Engine surging hunting while on highway

NatesGr8

Original poster
Member
Dec 13, 2011
9
Hey guys, this is my first post over on this site, and first post about this problem. Figured I'd have a better chance of getting the answer over here.

I've noticed recently that when cruising on the highway, with the engine slightly loaded (slight incline or slow acceleration) the engine rpms seem to fluctuate slightly and this is felt in the car. If i'm cruising along on the high way at about 2000 rpm and there is a slight increase in grade i will see and feel the rpms increase then decrease, this fluctuation goes on until i let off the gas or push down on it harder.

Wondering if anyone else out there has had an fixed a similar problem or if there are any ideas as to what could be causing this. Car is a 2004 Trailblazer LT with the 4.2L engine (auto trans). I have recently changed the front, rear and transfer case fluids due to scheduled maintenance, it has had this problem before and after the fluid changes. I also have a thermostat replacement in the works as the gauge is showing to be a bit on the cool side.

Any help is greatly appreciated, Thanks, Nate
 

AtlWrk

Member
Dec 6, 2011
674
First, what does your transmission fluid look like? is the level adequate? and what kind of service has been done to the tranmission?

:twocents:Fluctuating RPMs while cruising immediately leads to me to a worn Torque Converter Clutch (TCC) valve. When this valve wears it makes it difficult for the clutch to stay engaged under moderate loads like hills or light acceleration. When you give it more gas the clutch disengages completely anyway.

There is an inexpensive replacement valve designed specifically to cure this problem but last time I checked it required removing the valve body.
 

NatesGr8

Original poster
Member
Dec 13, 2011
9
Transmission fluid looks clean and level is good, it was changed about 50,000 miles ago, maybe less and i'm not sure if it was the pan only or the entire flush. (this is my GF's car and she's not sure about what exactly was done but she has a garage receipt that says "transmission fluid change"). I also have that on my list of things to do.

Where exactly is this valve located? I'm not afraid to work on anything as I have many engine rebuilds (boats and jet skis mostly) and lots of time under cars under my belt. Any links to the specific valve in mind?

Thanks so much, Nate
 

Regulator

Member
Nov 20, 2011
2,496
First off, Welcome to the Nation!

When was last time that you cleaned your throttle body? I would probably start there.
 

NatesGr8

Original poster
Member
Dec 13, 2011
9
Cleaned the throttle body 2 weeks ago (completely removed it), as well as changed the air filter, fuel filter, oil and oil filter. Spark plugs were changed over the summer.
 

NatesGr8

Original poster
Member
Dec 13, 2011
9
Need to bump this back to the top, anyone else have suggestions? Or know where i can find said problematic valve?
 

NatesGr8

Original poster
Member
Dec 13, 2011
9
NatesGr8 said:
Need to bump this back to the top, anyone else have suggestions? Or know where i can find said problematic valve?

another bump please anyone out there have any suggestions?
 

djthumper

Administrator
Nov 20, 2011
14,956
North Las Vegas
It is still hunting and not throwing any codes? If it wasn't doing it before they did the fluid change I would take it back to them and let them deal with it. At the least find out what they did to service it.
 

NatesGr8

Original poster
Member
Dec 13, 2011
9
djthumper said:
It is still hunting and not throwing any codes? If it wasn't doing it before they did the fluid change I would take it back to them and let them deal with it. At the least find out what they did to service it.
Service was done about a year and a half ago, and to my knowledge it only developed this problem within the past 6 months. No codes are being thrown. I still haven't replace the t-stat which i hope to get done next weekend. Could this possibly be a fan clutch issue?

Nate
 

djthumper

Administrator
Nov 20, 2011
14,956
North Las Vegas
NatesGr8 said:
Service was done about a year and a half ago, and to my knowledge it only developed this problem within the past 6 months. No codes are being thrown. I still haven't replace the t-stat which i hope to get done next weekend. Could this possibly be a fan clutch issue?

Nate

Unless the fan sounds like it it running like a jet it is highly unlikely.
 

NatesGr8

Original poster
Member
Dec 13, 2011
9
Would like to bump this to the top. Really want to get this problem fixed. Have noticed the surging/studdering more on the highway, at very light loading (lightly accelerating). T-stat has been swapped. It's almost as if its searching for the right rpm. I'm wondering if i could have a bad torque converter?

Anyone else having this problem? Any info or ideas are greatly appreciated.

Thanks, Nate
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Or a TCC lockup clutch. Take a drive with a mechanic using their high-end scan tool to diagnose things while the problem happens.
 

gmcman

Member
Dec 12, 2011
4,656
Likely not your issue but what plugs did you put in? were they the AC Delco?
 

jham

Member
Nov 20, 2011
1,334
Maybe this is just me but it sounds like a misfire. My had a lugging/surging feeling a while back i checked the plugs and had a carbon track on #3 so i changed all the plugs with ac Delco and changed that boot since it had some melting damage on it. Just a thought for you
 

borretje

Member
Jan 1, 2012
94
Torque Converter Clutch (TCC)!:yes:
 

NatesGr8

Original poster
Member
Dec 13, 2011
9
Changed out the plugs with delco's. I hope its not a TCC. Is there any way to diagnose a bad TCC? Not sure if i mentioned it earlier but the car also hunts while idling, the rpms dip and rise, both with and without the a/c or heat being on. I'm going to borrow a buddies scan tool to check things out while this problem is occurring, any suggestions on what i should be looking for?

Thanks, Nate
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
NatesGr8 said:
Is there any way to diagnose a bad TCC?
Pay a mechanic with a high end scan tool to drive around with you and look at the TCC status. They can also do other things. Not every problem is going to respond to free DIY diagnoses. But if it happens at idle, it's not the tranny.

If you pulled the battery cable to reset the PCM while you cleaned the throttle body, then it's probably not that. Be suspicious of the CPAS next. And check compression and vacuum.
 

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