Suburban shudder/surging

Hi, I have a 99 Suburban with very high miles. Going on 306K mostly highway miles. This is our baby and the best vehicle I’ve ever owned. I hope someone can help because this is a solid vehicle that I think we can get more miles out of.

The problem is that it will intermittently surge or shudder between 40-60mph during acceleration. It’s hard to pinpoint because it doesn’t happen all the time. It seems to be worse if the vehicle is low on gas or the engine is cold but I’m not certain about that. It’s not hard enough to call a buck, jerk, or sputter though. Maybe I’ve over-analyzing but basically it will shift to the correct gear and then back down then back up etc., and back and forth for very brief seconds. Then levels out after 60mph and there are no other problems. If the gas is low or engine is cold is does this in lower gears as well but usually only around 4th gear. I hope this description makes sense.

Here’s where I’m confused. The transmission was rebuilt 2 weeks ago specifically the torque converter and rear planetaries. The day we got it back we performed a tune-up so all new plugs/wires/cap/rotor. The fuel filter was replaced about 8 months ago and the fuel pump 2 years ago. All regular maintenance has been performed on time since we bought it 4 years ago. Yesterday the Service Engine Soon light came on and it only showed one code (OBD II), PO161, so we replaced the 02 Sensor (Bank 2, Sensor 2).

However the problem persists and there are no other codes providing any direction. I’ve tried to search the internet for drivers with the same issue but the fixes vary too much and seem uncertain.

Other known issues in case that helps diagnostics: four wheel drive hasn’t worked for years (haven’t needed it in our flatlands) and ac compressor went out last spring.

Lost, not that I know very much about engines and transmission. Any thoughts? We bought it at a bargain and even with these recent fixes we have gotten our moneys worth. I really wish I could see this resolved. Thanks for reading and thanks in advance for any direction!

It sounds to me like you are having having torque converter clutch lockup problems.

You need to back up to the recent transmission work. What was done exactly? And why? What kind of place did it? A torque converter would be replaced, not rebuilt. The transmission might have been rebuilt, but all you mentioned was converter.

Ok, so they rebuilt it, replaced the rear planets and TC, do you know whether they did any valve body mods, specifically, the converter clutch regulator and isolator valves?? These transmissions are well known for their wear in these areas. This valve is a very busy valve, constantly moving in its bore and they tend to wear the bore in the valve body which causes leakage. Leakage in this area will cause converter clutch shuddering or the clutch falling in and out of lockup. They would have been crazy not to address this area during a rebuild…

I would also have someone scan the computer and monitor the converter clutch operation while driving the vehicle to see whether the computer is commanding it on and off. This will tell you whether the problem is computer related or mechanically related.


Thank you both, sounds like we need to go back to the transmission specialist and find out if something that was thought to be fixed really wasn’t and get a clearer picture of everything that was done during the 3 days it was being worked on because we were told the TC issue was fixed during the rebuild. If it ends up being the TC or TC related still then that explains it all because we were looking in other areas.

Thank you again for the direction, and quick responses!

And thank you transman for the scanner advice. Could be computer related, I don’t mean to doubt the specialist first. I’ll look into that as well but start with investigating the recent fix.

Having it scanned isn’t about finding out if the computer has a problem. Its because the computer will set error codes if it detects a problem. Some of those error codes will light up a check engine light. But on GMs of this vintage there are many codes that can be present without the check engine light. Some of those would be set if you have a TCC lock up problem.

The other thing a scanner can do is find out exactly what the computer is doing in terms of transmission controls. E.g. IF this is a TCC lockup problem you could get one from a transmission problem - or some other kind of problem. What transman mentioned was finding out if the computer is telling the TCC to kick on/off when it really shouldn’t be. That’s not likely a computer problem. It could come from something as a simple as a faulty brake light switch - or a number of other things. But those things keep one out of the transmission as the actual cause of the issue. So putting a scanner on it will allow a tech to find out exactly what the computer is “seeing” and doing.

I assume by your followup that one of the issues you had that led to the transmission work was diagnosed as a TC problem? Was it similar to what is happening now?