Intake Backfire after Head Gasket change...

[LA1 J Body]

I changed the HG but didn't send the head out to be fixed. When I put it back together and started it, you can hear it backfiring through the intake, and I'm also getting positive pressure in the manifold when it does this. Any idea what's causing this? Do you think a re-built head will cure this?

[93ztwo4]

i had a probelem in the summer when the car would backfire out the intake. it turned out to be the map sencer well not the map sencer itself but one of the wires goin to it. so ur problem could be the map

[SappySE107]

Pushrods in the wrong position?

You should have checked the heads.

[sharkey]

im gonna guess either a pushrod in the wrong spot or a rocker set to tight.

[1988GTU]

Ign. Wires not placed in the correct order? I would not think rockers would the cause since it should be torq and go preload settings, no lashing required.

[Carlspike]

It's always a good idea to have heads checked while you have them off. I'd do a compression check and see if you've got a dead cylinder.

[Superdave]

+1 for the pushrods in the wrong spot... that'll do it every time

[geoffinbc]

I +1 for the plug wires being wrong.

[Chris88CL]

Im in on the plug wires being wrong too.

[SappySE107]

How to plug wires cause intake manifold pressurization?

[geoffinbc]

It doesn't. But if the plug fires when a fuel charge is in the cylinder and the intake valve is open a bit then you will get a backfire in the intake.

[Joseph Upson]

Wouldn't you have excessive rocker play if you installed the short pushrod on the wrong valve? There is only about .070 preload on them based on an estimate from the preload I got from my reground cam and from what I remember there is considerably more difference than that in the rod lengths.

I'd go with crossed wires unless you have adjustable rockers in which case it could be a loose exhaust rocker which I had after a cam install. Combustion and exhaust through the intake valve. Sounded like a wrench being tapped on top of the intake. A Jeg's rep told me that I had a bad rod bearing just that quick and argued with me about it on an engine that had less than 15k on its rebuild, and no test drive after the cam install, jerk.

The rod came out of the rocker seat so I didn't hear the characteristic rattle.

[1988GTU]

I'd go with crossed wires unless you have adjustable rockers in which case it could be a loose exhaust rocker which I had after a cam install. Combustion and exhaust through the intake valve. Sounded like a wrench being tapped on top of the intake. A Jeg's rep told me that I had a bad rod bearing just that quick and argued with me about it on an engine that had less than 15k on its rebuild, and no test drive after the cam install, jerk.

The rod came out of the rocker seat so I didn't hear the characteristic rattle.

Was the pushrod bent from that also?

[SappySE107]

.020-.040 preload is what you want.

[Joseph Upson]

Was the pushrod bent from that also?

No, the nut backed off until everything went limp. When reusing rocker lock nuts I usually give them a ding with a ball-peen hammer, I was in such a hurry to try out the new cam I didn't do it.