I have a test vacuum gauge connected to my '93 GTS 5-spd.

When cruising I get a constant 24 lbs (seems high?).

When accelerating I get 15 lbs when the rpms pass 4000 (seems low? could be caused by cam timing?).

What lbs should I be looking for if everything is normal?

The engine runs and idles fine, just smokes when I deccelarate and downshift. It does not smoke when I rev the engine in neutral or at start up.

 

Where are you measuring the vacuum?

From what is described IMO the smoke sounds like piston ring blow by. If the valve giudes were leaking it would smoke at start up.

 

I tapped in where the cruise control vac line ties into the 4 way by the brake booster.

 

Sounds like your vacuum gauge might be reading 2-3" too high. Normal vacuum at idle is 19-21"; throttle to the floor and it should go to 0".

Based on my experience, oil smoke when you lift the throttle (high vacuum) usually means oil is being sucked past the valve guide seals. Oil smoke when on the throttle is often the result of worn piston rings.

How many miles are on the engine?

 

I don't know what would be considered full vacuum on a NA car, but 15 inHg (that's how vacuum is measured vs. pressure which is psi) is probably just fine.

Edit: Okay, now that I re-read your OP, 24 does seem high while cruising. Usually it's around 21. However, as Adam mentioned if your go WOT, there should be almost zero vacuum. But anything less than WOT will show some vacuum and the higher you go in your RPS, generally the less vacuum you'll see. At least that's my experience - so 15 at 4k seems okay to me.

 

Engine has 44k miles.

 

Well there's yer problem right there... You need to drive it more. A lot more.

 

On my '86.5's at idle, both (NA) were 19.5-20". On Mike's '85, the vacuum reading at idle was 20" when we tested it last month.

Absent vacuum leaks, cams with higher intake/exhaust valve timing overlap will have a lumpy idle and lower vacuum readings.

 

At 44k, you should not have worn seals or rings. I suggest you validate the vacuum readings against another vacuum gauge. I do not own a GTS but I've read that they have a tendency to consume a bit more oil than their predecessors.

 

Its a '93 GTS..............wasn't there an oil consumption issue with this MY associated with piston rings?

Vacuum should fall when rpms rise................24 inHg does seem high.

 

But the car is 16 years old as are the valve seals.This engine probably spent more time off than running.Seals of any kind last longer when used than when not.It is not unreasonable to think that the valve seals would be toast if the car spent more time dormant than running in the past 16 years.Just my opinion.

 

Ed...................agreed, valve guide seals do wear with age.

My point, from what I've read over the years, is that the GTS engine of a particular MY had an issue with oil consumption which I believe was somewhat related to a piston ring design flaw.

I'm sure someone with hands on experience will advise accordingly

 

I think the problem was no oil return holes or not enough in the ring area of the pistons.

And higher crank case pressures amplify the problem, which is what I'm looking at with my engine.

Old valve seals may well be the issue, but who knows. Can valve seals be replaced with the heads still on the engine and cams removed?

 

Yes.............but I only know the trick as it pertains to large valved heads.

Remove the spark plug of the cylinder your working on, thread in a tube to input compressed air.

With the cylinder under air compression you can remove the valve springs without the valves dropping in the cylinder. Replace the guide seals and springs, release the air pressure....presto.

You'll need a special tool to release the valve springs.

 

Yes

You need the type of valve spring compression that bolts to the head and you need to pressureize the combustion chamber to keep the valve from falling.

https://rennlist.com/forums/6388992-post28.html


On a not so happy note - just because the engine has 44k doesn't mean the valve guides are 100% either. I've seen a couple 928's with around the same mileage with worn exhaust valve guides.