Hesitation after new muffler

scottmoyer · October 30, 2012

Is it possible that the slight hesitation I recently acquired is coming from the GMPP sport muffler I had installed? My truck was running fine and I took advantage of a great price on the GMPP sport muffler only. The shop that installed it used 2.5" connections since he was out of 2.75" pipe. He said that the muffler was more free flowing than the factory system so the small reduction of the pipe connections was still better than the original set up. I don't drive the truck much since I work from home now and about 3 weeks after I installed the muffler, I drove the truck daily for a 40 minute commute each way. That's when I noticed a slight hesitation on acceleration from a dead stop of at less than 1500 rpm in first.

The only real mods to the truck are a black bear 87 tune, the muffler and I gutted the bottom of my factory air box to allow more air in than the factory 1"x4" fender opening.

BTW, I did email Jenna and Justin and asked them if I need a retune and I haven't heard back from them yet. Any advice is appreciated.

scz71864 · October 30, 2012

you dont need a retune for a new muffler. the hesitation could be you getting used to the truck with the new muffler on. when i add new things i over analyze everything.

scottmoyer · October 31, 2012

No. This isn't over analyzing anything. There is a distinct hesitation for a second or two when the truck is cold and just starting out. It didn't use to be there and started after I got the muffler. It feels like the timing is retarding or like the fuel filter is clogged or a fuel injector, etc. I'm thinking that the truck is leaned out after the increased exhaust flow.

scz71864 · October 31, 2012

I dont think the flow of the exhaust would cause that. I would look at your injectors. now that you mentioned it, this was the first COLD morning we have had and my throttle has a lag too until the engine gets warm.

kstruckcountry · October 31, 2012

I highly doubt it was the muffler.

Jennabear · October 31, 2012

You should not have any hesitation from the muffler. Sounds mechanical just from your description....try cleaning your MAF, and throttle body. What gas are you running? Most places just switched to winter blends.

RJG183SA · October 31, 2012

No. This isn't over analyzing anything. There is a distinct hesitation for a second or two when the truck is cold and just starting out. It didn't use to be there and started after I got the muffler. It feels like the timing is retarding or like the fuel filter is clogged or a fuel injector, etc. I'm thinking that the truck is leaned out after the increased exhaust flow.

I had a similar issue with my 03 2500 HD 6.0. As the outside temps got colder it would sometimes stall / run rough after startup. A quick check of fuel pressure revealed the problem. IIRC it was low about 5 psi. Changed out the FPR corrected the issue.

6Sixathome · October 31, 2012

Ditto what Jennabear said. I just cleaned my MAF sensor and throttle body the other day and I could tell the diff. Also gas lately at least around me seems to be crapy, maybe you got a little bit a bad gas.

Also, scottmoyer your truck looks awesome, I think I like that body style a lil bit better than the 900's.

scottmoyer · October 31, 2012

Ok. I guess I didn't provide enough details for people to know the timing of this problem. I installed the muffler in late June. The truck sits in the garage most of the time and gets driven only when I need to go somewhere. I work from home, so there is no daily commute. About three weeks after the install, still during the warm Florida summer, I needed to drive daily to a job site about 40 minutes away. I noticed it then that I was getting this hesitation. The Chevy dealer told me they couldn't reproduce the issue so I have been researching it myself.

Three weeks ago, I bought a can of MAF cleaner and cleaned the MAF. I also bought a can of throttle body cleaner and took care of that. I did the Sea Foam through the brake booster treatment and added some to the gas tank. Still no different.

When I refer to the truck being cold, I'm not referring to cold mornings, but just after sitting for a long time like over night or even after a few hours. Again, this issue was first noticed in the middle of a Florida summer, so cold weather isn't an issue. I do run 87 octane and it's been fine with the tune for years. I have had plenty of fill ups since this issue started, so I don't believe bad gas to be the issue. It might have caused this issue though if I have a clogged injector or fuel filter.

I replaced the spark plug wires last night and have not driven the truck yet. I'll let you know if that resolves the issue or not. Thanks

scz71864 · November 1, 2012

how many miles do you have?

SILVERADOSC540 · November 4, 2012

First, nice truck. The black flares look awesome with your color.Second, if it's only when it's cold (the engine not the temp) then it's probably not the muffler. Third, and I may be mistaken, but I think that your muffler shop sold you a bill of goods with the small couplings and "better overall" line. For the sake of discussion , if a 2 inch water pipe flows at twenty gallons a minute and a 1-3/4 inch water pipe flows at 18 gallons a minute, doesn't a 1-3/4 inch fitting in the middle of a two inch pipe reduce the flow of that pipe to 18 gallons per minute? Again, I may be wrong, but it seems to me that while your truck probably sounds better, they didn't do you any favors as far as fuel efficiency or performance gains from better flow as they reduced your system from a 2-3/4 inch system (if that's what your stock system is or was) to a 2-1/2 inch system with the addition of the smaller couplings. Again, I may be way off, but I think that I learned that a whole long time ago in an engineering course for work somewhere...

scottmoyer · November 4, 2012

The truck has 118k miles now.

There is no difference in the hesitation after the spark plug wires were changed. Before somebody says that I'm just throwing parts at a problem, I need to state that the parts being thrown at the truck are parts that probably need replacing now anyway. This was the first plug wire change. I may do the 02 sensors because they are the originals also.

I agree with the statement that a smaller hose connected to a larger one will restrict the flow to that of the smaller hose. The factory muffler is not a straight through design, so the air flow might enter the muffler at 2-3/4", but once it hits the muffler, it gets chambered and obstructed before it exits the rear of the muffler. The new GMPP muffler is a free flowing 2-3/4" straight through design. Yes, my exhaust has been restricted to 2.5" at the couplings, but isn't that still better than the factory obstructed muffler?

SILVERADOSC540 · November 4, 2012

You are probably better off than stock, but if my math is correct you're getting just under 91% flow out of your 2.75" system. Those couplings would make a difference in flow by nine or ten percent. I'd ask them to order and install them or get a nine percemt refund.

On the hesitation when cold, I think that I recall my '07 vmax owner's manual saying that a sliight hesitation when cold was normal as the sytem went through checks or some yada yada. Maybe check you manual for the same type lingo. Anyone else have that in their manual?

scottmoyer · November 4, 2012

If the owner's manual states that it's normal, it would have been there from day one or soon after. Three years short of seven years and 116k miles for a hesitation to start is not something that an owner's manual would acknowledge as normal. It feels like the timing is retarding. I guess I will need to keep researching this.

scottmoyer · November 19, 2012

I just reread my last post and found that I can't edit it anymore. It should have read, "three months short of seven years", not three years short. Sorry for that confusion.

I did some more digging into work I've recently done and wonder if the following could be an issue. About two weeks prior to noticing this hesitation, I bled my brakes. I did a gravity bleed starting at the standard furthest line 1st and ended with the driver's front. Is it possible that since I didn't use the computer bleed system, that activates the anti lock system, that I may have a brake hanging up causing what feels like a hesitation? Can bleeding brakes cause issues like this? Tomorrow, I'll lift the front end off the ground, start the truck, apply the brakes and see if I can rotate the wheel immediately after releasing the brake pedal. I'll let you know.