99 GP GT P0305 Missfire only at Hwy Speed Climbing Hills

[71gs3504sp]

Guys

This is driving me crazy!

Car misfires only on #5 Plug under load climbing hills, when downshifting on the Interstate.

Some info
- Car is all stock. with 122k miles on it
- Did the LIM at around 100k miles including
- Plugs - factory iridium
- Wires - ACDelco
- Hoses/thermostat/coolant/aluminum elbows

At 120k Replaced both O2 Sensors

Still running original coils.

Checked the #5 plug gap perfect.

I still have the factory metal shield around all the plug wires.

Can runs great all other times.

 

[derpyimp]

Swap #5 plug and wire with another and see if the misfire moves to the other cylinder. If so, bad plug or wire. If not, try swapping ignition coils too. Could also be #5 fuel injector, so you could try swapping injectors after that if the ignition components turn out to be good. Just start with the easy, common stuff and move on from there.

 

[Fivefingerdeathpunch]

Word.

I'd move the coil around and see if it follows.

 

[BillBoost37]

Agree..move coil. Seen it a few times.

 

[71gs3504sp]

Bill and everyone else!

1st - I double checked the plug today and it look and gaped perfect.

2nd - I switched coil packs today to see if the misfire happen to the same plug or moved to the new coil location.

Tomorrow I will try to see if this happens and get back to you guys!

Thanks all of you who gave me advice!

George

 

[71gs3504sp]

OK guys it finally miss fire again.

I got the same code as always P0305, miss fire on cylinder #5.

This is what is did so far prior to miss fire:
- Checked plug #5 and gap and it was clean with a perfect gap.
- Switch coil packs

This is what I am going to do next:
- Check the spark plug wire for plug #5.
- Check fuel injector #5

Let me know your opinions.

BTW the miss fire only happens climbing a steep hill on the interstate when the RPM starts getting low and before it downshifts, also the engine light starts flashing during the miss fire.

While I am writing this it seems to me it could be the injectors. If it is the injectors should I get DIY fuel injector cleaner that connect to the injector rail .

Advance thanks for any suggestions!

George