1998 Chev K1500 with vortec misfire - how best to troubleshoot

Dear friends, Did a rebuild on a 1998 L31 vortec K1500 - I also added a slightly larger camshaft with 203 - 203 duration - 450 lift with 1.6 rockers and a 112 Lobe sep. ( Basically mid 1990s corvette intake lobe on intake and exhaust - Delta Cam recipe from tacoma wash) ( last motor before rebuild broke a piston from water lock - leaking intake manifold)
1) I have had a random misfire code - looked at it on HP tuners and noticed I have a couple cylinders misfiring intermittently , also got a catalyst code on #2 on bank #2 - which would seem to confirm the misfire.
2) truck also stalls when cold and selecting from park to drive - like a Idle Air control sticking ( replaced that to no effect) Cold start issue goes away with warmup.
3) The MPFI was replaced 5 year ago before the rebuild - believe was the electronic california kit
QUESTION:
4) thoughts on the misfires ( have replaced cap, rotor, distributor, and wires - going to do plugs next ) ? MPFI ? Leaking intake ? intake manifold gasket was replaced with rebuild - IE claimed novel intake fix feature for the vortec. But I just topped off coolant which was low to my suprise - so seems to suggest a very slow leak.
5) replace intake and redo gasket?
6) Thoughts are the best approach for tuning this new camshaft - runs fine enough - good power - could never light up tires before - can now ( thinking would like to get as much as I can out of this cam ) .

any thoughts appreciated - thanks

Have you changed anything yet?

cap, rotor, wires, IAC and plastic OEM distributor was replaced with an Amazon chinese aluminum distributor ( which is on my suspect list ) - Im thinking that the intake manifold seal is leaking - going to check torque again - try the WD40 trick .. Im getting some misfires on multiple cylinders .. also noticed a very long term slow slow coolant leak .. this motor had a hydro lock from a previous coolant leak - the motor was rebuilt - broke piston and rod - hole in oil pan - motor was excellently rebuilt - but previous owner never fixed intake issue and was using silicone to goo up the leak - the gasket was good when I took this motor apart - so chance manifold is warped ..
but willing to entertain any ideas - thanks

Needs another spider

YEa I was wondering that - previous owner did the spider 10-15K before engine blew - the engine rebuild was held up for almost a year during covid waiting for parts & weather - the intake was exposed to weather under a tarp in back of pickup box and was later exposed to snow .. IE like a sh__ pile stew of car parts under 1 ft of snow for a month or more ..

hardly EVER change spiders on the vortecs , usually the intake gasket ( mostly upper gasket or warped upper plenum) causes p0300s. another suspect is cams in these but stated was a different cam already, have seen sum NASTY engines have valves stuck from carbon build up...lastly handful needed a shim on crank sensor as worn crank bearings let tone ring touch sensor and cause misfire...also do a crank sensor relearn, have seen that clear p0300 up also, these are ungodly simple engines, dont think too hard lol

Quote:

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Originally Posted by ehall8702

hardly EVER change spiders on the vortecs , usually the intake gasket ( mostly upper gasket or warped upper plenum) causes p0300s. another suspect is cams in these but stated was a different cam already, have seen sum NASTY engines have valves stuck from carbon build up...lastly handful needed a shim on crank sensor as worn crank bearings let tone ring touch sensor and cause misfire...also do a crank sensor relearn, have seen that clear p0300 up also, these are ungodly simple engines, dont think too hard lol

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There was a GM TSB for valve guides being too tight, causing valve(s) to hang and a misfire or bent valve(s) as well. 10 years ago I got my hands on a Tahoe and 1 ton SRW truck, I got cheap because the mechanics insisted they needed engines because the pistons were done. Both had valves that had hung open and the Tahoe had a bent valve but no piston damage.

The other problem I have run into was the distributor needs to be vacuum vented like a later opti spark. The plastic base ones are garbage and the gears prone to knife edging. Vacuum vent an aluminum distributor base with a good drive gear, use a MSD cap and rotor and the cap and rotor life went from less than a year to years. I am running that setup on my 383 and it is misfire free to 6,200 rpm. Moisture and ozone will quickly corrode the factory aluminum terminals in the cap, overheat the aluminum and break down the insulating gap of plastic in the cap causing the cap to crossfire. Vacuum venting the cap purges it of ozone and moisture and the brass terminals will last alot better. MSD should have fixed this either with the complete distributor or their caps. United Motor Products makes a brass terminal cap with a vent that is easy to plumb vacuum too. I have not used that cap to comment on the quality but vacuum venting the housing works well and it would allow the housing to be vacuum vented without modifying the base as I have. I drilled the base with a 27/64 drill bit, tapped it to 1/8" NPT, then screwed in a 90* barbed fitting. Route a hose from that to a briggs & stratton 1/4" fuel filter with the metal screen inside, then hook that to a Dorman 47311 vacuum restrictor fitting. Then it simply hooks to the vacuum source nipple under the PCV that goes to the HVAC controls with a T. I added a second barbed fitting and routed fresh air from the air intake duct behind the MAF. Seal the cap to the base with a thin bead of RTV. 3/16" hose fits the 1/4" barbs very tightly, preventing the hoses from pulling off. Add a couple of zip ties to route the hoses neatly and you are done. PS the HEI distributors have the same ozone/moisture build up problems. I have seen guys knock out the screens in those that are completely corroded closed. Those enjoy being vacuum vented as well.

Quote:

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Originally Posted by ehall8702

hardly EVER change spiders on the vortecs , usually the intake gasket ( mostly upper gasket or warped upper plenum) causes p0300s. another suspect is cams in these but stated was a different cam already, have seen sum NASTY engines have valves stuck from carbon build up...lastly handful needed a shim on crank sensor as worn crank bearings let tone ring touch sensor and cause misfire...also do a crank sensor relearn, have seen that clear p0300 up also, these are ungodly simple engines, dont think too hard lol

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How do you perform the relearn?

I'm having a P0300 code on a 1996 5.7 in a c1500 I'm working on. It came to me with P0171, P0300, P0420 and P0430 codes. Found the elbo leaking badly for the PCV valve, so I replace both rubber ends as well as the valve. The P0300 came back after about 4 drive cycles. Sprayed coil with water, no stumbling. Sprayed the intake to check the gaskets, no change. Started a log and the timing is Negative all the time. -20 to -22 at idle. Makes me think the distributor was put back in a tooth off because someone replaced the intake before me.

i had the p0300 code come up on my 98 k2500 L31 and it needed a crank relearn then it was fine after that

Quote:

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Originally Posted by ehall8702

hardly EVER change spiders on the vortecs , usually the intake gasket ( mostly upper gasket or warped upper plenum) causes p0300s. another suspect is cams in these but stated was a different cam already, have seen sum NASTY engines have valves stuck from carbon build up...lastly handful needed a shim on crank sensor as worn crank bearings let tone ring touch sensor and cause misfire...also do a crank sensor relearn, have seen that clear p0300 up also, these are ungodly simple engines, dont think too hard lol

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When they first came out we could not get enough of the injectors. On the rate of over 100 a week. This was a dealer. The ball at the end of the poppet would stick. Sometimes we would replace them and they would be back within a month. When they finally fixed it by using a single multec per cylinder that fixed the problem. Not that I haven't had the new style fail including the china spiders. Talked with an engineer at the time doing a report on a warranty job and he said that they believed it was due to the change to E fuel. Not just that the ethanol was "overcleaning" the poppet ball and seat but that when fuel came down the pipeline different "lots" were separated by water. Of course water, fuel/oil don't mix so the separation was good. Enter ethanol. Sucked the water up so they had to use a different formula "stuff" to separate the loads. They thought that some of this substance was getting in the fuel and making the fuel "sticky". Never really followed up on it as GM was remaining very quiet about the whole thing.

So on any misfire code your best friend is Freeze Frame Data. It will tell you what conditions you need to either repeat of systems to look at.
Here is an example.
If I have a misfire at idle I will look at high voltage leaks, vacuum leaks, burned valves, and compression. Each is its own daig
mid throttle mid RPM mid load I look at high voltage leaks.
WOT or high rpm I look at valvetrain. Usually weak springs, weak high voltage output or low fuel volume.
Almost all issues are pretty easy to diag.

It ended up being that the CMD was -12 when my friend at the smog shop said it's supposed to be within 5 either way. we adjusted it to -2 and it runs great now! I have no idea why the timing shows negative in the scanner though.

The timing is supposed to be +/- 2 degrees. Sounds like you got it figured out.