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

sdtim

Active member
Feb 21, 2021
26
10
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

Active member
Feb 21, 2021
26
10
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

Active member
Feb 21, 2021
26
10
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
6,278
441
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

Active member
Feb 21, 2021
26
10
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

Active member
Feb 21, 2021
26
10
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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