99 5.7 vortec

Asked by Sep 28, 2017 at 12:55 PM about the 1999 GMC Sierra 2500 3 Dr SLE Extended Cab SB

Question type: Maintenance & Repair

I have a 99 gmc c2500 with a 5.7 vortec. I pulled
the motor and dropped 4000$ into parts. Corvette
crank cam pistons and heads. Got it running great
till last week when I cold start and try and take off
it starts to sputter real bad for a couple seconds
then launchs, the second issue when I'm giving it
gas and let off at my cruising speed it pops and
back fires every time I left off the gas. Any ideas
what it could be? My MAS just went out  ordered a
new one. Do you think this is related to my MAS?

6 Answers


Could have been, Mass Air Flow sensor can cause a bunch of varying driveabality problems, replace it first clear the computer run the drive protocol and see what happens after that. You may want to consider a reprogramming unit like BullyDog so you can take advantage of all that money you spent one the engine. HTH

The mas will be here tomorrow and yes I was planning on clearing before I put the new one on. I had my buddy erase the ECU and completely rewrite everything to this new motor from the old ecu to the new it's night and day between the two. But after going threw everything the mas is the only part faulting and not sure if it could cause an issue like this


I have personally felt a MAF sensor causing a lurch during gradual throttling up, but not a backfire, so I'm with you on replacing it, backfire is usually fuel introduced when it should not be but there too it could be the ECM getting a false input from the MAF. Hope that's all it is.

Shoot if this mas only fix s the sputter I'm happy with that hopefully it will fix the back for too if not I'll have to go threw it again and see if I can figure out what s going on causing the back fire. I will keep you posted tonight and let u know what happens with the new mas

Put the new mas on cleared n reset ecu, it's still doing the same thing but not all the time only half the time is doing the popping and sputtering


The only thing I can think of right now is check the TPS sweep, it could still be sending a ohms signal when in the closed position so the ECM thinks it still has some throttle and is still delivering fuel instead of shutting off, now this might be intermittent so test it a number of times. Also rechecking the systems grounds that were all undone when your had all that work done might also be a good idea.

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