We keep getting an engine misfire code on bank 2-4-6 (3.0L V6 manual trans 2003 S-Type), but we've already replaced the coil packs, spark plugs, etc., as well as the VVT solenoids, all relevant gaskets and seals.

We're also getting a catalytic fault code for the exhaust system that serves that side of the car. Our opinion is that the car was run for too long with poor coil performance, which caused too much unburnt fuel to go into that cat, and has ruined it. This theory is somewhat backed up by the fact that since we've replaced the coil packs, plugs and such, gas mileage has gone up by about 30%.

Even though we keep getting misfire reads, there is no discernable misfire or hesitation. However, the car is going into Restricted Performance, and on a stick-shift car, that means a top allowable RPM of 2000 and a top allowable speed of 10 mph. This is no good when we get a fault on the interstate.

My question, in lieu of replacing the cat right now (the car needs to be able to travel before the new parts can get here), is this: Is it possible to disconnect whatever sensor is sending the cat fault back to the computer, and if so, what's going to happen when we do that? And, if we can disconnect it, is there anything else we'll need to do (i.e., jumper the connection to close it, put a resistor across the switch, etc.)?

The reason I'm trying to "shadetree" this for the time being is that I'm in the process of moving, changing jobs and the cost to replace the cat and exhaust is a bit prohibitive. I'll order the correct parts as soon as I can, but I really don't need to be laying out a bunch of money at the moment. Thanks for any advice you guys can give.

Jess

 

Not sure of this relationship to your car and/or market.

Our S Types (2). Both 2001 V6 units.

Hers, (126000kms) constant misfire on RH bank. SAme as you, coils/plugs etc, better, but sometimes that annoying stumble at idle.

Mine (95000kms), very intermitant stumble at idle.

I changed the Pre Cat O2 sensors on both cars. NO idea why I went that path, I just did.

FIXED.

Both cars never threw a code (bizzare), and never went into limp mode. The dealer was confused as to why no codes, and I also have no answer for that??

I have since found that O2 sensors have a life of Approx 100K kms (Bosch and NTK), and that is shortened in some markets due to fuel quality etc.

I will be changing the Post Cat sensors very soon, just because.

 

2003 has wideband O2s upstream, unlike 2001. Also has radically different PCM (32-bit Denso). I really doubt it's an O2 sensor. Far more likely the cat but some peering at fuel trims and O2 readings at various revs / engine LOADs may show.

To cause misfires, if it's the cat it pretty much has to be there and reducing flow. (If it was missing due to breaking up then you'd get a cat efficiency code.)

Freeze frame data may hint at what's wrong - you'd tend to see a blocked cat hit misfires other than at idle but I wouldn't rely on that as a diagnostic.

By all means remove an O2 and look in with a borescope. Also, sometimes an IR gun can show the temp drop is different side-to-side. Heck, sometimes you can hear a cat is loose and rattling around.

If it's there but blocking the flow I don't really see a workaround other than removing it. Maybe there would be a way to drive that never demands big flow?

 

On your last paragraph: You can cause a fault by driving it at a high RPM, but it doesn't even take that to cause a fault. Simply coasting downhill without pressing the gas pedal makes it trigger a fault about four out of every five times you perform that activity (coasting). When the engine starts backspinning while going downhill (let's say it's coasting at 2500 RPM, pulling a load on the motor but without receiving any fuel), it triggers Restricted Performance and if you jump back in the gas right away, it will go into full-on limp-home mode. You have to feather the throttle a little, give it just a little bit of gas at a time to make the car snap out of it. Where this becomes a problem is if you happen to trigger a fault just before going UP a hill, you can't give the car any gas to actually get up the hill. The car becomes an instant roadblock.

I was hoping for a way to unplug whatever sensor goes to the cat to keep the computer from reading a cat fault, until I can permanently fix the problem with new parts. Removing/replacing the cat is my second option, it just happens to be an expensive one.

Jess

 

The whole point of misfire etc codes is to get people to act early before the cat dies. If it's a dead cat it's a dead cat.

 

Understood, but I need to milk 1-2 weeks of driving out of the car before I can replace it. I was hoping to find out whether I can unplug a sensor for that short amount of time and get it to quit throwing faults, thus get it to stay out of Restricted Performance for a short period of time. Possible?

Jess

 

I highly doubt it but just go playing if you're so determined. Worst case the cat is OK now and you wreck it.

 

Thanks. The problem has since changed in nature; I'm going to make a new thread.

Jess