Misfire after head gasket replacement

[Hmj7819]

Hello! I have a 2007 chevy tahoe 5.3L Z71. I recently replaced the head gasket, coil packs,spark plugs and wires. I upgraded to a cold air intake also. Now when I start it it stuggles to turn over and has a significant misfire on cylinder 1 and 8. I switched out the coil packs, plugs and wires on both cylinders and it made no difference. I dont remember the codes it was throwing at the moment. I also replaced injectors and upgrade to a 4 port from a 2 port injector. Cylinder 1 and 8 are loading up with fuel. Please help.

 

[swathdiver]

Take the time to pull the codes and then we have somewhere to start; accurately.

 

[Fless]

Now when I start it it stuggles to tuen over and has a significant misfire on cylinder 1 and 8.

If the crank speed is slow, verify your battery cables and connections to the starter. If you mean it cranks but takes time to fire, that's different.

Ditto on getting the codes.

 

[rdezs]

Rule number one, don't replace multiple things at the same time. I would start by reinstalling your OEM air tube and filter. Quite possible it's simply running too lean from the cold air intake....

 

[rdezs]

Just realized you mentioned it struggles to turn over. Ditto on checking the battery connections. Also the ground to the cylinder head.

 

[B-train]

Have you lost any coolant? I also second the battery cable connections and ground. You could try a voltage drop test on the cables and see if you have high resistance internally.

Were any other sensors replaced when you did this work?

 

[Hmj7819]

Rule number one, don't replace multiple things at the same time. I would start by reinstalling your OEM air tube and filter. Quite possible it's simply running too lean from the cold air intake....

This started as a top end rebuild due to a blown head gasket

Rule number one, don't replace multiple things at the same time. I would start by reinstalling your OEM air tube and filter. Quite possible it's simply running too lean from the cold air intake....

That would cause 2 cylinders not to fire?

 

[Hmj7819]

Have you lost any coolant? I also second the battery cable connections and ground. You could try a voltage drop test on the cables and see if you have high resistance internally.

Were any other sensors replaced when you did this work?

No I’m not loosing any coolant now that I’ve replaced the head gasket. I did replace the negative battery cable back to an OEM cable. I scraped, sanded, and cleaned the ground connection..

 

[Hmj7819]

Fuel is loading up in the 2 cylinders also.

 

[Hmj7819]

Just realized you mentioned it struggles to turn over. Ditto on checking the battery connections. Also the ground to the cylinder head.

My starter was going out. It went out right after posting this.

 

[Hmj7819]

Have you lost any coolant? I also second the battery cable connections and ground. You could try a voltage drop test on the cables and see if you have high resistance internally.

Were any other sensors replaced when you did this work?

No sensors replaced

 

[mikez71]

No way the ignition coil wires could swap right?
I figure the're on opposite sides and ends.. but they all go to ECM..
The wire is purple for coil 1, and purple/white for coil 8. (ECM ground trigger)

Maybe check all the wires at coil connector, grounds and power.

BTW did your old starter feel like a case with loose parts inside?

 

[rdezs]

That would cause 2 cylinders not to fire?

I would reinstall your OEM air intake and filter simply to rule it out.

When you have the heads off, did you have a machine shop skim them and make sure they're flat? I would think that most likely it's going to be one of the harness plugs. (Or a broken wire in the harness from disturbing it. Our harnesses are starting to show their age)

 

[Fless]

I would reinstall your OEM air intake and filter simply to rule it out.

When you have the heads off, did you have a machine shop skim them and make sure they're flat? I would think that most likely it's going to be one of the harness plugs. (Or a broken wire in the harness from disturbing it. Our harnesses are starting to show their age)

^^This. A lot of new (questionable) parts have been shotgunned.

What was wrong with the coil packs?

 

[mikez71]

I just noticed you said "upgraded injectors.. 2 port -> 4 port"
Maybe you're talking about the steam ports in head?

Tuners want very detailed info on the injectors.
They won't run anything without complete injector data..
And it needs to be input into the tune..

Is this a known good injector upgrade?

You have tried swapping plugs/coils/wires around, but not the injectors.

 

[j91z28d1]

curious of why it blew a head gasket. ls engine isn't really known for doing that.

did it maybe over heat badly before?

 

[RET423]

If 1 & 8 are loading up with fuel, those injectors are leaking

Yes I know their new, that increases the likelihood of them being bad it doesn't reduce it

 

[rdezs]

You might put your old fuel injectors in and see if the problem disappears

 

[West 1]

Pull the plugs that are loading up, I suspect they will be very wet. A leaking injector causes hydro lock in the cylinder. Don’t mess with it or you will bend a connecting rod or break a piston. With two plugs removed spin the starter, I bet it spins normally.

If the injector is over fueling you will need to find out if it is computer related or a bad injector.