Fuel trims

98 Silverado, 383ci, upgraded spider, full length headers, no cats, no egr, black bear ecu. Long term fuel trim bank 2 is 25-30 at idle and I get po305 code or po300 code. New plugs, wires, distributor. Any suggestions?

Seems like your engine is running lean so the ecu is dumping fuel onto bank two. I bet your spark plugs are getting soaked with fuel, causing misfire(s).

You need to check your fuel pressure, regulator and both the where the injectors plug into the manifold themselves and where they meet the metering body.

BCREEL said:

Long term fuel trim bank 2 is 25-30 at idle

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What does "25-30" mean? Is that a percentage?

I'm used to seeing fuel trims as a number with 128 as the "neutral" or mid-point. 0--256 theoretically possible. Lower numbers represent less fuel delivered (lean command) while numbers higher than 128 represent fuel added (rich command).

So if you're getting a "25" in that system, the computer is farther into lean-command than I've ever seen before. I didn't know it was possible to have a number that low.

But if that "25" represents something else, like +25% additional fuel, the computer would be in rich-command.

BCREEL said:

and I get po305 code or po300 code. New plugs, wires, distributor. Any suggestions?

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Pull the plug out of #5 to check for fouling?
While the plug is out, what is the cranking compression pressure of cylinder 5?
Have you tested the plug wires to assure they're not defective?
Any chance of a failed or electrically-disconnected injector?

What is the upstream O2 sensor showing on that bank?