KChevy75

Help! I need fresh eyes on a problem i'm having with my vehicle. I have a 2007 Suburban LTZ with 290,000 Miles. I've always run Mobil 1 but for the last 2 oil changes, I used Valvoline MaxLife. I personally rebuilt the transmission at 202,000 Miles due to a broken forward sprag and replaced the torque converter at the same time just in case. Recently, it started experiencing a shudder starting around 45 MPH when the torque converter clutch is engaged. Once the TC is locked, when you slowly press the gas pedal, the car will start to shudder. if I keep steadily pressing the pedal and it doesn't unlock, I can cause it to shudder all the way up to 70+ MPH. Also, while it's shuddering, the speedometer dial with bounce up and down about 1-3 MPH. It almost feels like lugging such as when you have a manual trans in 5th gear but are only going 20 MPH.

I noticed a pretty rough idle last September along with some misfire codes. Did the cylinder soak due to oil consumption while I had the plugs out. I replace the plugs and wires and the problem went away. That was 15,000 miles ago. Over the last couple months, the shudder started. It was really bad at first so I replace the ignition coils which made a significant improvement but the shudder was still there. I also noticed that it seems like the engine is losing all power at low speed occasionally for about 1/10th of a second.

Things I did after September but before the shudder started:

• Erratic oil pressure showing on gauge. Replaced oil pressure sender. This required removing the intake manifold. While removed, I checked the injectors. All looked good. Created an arduino sketch with relays to clean and test each one. All produced the same volume and spray pattern over duration of the test. Reused the intake gasket as it was relatively new from another bad oil pressure sender.

Things I've done so far to troubleshoot:

• Replaced ignition coils.
• Checked the U-Joints and they are nice and tight. There is slight movement of the yoke where it enters the transmission but it's tiny and has always been that way.
• Ran 93 octane gas through it along with injector cleaner. No change.
• Checked and found no vacuum leaks.
• Replaced the TC PWM solenoid in the transmission. No change
• Installed a Sonnax tcc regulator and isolator kit. No change.
• Read about counterfeit spark plugs and decided to pull a plug. The supposed iridium tip was now a nub after only 15,000 miles. They were all like that. All counterfeit. .080 to .100 gap. Replaced them and the wires. Thought this was going to be it but...No change!
• Used some shudder fixx as a diagnostic aid to see if there was any change to the shudder. There was no change. If it was torque converter clutch, surely, I'd notice a reduction in shudder but not in this case.
• Scan tool shows no misfires while driving or while the shudder is occurring.
• GM Scan tool shows no transmission codes and only P0449 ECM code which is unrelated to this problem.
• When I did a compression test back in September, everything was fine.
• Checked ground connections. Unscrewed, cleaned and screwed back in. No change.
• Checked positive connections. Unscrewed, cleaned and screwed back in. No change.
• Checked battery connections. positive wasn't very tight. Tightened. No change.
• Checked mega fuse. Unscrewed, cleaned and screwed back in. No change.
• Checked Fuel pressure and it's perfect. Fuel pump is original to the suburban.
• Replaced knock sensors.

Things I plan on doing next:

• Compression test on all cylinders. Should have done this before installing new plugs and wires. Hope I don't pull the ends of the new wires!

What are your thoughts on this?

• Compression issue possibly caused by cylinder soak 15,000 miles ago?
• AFM lifters going bad? There's been two or three times recently where I did hear one lifter tap on first startup after sitting overnight.
• Torque Converter? Shudder Fixx made no difference so I'm not so sure but maybe the clutch is too badly worn. I tried the trick where you press the brake very slightly to disengage the TC clutch but it stays engaged and continues to shudder.
• Electrical issue? Ground loose or corroded somewhere?
• One or more of the new ignition coils are bad? These are inexpensive Chinese versions. I may put the Originals back in.
• Bad brake switch? I've hear of others who had a bad switch which was causing the same symptoms. I replace the original a while back because it was bad and was causing the cruise control to not function.
• Too much free play in yoke? It's maybe 1/16". My truck experienced a similar shudder caused by a bad u-joint. This is different though.
• Bad fuel pump / clogged fuel filter?
• Bad fuel injector(s)?
• Intake manifold leak?
• Other?

I'm hoping this is a simple electrical issue that I'm missing. I don't want to decouple the trans from the engine yet again to replace the torque converter. I'm too tall to be crawling under there. I really need a lift! I also don't want to have to remove the heads and replace lifters.

After installing new plugs yesterday, it started throwing an air bag code. The right front sensor is showing erratic data. I was nowhere near that sensor or wiring that I know of. Hope it's just a wire issue, otherwise, there goes another $70.to the auto parts supplier.

Thanks in advance!

KChevy75

05/30/2019 Update:

Switched back to the original ignition coils. Problem persists.

Performed compression test. Did two dry tests. All looks good to me.


Also checked the 4L60e line pressure. Specs say 50+ PSI at idle. I'm at 59 PSI. Looks fine to me.


I'm leaning toward torque converter at this point.

KChevy75

Today, I decided to do one more troubleshooting step to hopefully eliminate the engine or something else electrical as the cause. This involved modifying the TCC PWM Solenoid to essentially be an on / off switch instead of modulated. To do this, you remove the o-ring on the end and the screen so you don't damage it. You then grind a notch in the two raised areas on either side of where the o-ring was. Reinstall the screen but not the o-ring. Install in transmission.

Took it for a test drive and absolutely no shudder now. You can feel the torque converter engage and disengage now but it locks up fully. No shudder and no slipping while engaged. This eliminates the engine as the cause.

The question now is, what's causing the torque converter to not fully lock up with the TCC PWM solenoid in stock form?

• Wrong friction material in the remanufactured torque converter causing it to wear out quickly?
• Worn lockup o-ring?
• Restriction in the transmission somewhere?

I'm at a loss. Just glad I was able to prove it's definitely not the engine.