My 92 Vette LT1 stumbles at partial throttle slightly and occasionally backfires (sounds like through the intake). It accelerates clean and strong.
Temp gun showed right side cat intake very hot. We assumed that it was lean and replaced all injectors but it made no difference.
Could a clogged cat on that side cause the backfiring and stumble at partial throttle without hampering heavy throttle acceleration? This engine is built with cam, heads, ported intake, big TB, custom chip, headers, etc. and runs exeptionally strong.
I have had the water pump fail in the past (water leaking for a few months) and know of the Optispark's tendency to fry but how could this impact cat temp on one side and why would this engine spin to 6K+ RPM smoothly if it was bad?
I'm at wits end and am tired of fighting this thing. Any help would be appreciated!
This engine is freshly rebuilt with new pistons, cam, etc. etc. and was doing this before the rebuild as well. :sad:

 

Do you still have the AIR system on the car? If so, look at the 2 check valves, and diverter valve to be sure they are working correctly.

 

last summer, I had a hose leak on the optispark. It dried out, but weeks later the car started to stumble and backfire while driving.
The symptoms were that it would stumble bad if I put a load on the engine.
Say, like going 30 MPH in sixth gear, or taking off in second gear kind of load.
Once the RPM got up, it ran like a champ. Does this sound like your issue?

I had to replace the Opti of course. I opened it up after I removed it and there was rust covering all the metal inside.

I'm betting the opti is going on yours too.

 

Mine started that a couple of weeks before the coil went out.

 

yes.... my 94 was acting very similar.... turned out there was some corrosion on the coil.....
:iagree:
check the coil..... and or replace....
just my 2 cents

 

My guess is that you have a ignition problem, that could include the Opti unit. You best bet is to get scan tool on it and see if there are any codes set for either the low or high resolution Opti signals. If you don't see any codes, then the next thing to do is to get the car to a shop where they have an ignition scope and check the primary and secondary wave forms, this will tell you if the plug wires, coil and spark plugs are up to par in your engine.

 

Since your rebuild, has it ever run the way you want it to? If not, I'd wonder about the cam and if it were degreed properly. BUT I'd check all the easy things first; connections, vacuum leaks, etc. Any codes? As far as the difference in cat temps, maybe one bank is leaner than the other. We'll find it.

 

Thank you all for your feedback.
In answer to some of your questions: No codes have been set. The car formerly had (they thought) a bad lifter bore that was causing one lobe to wear. Also since the last shop put in a 110 degree lobe center cam, the car barely made it through emissions. The car started acting up so they determined that the cam was bad and needed to be changed over to a 112 degree model anyway so they went in to do that. Turned out the lifter bore was OK and the cam was OK but bearings looked bad from dry starts (28K mile 92 Vette) so I had them do total rebuild.
Put in different cam and lifters and did complete overhaul with all good stuff but afterwards still had the occasional backfire and stumble at light throttle.
Cam was and is degreed properly and heads are freshened up and perfect.
This seems to only happen in closed loop.
The shop found corrosion on coil connection and tried new coil but seemed to offer only slight improvement.
In the dark I could see no sparks / shorts anywhere around plug wires or coil.
The obvious answer would be the Opticrap but how does that explain the overhot cat on the passenger side?
What would cause a one bank lean condition? (All new fuel injectors, too)
Like I said earlier, the car runs super strong and with moderate to heavy throttle is smooth.
I want to sell this car but will not do so until it is perfect and have spent about $5K in the last couple of months to this end without finding this bug.
:cry

 

A lean condition wouldn't necessarily cause the cat to be hot. It could be a rich condition caused by a leaking injector. Excessive fuel getting into the cat would cause it to get hot because it would have to process all that extra fuel. Do you have a scan tool? If so check the fuel trims for the left and right banks and see if one side has a lower trim number. This would indicate that the one side is rich and the ECM is adjusting for it.

It also could be hot because during a misfire all that unburned fuel is going into the cat as well to be processed. Another reason for a lean or suspected lean condition is perhaps a exhaust leak that sucks in air and the O2 sensor thinks it lean because it sees O2 in the exhaust stream, and then trys to compensate by adding fuel, there again making the cat work harder.


[Modified by tjwong, 9:08 PM 1/5/2004]

 

Thanks for the input.
Since the right side cat had very high input temps relative to output, could this be a clogged cat and could a clogged cat cause occasional backfire through the intake with a moderate overlap cam?
The other cat had normal temp variation between input and out sides.
I have run lots of octane boost additives through this motor over the years.