Engine misfire DTC codes P0300 and P0420

[alanb]

My 1996 5.7 Tahoe has an intermittent engine misfire, the truck drives great then occassionally there is a loss of power which then clears, the SES light is up on the dash, I checked the codes and found the following;

p0300 random/multiple misfire detected
p0420 catalyst system efficiency below threshold

So far I have changed the spark plus and fitted AC Delco 41-993 gapped at 60 thou which seemed to make the problem slightly worse.

I have tapped both the cats and there is a very slight rattle on both of them.

Any ideas ?

 

[SunlitComet]

You most likely have catalytic damage most likely from:

• • Misfire.
  • High engine oil consumption or coolant consumption.
  • Retarded spark timing or weak spark.

Make sure you solve any of those problems first then replace you catalytic assembly. High performance ones are available from Magnaflow.

It would be advisable to get your PCM scanned for misfire history data and other live perimeters as soon as possible. You got pics of old plugs for condition reading? Also was the SES light just on or was it actively flashing.

 

[alanb]

The SES light is just on not flashing, the old plugs varied in condition between just right (biscuit brown) and lean (light brown / white), and plug no 7 looked dry/hot ? I will upload photos.
Also forgot to mention the truck has a full mulipoint LPG conversion (Petrol is expensive in England but LPG has big tax cuts - petrol approx 9 dollars a gallon and LPG 5 dollars a gallon)

 

[SunlitComet]

Look forward to the pics but perhaps you should get your regulator/vaporizer checked out. It sounds like it is lean your mixture.

 

[alanb]

I have taken pics and attached

---------- Post added at 05:45 PM ---------- Previous post was at 05:43 PM ----------

I can't seem to upload from photo bucket please see link
http://s1141.photobucket.com/albums/n590/axbreck/?action=view&current=DSCF4590.jpg

 

[SunlitComet]

Actually they don't look that bad at all. Can you get your misfire history checked? In case it is a specific cylinder malfunctioning.

 

[alanb]

The only plugs that worried me was the one on the far left which came from no 7 cylinder which looked like it hadn't being do much work and the one far right which looked at little lean.
I will see my pal with a decent OBD reader at the weekend and try to have the PCM scanned and report back
I haven't checked the cap or rotor yet, so I will do that next.

 

[alanb]

I replaced the dizzy cap rotor and spark plug wires which seems to have fixed the missfire.
I have not changed the Catalytic converter yet, the smog test is not due for a while, so I will so if the SES comes back on

 

[SunlitComet]

Sweet if the misfire is gone and the only code is for converter efficiency then leave it be as long as possible as long as no other issues show up.