Yes this is a newby 1st post, but I have searched forum and unable to find post to help further, maybe someone here can offer some suggestions.
I have a 1997 Z71 with 5.7 vortec. Engine is a new crate (longblock) 5.7 with 10,000miles now. Truck has 366,000. Before finally losing motor and replacing it was sometimes hard (slow crank) to start when warmed up.
After motor replacement same "slow crank when warm" existed, however new engine also had new radiator,water pump, thermostat, hoses, belts, tensioner, distributor cap, rotor, plugs,wires, and battery. After some suggestions by mechanics a few more parts were replaced to try to eliminate warm cranking issue which included- starter, alternator (both new - not reman or refurb units) another new battery, and a new billett distributor . Issue seemed better but not cured.
Now the other day it suddenly got a misfire, got the dreaded P0303 code. I have since replaced all plugs, wires, cap, rotor, and checked cylinder pressures (all between 185 and 190) fuel pressure @ 64. Misfire is still present and seems better @ 55-60mph, no differences noted on hills or under load.
Any suggestions or help how to fix is appreciated.
Thanks!

 

Forgot to mention new fuel filter, and air filter when engine replaced. Another fuel filter about 3,000 miles ago by another suggestion of a mechanic. Also run 3 different types of fuel additives (at different times of course) after suggestions by mechanic for the warm start issue. Before all this truck ran great with new engine.

 

Try cleaning maf. Also could be coolant temp sensor.

 

A code P0303 may mean that one or more of the following has happened:

Faulty spark plug or wire
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector
Burned exhaust valve
Faulty catalytic converter(s)
Running out of fuel
Poor compression
Defective computer

 

Thanks guys for the input, I'll eliminate what I can from these lists.
MAF - cleaned - no changes.
Plugs and wires replaced with new(today) - no changes
Compression checked (185-190 on all cylinders)
Cap and Rotor new today also.

I'll try the temp sensor, and o2 sensors checkewd tomorrow and let you know.
I'll even replace coil, but would a coil only make 1 cylinder misfire? I guess I was just figuring since it only has one coil, and if it was breaking down it would misfire on several cylinders or throw the P0300 code of random misfires.

 

Slow crank.... after warmed ... Sounds like timing .
Can you feel the misfire/ notice?
Did you clear code , and came back?

 

Yes the miss is noticeable until about 55mph, then just barely. Code has been cleared 4 times and each time it comes back before leaving parking lot. While driving the SES light flashes, but when letting off accelerator and coming to a stop or making a turn in goes backto a steady on state.
As for the timing, when new distributor (complete billet) went in it was timed by external computer and verified dead on 0 deg. It was verified again the other day, still @ 0 deg. which was a day or two before the miss problem started.

 

If you got the p0303, then its misfiring in cylinder #3. Pull the spark plug in that one and see what it looks like.

 

Ruffneck - i have replaced the #3 plug and wire twice in last 48 hours, yesterday was second time, at which time i replaced all plugs, wires, rotor and cap, as well as checked all cylinder pressures.

update for today- New Coolant temp sensor installed, MAF cleaned (Again) and checked, O2 Sensors checked, new PCV valve. Code Cleared, and reappeared shortly after, miss never went away.
Did find more news, that i did not know before. Mechanic says in code "history" it shows a previous code for "Low Flow" on both rear bank O2 sensors! But that code is not on now, only code found is the P0303. So i guess my next step is new Catalytic Converters, and after looking up some it seems to be several choices. I have the 2 Cats, and 3 of the O2 sensors in that section of exhaust setup.

Does anyone have suggestions on which ones to choose or stay away from when replacing Cats?? would like to purchase the entire bolt in style setup, instead of just converters alone. Also would like if possible in stainless, but i am unsure what are good brands and what are bad - mechanic suggested any would be fine, except "Eastern ??" i believe is what he said. Any suggestions would be welcomed. Thanks!

 

My buddy replaced his with Magnaflow cats and said it made a huge difference. It ran even better than when his chevy was new.