Exhaust leak found
The exhaust leak causing the loud tick, tick, tick, tick has been found. I was expecting a loose header or failed gasket letting exhaust gasses escape from the head. What I was hoping not to find was a cracked header as that would mean removal and repair – a difficult and time-consuming job with the limited access on a 308. What I actually found was quite surprising…
The emissions sampling port plug on cylinder 2 of the rear bank was missing (arrowed). The US 308s had ports welded in to the exhaust headers that allowed exhaust gases to be sampled, per-cylinder, for emission-tuning the carburetors. When not in use, these ports are supposed to be plugged up. With one plug missing, my header effectively had a 2mm hole in it. My $20 Chinese USB wifi endoscope took a photo of the hole… not a very good photo… but a photo and enough to see the hole at the bottom of the port.
So if there is no plug, where did it go? The exhaust leak has only been there for the last 50 miles or so and only got really noticeable in the last 10 miles. Unfortunately I don’t have any photos of that header from before the leak. The other three ports have some unknown part threaded into them with what looks like a wood screw in the middle. Paging the Chinese USB wifi endoscope… Time for another fuzzy, poor quality photo!
This appears to be the remains of one of the sampling tubes that attached to the port. I’ve heard they were prone to corrosion and once broken, difficult to remove. One of mine appears to have corroded so much it completely fell out of the port over the last few drives. Time to find something to plug that hole with.
Two days later, part number 110860, “plug” and appropriate copper gaskets are on my workbench waiting to be fitted. This M12 1.25 plug is what previous owners/mechanics should have used to plug the other ports.
I did wonder if I should order four instead of one and replace all of them but I suspect it will be close to impossible with the header in place to change the others. That job can wait for until I have the engine out and headers off sometime in the future. Maybe the others will disintegrate in the same way in another few thousand miles and save me the trouble!
With the plug installed and torqued down (no point in adding thread lock or sealant as they will breakdown with the high temperatures), the tick, tick, tick, tick noise has gone and the overly-loud crackle and pop on overrun has reverted to its regular burble… job done!
Disclaimer: 308restoration.com describes the restoration work I perform on my car and only my car. I am not a professional mechanic. The website content is presented for entertainment purposes only and should not by seen as any kind of advice, information, instruction or guidance for working on any other car. The opinions stated here are my own and no-one else’s.