1998 k2500 5.7 miss / p0300 tune up

Two weeks ago I had my first issue with 1998 k2500 - 5.7. Owner before me replaced engine with one from Jasper, jt has 60k +/- on it.

I had the truck at WOT, felt a bit of a miss and CEL came on immediately. When ai got home, I popped the hood and started looking at the spark plug wires, etc.. I figured it would be a good time for a tune up anyway and that may fix my issue... it didn't.

Parts replaced :
Ignition module - AC delco
Coil - NGK
Cap and rotor - standard ignition
Spark plug wires - standard blue streak
Spark plugs - NGK platinum
Both front o2 sensors - Bosch (BB tune, rear o2's not active)
Fuel filter - K&N
Air filter - K&N
Changed oil - Castrol/ Baldwin filter

I also cleanes the MAF and TB, TB was filthy.

Truck has 3" exhaust true dual, to 2- single chamber mufflers, came with the air raid CAI and I put a new K&N filter on it. Has a BB tune.

Problem ha sprogressivly gotten worse. Its strange that my MPG has gotten better. The miss is much more noticeable and at times it stalls and will not get above 3,000rpm. I can't hold the gas pedal to the floor and eventually it starts making power. Acts likes its flooded and then clears it out. I'm suspecting that the spider is my issue. Maybe the day of the first miss it had was an injector/poppet or the regulator going out. I plan on pulling thr upper plenum and have and mpi ordered. Does that make sense? I wasn't completely sure it needed all the tune up parts when I did it, but I figured it was about time anyway. I would like to the get this fixed, though. My Longft are negative 3-5 on both banks. MAF and MAP seem to be reading correctly.

On the scanner the RPM will intermittently jump to 12,800 when idling. No change in actual rpm just on the scanner, which is a low end code reader type. The only code it has thrown this whole time was the p0300, I don't believe it will do specific cylinders.

Any advice would be appreciated.

Verify fuel pressure under load. Most fuel pressure testers have a push-button connected to a length of crappy clear-plastic tubing that you'd route to a drain pan; push the button to simulate heavy fuel demand. Verify that the system holds pressure when shut off.

Get a better scan tool, check misfire counts for each cylinder.

If the better scan tool also shows 12,800 RPM, (!!!) start investigating the crank sensor and wire harness. There was a guy on this site that had an ignition module failure; drove his TBI injectors to spit like a crazed Llama--false RPM signal with key on, engine not running. But I'm not sure the Vortecs are wired like that--and you've already changed the ignition module.

Not going above 3K rpm, no power, etc. sounds like the computer has maybe entered "Limp Home" mode.

Thank you for the response, Shurkey !


I checked the fuel pressure. Key on it went to 52+/- then key off slowly kept rising to 58psi, which seemed odd and made me wonder if it was the fuel filter. The fuel filter is new, though. Pressure was still 58psi after 30 minutes. Most likely will change the fuel filter again today, but only because I have an extra one in the back seat. The one on it is K&N and the extra is WIX, any preference between those two? Or suggestions on different brands? Also have a K&N air filter, any advice on that? The truck had the old Airaid CAI on it when I got it. I personally thing the stock box would be just as good, if not better and wouldn't have made that change.... but its on there.



I cranked the truck and fuel pressure was all over the place. it would rise to around 55-60psi then fall back to 40psi.

I pulled the tb and shined a light under into the plenum. The upper intake and bottom of the tb was NASTY. but... looking towards cylinders 5 & 7, it was clean as a whistle.... Making me suspect that gasoline was keeping it washed off. So... off came the upper intake and old spider and in went the new MPFI. It seems like the problem is solved. The truck runs the best it has since i have owned it.

It does seem to still have a sight miss at idle. Nothing significant, but... there, if that makes sense. No CEL and when accelerating at all it will disappear. Still has it though. Any thoughts on what could cause that? Nothing serious, just a small, intermittent drop of 25-30 rpm.

As far as scan tools. I saw on here where people are having luck with dash command and car gauge pro. Would that be sufficient for me?

jhncrwfrd2 said:

The one on it is K&N and the extra is WIX, any preference between those two?

Click to expand...

Not really. I've seen that K&N and Wix are both offshoring (some of) their filters. Disappointed in both companies. Wix has been my Go-To filter brand for decades. I don't know what I'm going to switch to. Baldwin, maybe. I've got enough older Wix filters on-hand to last a couple of years.

jhncrwfrd2 said:

Also have a K&N air filter, any advice on that? The truck had the old Airaid CAI on it when I got it. I personally thing the stock box would be just as good, if not better and wouldn't have made that change.... but its on there.

Click to expand...

Agree about the stock airbox.

I have installed K&N filters. Several of my vehicles have K&N filters right now. I'm pretty-much locked-into K&N for the Tri-Power (no stock filters available any more) and the motorcycles; but I'm unlikely to buy more K&N or other oiled-gauze filters for my own use.

The worst thing you can do to an oiled-gauze filter is to clean it before it genuinely needs cleaning. K&N used to recommend you leave it alone until there's an eighth-inch buildup on the outside (short story: If you can still see the pleats, it's not dirty enough to need cleaning.) Oiled-gauze filters work best when there's dirt on them. The dirt becomes part of the filter media, making the filter more effective.

Now, K&N says to use a filter restriction indicator, and not clean the filter until the restriction indicator shows restriction.

jhncrwfrd2 said:

It does seem to still have a sight miss at idle. Nothing significant, but... there, if that makes sense. No CEL and when accelerating at all it will disappear. Still has it though. Any thoughts on what could cause that? Nothing serious, just a small, intermittent drop of 25-30 rpm.

As far as scan tools. I saw on here where people are having luck with dash command and car gauge pro. Would that be sufficient for me?

Click to expand...

I have ZERO experience with scan tools except for the older Snap-On units. That said, I lean away from the consumer-grade tools, and the "connector cable + laptop computer software" solutions. But others disagree.

When it's my money, I bought a used-but-usable professional-grade tool system from eBay. Craigslist, pawn shops, and such are other sources.

Thank you