Aftermarket exhaust responsible for low RPM bucking?

[Danneman]

Has anyone replaced their stock exhaust with aftermarket and then a few months later started experiencing a "bucking" at low RPMs in overdrive?

I've eliminated nearly every other culprit I can think of. No codes...I've done the torque converter test where I tap brakes with throttle open, but the revs increase as expected...full tune up...every sensor I can think of, I have replaced...fuel pump and regulator all check out great...all the usual stuff you'd think of.

It's at its worst in the high 30s/low 40s MPH and it drops from 3rd into its "comfort gear."

I ask about the exhaust because it's the only thing I can think of, based on both my knowledge of what was done to the vehicle over time (just basic maintenance and stock part replacement!) and because of an article that suggested changing exhaust pipe diameter could potentially affect valve timing due to backpressure change. I did full flowmaster, cats (FLO-2010023) and cat-back (FLO-817166).

 

[willxfs]

Kinda sounds like the torque converter locking and unlocking. I think I remember reading the converter is supposed to lock up around the 35-40 mph range.

 

[east302]

Were the catalytic converters changed? When mine were bad, it would lug at around 40-mph though the engine speed wouldn’t change. It felt as if you had a manual transmission and shifted up too early.

You can do a backpressure test, installing a pressure gauge in place of an upstream oxygen sensor. If the pressure is too high, it indicates an exhaust restriction. I’ll look for that number if you like.


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[Danneman]

Yep, it was from the manifolds back. What did you do about it?

That's exactly it, like you shifted up too early on a manual, except rough bucking/jerking. Revs seem way low at the bottom end of 3 overdrive...starts to buck...but punch it and it goes away. Driving in 3 rather than O, doesn't happen (or isn't noticeable anyway), I've theorized because the revs never get that low in 3. Wasn't like this as soon as I got the new exhaust, it was a few months later (bearing in mind I don't put many miles on, even though it is daily driver)...but nothing else changed in the meantime.

Revs don't go up and down when this is happening, and there are codes - both of which it seems like happens if it's a TC thing, based on a little research?

What about the cats could be the problem? Cheap, defective? Natural and more likely for aftermarket / flowmaster in particular? Can catalytic converters get "clogged?"

 

[east302]

Sending fuel into the exhaust can damage the catalyst “stuff” inside the converters, so they usually don’t just fail. Something usually causes them to fail.

So there are no trouble codes?

Mine got so bad that if I hit the gas pulling out in traffic and going uphill, it would actually slow down to a stop. Otherwise, it just hated being in 4th while doing 40-45 mph. I’d either have to speed up or go to 3rd to bring the revs up.

Here’s that pressure test that I mentioned.

Any issues with starting (fuel pressure)?




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[Danneman]

Thanks very much! No codes, no issues starting - fuel pump builds and maintains pressure within spec.

 

[bigfootchiro]

Possible TCC solenoid? Spark plugs? Wires? Cap/rotor?

 

[Danneman]

So, the plugs, wires, cap & rotor are all good...all the typical (and even some of the non-typical, like coil) tune-up type things have been ruled out. The TCC solenoid keeps coming up in all of my searches on anything related to bucking. But doesn't the brake test rule that out? Tap brakes while holding throttle open, revs increase. And the revs don't change while i'm experiencing this bucking/jerking.

The frustrating thing here is, no codes. And it can be a little difficult to get driving conditions perfect in the area surrounding a mechanic in the busy chicago suburbs, so as to replicate. Just wish I could take someone on my commute some day, happens at least three times on the way.

 

[drakon543]

i cant find the diagram im looking for maybe someone else has it on hand. the brake test your referring to may not be usable in this situation. these generation tahoe's have a feed for the torque converter going through the brake pedal switch. so if there is something else causing it to not work correctly the brake pedal switch might be bypassing that issue.

 

[Danneman]

Gotcha, that makes sense. I'm wondering...the engine and trans are new as of 120K (thrown rod), done by dealer. I've got 10K on them since. I would like to think that the TC and related components were part of that, maybe not? This might give me a clue as to where to have them start looking. Again, without codes, I'm sending the mechanic on a mission without much to go on.