Clogged catalytic converter?: Grassroots Motorsports forum: Grassroots Motorsports Magazine

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Oct. 31, 2011 10:27 a.m. PseudoSport Reader

I’ve been trying to get me Jeep Comanche running on MegaSquirt with a new 4.7 stroker. Now I’ve already installed MS on our other Jeep plus a few different vehicles and I’m pretty sure everything is set up correctly. Problem is it runs rough at idle and sounds like it has a random misfire. When I rev it up it’s still rough and sputters but it does rev pretty freely. I’ve tested fuel pressure, swapped injectors, checked plugs, coil/cap/rotor/wires are new, messed with fuel are spark settings, messed with dwell, logged signal from hall sensor, confirmed timing, and checked for vacuum leaks. I’d like to say that when I first got it running it idled ok but very shortly after it started to get worse. It runs so poor now that I’m questioning if my memory is correct or I was just excited it started up.

The converter is aftermarket and has about +60K and the exhaust system was removed from my old Jeep which ran perfect. I’m guessing that when I went to start the Jeep for the first time I was dumping fuel into the exhaust trying to get my cranking settings close enough to start the Jeep. When working on the Jeep Sunday I noticed the cat was extremely hot but not red. The floorboard inside the truck above the cat was also too hot to touch. This could be normal but I was curious if a clogged catalytic converter could cause the symptoms I’m having. It’s cold and wet out and I’d like some input before I crawl under the Jeep to mess with the exhaust. I also hate swapping parts until I’ve done some trouble shooting or have a very good reason.

Thanks
Oct. 31, 2011 10:32 a.m. jimbbski Reader

I have heard that a matrix inside of a cat can melt from excessive heat due to to rich of a fuel mixture. Something that you say may have happened on your car.
Oct. 31, 2011 10:33 a.m. Streetwiseguy Dork

Plugged cat won't affect idle unless its monstrously bad. all you are doing is restricting flow, and there isn't much flow at idle.

The part about the cat getting hot implies you are dumping fuel in, or have a dead ignition miss sending unburnt fuel down the pipe from that cylinder.
Oct. 31, 2011 10:36 a.m. Ranger50 Dork

Vacuum gauge. IIRC, gradual decrease in vacuum will be a plugged cat.
Oct. 31, 2011 11:06 a.m. PseudoSport Reader

Ranger50 wrote:

Vacuum gauge. IIRC, gradual decrease in vacuum will be a plugged cat.

Well I can log vacuum with MS. I’ll have to give that a shot when I get home.
Oct. 31, 2011 11:15 a.m. alfadriver SuperDork

The consistent misfire will plug the cat, and depending on how bad it is, it will do it very quickly. All of that fuel and air that would be used to move your foreward would burn in the cataylst just to generate heat.

Gotta figure out what's causing the miss.

For a case like this, I would go as far to suggest removing the catalyst to fix the misfire, and then put it back in.
Nov. 1, 2011 12:02 a.m. amg_rx7 HalfDork

If you are able to have a peek at the cat core, it looks like a small honeycomb. Clogged cats often look like a clogged shower drain b/c the honeycomb gets full of "soot" and eventually melts into a mess.
Nov. 1, 2011 9:33 a.m. Gearheadotaku Dork

Are you required to have a cat? Once it's off the truck, does it rattle when you shake it? I'd just be rid of it if possible, even if it's not the trouble maker.
Nov. 1, 2011 9:48 a.m. AngryCorvair SuperDork

does the engine the crank came from have the same firing order as the engine the cam came from? i know you said you checked plugs etc, but did you check that the wires are following the correct firing order?

i will not lie. i have made this mistake.