I'm stuck and need some expert help here. 2001 Suburban 1500, 5.3V8, 166k, one owner so I know all of the history.
Changed the radiator, refilled with fresh Dex-Cool. I've always changed the Dex-Cool every 4-5 years, no sludge in the tank, etc. While it was idling during the refill and recharging the AC, check engine light came on. P0304 misfire on #4. It picked up a rough idle but pretty slight. Rolling to a stop it gets a little surgey in the last 1-2MPH and then a rough idle like it has a big cam. Rolling from stop through acceleration and all driving conditions it shows no problems at all. Just at idle. So far I have:
- swapped coils around
- swapped plug wires around
- new spark plug
- swapped fuel injectors around
still P0304
compression check. 140-150 on all cylinders except #4 is 110. I forgot to do the oil in the cylinder trick to see if compression would change.
Before I tore into it I wanted a second opinion. Took it into a shop and they claimed there was a TSB for wiring in the main coil harness and didn’t think the compression was a big deal. I replaced the coil harness. No change.
At this point I still get a P0304, but sometimes it takes a few idle periods before it goes from a pending code to turning on the light. It will also flash the check engine light at the end of a long traffic light cycle. It will continue to flash for 3-5 seconds of acceleration and then goes out. Sometimes the check engine light will be off and there is just a pending code when this happens.
What do I do next? Is there anything a really good scan tool could tell me here? If so is there a scan tool I can buy to do this or is it GM Tech2 level?
I would make sure it doesn't have a particularly big vacuum leak on that cylinder, and then I'd take a quick look at the valve springs.
Also, a scan tool won't tell you dick. It gives you information that you have to interpret.
It could be carbon hold a valve very slightly open. If you don't feel like pulling the valve covers I would put a dose of your favorite carbon cleaner in the gas. If that fixes it problem solved.
I have seen a couple 5.3s with intake leaks. Aftermarket replacement manifold are available but they kinda suck.
Don't swap wires/plugs around - just put new ones in. At 166k the wires owe you nothing and plugs should be changed every 50-80k not 100k like GM claims.
have you tried swapping spark plugs with a neighboring cylinder to see if the misfire moves?
Knurled wrote:
I have seen a couple 5.3s with intake leaks. Aftermarket replacement manifold are available but they kinda suck.
Don't swap wires/plugs around - just put new ones in. At 166k the wires owe you nothing and plugs should be changed every 50-80k not 100k like GM claims.
Spark plugs and wires were changed 15k ago as part of periodic maintenance. Intake gaskets were done to get to the failed knock sensors about 20k ago.
I know neither one is a guarantee to not be a problem but both have been changed since new.
Good to know, then.
The intake gaskets themselves aren't failing, the manifolds warp.
Thinking about it further, I would definitely look into why compression is low. Reason being, they find misfire by watching for RPM variance. A weak cylinder may show as a misfire under the right conditions, even if it isn't actually misfiring. It would be odd because the tendency is to err to the side of NOT calling a miss, but it can happen.
