454 intermittent rough running and CEL's 97 K2500

Start it up and it runs great for a month or 5 minutes. Then it starts running very rough and sometimes Service Engine Light comes on and burns steady. Sometimes for a short period it will come on and flash. It will eventually (so far) start running smooth and the lights go out by them selves. It will run smoothly for 10 mintues or for a month and then it will happen again. If I try to scan after light goes out nothing comes up. Once I was able to scan while light was on and it showed #5 cyl misfiring. I can sometimes get it to clear by hard accelerating. It almost always developes the rough running and SEL when I brake hard. The rough running and SEL can last a few minutes or for a few days. I replaced the #5 spark plug wire, didn't make any difference. Possible valve spring or bent valve? I'd rather not go that deep, I'm getting too old.

No other codes?

jrtparrot, have you stayed on top of the maintenance? Changed the plugs etc.? When was the last time the 02 sensors were replaced? How many miles are on it? Probably nothing internal, hope this helps.

Thanks for getting back to me. Only code that comes up. Subruban has 118k on it. Haven't changed plugs or done tune up in a while. Could be that O2 sensors are original. Thanks guys

Any time the CEL comes on (or goes off) the VCM will store a "history" code.

Thanks for getting back to me everyone. I'm going in tomorrow for a new waterpump and installation. I'll have the shop run the codes to see if anything else comes up. I'll let you know.
Thanks again,
Jim

have the fuel filter replaced and fuel pressures measured using a pressure guage for the correct psi with this engine. you need to replace the fuel filters more often with the fuel made now.

check vacuum. check that the brake booster has no leaks. also replace the PCV and make sure hoses are not cracked poor fitting etc..

the O2 sensors will throw codes when bad. also at this mileage I doubt they can cause the bad idle.

running the vehicle with missfires will damage the cat converters.

plug wires need to be removed and inspected and measured for the correct resistance . any defects new wires. get ac delco plugs and do not attempt to adjust gap . plugs should be replaced at 50,ooomi. going over that they can have the platinum/irridium wire fall off.

make sure the plug wires are properly secured and routed .

this is not to hard to do for an old guy.