid & filter changed at 100K. Drop pan, drain & refill method. Didn't drain the TC of fluid. Am not inclined to do the procedure outlined for draining all fluid. Do shops drain all fluid if they don't do a flush, or is it SOP to just drain the pan? I just want this fixed & want to make sure I am not causing any more damage to tranny, so if it makes sense to do several things at same time, I will do that.

Thanks,
kev

 

[ChrisinVictoria]

If it is your tcc it is inside the torque converter. There is not a magic additive or service that will fix it. Typically if it is failing you will get a bad smell from your fluid. If you ignore it, you may experience catastrophic failure. The converter failst internally and sends shrapnel throughout. It probably needs a new Torque Converter, which means removing the transmission. I would get a proper diagnosis from a trusted tech. and I would do it soon. Sorry to be the messenger.

Good Luck,
Chris

 

[bugeyed]

Thanks for the reply. I have read several cases with similar symptoms & wonder how, short of a mechanic with scanner, to get a better idea of what is going on. The only symptom I have is the hesitation when in 4th gear at steady speed. If I touch the brake the RPMs jump up about 300RPM & the hesitation is gone. I am trying to understand if there is anything short of a complete torque converter replacement I can do. I am not trying to put a bandaid on it, I will have the TC replaced if that's what it takes, but I don't want to do it if it could be something else. I have read about things like TCC Valve, Solenoid etc. I also read about a Transgo shift kit. If anyone can educate me a bit more, I would appreciate it.
Thanks,
kev

 

[gmcman]

This is what I did to mine because I was experiencing a slight buck or jerk when pulling a trailer in 4th when the converter was locked, very similar to your symptoms.

If you are slightly mechanically inclined then it's fairly easy, just takes a couple hours. You need to pull the pan and then the valve body which is just a bunch of little bolts and a few connections and install a PWM TCC eliminator valve. The factory lockup is a "variable" pressure setup to smoothly lockup the TCC, this inexpensive fix will make the TCC more of an "off/on" setup and also improves holding pressure on the clutches. This along with a fluid flush and I think you may be good to go.

Here's a thread I did on mine.

The pictures loaded backwards so on post #1 just reverse the order of the images posted.

http://gmtnation.com/forums/topic/7238-removed-pwm-control-from-tcc/?hl=%2Bpwm+%2Bcontrol

 

[IllogicTC]

This is what I did to mine because I was experiencing a slight buck or jerk when pulling a trailer in 4th when the converter was locked, very similar to your symptoms.

If you are slightly mechanically inclined then it's fairly easy, just takes a couple hours. You need to pull the pan and then the valve body which is just a bunch of little bolts and a few connections and install a PWM TCC eliminator valve. The factory lockup is a "variable" pressure setup to smoothly lockup the TCC, this inexpensive fix will make the TCC more of an "off/on" setup and also improves holding pressure on the clutches. This along with a fluid flush and I think you may be good to go.

Here's a thread I did on mine.

The pictures loaded backwards so on post #1 just reverse the order of the images posted.

http://gmtnation.com/forums/topic/7238-removed-pwm-control-from-tcc/?hl=%2Bpwm+%2Bcontrol

A note on this mod, which some people comment on as enjoying, is with the strict on or off system, you'll feel the TCC lockup to be more like a gear shift, like a "hidden fifth gear." Just something to be aware of when changing the operating characteristics, it's not harmful.

 

[bugeyed]

I am intrigued to try this DIY, but don't have any experience inside the tranny, except for dropping the pan & changing the filter. I am competent mechanically with many repairs, but mostly bolt-on jobs. For example, I chose to have a shop change my motor mounts 'cause I didn't want to lift the engine. I did order the mounts from GMParts direct to save a bit. I read one member almost lost a spring when disassembly, so just a bit nervous. Don't have a lift, so jack stands are best I can do. If I do this myself, can I just replace the lost fluid & not worry about what is in the TC? That's what I have done when changing filter.

 

[Mattinla]

Bugeyed! I’m not only having the exact same issue, but it started immediately after having the motor mounts replaced. 4th friggin’ gear…

Any advice you can give would be wildly appreciated!

 

[Mooseman]

You are replying to a 9 year old thread and @bugeyed hasn't logged in since 2019. I doubt you will get a response from him.

However, as indicated in this thread, it could be the TCC. When it's doing this, tap the brake with your left foot while continuing to hold the gas. If it stops, there's your answer.