fics

2004 Silverado with 4.8L engine..... 82,000 miles..... hit the highway for a 150 mile ride, and the check engine light started to flash. it will flash anywhere between 3 and 45 times..... it flashes, then goes away for a few miles, then flashes again, etc... the whole way. it never stays on steady. in town driving, it never comes on..... got home and it was idling rough....started it the next morning and it ran great and idled smooth.....took it to the shop and had the codes read there were none......new plugs and wires....and it did it again yesterday, but still no codes...... any ideas???

FFDP

iTrader: (2)

If I was to diag the vehicle, I would have a scanner with me when I hit the freeway. That way I could watch the live misfire data and see what cylinder the counts are coming from. Then go from there to see what may need fixing, be it a coil or injectors etc.

Suburbazine

Since you've not got any codes, pull all the plugs and check them for crud/damage/burning oil/etc.

ChevyThunder17

iTrader: (45)

Failing coil pack(s)? Like FFDP said, you'd want to log it while it is happening. Does it come on when you are going up an incline or attempting to accelerate? I'm not sure why it wouldn't log a code. I've seen the misfire code come up stored in the computer before, but didn't remember seeing anything wrong at the time. If you can log it and find the specific cylinder, you can try swapping the coil to another cylinder.

Good luck.

1FastBrick

iTrader: (17)

Not sure about the shop you are taking it to but they should be able to read the codes stored in the history.

Any decent scanner should be able to read current codes as well as codes stored in the history.

Some of little hand held code readers (Especially old ones) may not be capable of reading codes stored in the history.

If the mechanic You used is not smart enough to check the History codes or is not using a scanner that is capable of checking the history, you should be looking elsewhere for a real mechanic.

1FastBrick

iTrader: (17)

With that being said,

If you can get the stored history codes, that can point you in the right direction.


Check Fuel pressure at Idle and Under Load. I have seen failing pumps dip under load.

With that being said, there isn't a whole lot else you can check without a scan tool.

You need to be able to monitor misfires current and history, sensor readings like the O2 sensors, RPM, and so on.

trxmxzx

iTrader: (7)

My 02z71 does this when towing in 3rd gear converter locked at 65mph - my code said muiltiple misfire detected-

I think knock sensors are failing

Plugs and wires are new- new pump- new intake gaskets too

1FastBrick

iTrader: (17)

Failing knock sensors will give you a code.

As far as the Misfires, The factory parameters are not set up to lock in 3rd so if you're locking in 3rd you probably need to adjust the tune some more. Better yet get some better gears in it.

2xLS1

iTrader: (25)

You can log misfire history and see which cylinder/s is/are misfiring.

bark-79

Mine was flashing, and running rough, put in new primary o2 sensors and fixed it rite up.