Old182

Hi folks. Some time ago, my 91 vert began misfiring and billowing large amounts of white smoke from the right side tailpipe. I nursed it home and thought I blew a cylinder head gasket, but later figured it was an intake manifold gasket as there were no bangs or hisses that a head gasket with a hole in its side might give. But when I pulled the intake I found no broken intake gasket - uh oh. I did NOT do a compression check before disassembly (duh!). Since the engine has over 50,000 miles since a spark plug change (122k total), I decided to pull the spark plugs and found 7 to be burning normally but No. 4 completely lost its hot electrode (see pics). I bought the car with 51,000 on it and it has never been driven hard nor overheated. I've turned the engine over several times by hand and all the lifters are moving where they should and there are no clicks, groans or rubbing sounds from any cylinder. I'm reluctant to button it back up for fear that I missed something (like, why did the spark plug cough up a lung, do I have a cracked head from electrode debris?), but I really do not want to pull the head unless there's no alternative. I'm planning to do a leak down test on that cylinder but wanted to hear any advice or tips from the experts. Any takers? Thanks in advance! (Gosh! Sorry for the HUGE pics!)

Tod Stiles

First thing I'd do is a compression test, that can tell you a lot about what is going on. Is the oil milky or is there oil drops in the coolant? That'll help you on the blown head gasket question.

I've never seen a plug lose the whole center electrode and insulator. I would think that it could possibly be defective or it cracked when it was gapped. I've seen engines ingest all kinds of stuff, sometimes it just goes through other times it digs into the valve seat. Compression test will help you find out what happened there too.

If all that works out, I'd button it back up and see how she runs. That's all my opinion I'm sure others may have some ideas too. Good luck!

9T3VETTE

Old182

Thanks for the advice. Drained coolant looked normal (no oil) and no coolant in the oil (checked dipstick and valley under manifold).The spark plug was a Bosch Platinum (but it has no numbers on the insulator), and had been in there for over 50,000 miles. No plugs were burned but all had signs of wear (large gaps) and a couple were beginning to foul. In fact, the car ran great - until it didn't. I'll post findings after compression test - thanks again.

JCrock

Ah, Bosch platinum plugs - don't get me started on these and my less than happy experiences with them. Iv'e ask around and it appears I'm not alone. Put in new plugs and run it. There's very little chance that there is any damage from the loss of the electrode tip.

C6_Racer_X

Before you do any more damage, stick a borescope down that hole and see if the cylinder walls are scored and/or there is still debris in the cylinder.

I suspect at minimum you'll need head work on that head, and it's possible the junk that dropped in scored the cylinder.

billschroeder5842

If it were me, I'd pop a new plug in and see what happens. You will know in about 10 seconds after starting if the engine is running smooth.

50k on a plug is enough miles. I'm not sure I'd over engineer this thing too much; with 122k on the motor your can easily justify a rebuild, so swap the plug and see what happens.

cv67

x2 on the borescope & new plug...if the electrode came out and banged things up some but nothing broken get the plug back in and motor on. Make sure they are the right ones/heat range as said.
A plain old Autolite/AC or? Is all you need.

ddahlgren

Do a leak down and my bets are you might have hurt the head. A dead cylinder does not make a bunch of white smoke. Coolant does.