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).

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?
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I would start with ensuring that all your new wires are fully seated on the coil packs. When I did mine, It started up just fine but not too long after start up I got similar messages your getting. Went back a and checked seating on coil and the one acting crazy was not properly seated.

 

It's possible the misfire is not caused by an ignition system problem, especially since the misfire returned on the same cylinder. A coolant leak into a cylinder can cause a misfire. When you pulled the old spark plugs did the plug from number 8 look different than the others? Was it, for example, noticeably cleaner than the other seven?

 

I pulled #4 and #8, they looked nearly identical (I will check a couple from the opposite bank). I'll add a coolant system pressure test to my list of things to check. Thanks.

 

The process of elimination using known good parts is always a solid troubleshooting process. Swap #8 plug with a different one (not #2) to start, then the injector if that does not work.

 

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

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.

 

Those TBH plates constantly leak or will leak. **** poor design IMHO and I bypass em all vs fry a perfectly good engine from it slowly bleeding itself to death.

 

The plan is to leave it bypassed. I live in San Diego so there's zero need to heat the TB. If I end up somewhere below freezing I'll let it heat up for a few minutes before driving.

 

People have mixed idea's on them, but the fact remains a lot of vehicles from Jeeps to Hummers = DO NOT have TBH's and plenty of them are in Canada, Alaska, or all over the world with zero issues. Is it a bad idea? No, but it was poorly executed on the Bosch platform, and can cause way more harm than good. 14CUX/GEMS = much much better design with a much stronger gasket/bolt design. I know plenty of people with Jeeps & Hummers up north and zero issues. However I can't count how many LR's have slowly bleed to death and been scraped due to the TBH slowly leaking. I'll take my 1,000,000,000 to 1 chance of it freezing up vs knowing it will leak n slowly guaranteed.....

 

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...