P0306 - Remains after tune-up

[98K1500Utah]

Greetings - Looking for some common suggestions on fixing an issue I have. On my 98 K1500 5.7, I was getting a P0306, and a very few times a P0430.

I replaced the Cap, Rotor, Plugs (Double Plat) and Wires with AC Delco Professional stuff, also replace the fuel filter as well. The truck runs good, so significant misfire sounds. But, I am still getting a P0306... read below:

If the P0306 code comes up... 99% of the time it is on the highway at high speeds. Driving around town, running errands will typically make the light go off the same day.

To me, it doesn't run rough... but feels a little sluggish when picking up speed. Takes a good pedal to get it to downshift.

A friend says I should be getting closer to 18 MPGS on this beast. I typically get 12-14 if I am lucky.

Thoughts and or suggestions?

 

[Supercharged111]

I get 12-13 mpg in mine, STILL fighting a misfire myself. You didn't mention intake gaskets, injector spider, or distributor gear/position/CMPRET.

 

[98K1500Utah]

I get 12-13 mpg in mine, STILL fighting a misfire myself. You didn't mention intake gaskets, injector spider, or distributor gear/position/CMPRET.

Intake gasket was replaced a couple years ago - been looking into the conversion kit to MSFI spider... but wanted to rule out fuel pump, fuel pressure regulator, vacuum leaks, etc. I was hopeing to find another culprit as the spider can be pricey.

 

[mistaake]

I'd lean towards injector issue... It's not that bad of a job, I did mine in about 4-5hrs taking it slow and including a lunch break and some other distractions. And I don't think 18MPG is realistic, unless you have a 4.3 v6, manual, and are going downhill 100% of the time, I've only ever got 11-14 mpg across half a dozen GMT400s.

 

[BOTTLEDZ28]

Id replace the spider injectors and get yourself a new distributor gear while youre there. If you plan on keeping the truck for a while then those new parts will be well worth the time and oney spent on them. DO NOT buy the chep spider injectors from the local parts store. nly use AcDelco or Dephi.


You will need to set the timing with the scanner when youre done so make plans for that. Maybe a friend has a Snap On scanner you can borrow for a half hour or so.

 

[skylark]

Do compression test first. If you have low compression in a cylinder you will always have a misfire.

 

[smdk2500]

[QUOTE="mistaake, post: 990874, member: 13643" And I don't think 18MPG is realistic, .[/QUOTE] I got 18 out of a 94 k1500 5.7 auto with 1800 lbs of iron in the bed. Sorry to derail the thread

 

[slowburb]

Do compression test first. If you have low compression in a cylinder you will always have a misfire.

What he said, as usual.

 

[mistaake]

Id replace the spider injectors and get yourself a new distributor gear while youre there. If you plan on keeping the truck for a while then those new parts will be well worth the time and oney spent on them. DO NOT buy the chep spider injectors from the local parts store. nly use AcDelco or Dephi.


You will need to set the timing with the scanner when youre done so make plans for that. Maybe a friend has a Snap On scanner you can borrow for a half hour or so.

I used the BWD unit from O'Reilly when I did mine and no issues, plus lifetime warranty at 5k locations around the US, ACDelco can't match that. And you can use DashCommand with the GM specific data (cost like $10) and a cheap Bluetooth OBD thing, no expensive scan tool required.

 

[Steve A]

RockAuto has the MPFI setup for ~$250 plus you can use the discount code on this forum for 5% off. Be careful about what Bluetooth adapter you get, some of the really cheap ones don't work with DashCommand, experience speaking here. Don't rule out the PCM until it's checked either.