Misfires and Sputtering on acceleration

[John03Z71]

About 10 days ago I replaced the knock sensors on my '03 Tahoe Z71, and replaced the intake manifold gasket while I had it off with a genuine Fel-Pro gasket. After I put everything back together and took it for a quick test drive down the street and back, I could immediately feel an increase in throttle response and overall performance. However, beginning the next day when I started "really" driving it I began noticing misfires, hesitation, sputtering, etc. when I accelerate, especially from a stop or when under load (AC on & going uphill for example). If I gently accelerate from a stop and get up to speed it's not too noticable, but if I really step on it from a stop it hesitates and sputters so much it takes almost 5 seconds to get a little speed/momentum up to where it'll not be so bad. Really dangerous when turning onto busy roads.
Also when pulling into a parking spot or just coming to a stop in general, if the AC compressor is on when I finally come to a stop the idle dips so low it sounds like its going to stall, but then goes back to normal after a couple seconds.

Does anyone have any suggestions what this might be caused by? New plugs & wires about 10k miles ago. No check engine light.

Edit- also have noticed some black smoke right after starting up when the engine is hot (not from cold start). Not sure if it's related.

Thanks in advance!

 

[OR VietVet]

During your previous work you may have damaged wires when taking apart and reassembly and you also may have not got all coils plugged back in correctly. You need to check those plug ins and make sure a wire in the connectors did not push out backwards when trying to plug in. Also, make sure any ground connections are back in place and clean and tight.

The black smoke could indicate an over rich condition and now you have fouled plugs. Remove plugs and inspect. Post pics of them if can.

It sure sounds like you caused this problem during the recent work. I am surprised no CEL or codes from a misfire but you can plug in a scanner to check for history. No CEL does not mean no codes.

 

[John03Z71]

During your previous work you may have damaged wires when taking apart and reassembly and you also may have not got all coils plugged back in correctly. You need to check those plug ins and make sure a wire in the connectors did not push out backwards when trying to plug in. Also, make sure any ground connections are back in place and clean and tight.

The black smoke could indicate an over rich condition and now you have fouled plugs. Remove plugs and inspect. Post pics of them if can.

It sure sounds like you caused this problem during the recent work. I am surprised no CEL or codes from a misfire but you can plug in a scanner to check for history. No CEL does not mean no codes.

Thanks for the reply. I just scanned for codes a few different times and all of them returned 0 stored codes. Misfire monitor "OK".

I just took a quick look and did notice the very small vacuum line from the fuel pressure regulator has a little crack in the rubber boot at the top connection, looks like when I pressed it back down the rubber may have bent a bit to the point where it cracked. Still intact though. Could this be a possibility or would it have to be completely broken or disconnected to cause problems?

 

[OR VietVet]

It very well could be the problem. If it is cracked, replace it and also check thru the things I talked about earlier. If you have the means and the equipment, get a fuel pressure reading as well. Best if you can monitor while driving with a gauge attached to windshield while driving or with a Tech 2 scanner while driving.

The thing that is the common denominator here is that the problems started after you worked on it. That is why it always takes me more time to reassemble work than to tear it down. I check and double check all my moves during the reassembly.