P0300 - Cyl 4 - When under higher load

rvsmith70 · January 18

2001 Silverado 1500 - 4.3L 380k miles

1.5 Years ago - I finally decided to put the truck in a local shop to chase the CEL that illuminated LITERALLY a day before I was going to take it in for annual state inspection.

Several things were addressed and fixed by the shop (intake resonator box had air leaks causing lean conditions, fuel pump & FPR changed, evap canister, injectors changed & upgraded to the newer style)... I myself changed plugs, wires, distributor cap, ignition coil, water pump, and fan clutch (All were suspect - and way past time to have replaced them...

A P0300 still is a ghost that occasionally appears, though. I've narrowed it down to happening generally when the engine is under higher load. (i.e. driving up a slight hill if it's a mile or longer - the check engine light will flash, then turn off once the load has been reduced on flatter (or downhill) terrain) I picked up a scanner to read live data for the misfires, and it looks like Cyl 4 is the main culprit. (see screenshot - and I did reset the numbers while on my trip)

With cyl3 & 5 also having some misfires along with 4 - but not nearly as bad. When this happens - I feel NO change in the engine performance. No shake or roughness at all that I've noticed.

I'll do the obvious quick checks this weekend to check plug/wire and distributor cap (since firing order is 1,6,5,4,3,2 - with 4 having the other offenders (3 & 5) on either side of it's firing order... but was curious if anyone had other ideas for me to check.... Could it be PCM, if it's not the obvious spark plug/ dist. cap ?

asilverblazer · January 18

16 hours ago, rvsmith70 said:

2001 Silverado 1500 - 4.3L 380k miles

A P0300 still is a ghost that occasionally appears, though. I've narrowed it down to happening generally when the engine is under higher load. (i.e. driving up a slight hill if it's a mile or longer - the check engine light will flash, then turn off once the load has been reduced on flatter (or downhill) terrain) I picked up a scanner to read live data for the misfires, and it looks like Cyl 4 is the main culprit. (see screenshot - and I did reset the numbers while on my trip)

With cyl3 & 5 also having some misfires along with 4 - but not nearly as bad. When this happens - I feel NO change in the engine performance. No shake or roughness at all that I've noticed.

I'll do the obvious quick checks this weekend to check plug/wire and distributor cap...

Could it be PCM, if it's not the obvious spark plug/ dist. cap ?

I wouldn't even look at the PCM as a potential culprit.

You're on the right track - and if memory serves correctly - the cap and rotor style can foul up pretty quick.

In any case using plug wires other than AC Delco is a risk. In that generation the engine weren't so sensitive to plugs as the newer ones.

rvsmith70 · January 20

Thanks! I'll definitely go that route for checking. It was also suggested to me that it could be some lash from a slack/stretched timing chain. I know that the chain has never been changed... and with almost 400k miles on it...

asilverblazer · January 22

On 1/19/2024 at 11:02 PM, rvsmith70 said:

Thanks! I'll definitely go that route for checking. It was also suggested to me that it could be some lash from a slack/stretched timing chain. I know that the chain has never been changed... and with almost 400k miles on it...

I would expect more widespread miss fires if that were the case.