350 TBI tapping noise


#1

Ok, so I have a 1992 GMC Suburban with a 350 TBI with a full dual exhaust and long pipe headers. The problem I’m having is that there is a tapping noise that comes from the driver’s side of the engine when you push the gas pedal down. It only makes the noise when you push the pedal down more than about a quarter of an inch. I have tested it down hill with the transmission out of overdrive and the rpms up and there is no tapping. I’m thinking it might be pre-ignition in one of the cylinders but I have no idea how to test for it other than just disconnecting plug wires one at a time until it stops. Much more I don’t know what would be causing it to happen if that is the problem. Any advice would be helpful.


#2

Does this vehicle still have the EGR system on it? Pre-ignition can be caused by an EGR fault, too much advance in the ignition timing, and to a lesser degree gasoline octane/quality or carbon buildup.


#3

No there’s no EGR. It seems to be only happening on one cylinder judging by the timing of the tapping. I’ve had ignition timing problems on other vehicles but never had anything that works like this. As I said before the noise is only there when you open the throttle. I’ve treated my fuel a couple times with Lucas with no change.


#4

Is it possible what you’re hearing is an exhaust leak when under pressure? Maybe one of the headers has a partially blown gasket. Exhaust leaks can make all sorts of strange noises.


#5

I’m going with @oblivion here. Headers are notorious for leaking and the leak generally sounds like the ticking of a valve tappet.


#6

Putting premium fuel in it would quiet a pre-ignition miss fire. I’m with the exhaust leak crew.


#7

Inspect the distributor cap and rotor and confirm the base timing is correct. It is possible that arcing in the distributor results in one cylinder firing prematurely.


#8

I also vote for a exhaust manifold/header leak. I had the same issue with a 71 Buick.

Ed B.


#9

Come to think of it, those headers were notorious for blowing gaskets.


#10

What about with aftermarket long pipe headers?


#11

Have a friend push the pedal down while you listen under the hood. You may be able to tell exactly where it’s coming from.

Post back with the results.