Torque converter or engine misfire?

[Old Salt]

Hello,

I have an issue that has just started with my 2003 Trailblazer Ext 4.2L engine and 4wd with 252,000 miles. I cruise along at between 45 and 55 mph with the transmission in OD and torque converter locked up, rpms between 1400 and 1600. Then I step very slightly on the gas to accelerate and I feel a slight bobble in either the engine or transmission. I have to accelerate slowly enough that it does not unlock the converter or downshift. Some of the time I just feel a bump, but the rpms don't move. Other times I see the rpms jump slightly then drop back. Once I get to 1700 rpms, a little over 55 mph all is smooth again. From 60 mph on up there is no evidence of problems. At hiway speed the transmission will unlock and then downshift if I step on the gas hard enough and feels normal. Below 45 the transmission is not shifted into OD or locked up. There is no check engine light on. The bump is very slight, such that if I'm on a bumpy road I can't tell the difference between the road and the problem. On a smooth road, it is very noticable however.

Can this be the torque converter? It feels like the engine is misfiring occasionally (just a subtle bump at random intervals) but it only occurs at the very slowest speed that allows the torque converter to lock up. It does not happen at slower speeds in a lower gear, but the same rpms. It does not occur at slightly higher speeds. I would have thought that if the engine were misfiring the computer would detect it and set the check engine light. I don't feel any evidence of slippage in any of the gears.

To give a full history, about 4 months ago, at about 240,000 miles my trans had gotten really soft in the shifts, especially the 2 to 3 shift. Under light to moderate acceleration the rpms would flare as it made the shift. I did a complete fluid flush and changed the filter, but it had almost no effect. I read on the forums about the corvette servo hardening up the 1-2 and 2-3 shifts, so I got one and tried it. It worked very nicely. The lower shifts are nice and solid now. The shift to 4 is still rather soft but once engaged it is solid, and the TCC lockup has seemed to get even slower than it used to be. A very soft slide into lockup which I hate. I've read here about the problems with the TCC lockup valve getting worn, and the transgo shift kit fixing these kind of problems.

If it is the torque converter, will a shift kit likely fix it? If it is the engine misfiring, wouldn't the computer record the misfire and set the check engine light? Any guidance would be appreciated.

 

[jonbo2002]

my guess would you're correct with the missfire not being your problem, when mine missfired it threw a check engine light right away. the kit may help your trans but it may just be too late, our friends the Keys did a shift kit to their old trans and it just killed it quicker. at your milage it may just be the end of the trans's life. So you might have to figure if its worth the cost to put a reman in.

 

[strat81]

How much and what brand and type of ATF did you use?

Is the vehicle used for towing or hauling?

 

[Old Salt]

I used the newer Dex VI fluid and did a total flush. I used about 14 or 15 quarts in the process. Changed the filter and pan gasket too. I was amazed at how clean the inside of the pan was. Only a light smear of darkness in the pan that wiped right out. No metal or other solid material. Previous changes were just the flush you get at an oil change place.

Most of my mileage is running around to job sites at work and that is probably 2/3s hiway travel. Occasionally I carry some weight but not often and once in a while I may tow a small 2 wheel trailer, but that may only be 2 or 3 times a year. When I do tow I usually put it in 3rd because I hate the way it hunts in and out of overdrive if I let it shift to 4th. I'm not a heavy footed driver. Overall I'd say I am very easy on the transmission. The only reservation I'd put on that statement is that I live in the Texas hill country and when I drive on the hiway it shifts in and out of OD and lockup with every hill climb if I use the cruise control.

Driving around today I was becoming more convinced that it is the transmission coming out of lockup that I'm feeling. I was seeing a definite jump in engine rpms and then a quick drop again. It is still all in that narrow speed band where it happens though. I'm still left wondering if a shift kit will fix it. I feel like the fact that it works fine at higher speeds indicates that gear clutches and torque converter are holding ok but at low rpms the pressure is not high enough to keep the torque converter locked. I may be applying too much wishful thinking though.

 

[strat81]

If you installed a drain bolt in your pan (or if your trans isn't completely full), drain off some fluid and add 12 ounces of Lubegard Red:
Amazon.com: Lubegard 60902 Automatic Transmission Fluid Protectant: Automotive
Lubegard 50902 Automatic Transmission Fluid Protectant : Amazon.com : Automotive

If you did not install a drain bolt and your trans is full, add some Shudder Fix and chase with 1oz of ATF to wash it down the dispstick tube:
Amazon.com: Dr. Tranny 19610 Instant Shudder Fixx Friction Modifier: Automotive

I have not used the shudder fix, but it does review very well. Both products are from the same company. I've been using the Lubegard Red at the recommendation of a friend that's a trans mechanic and it seems to be working very well. It's funny, I originally asked him about shift kits and such for my trans and his recommendation was that unless I race not to bother and to put the money I'd spend on a shift kit and other trans parts into a savings account for a rebuild with $20 going towards some Lubegard. If I towed, he said to get a trans cooler and avoid overdrive.

At 250k miles, I'd start saving for a trans rebuild. You've certainly done well with the 4L60E, assuming it's the original transmission.

 

[gmcman]

I would say 250K is a remarkable life for a stock 4L60E. If that were mine I would possibly go with the TCC PWM eliminator valve which includes reaming the separator plate for the orifice that supplies pressure for the TC clutch. Couple that with a new separator plate and see if will hold the lock.

Have you tried to duplicate the symptoms in 3rd?

 

[Old Salt]

Thanks for the tips on the Shudder Fix and Lubegard. I'll definitely check them out.

It is the original transmission. The main thing I'd be looking for with the shift kit would be restoring pressure due to worn valves. I would also like the PWM eliminator. Beyond that I'm not trying to upgrade the transmission. I do understand that I've taken this trans a long way and it may be about at the end of its life but the thought of spending a couple grand or more gives me a real pain in the wallet.

i was amazed at the improvement when I put the Corvette servo in. Before I did that, I thought the trans was about to blow up. It had gotten real bad about letting the rpms surge while it made the shifts. I figured it was just a matter of time before it didn't engage a gear at all. The Corvette servo fixed all that. I guess I'm hoping I can get a similar rescue of a lost cause here.

I'll have to try it with the selector in 3rd.

Thanks again for the input.

 

[gmcman]

Old Salt, I forgot to mention about the Sonnax super hold servo. This is what I would do first and in the end it's a win-win situation.

Providing you plan on keeping it, this can be swapped to the next tranny and honestly should be in there anyway IMO. Replacement is the same as the vette servo but replaces the 4th apply piston.

The 3-4 clutches are the weak link next to the sunshell and are generally the first to go out. After you added the super hold servo, and IF the problems still exist, I would then go for maybe the .500 boost valve and the TCC PWM eliminator.

What you could be experiencing is the 3-4 clutches going and just like the 2-3 flare the vette servo fixed, this should hopefully do the same.

I would change the fluid to Valvoline max life on your next flush. Also if you are getting clutch slip, maybe check the filter and bottom of pan as well for excess clutch material.

Generally you will get a code if the TC slips under lock, but could be just barely slipping not to throw one.