So I have a weird issue. As many of you have read I did a full rebuild of my 997.2tt engine with all serviceable parts replaced (left the heads alone as they were good) and forged rods, pistons, crank, etc. Everything was good and well until one day it developed a misfire with any amount of boost. It had half a tank of gas in it so not fuel, plugs and coils are 2,000 miles old. It was on cylinder 5 so we changed the coil and all plugs, then it moved to 6, so we changed that coil, now it’s on 4.

These are all on the same bank. The coils came from RM European (wondering if they are knock off beru’s from China). The car is running Cobb stage 3 (has their map sensors). When checked with a cheap code reader it shows a throttle/map code (basically reading two different values) but doesn’t throw a CEL, the durametric, snap on, and other readers don’t show that code at all.

All cam silonids are new, as are timing chains and such.

Any ideas what could could be causing a roaming misfire on 1 bank of cylinders?

Hello I am sorry to hear about your troubles. Just a few questions to help me get a grasp of what’s going on so maybe I can help.

1. Have the heads surfaces been checked?
2. Have you checked the camshaft deviation?
3. Have you checked to make sure VarioCam is working properly on a scan tool when actuated and monitored it when boost comes in?
4. Have the valves been replaced or seated if not?
5. What mileage is on this block?

I would remove the tune and see what happens? If you said nothing but listed the symptoms, I would guess coils. But based on the history of the car, it could be anything...

Another thing I forgot to ask what is the boost levels and what is the spark plug type and gap?

One coil and all plugs at first, then another coil, and on Monday all coils are going to be changed (wondering if RM European sold me a set of Chinese knock off Beru’s).

The car ran perfectly until it suddenly started misfiring with any amount of boost.

The car is running cobb stage 3.

They are stock plugs with stock gap.

I don’t think the plugs are the issue because the car ran perfectly for 2,000 miles after the build and with no changes, suddenly developed a misfire with any boost. Since that all the plugs are new and the problem persists. The tech working on it is out today but the shop owner is checking over all the connections and smoke testing it to see if something is leaking.

This was one of those times that the solution is easy, thankfully. We changed all 6 plugs and 6 coils, redid any other connection to make sure it was tight, and put dialectic grease on the connectors.

The car runs far smoother and feels like it’s making more power than before. Plugged in the computer to check fuel, timing, etc and all was spot on.

Lesson of the day is be very careful where you buy your coils from. Mine came from RM European and were silly cheap (as in $37 a coil) and were likely Chinese knock offs of the Beru. Given how much of a PITA it is to change coils, I’ll only buy from a dealer from here on out.