P0308 Cylinder 8 (only) Misfire - New Plugs & Wires

sdtim

Member
Feb 21, 2021
20
6
Oceanside, CA
Truck had been running strong for months. Last weekend I got a P308 - Cylinder 8 misfire detected. Yesterday, I replaced the spark plugs and installed a new set of Kingsborne 8mm spark plug wires. After the repair, the truck fired right up and ran fine on a short test drive. However, took it out today and codes came back after ~9 miles of driving. Service Engine Soon light flashes periodically then stays on. Engine has a noticeable miss / power degradation.

P0300 - Random/multiple cylinder misfire detected
P0308 - Cylinder 8 misfire detected
P0327 - Knock/combustion vibration sensor 'A' circuit low

During the repair I tested cylinder 8 injector with a 9v battery and it seemed to be working (could hear a click and fuel flowing from residual pressure). I replaced the head gasket and had the heads rebuild about a year ago (+/- 1,000 miles).

I'm going to check compression and vacuum in the morning. Provided they both look good, should I replace the coil packs and/or injectors, or is there another logical troubleshooting path before pulling the intake and throwing more parts at the beast?
 

sdtim

Member
Feb 21, 2021
20
6
Oceanside, CA
Finally had a chance to pick up a compression tester and coolant system pressure tester (loaners from Autozone). Looks like the misfire may be the result of a coolant leak. Here is what I found.

Compression test looked good
1 - 150 -- 2 - 155
3 - 150 -- 4 - 160
5 - 155 -- 6 - 150
7 - 160 -- 8 - 155 (cyl w/ misfire)

Coolant system pressure tests
1st test: 10 psi, very slowly dropped 2-3 psi over ~45 mins - fail
2nd test: 15 psi, immediately started dropping 2-3 pst after a couple minutes - fail
3rd test: 15 psi with throttle body lines looped together, held pressure for 10 minutes without any drop! - pass
H5uKDcM.png


I'll get everything back together tomorrow and take it for a test drive. HOPEFULLY the throttle body coolant leak was the culprit. I'll post the results.
 
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sdtim

Member
Feb 21, 2021
20
6
Oceanside, CA
Well...this sucks. Got the bastard back together and took it for a spin around the block. I'm still getting a P308 - Cylinder 8 misfire detected code. This is getting annoying.

I guess a new coil pack and set of injectors are up next...
 

discostew

Well-known member
Sep 14, 2010
6,169
402
Northern Illinois
Well...this sucks. Got the bastard back together and took it for a spin around the block. I'm still getting a P308 - Cylinder 8 misfire detected code. This is getting annoying.

I guess a new coil pack and set of injectors are up next...
Why not check for spark at the wire? I thought you said you used a 9 volt battery and the injector worked? I would put a noid light on the injector connector and make sure the car is opening it. And check for spark on that wire.
 

sdtim

Member
Feb 21, 2021
20
6
Oceanside, CA
I've already replaced the spark plugs and installed a new set of 8mm Kingsborne wires. I've confirmed there is spark using two types of basic testers. And yes, I did hear the injector open and spray fuel from the residual pressure.

However, I don't really know if either are working within normal operating tolerances (i.e. I can see the spark with the tester, but is it the correct strength/frequency/and consistent? Similar with the injector).

I'm trying to minimize the number of times I have to pull the upper intake. The idea of disassembling, swapping injector positions, reassembling, testing, ... makes me want to roll it off a cliff into the ocean.

I will try the noid light to make sure the injector is getting signal. Thanks for the great idea.
 

sdtim

Member
Feb 21, 2021
20
6
Oceanside, CA
I ran a lazy vs cheap analysis and decided to replace the coils and injectors instead of troubleshooting further.

I just wrapped up installation of the new coil packs and six* refurbished injectors. Fired it up....and took it around the block a couple times. The Beast is running great!

I'm still a tad bit hesitant. After I replaced the wires it took a ~10 miles for the P0308 to return. After repairing the coolant leak I could immediately tell it wasn't fixed. However, this time around it's running much smoother and has great power.

*I ordered a set, 8, but only received 6. Fun times. Since I was only having trouble on cylinder 8, I went ahead and installed the 6 refurbished injectors. I'll try to get a refund for the two missing injectors....or maybe have a couple around for spares / next time I pull the upper intake.
 
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