3/4 misfire and dreaded P0420

[bagtagley]

98 Leg GT, 2.5 Litre.

 

I had a misfire code (cyl 3) about this time last year, as a result I replaced the plugs w/ NGK and the wires w/ OEM. I reset the ECU and hadn't seen the CEL until a couple weeks ago (and about 15k miles after the plug/wire swap). Being a slacker, I've let it go until last week when I got the errors checked at Autozone. They guy read misfires on cylinders 3 and 4 and a P0420 catalyst inefficiency code. He said the P0420 was probably a result of the misfires and to address that first.

 

I reset the ECU tonight, but I feel certain the light will come back. I think so because occasionally when the engine is cold and I'm on the interstate, the CEL flashes briefly before going back to solid.

 

I've gone through numerous threads regarding these codes. Before I tackle the cat problem, I wanna take care of the misfires. Most problems seem to involve a single cylinder misfire, or a 1/3 misfire. Is there any connection between the 3/4 misfire, or am I double screwed? Any help is appreciated.

[Legacy777]

Let's forget about the codes......how does the engine run? Do you notice misfires, or does everything seem to run fine?

[vic]

98 Leg GT, 2.5 Litre.

 

I had a misfire code (cyl 3) about this time last year, as a result I replaced the plugs w/ NGK and the wires w/ OEM. I reset the ECU and hadn't seen the CEL until a couple weeks ago (and about 15k miles after the plug/wire swap). Being a slacker, I've let it go until last week when I got the errors checked at Autozone. They guy read misfires on cylinders 3 and 4 and a P0420 catalyst inefficiency code. He said the P0420 was probably a result of the misfires and to address that first.

 

I reset the ECU tonight, but I feel certain the light will come back. I think so because occasionally when the engine is cold and I'm on the interstate, the CEL flashes briefly before going back to solid.

 

I've gone through numerous threads regarding these codes. Before I tackle the cat problem, I wanna take care of the misfires. Most problems seem to involve a single cylinder misfire, or a 1/3 misfire. Is there any connection between the 3/4 misfire, or am I double screwed? Any help is appreciated.

 

I have a similar problem with a cylinder 4 misfire but no P0420 catalyst inefficiency code. Have done the plugs, wires, coil stuff but no help. Also tried the Sea Foam routine I read about here. It only helped for about a month. Engine seems to run well. Do not notice physical signs of misfire. No loss of power nor MPG. Does not seem to run at higher than normal RPMs for known speeds and conditions. Dealer can't figure it out. Wants to replace all sorts of expensive stuff.

[SubeeTed]

Hi! My .02.

 

I had a misfire code for cylinder 2. I have a 2.5 Dohc, 1997 GT. I changed the plug wires, plugs, coil pack, ran Subee Cylider head cleaner through car! No dice! These are the things the dealer would do, For me it ended being one of my Valves was TOO TIGHT! This is an old occurans! Than after that was taken care of I got a code for my Cat. Before I bought the car, previuose owner had a 100K mile service done. They installed 4 NEW plugs! They FAILED too seat them properly! This killed my Cat.

 

SubeeTed

[color-blind]

Have you replaced the coil? Most mis-fire codes are from ignition wires and coils. You will then need to replace the front converter chances are the damage is done. good luck,

[friendly_jacek]

They installed 4 NEW plugs! They FAILED too seat them properly! This killed my Cat.

 

Can you explain? I don't understand.

[bagtagley]

Let's forget about the codes......how does the engine run? Do you notice misfires, or does everything seem to run fine?

 

Gas mileage is down a bit, and it runs a bit rough in the morning, just at first.

 

color_blind: I had a feeling you'd post and tell me the cat is toast. What are the ramifications of running w/ a burned cat? Also, I've only replaced the plugs and wires, nothing else. Any suggestions on where to score a coil? Is this a necessary OEM purchase or can I go for a refurb or aftermarket part?

 

Should I go ahead and replace the plugs again too? 15000 miles on a set of plugs doesn't seem like much.

 

Thanks for the help so far.

[color-blind]

Purchase your coil from the dealer-they're not that expensive. If the plugs are ngk or nd then leave them in; if they any other brand change them out.

 

 

Their is a chance you have a tight valve; yet this is rare. Start with the coil if you still have problems have the valve clearances inspected.

 

The converter really isn't that big of deal, unless you need to pass a state emissions test. Since they don't tend to plug you should not have any driveability problems. You'll just be politically incorrect by driving a car that is polluting the air we breathe, LOL. Have a good weekend.

[SubeeTed]

After the valves were adjusted. And that code went away. (Basically I had 2 codes going on at one time, one over road the other) Soon the Cat code appeared. Previouse to this we replased all four plugs, during that we saw that the 4 plugs that the dealer installed at the 100K maintenance period, were NOT seated at all. I.E., the crush washers were not crushed. Because of the running the car the the miss seated plugs, they didn't burn the mixture all the way, the Cat did it's job and died finishing the combustion. Because that's what there supposed to do.

 

 

 

 

Can you explain? I don't understand.

[Legacy777]

Gas mileage is down a bit, and it runs a bit rough in the morning, just at first.

 

 

Hmm....the rough running conditions and poor gas mileage could be due to coolant temp sensor. Or possibly IAC valve....but it wouldn't necessarily explain the poor gas mileage.