Hey everyone, I need help fast!!! I sold my 79 F250 with a 400 in it (75,000 original miles) and it ran great. They buyer is picking up Monday at 4:00 pm. I parked my truck that night, then the next morning fired it up, and it ran aweful, misfires left right and center, and eventually blue smoke. When idling in gear it want's to die, the whole truck shakes and moves. After work yesterday, all my co-workers and I looked at it and came to the conclusion of a vacuum leak. My co worker simply pulled out the breather hose from the oil fill cap and plugged it with his thumb, when he did that, the truck no longer burnt oil (smoking blue) or misfired. Now, my question is, 1 could it be the oil filler cap? 2 breather hose, or my co worker said it could even be a booster leak.... Where do I start from knowing that it a vacuum leak and can be stopped by plugging the breather hose with a thumb!!! Please help! Truck is selling in 5 days!!!!

 

Bad PCV valve?

 

I will look into it! Thanks!

 

An easy way to check for vacuum leaks is to spray carb cleaner around your vacuum hoses and carb/intake gaskets while the engine is running. You will be looking for a change in rpm when the leak sucks the cleaner into the engine.

 

The pcv valve is still good...

 

Did you put a new one in, or somehow check the old one?

I ask because I can't think of what else would cause plugging the breather line to make a difference. The system draws air through a filter (either in the air cleaner or built into the cap on an aftermarket breather-cap w/o a hose) into the crankcase, then through the PCV valve and into the intake manifold. The PCV valve is a double-acting check valve. It will only allow flow through in one direction, from the crankcase towards the intake manifold, but it also closes if the vacuum is too high, blocking off flow at idle so you don't have a big vacuum leak.

 

I'm lost now... Beginning to think the vacuum leak was coincidence. Starting to think valve seal or piston rings...

 

If the PCV valve wasn't closing properly at idle, it would cause a significant vacuum leak, which plugging the breather line would stop.

That's not the only possible source of a vacuum leak, but it is the one that would have anything to do with the breather line.

 

When I pull the PCV out, it rattles around freely, and when I plug it, the idle drops slightly but gets smoother. It still is burning oil, and I can hear a tick tick from the passenger side and it's not like an exhaust leak tick, I talked to my dad and he said I may have broken a ring, or maybe even dropped a valve... Regardless of what it is, blow-by is happening very badly, and it's burnt 2 quarts of oil in the past 3 days with about 30km on the engine since it's been runninh rough...

 

OK, yeah, that sounds like a more serious problem. Have you tried pulling the spark plugs to see if one or more are oil-fouled? Would at least help you narrow down which cylinder has the issue. If you pull the valve cover, you'll be able to see if you've dropped a valve or have a collapsed lifter or something there ... won't give you much info about the rings/pistons though.

 

I pulled all the plugs, the truck is running very very lean.....

 

Running very lean on all cylinders? That does sound like it could be a vacuum leak then. If plugging the breather didn't really fix it and it wasn't PCV, have you checked other sources? Try squirting starting fluid around and see where it makes a difference? Try pulling vacuum lines and plugging the ports?

 

Why not just replace the PCV valve for like $4 and be sure that its not the problem? I'd start tracing all of your vac hoses, they're easily and cheaply replaced. Good luck on the sell.