My truck is running rough and has very weak acceleration (over about 20% throttle it just dogs out and slows down). I checked the codes and it has a P0305 (misfire cylinder 5) and a P0106 (MAP/MAF problem). So I contacted a mechanic, and he said it sounds like a clogged catalytic converter. I asked if a clogged catalytic converter could cause a misfire and map problem, and he said yes.

So, does that sound reasonable? I'm having trouble seeing how a clogged catalytic converter can cause a single cylinder to misfire or cause MAP problems. But I am not an expert.

Steve

 

More likely, the misfire caused the clogged cat.

It also helps IMMENSELY if you specifically identify your vehicle, especially the year, model, engine, and transmission.

 

And I jumped the gun on my comment. I was thinking the mechanic told you it had a clogged cat after seeing the vehicle. Since this was based on a phone conversation, my comment was not sensible. Any 2007 could very well have a clogged cat though, so you should at least verify that it doesn't before assuming it.

 

If misfires and a MAF problem is IN FACT caused by clogged cats... (excessive backpressure), it can be quickly and inexpensively verified.


Remove BOTH upstream O2 sensors, clear the codes, and go drive the vehicle.


Front ones are sometimes hard to get out, and it will sound AWFUL and be embarrassing for your friends and neighbors seeing you drive by. But just wear ear plugs and a Halloween mask. It will relieve enough backpressure that you can tell immediately by improved accelerator response if THAT is in fact the problem.


THEN go from there.

 

so you have a code of miss fire on #5 cylinder and instead of fixing that, you want to buy a new converter ? You need a new mechanic.

 

I bet 80% that the problem is a bad COP ........... if you have COPS ?

 

You know - it occurs to me that we have ALL missed the whole point of Steve's question. We have ALL concerned ourselves with what caused a clogged cat, description of his vehicle, what he should fix first, how to verify if cats are clogged, or his needing a new mechanic. NONE of which he asked.


"How can a clogged cat cause a single cylinder misfire or MAP problems?" - In the same way the weakest link of a chain will break first.

 

I understand that backpressure can cause misfires and problems of different sorts, and it is a simple thing to pull the o2 sensor and see what happens.

My confusion is this. I have a misfire on #5, constantly, and just #5. I might think a clogged cat would cause more random misfires? But there's no direct cause of misfire I can find; I replaced the plug twice, replaced the coil pack twice, and tested the fuel injector harness with a noid light (it flashes ok) and tested the injector itself by listening to it with a stethoscope (it is clicking away just like the others) and by checking resistance with an ohmmeter (13.9 ohms, same as two working fuel injectors I also checked). So, the misfire seems to be caused by something else.

And I don't know for sure the cat needs to be replaced, I'm not getting any error codes on it, just the P0305 (misfire on #5) and the P0106 (Manifold Absolute Pressure/Barometric Pressure Circuit Range/Performance Problem).

Oh, and I forgot to say, its a 2007 F150 4.6l V-8.

Steve

 

you may want to do a compression check... valve leak can give the same symptom

 

Confucius say "Fuel + Spark + Compression = combustion". Seems like you have verified the first two - except that I have VERY LITTLE faith in the flashes of a noid light.


They only test continuity of the circuit - and won't even expose a high resistance connection because they represent little to no load to the PCM output circuit.


If you have determined a hard misfire - I prefer to test spark / ignition problems in a more live environment, ie: with a high voltage spark tester. (< $15 @ ). This verifies the wiring harness, connections, PCM, and COP primary & secondary. If the PCM output circuit will not carry ('sink') adequate current to pull the negative side of the COP primary to ground, firing the plug, there could be a high resistance connection between PCM & COP or the PCM output could be bad or shorted to ground internally. You can narrow that down to the PCM output circuit by "shorting" the PCM pin for that COP to ground and see if the plug fires (this won't hurt the PCM as that is all it does anyway is pull the negative side of the COP to ground).


If this test passes - and you're comfortable the injector is "passing fuel" - then as I see it, @steve(ill) 's suggestion is all there is left.