Couple of months ago I bought a 1999 S70 (5-speed, non-turbo) with 100k miles. After driving it about a thousand miles I heard a tappity sound coming from the head and discovered that the oil level was 3 quarts low. Topped it up and now every 300 miles I have to add a quart. The valve noise went away but the engine also makes a knocking sound at idle that is most noticeable around the driver's side wheel well.

How screwed is the engine? Can I drive it like this for a while or will it grenade on me in the next couple of thousand miles? Really like the car and was hoping to keep it but there's no way I'm sinking $3,000 into an engine rebuild.

 

Sounds like it was run low on oil before. If you like, you can keep adding oil as necessary and drive it until it gives up the ghost. Could be 100 miles, could be 20,000 miles. Hard to say. You can do an engine transplant with a good donor engine for around $1500 out the door.

 

yep the tapping can be either a lifter or a rod that got scarred by low oil pressure. The bigger issue is where is the oil going? If you have no oil in the coolant, outward signs of leaks and don't have a turbo to blame, you probably have ring damage (a wet compression test can give some clues here) which means it may be time for a transplant. No harm driving with an oil burner - but you will eventually set off the check engine light if the catalytic converter starts clogging up (or runs below "efficiency"). If you have weepy seals (ie leaking all over the place) then you probably have a clogged PCV system.

 

It's both. You can drive it like that for a while AND it will grenade on you. You got nothing to lose by continuing to drive it.

Good 5-cyl engines are everywhere; you can engine swap pretty cheaply if you decide you want to.

 

The tapping goes away when I push in the clutch pedal - sounds like it's not internal to the engine after all. Might be related to the difficulty engaging gears when the transmission is cold, but regardless, it sounds like the pistons / rods / crank are probably solid.

So the engine will probably survive as long as I keep the oil topped up, which basically means adding a quart with every tank of gas. Can worn valve guide seals really cause this much oil consumption? And is replacing them a DIY job for someone with tools and a good amount of shadetree repair experience?

 

A quart every 300 seems a lot for just valve seals to me but if say the engine was over-revved and a seal got mashed then yeah I suppose its possible. I'd still do the wet/dry compression test for starters to see if the rings are sound. That would give you the option of simply doing the head vs the block. Now considering you suspect the tapping is not in the engine, you have to consider that should you replace the block, you'd want to do the clutch at the same time. If you suspect the transmission, then you can check fluid levels and consider a drain/fill and use something like Redline MTL or Volvo OEM 1161423 (also synthetic). As noted these engines are fairly easy to find.