I have a 1992 Camaro with a 305 TBI motor in it. My problem is backfiring coming down off of 2500rpm and above. This comes from the exhaust tips and not the cats. What could be the problem? I was thinking timing, but am not sure.

 

Hmmm....depending on how loud your exhaust is, and engine RPM (like downshifting to slow the vehicle with a manual or automatic trans), you may here some popping on deceleration. This is a lean condition, and can be a normal part of engine operation. The "louder", or more open your exhaust, the more likely you are to hear this.

Hard to tell what you have from a brief verbal description.

 

Do you have an AIR pump? Failed diverter valve for AIR pump is a classic cause of exhaust popping. I had a Chevelle with an AIR pump that blew both mufflers to bits due to a failed diverter valve...pop pop pop pop K-BLAM!

 

Schurkey, I'm going to label you as the scan tool King (in a nice way), and I'm still learning (trying to remember) what all you can get out of an OBD I system.

Is diverter valve failure something you could see with a Tech 1 or similar scan tool? If it doesn't set a code, would you be able to "see" the status of the valve....see it NOT switching?

 

I don't think there's a position sensor on any diverter valve. At least, none I've ever worked with. No, you wouldn't see a code. Depending on the vehicle, you might be able to "see" the diverter valve switching based on the O2 sensor. "Upstream" could drive the sensor to report lean, "downstream" air bypasses the sensor so it wouldn't interfere with sensor operation.

MAYBE there'd be an acknowledgement that the diverter is commanded "upstream" or "downstream"; in the same way that you'd get a command for the torque converter clutch to engage, or for the A/C to engage. Point is, just because the diverter is commanded "downstream" doesn't mean it's actually pushing air downstream. The valve could be sticking.

The ancient diverter valves--like the one that blew my mufflers--worked off of ported vacuum. Throttle closed, no vacuum, air diverted to the atmosphere. Throttle open, vacuum, air goes upstream to react with the exhaust gas in the exhaust manifold. No ECM to "command" anything.

If the AIR system is pushing air "upstream" when it shouldn't--like when you decelerate--it would drive the O2 sensor lean, the computer would respond by richening the mixture, and the additional air in the exhaust manifold makes the unburned fuel in the exhaust "pop". Or K-BLAM as the case may be.

 

Good info Schurkey. Thanks.

So how do you test for this? Use the scan tool to command it on/off, and with the hoses off, see if the valve works?

 

My scan tool will command the diverter valve on my '88 K1500. Seems likely that this function would be built into a '92 F-body computer also. All you'd need is the compatible scan tool.

I suspect that diverter valve diagnosis (with a compatible scan-tool) will be built-into the ECMs going back to '80 1/2, but it's been so long since I've worked on anything that old that I don't remember. At any rate, I suspect you can switch the diverter valve manually using jumper wires or some-such, but you'd need to see how the thing is wired so you don't let the smoke out of it.

 

If equipped with pin c you can ground it to enable the diverter valve.