I wanted to let everyone know what was causing my 92 LT1 backfiring at "startup" with the engine hot or cold. After a couple of minutes the engine ran perfect and I could not diagnose the problem in less than 2 minutes while acting up. The computer never had any codes so I replaced the usual suspects plugs ,wires, Icm, coil , FIC injectors,testing sensors and replaced Opti. This problem has been going on for years and been to a half of dozen good shops as well. I was asking our forum members for advice of where to get my ECM repaired or replaced out of desperation. Thats when one of our senior members (Toptechx6) told me it sounds like the electric air pump system (emissions ) was not working properly. Nobody ever suggested that to me before after all these years. I took his advice and just disconnected the electric air pump for now before cking the diverter valves. Finally after all these years my Vette is running as good as it looks thanks to a senior CF member sharing his knowlege and expertise with me !!! Toptechx6

 

Thanks for sharing your fix!

 

I wanted to be one of the guys that shared what fixed his LT1 misfire (exhaust backfire). It always drove me nuts when members take the time to help someone and they don't report what fixed the problem !

 

The AIR pump has a valve to prevent operation under certain vacuum conditions. Under high vacuum the AIR system can cause backfiring because there is too much fresh oxygen at the exhaust. I would check out the check valve that is part of the system.

 

Glad to hear you have isolated the problem area. Especially, one that is not one of the "usual" suspects.

 

Glad you some Great advise from Jim.....He's helped me out in the past and I'm very grateful to him.....He knows alot about these cars...

 

Thanks to Drew and Rick for the kind words, always my pleasure to help either of you at any time. You both have incredible C4's that deserve to run as good as they look.

 

Thank You very much Jim. It's always very comforting to know if I have an issue I can always drop you an email and you always help me out, it's much appreciated......And by the way, your '93 is pretty darn incredible as well.......TTYS, D

 

OK, Hot Rods, you isolated the problem by disconnecting the air pump before checking the diverter valve. There must be more to the story.
What did you actually fix, or is the disconnected pump the fix?

 

Thanks for letting us know about the fix

 

Seabright you are right I did not fix the problem I just eliminated what was causing it for now. By disconnecting the electric air pump it is no longer sending air through one or more of the valves that are not working properly. I will trouble shoot which valve or diverter is bad later. I'm just enjoying my car running without backfires through the exhaust anymore !

 

Hot Rods, the following words are not mine. I copied them from this link: http://tech.corvettecentral.com/2008...tion-overview/

It describes your problem, and cautions about a 'lean' condition that
can cause catalytic converter failure.

The Air Injection Reaction (A.I.R.) system consists of a pump, control valves, and solenoids. The A.I.R. system has a diverter valve control system that controls flow to the exhaust manifolds or catalytic converter. The A.I.R. system diverts air to the exhaust manifolds for the first two minutes of operation to heat the exhaust and speed up the oxygen sensor on time. After the engine warms up the A.I.R. flow is diverted to the catalytic converter or atmosphere. The A.I.R. system should divert to atmosphere when the throttle is closed on deceleration. If the A.I.R. system is not controlling the airflow properly a lean condition (misfire) may occur or catalytic converter failure is possible.

This lean misfire can occur at the same speed (45 MPH) as the EGR misfire. If misfire is a problem remove the hoses at the check valves to see if air is flowing at all times to the exhaust manifolds. Then drive the car with the hoses disconnected to see if the misfire has been eliminated. If the lean misfire has been eliminated the diverter valves or control system has a problem that will require a service manual for diagnosis.

After-fire (popping in the exhaust system on deceleration) can be caused by an improperly operating A.I.R. control system. The after-fire is heard on deceleration if the A.I.R. pump is incorrectly pumping air into the exhaust system. After-fire can be also be caused by cold air being introduced into the exhaust system. This condition can cause drivability problems because the fuel system will be lean, especially if the exhaust leak is before the oxygen sensor. This popping in the exhaust is often mistaken for backfiring.

 

Thanks Seabright the only problem I was having was during the first 2 minutes after startup. The way I understand it is one of the valves in the AIR system was not working right causing way to much air going to the O2 sensor during startup.with the engine in closed or open loop. The sensor sees a really lean mixture and tells the ECM to send more fuel. During this time my engine did not need all this fuel and would load up very bad and was not able to burn it all within the cylinders . The excess fuel or vapors got into my exhaust and exploded in the pipes during these 2 minutes. I never had any issues other than the first 2 minutes. I was surprised it did not damage my mufflers with such loud back firing.

 

Can I hear an amen! I don't know how many threads I have read that just deadended with no resolution of the problem reported back. Thanks for posting the solution to your problem - I'll file it for future reference (but hopefully I won't need it).

And a big to toptechx6!

 

Keep in mind that those valves are one way check-valves. If they're bad, ultimately the valves can completely fail such that hot exhaust gas will escape and burn through the rubber hoses and potentially damage wiring and other "soft" items in close proximity.

Had this happen on mine....burned through on the driver's side and then took out the vacuum line going to the power brake booster.

 

Thanks 96GS I can imagine that happening. I'm not sure if I'm going to fix or remove this system. This is not a daily driver. I would really like to remove and beat it with a big sledge hammer !!!! Then tell my wife I need headers.

 

You may just want to remove the fittings from the manifolds (or remove the check valves) and then plug the manifolds or cap the lines. That way you won't have any worries.

FWIW, the fittings can really be a challenge to remove from the manifolds and IIRC they're no longer available from Chevrolet (and used are quite expensive). The valves can be a pain as well...be sure to use good quality tools for the removal.

 

Keep in mind different Years use different means to provide clean air to the Headers and CAT. The LT Motors use an Electric Pump which is commanded to run at start up. L98's use a belt driven pump which not only runs at start up, but uses a switching valve to divert air to the CAT or overboard during WOT and decel. Earlier L98's used vacuum to accomplish the same thing.

When air pumps were first mandated back in the 70's, the crudeness of the vacuum actuated solenoids was such that most cars backfired under decel - particularly Chevy Vegas and Chrysler K-cars. That happens when unburned fuel or exhaust laden with HC's is suddenly met by fresh air which in turn sets off an uncontrolled burn or backfire.

Air pumps are like an afterburner and are particularly helpful when the engine first starts in that they keep your exhaust from smelling like crap. They also help light off the non heated O2's and the CAT and for earlier Years, make the CAT more efficient after warmup.

Checkvalves are necessary to keep the air flowing in one direction only and to make sure that when it's shut off, no air gets sucked in which will cause the O2(s) to sense a lean condition. When that happens, the ECM will dump a bunch of fuel it doesn't need and driveability can go to hell.

To the OP, your condition sounds like a motor that's running when it shouldn't; ie, under Decel. There is no reason for an Air Pump, at Startup without any load, to do anything to cause a Backfire unless air is already present in the exhaust stream; ie, a broken Checkvalve - or it's just dumping a ton of fuel. If that's the case, a closed loop scan should show that it's lean or replacing the Checkvalve(s) should fix it (you've already replaced the Injectors).

 

Thanks guys

 

SunCr, for info, ..... Florida does not have emissions testing......
are there other concerns?