Transmission issue??

I have a 5.3 Z71 150k w/ 85k on tranny.

Truck runs fine on hwy but when sitting in stop light I get a real rough feeling, I don't think it's the idle, seems more like the transmission. If I put the truck in N or P, the roughness goes away. I only feel it when it's in gear. Also, it made a whining noise briefly, but it was not persistent. And it seems like the shifting is a little rough.

Any ideas? Is my tranny shot?

Thanks in advance.

Is transmission engagement normal, or slow to shift firmly into REVERSE and DRIVE?

Engagement is normal, it's after engagement that the rough idle begins. If I shift to N, the idle will spike up to about 2k rpm and then be smooth.

Is it possible that a vaccuum leak could cause this? But, why only when the truck is in gear?

GMMerlin?

GMMerlin?
You rang ?
Crystal ball alert went off this morning...so whats up?:lol:

Engagement is normal, it's after engagement that the rough idle begins. If I shift to N, the idle will spike up to about 2k rpm and then be smooth.

Is it possible that a vaccuum leak could cause this? But, why only when the truck is in gear?

The reason you feel the rough idle in gear is because the engine is under a load.
We will need some more detail as to what is happening on your end, but I have a couple questions.
Is this a rough idle or does it feel like the engine vibration is being transfered into the passenger compartment?
Does the truck have a lower then normal idle speed?
From what you are describing here , it sounds like you have a low idle and the IAC tries to compensate by raising the engine rpm...this is where you get the rpm spike.
SO many things could cause this its hard to know where to start.....
A couple ideas would be, check the TB and clean it. Check for vacum leaks especially at the area where the intake meets the heads.

RE: GMMerlin

1. After messing with it again this morning it does seem more like engine vibration rather than rough idle. The truck will idle fine in P and N, and even in D when I'm not on the brakes. Once I press the brakes, the RPM's take a dive, and then the vibration begins within about 2 seconds. It's almost like the engine is missing...It's a sputtering feeling.

2. Idle speed seems normal. maybe a tad high, but probably just my imagination.

I will clean the TB and check for a leak today...any other ideas/opinions would be greatly appreciated.

Thanks.

Just my:2cents: but yes a vacuum leak can absolutely do this. My wifes car had the exact same symptoms but it was also posting a misfire code. So, I changed plugs and wires all to no avail. When I went around the vac lines and sprayed carb cleaner, I finally found the leak b/c the engine revved up. Plug the leak and it ran perfect. Just goes to show, don't always assume that the code tells you what is wrong direcly huh? If you clean the TB, go ahead and spray some of the cleaner around the lines to check for a leak. Or you could use the tube in ear method of checking for leaks. It's cheap, and good insurance before you go spending money.

Tube and ear method??

I've got tubes and ears...please explain the method...this is my first go around with tracking a leak

Also, since the problem occurs when I'm on the brake, should I look around the master cylinder and brake lines for a leak first?

Thanks again.

RE: GMMerlin

1. After messing with it again this morning it does seem more like engine vibration rather than rough idle. The truck will idle fine in P and N, and even in D when I'm not on the brakes. Once I press the brakes, the RPM's take a dive, and then the vibration begins within about 2 seconds. It's almost like the engine is missing...It's a sputtering feeling.

2. Idle speed seems normal. maybe a tad high, but probably just my imagination.

I will clean the TB and check for a leak today...any other ideas/opinions would be greatly appreciated.

Thanks.

Sounds like you have a vacum leak at the booster. When you put your foot on the brake pedal, do you hear a hissing sound inside the truck?

Sounds like you have a vacum leak at the booster. When you put your foot on the brake pedal, do you hear a hissing sound inside the truck?

I don't hear it inside, but with the hood up it makes a sort of clucking noise...But it's definately only a problem with brake on and trans engaged.

thanks.

I've never tried the tube method, but from what I hear, just stick a piece of tubing in your ear and take the other end and run it around the vac lines and you can hear a leak as a whistle. Again, never actually tried that one, but I know the carb cleaner works. Does it cause problems when it's in gear and the parking break is set but the brake petal is not engaged?

I've never tried the tube method, but from what I hear, just stick a piece of tubing in your ear and take the other end and run it around the vac lines and you can hear a leak as a whistle. Again, never actually tried that one, but I know the carb cleaner works. Does it cause problems when it's in gear and the parking break is set but the brake petal is not engaged?

ha..I'll do it while nobody is around...i don't want my neighbors to see me stethoscoping my engine!

My parking brake is weak anyway...so I can't test that.

I've got some carb cleaner so I will start with that method. I'm just looking for bubbles right?

If it is the booster, it seems like it shouldn't be too hard to replace...even for a rook like me.

Thanks again to everyone..I never would have even known where to begin, at least now I have a good solid starting point.

"I've got some carb cleaner so I will start with that method. I'm just looking for bubbles right?"

the engine will rev up if you find a leak using carb cleaner

"I've got some carb cleaner so I will start with that method. I'm just looking for bubbles right?"

the engine will rev up if you find a leak using carb cleaner

Okay. I will check this afternoon and post the results.

stupid question...

Since the problem only occurs when the brake is applied and the trans is engaged...Do I need to do the vacuum test under those conditions?

No, if there is a vacum leak on the engine, you should be able to pick it up with the carb cleaner.
Please be extremely careful using carb cleaner....it is highly flammable.
To see if the booster is leaking, plug off the vacum line to the booster and apply the brakes..if the problem goes away, the booster is the culprit

re: GMMerlin


I was careful...fire extinguiser nearby! Kids far away...

Will try your method on the booster next, prolly tomorrow b4 i get to it.

Also, on my way back from the auto store I noticed rough shifting, very jerky..did it on the way there, not the way back...the rough idle while braking has been consistent for two days, but this jerky shifting was a new problem.

My dad, who knows less about cars than me! Said it could be the torque converter...causing the rough shifting, and the roughness while stopped and tranny in D...

Like I implied, my dad is no expert and nor am i...just thought I would throw that out there to see if he's even warm....

Thanks, and I'll post again after properly testing the booster.

Cracked flywheel?

The booster is working properly.

The truck is now shifting really hard. It doesn't happen all the time, but after a little highway driving, the shifting gets real jerky.

I am going to give it a tune up today, but I'm starting to think the problem is beyond what I can diagnose or fix. I just want to get the tune up out of the way, so the shop doesn't charge me for that...

Anyway, thanks for all the help. I will post the proper diagnoses once an expert looks at it.