miss fire when cold please help

[ocpman]

2000 5.3 litre only miss fire when cold. It has codes p0322 knock sensor, and p0420 Catalyst System Efficiency. Freez frame data say codes where set when warm are they related, or what would cause a cold miss fire. I check for vacuum leaks when cold found none. and the miss fire are all over the board. I would be grateful for any help Thanks

[danielsatur]

1) I would start with a New gas filter, Air filter, and clean MAF/IACT sensors.
The P0420 code, sounds link a blown, or bad catalytic converter on bank one.

We have weeded out plenty of bad MAF's, Air filters, and Gas filters, causing random misfires.

[ocpman]

I did all of that, because it had a K&N Filter, so I removed it, and cleaned everything. The maf code did not come back.

[danielsatur]

The trick is knowing if you got a consistent misfire on a single cylinder, or random misfires.

2) Do a sound test on your catalytic converters, and compare bank 1 with bank 2,
a blown cat will sound like a empty tin can.

[ocpman]

Its random miss fire in every cylinder. I watched the missfire counter on my scanner. I'm not concerned with the cat at this time, mainly the cold missfire. I will figure out the cat afterwards. The missfire only when its cold. It also only missfire at idle. Off idle the missfire counter go to 0, and the engine runs smooth.

[ocpman]

Do you think I could have a problem with the valves? Maybe they are leaking, then seal after warm up.

[danielsatur]

If it was valves, it would be more like a consistent misfire on the same cylinder.

3) Do a good throttle body cleaning next.
4) Fuel pressure test.

We have seen plugged cats causing random misfires, but yours is blown!
Could broken pieces cause blockage down stream on your exhaust system?
The blockage down stream would act like a plugged cat, causing random misfires.

Was the MAF/IACT sensors cleaned, or was it replaced?
Doe's the random misfires occure on bank 1 or bank 2?

[777stickman]

Also check ECT temp with your scan tool when cold. It should show something close to ambient temp.

[mishalah]

Wouldn't a bad cat code show up as one bank (head) or the other?

[j cAT]

Originally Posted by ocpman

Its random miss fire in every cylinder. I watched the missfire counter on my scanner. I'm not concerned with the cat at this time, mainly the cold missfire. I will figure out the cat afterwards. The missfire only when its cold. It also only missfire at idle. Off idle the missfire counter go to 0, and the engine runs smooth.

fuel pressures and plug cat converter are known to cause radom miss fires...if only when cold most likely the fuel pump is weak...need more fuel when cold...

[usamarshal]

What was the cause of the miss fire?