Passenger cylinders not firing correctly, right cat glowing after 2 mins

davidbr · Jul 15, 2025

Hallo from Germany,

my 1997 Viper GTS gives me some headaches. I have it in an auto workshop right now to get a new exhaust installed with legal catalysts. After they installed everything, the Viper turned out to not run correctly on the passenger side cylinders. The cat on the passenger side glows after idling for just 2-3 min, so we think there is fuel burning in the catalyst. The driver side cat stays cold. The ignition works fine according to the workshop. They already changed both ignition coils, all spark plugs and cables. Compression is okay too, on all cylinders, they also checked rocker arms, valves and springs, and these parts seem to be okay too. As they're out of ideas right now, I will have the car delivered back to me in a few days to look for the problem myself. Apparently it is multiple cylinders not running correctly, so I guess there could be some kind of gas leak or the PCM is fried badly. Does anyone here have an advice for me where to look first?

Thank you very much

David

Viper Specialty · Jul 15, 2025

Check for fuel leaking into manifold, exhaust leaks, O2 sensor issues, plug wire incorrect routing. If nothing found, replace PCM. We do stock controllers if needed.

kblake905 · Jul 15, 2025

davidbr said:
Hallo from Germany,

my 1997 Viper GTS gives me some headaches. I have it in an auto workshop right now to get a new exhaust installed with legal catalysts. After they installed everything, the Viper turned out to not run correctly on the passenger side cylinders. The cat on the passenger side glows after idling for just 2-3 min, so we think there is fuel burning in the catalyst. The driver side cat stays cold. The ignition works fine according to the workshop. They already changed both ignition coils, all spark plugs and cables. Compression is okay too, on all cylinders, they also checked rocker arms, valves and springs, and these parts seem to be okay too. As they're out of ideas right now, I will have the car delivered back to me in a few days to look for the problem myself. Apparently it is multiple cylinders not running correctly, so I guess there could be some kind of gas leak or the PCM is fried badly. Does anyone here have an advice for me where to look first?

Thank you very much

David

Seems more like a plugged cat?

davidbr · Jul 15, 2025

kblake905 said:
Seems more like a plugged cat?

The cat is completely new, so that would be really surprising. It was parked in a collector's storage for eight years until I bought it in last winter, and I couldn't drive it so far either (it was straight piped with empty cats, just way too loud and everything but street-legal), so maybe there is some damage from just being unused.

Viper Specialty said:
Check for fuel leaking into manifold, exhaust leaks, O2 sensor issues, plug wire incorrect routing. If nothing found, replace PCM. We do stock controllers if needed.

I will do plug wires first and look for leaks, then maybe interchange the O2 sensors (or run it without?). With "manifold", do you mean the exhaust or the intake manifold? I know that the fuel lines are part of the intake manifold, is there any chance to reliably check them without taking the manifold out?
I hope that the PCM is okay, if not I'll get in touch, thank you.

daveg · Jul 15, 2025

My first thought was Plugged Cat. Did the car run normal before bringing it there? Did they re-use O2's?

davidbr · Jul 15, 2025

I cannot really say whether or not it ran normal before. I only drove the Viper about 20 km after I bought it, and as it was completely straight piped, it was just so loud that I didn't notice anything. They reused the O2's.

daveg · Jul 15, 2025

Same same amount of Exhaust exiting the Passengers side VS drivers side?

davidbr · Jul 15, 2025

No, it seems to be less on the passenger side or more specifically, it is not the same "regular" flow on the passenger side as on the drivers side.

daveg · Jul 15, 2025

Factory Cats?

davidbr · Jul 15, 2025

Aftermarket HJS cats.

Viper Specialty · Jul 15, 2025

davidbr said:
The cat is completely new, so that would be really surprising. It was parked in a collector's storage for eight years until I bought it in last winter, and I couldn't drive it so far either (it was straight piped with empty cats, just way too loud and everything but street-legal), so maybe there is some damage from just being unused.

I will do plug wires first and look for leaks, then maybe interchange the O2 sensors (or run it without?). With "manifold", do you mean the exhaust or the intake manifold? I know that the fuel lines are part of the intake manifold, is there any chance to reliably check them without taking the manifold out?
I hope that the PCM is okay, if not I'll get in touch, thank you.

Fuel leaks into Intake manifold. Exhaust leaks in exhaust manifold. Either of them can cause this.

Note: Once a converter overheats, its usually near instantly dead. Dont think just because its new it cannot be bad. Once the core melts, backpressure increases, O2's dump fuel, and the condition runs away.

Head/Cam cars w/cats are notorious for this.

davidbr · Jul 15, 2025

First of all thanks to all of you for your ideas.

Viper Specialty said:
Note: Once a converter overheats, its usually near instantly dead. Dont think just because its new it cannot be bad. Once the core melts, backpressure increases, O2's dump fuel, and the condition runs away.

Head/Cam cars w/cats are notorious for this.

Okay, I did not know this. I hope it is still okay. The engine should be stock (at least it was sold to me as stock lol).

Once I got it back in my garage, I will start with checking all the ignition wiring and contacts and have a look at the seats of the injectors. Before I start the car again.
If I do not find an error there, then I guess it would be best to take off the right cat, put on some pipe or something, and see what happens when it runs. Probably should also use an OBD scanner or an oscilloscope to see what happens with fuel and O2 sensors with and without cat. I will post an update here once I had time to check things.

That will cost a few nerves.

Viper Specialty · Jul 15, 2025

davidbr said:
First of all thanks to all of you for your ideas.

Okay, I did not know this. I hope it is still okay. The engine should be stock (at least it was sold to me as stock lol).

Once I got it back in my garage, I will start with checking all the ignition wiring and contacts and have a look at the seats of the injectors. Before I start the car again.
If I do not find an error there, then I guess it would be best to take off the right cat, put on some pipe or something, and see what happens when it runs. Probably should also use an OBD scanner or an oscilloscope to see what happens with fuel and O2 sensors with and without cat. I will post an update here once I had time to check things.

That will cost a few nerves.

Correct.

Best to troubleshoot this without the cat installed.

in a stock application, this can nearly always be tracked to a few simple things:

1. Misfire due to ignition fault.

2. Fuel leak into cylinder. via cracked manifold or bad injector.

3. Exhaust leak / cracked manifold. O2 introduction causes fuel trims to go wild, and mix of oxygen/extra fuel melts down converter.

4. Bad O2 sensor/wiring issue on fuel/sensor/ignition.

On an otherwise healthy stock engine, I doubt its going to be related to internal damage, tuning issues, or otherwise.

Goggles Pizano · Jul 16, 2025

If cat is not hot, then it is not working.

Fuel trims should have been looked at first.

Start first with checking all the cables and connectors to O2s, spark wires are in correct locations and any wires/connectors they removed to do whatever they tried to do to fix it.

GTS Dean · Jul 16, 2025

Be sure your plug wires are installed on the correct coil terminals. Many manuals are wrong.

yzf1999 · Jul 17, 2025

Does anyone know if the factory 1997 manual is wrong?..I plan on changing plug wires eventually and was gonna use the manuals routing.

Viper Specialty · Jul 17, 2025

yzf1999 said:
Does anyone know if the factory 1997 manual is wrong?..I plan on changing plug wires eventually and was gonna use the manuals routing.

I believe it is. Go by what the coils say, not the manual.

Steve-Indy · Jul 17, 2025

As Dan says, go by the numbers marked on the coil packs...period.
Use a mechanic's mirror when making the connections of the plug wires to the couple packs.

The diagrams printed in the Viper Service Manuals have been a source of misinterpretation since the very beginning.

davidbr · Friday at 6:01 PM

So, today I got the Viper back at my place and had a first chance to have a look at the engine.

- The new coils and spark plug wires were installed correctly, just like in the photos from GTS Dean (thanks!).
- The injector wiring seems to be correct too, same colors as in the manual.

I am still waiting for the OBD diagnosis tool that I ordered, couldn't look at actual ignition/fuel/O2 values so far. The workshop looked into the cylinders and said that all look the same on both banks, so I think the error is more likely to be in the exhaust manifold (but I could be completely wrong about that).

As I wanted to go on, I took off the heat shielding from the right exhaust manifold to have a first look. It's a bit burnt in the lower middle, looks a little weird to me, but could be absolutely normal too. It's a cast exhaust manifold. I'll test wiring connections tomorrow and see if I can spot any cracks or leaks on the exhaust manifold.

Goggles Pizano · Friday at 10:55 PM

Get a IR thermometer. That way you can hit each cylinder exhaust pipe and the cat too to get temperature reading.

davidbr · 2025-07-27T07:31:16-0400

Goggles Pizano said:
Get a IR thermometer. That way you can hit each cylinder exhaust pipe and the cat too to get temperature reading.

I decided to already pull the manifold because it looked odd to me. The pipes of cylinders 6 and 8 are in a dark grey (like annealing colors?), different to the others.
The manifold seems to be okay, I have to check for straightness, maybe have it grinded and then put it back in with new gaskets and a new O2 sensor.

Passenger cylinders not firing correctly, right cat glowing after 2 mins

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