Strange Exhaust Noise - Help with diagnosis please

[Gambit]

That first spark plug doesn't look like it is firing. It looks very clean.

Second plug looks like a slight lean condition. A touch of white ash forming on the tip.

Third plug looks like it may not be firing all the time, gas is washing carbon off the plug.

Fourth plug is what a spark plug should look like, slight brown color with a slight bit of carbon.

Now we know what they look like at idle, when you have an opportunity, take it for a short drive. Get it up to temperature and when you park it, immediately shut the engine. Don't let it idle, that will allow the gas to wash the first and third plug and skew the results.

How many miles on the engine?

The diesel engines are shim adjusted, I believe there is a valve adjustment on the gasoline engines. When you pull the valve cover to replace the seals, it might be a good idea to do a valve adjustment. Might be good to do a compression check as well.

Is this a carbureted or fuel injected engine? If fuel injected, you might check out the injectors. You might have 1 or 3 that aren't quite doing the job (at least at idle). Good candidates for cleaning or replacement.

I'm not denying the possibility of an exhaust issue, but I'm trying to think through other possibilities as well.

Thanks this is all super helpful.

On the plugs - wouldn't pulling each one and having the idle change mean that they were all firing? I'm happy to pull them again and take a look without letting them idle.

These were pulled with engine up to temp (180). Engine has 310k miles But had top and bottom end done recently.

Injectors were just cleaned while intake manifold was off for the Auxillary air valve replacement... so I'm hoping they are OK, but not ruling them out completely yet. I had removed the intake manifold and done the air valve - which didn't fix another issue I was having (cold start & high idle along with this exhaust sound), so I took it to a shop who thought it was all the same issues I had, they figured I screwed something up in there. So they also removed the intake manifold and while they were in there cleaned the injectors (all of this did nothing to remedy the current issues)

[Gambit]

Am I off base with the valve cover gasket and thinking it is leaking?

[bikeregg]

Am I off base with the valve cover gasket and thinking it is leaking?

Yes, the O ring around the spark plug tunnel is leaking, but doesn't normally cause an issue until the tunnel fills with oil, over the top of the spark plug boot and prevents a good connection.

Ideally, all the spark plugs should be in the same condition. If a plug looks different, there is a different condition occurring in that cylinder than the others. All of your spark plugs look different. I'll say it again, when you pull the valve cover, check the valve clearance. Nowadays, with hydraulic valves and computer controlled variable valve timing, it would be very easy to overlook the valve clearance as a culprit. And, just because the injectors were cleaned does not mean they are working properly. You need special equipment to check the injectors for proper operation. RockAuto has a couple of vendors that offer injector rebuild services or you could look for someone in your local area.

[Gambit]

Yes, the O ring around the spark plug tunnel is leaking, but doesn't normally cause an issue until the tunnel fills with oil, over the top of the spark plug boot and prevents a good connection.

Ideally, all the spark plugs should be in the same condition. If a plug looks different, there is a different condition occurring in that cylinder than the others. All of your spark plugs look different. I'll say it again, when you pull the valve cover, check the valve clearance. Nowadays, with hydraulic valves and computer controlled variable valve timing, it would be very easy to overlook the valve clearance as a culprit. And, just because the injectors were cleaned does not mean they are working properly. You need special equipment to check the injectors for proper operation. RockAuto has a couple of vendors that offer injector rebuild services or you could look for someone in your local area.

Thanks - really appreciate your patience, I am still learning about reading plugs and well, lots of other things. I wouldn't be shocked if it is the injectors. I'm having my exhaust guy take a look tomorrow AM. If that yields no luck, I guess I am pulling the intake manifold again...

[Flecker]

The bugger with the bad cat theory is that it's not throwing a code! Anything that has to do with emissions is usually throwing codes left, right, under and over!

I don't have a cat, and mine throws no codes. O2 Sensor was upstream of it anyways on my 87

[Gambit]

I don't have a cat, and mine throws no codes. O2 Sensor was upstream of it anyways on my 87

Unfortunately they started using 2 at some point - my '88 has one pre and post Cat

[Gambit]

Exhaust checked out as all tight with no leaks and they said the cat was good. Next steps I suppose is to get back into the injectors, it will be a little time until I am able to do so. Thanks again to everyone for the help - I'll report back.