P0303 Misfire only under load

[Jacobchevys]

98 K1500. Stock original motor. Have had truck for 15 years. Had this problem for a couple years now but just recently started digging into this to fix it.

So here we go- Truck runs smooth everywhere EXCEPT going up hill(cresting a hill), going to pass on the freeway, merging, anything above 55mph that has any load on the engine. I have to be really ginger and watch how long I hold the gas pedal so it doesn’t misfire. Misfire is strong engine will shake and won’t stop until I let off. I have to wait about 5 seconds otherwise if I get back on the gas it will start shaking again. WOT is ok from O to 45mph. Any longer misfire will occur. Can’t really go 25% Throttle for more than 5 seconds. Cruising at 70mph is great. If I push it to 75-80mph misfire will occur.

Any ideas or anyone had similar problem. I’ve done very thorough research and there seems to be a bunch of “fixes” from the TSB recall, to sensors, to MFI upgrade, to CMP Ret ETC.

A little busy right now so I will post the things I’ve done to fix here shortly.

Until then any input or recommendations?

 

[scott2093]

The similar threads below had this...Was the newer fuel pump...

High mileage 5.7 Vortec, Heavy sputter under load - no codes!!

Hi fam, long time Chevy owner, first time posting. I have a new to me 1996 5.7 Vortec 2 door 4x4 Tahoe. 300k on the ODO, swapped in motor has 180k. 40k on trans. Newer fuel pump, new fuel filter. Cap, wires and plugs unknown (they look heavily worn.) I’m in California and subject to smog...

www.gmt400.com

 

[Supercharged111]

If you have a P0303, then that helps narrow down your problem. But if your truck truly is stock, then I'm betting you have a P0300 which doesn't help you much.

 

[Komet]

Check fuel pressure when the misfire happens. I'm also curious what the plugs look like.

 

[Jacobchevys]

All stock. I just scanned again and I have P0303 and P0420. I have BAFX and dash command. CMP ret is at 3*. I know it’s slightly out of spec.

Things I’ve done within the past 3 months
-New plugs NGK TR55 (old ones were all clean including my cylinder 3)
-New AC Delco wires
-New AC Delco ignition coil and module
-Functional Dizzy out of clean running truck
-New Fuel filter
-New fuel pump about 1 .5 year ago
-New lower intake manifold gaskets about 3 years ago

I should also mention once I change the oil and filter it SLIGHTLY betters the situation- meaning I can run up the hill at 63-65Mph instead of 60MPH. I can also merge on highway a little harder for a few more seconds. It WILL still misfire though . I have freeze frames and data Logs through dash command. Nothing abnormal I’ll work on posting those.

Leads me to believe it’s a mechanical problem not electrical and not sensor related or ignition issues.

I’m hesitant on saying that cause I’ve read of so many different things fixing this similar problem. Or reading threads that went unsolved.

 

[Jacobchevys]

Haven’t verified fuel pressure yet, still some more diagnosing but I’m just looking for some insight from any of you who have more experience

 

[Komet]

I wouldn't try so hard to rationalize your way out of diagnostics.

 

[Schurkey]

Verify fuel pressure when it's misfiring.

Verify ignition system--in particular, check the #3 plug wire for cuts, burns, high resistance, loose terminals, etc, but check 'em all while you're in there.

Will the ignition coil fire a spark-tester calibrated for HEI when connected directly to the coil? Will it fire the spark tester when connected to the plug end of #3 plug wire?

Seems to me that other folks have said that #3 is the most-likely place for the Vortec distributor cap to burn-through. No actual experience with that myself.

 

[98yuk]

My 1998 Yukon just started doing almost the exact same thing. P0303 only under load up hills. No other codes showing. Any luck with the diagnosis? I'm going to start trying to track this down myself, I'll post updates as I go.