This problem is driving me nuts. My number 5 plug in the 355 keeps fouling. After around 20 miles the car starts to miss and gets progressively worse.
This is what I've checked so far and replaced.
Obviously that plug.
#5 wire I replaced with a known good one.
Replaced ignition module
Replaced pickup coil
The coil is testing fine.
The cap and rotor appear to be fine and are 6K miles new.
The injector ohms out good and I can hear it clicking with a stethoscope.
Compression in that cylinder is 184 while the rest are all between 175 and 180.
Spark appears to be strong.

Any ideas? I'm starting to think a valve isn't opening enough not allowing the correct amount of air to get into the cylinder. Currently I have a plug shroud on there to see if it's possibly a cracked oil ring.

 

i'd check and reroute my plugs wires first. might want to make sure the 5 wire is good and do a compression test on that hole.

 

Does this car still have the factory Ex. manifold? My 81z28 does that and i found that the manifold put tomuch heat to the plug. And was killing the plug. If it's in a tight place that may be the problem.

 

That cylinder might have a screwed up valve seal, leaking oil in only that cylinder.

 

The wire is good. I checked by using a known good one and the plug still fouled. The compression in that cylinder is 184.
It does still have the factory manifolds. Why would that suddenly cause a problem? There are ~5000 miles on this engine.
I'm thinking that valve seal or not it is an oiling issue. The plug shroud I put on is specifically for oil and wouldn't you know it the plug didn't foul today.
Is anyone familiar with these shrouds? If so, do they also protect against fuel fouling? They're made for cars that are burning excessive amounts of oil. I don't appear to be burning anything but gas.

 

what the heck is an oil shroud? I'm curious.

 

A plug shroud is a cap that gets screwed onto the firing end of the plug and then gets screwed into the head. There's a hole in the end to allow the spark to ignite the fuel air mixture but keeps the end of the spark plug covered except for the hole. It's supposed to keep anything from getting to the spark plug itself.

 

So, your plug is oil fouling? I had a similar experience when I had a bad injector a few years ago. The lack of fuel in the chamber at light loads allowed oil to collect and kill spark plugs. Incidentally, the injector solenoid resistance was fine and it "clicked" just like it should have, but total injector flow was off by 38%. A little cleaning and flow-matching later, and I don't think I've had the plugs out since.

 

Interesting. Is there any way I can test the flow rate without sending them to Rich?

 

Intake gasket

"The simplest explanation tha tfits all the facts is usually the right one".

-Occam, ancient Greek phiosopher, from the days when heads were just big flat chunks of cast iron and the plug almost wasn't even in the cylinder

 

You can perform a power balance test for the injectors. That's how I've identified bad ones. When I had a couple that were flowing way low, I would find clean (unburned) oil lightly coating the plugs on the "dead" cylinders.

RB also mentioned a good point. Check the intake flanges and injector bottom O rings for leakage. The O rings are easy, but the bottom sides of the intake flanges may be tougher. Are you losing oil level?



RB,

Did YOU get my new keyboard after I threw it in the trash? "Phiosopher"? Glad to see I'm not the only one with digital dyslexia. I've had a little less of that since I got the old "clicker" keyboard back - a little.

Just pickin' on you a bit. Thsi pro'lly means that pair of Dart heads won't be in my stocking, eh?

 

Intake gasket? O-rings? Leakage? For air?
Never thought of that.

Don't know if my oil level's changing. Even at 4 quarts my dipstick reads nearly the equivalent of a quart over full. At the next oil change I'll be adding 5 quarts and notch the dipstick at it's new full mark. Obviously I'll run it first and let it drain back into the pan first so as to make sure the filter is full.

 

I've pulled the engine back out. Only took 6 hours.
I'll be disassembling the longblock today. I need to get the rotating assembly balanced. Should've done it in the first place. Of course I figured since these 350's are supposed to be internally balanced I didn't need to do that. Wouldn't you know it but my engine vibrates WAY too much. Poly mounts or not there's definitely something wrong.
I'm also going to get the intake manifold and heads checked at the shop. This sucks.