97 stumbles when cruising

[Kevin Callahan]

I've read a ton of posts on here of people having issues but nothing quite fits the symptoms I have. My 97 tahoe with the 5.7 vortec will stumble while cruising. Speed doesn't matter much just needs to be a consistent speed and rpm. 65 on the highway or 35 it will act up. Rpm will drop a couple hundred for half a second then jumps back up. Zero codes thrown. Have done a tune up with zero change. Leaning toward a bad torque converter locking and unlocking. I've tried the lightly holding the brake test to prevent it from locking up and the shudder is considerably better but still there. Idles fine and hard throttle both are fine as well. With no codes I'm at a loss. I don't want to just throw a torque converter in it if that's not it. Trans was rebuilt by a reputable shop 4 or so years ago. Fluid is full and clean

 

[WICruiser-97]

Can you determine if it is an engine misfire induced stumble vs. drive train influenced engine reaction? My '97 has a less severe missfire/stumble that I am atributing to fuel injectors but not bad enough to actually change the injection system out.

 

[Kevin Callahan]

Can you determine if it is an engine misfire induced stumble vs. drive train influenced engine reaction? My '97 has a less severe missfire/stumble that I am atributing to fuel injectors but not bad enough to actually change the injection system out.

I don't believe it's an engine misfire but I'm not 100 percent on that. I've had a misfire issue in the past and this doesn't feel the same. And I would get a misfire code then and have no codes now

 

[scott2093]

Possibly electrical?.... wonder if you could catch something on a scanner....

 

[Kevin Callahan]

Possibly electrical?.... wonder if you could catch something on a scanner....

I drove around with scanner hooked to it one day and I did get a crank position sensor code pop up for like 2 seconds then it was gone. Never even set the check engine light. And from what I understand crank sensor only affects start up. I should add it runs perfect for the first 10 minutes or so. Coolant temp doesn't matter. It can be cold or hot but after the 10 minute mark is when it acts up. Not sure if these have a timed warm up cycle they run on or if it's all temp related?

 

[Schurkey]

I did get a crank position sensor code pop up for like 2 seconds then it was gone. Never even set the check engine light. And from what I understand crank sensor only affects start up.

Absolutely not. Crank sensor provides RPM signal to computer, is responsible for keeping the fuel pump running and provides info so the computer can provide fuel injection/spark timing.

I should add it runs perfect for the first 10 minutes or so. Coolant temp doesn't matter. It can be cold or hot but after the 10 minute mark is when it acts up. Not sure if these have a timed warm up cycle they run on or if it's all temp related?

Connect a scan tool, find out if that's when it changes from open loop to closed loop.

 

[Kevin Callahan]

Absolutely not. Crank sensor provides RPM signal to computer, is responsible for keeping the fuel pump running and provides info so the computer can provide fuel injection/spark timing.


Connect a scan tool, find out if that's when it changes from open loop to closed lo

What type of scan tool do I need to test that? I don't believe mine have that ability but I'm not sure where to look to see that

 

[Schurkey]

I'd expect that ANY scan tool would display open-loop/closed-loop info. That's entirely "basic" info.

The only scan tools I've worked with have been Snap-On products. Can't advise on other brands.

What scan tool do you have?

 

[Kevin Callahan]

I'd expect that ANY scan tool would display open-loop/closed-loop info. That's entirely "basic" info.

The only scan tools I've worked with have been Snap-On products. Can't advise on other brands.

What scan tool do you have?

I have an Innova scan tool as well as one of the Bluetooth ones that connects to my phone. I'll look at the Innova tonight and see what info it can give me

 

[Kevin Callahan]

So got home from work today and started to investigate with my scan tool. Haven't been driving the Tahoe because of the issues and have a spare vehicle. The battery was dead so I hooked my battery charger up to it. As soon as the battery had power my scan tool turned on even though the key was off. If I'm not mistaken I shouldn't have power at the obd2 port unless the key is on. Could that be a bad ignition switch and could that explain the stumble on highway?