that blue puff

[peterb]

1975 Sierra 15 with an original 350 engine I think rebuilt.  A quart of oil every 300 miles. Engine seems strong. Compression was good. Had valve stem seals replaced. Replaced leaking front seal. No more leaks..but i get that blue puff at startup. Ideas?

[jetmech]

I would say worn valve guides. New seals probably would not seal badly worn guides. 300 miles to a quart of oil you should be seeing that on the plugs. Does it start to smoke more after idling for a few minutes?

[bd]

What were the cylinder readings on the wet/dry compression test?  How do the spark plugs look, especially #4, #6 & #8?

Generally, blue smoke on start-up is caused by valve guides and/or seals, as stated.  What style of valve seals were installed: umbrella or Viton press-on?  The Felpro Viton press-on seals used in conjunction with the valve spring retainer o-rings provide the best results.

Other sources of oil consumption are the intake gaskets.  They tend to bake and contract allowing oil to pass directly into the runners from the valley.  It is worse with EGR, because of the concentration of heat where the valve bolts to the intake.  In some cases the intake will actually warp.

[peterb]

Thanks for responses, gentlemen. Valve guides, eh?  And intake manifold. I am just learning my way around the engine of my truck. Engines, period. I plan to book a 2 hour session with my trusted local mechanic/drag racer....will start by rechecking compression.....he did the valve stem seals last year so presumably would have checked guides, but will check again...and look into manifold issues....the main thing I want to re-establish is that compression is good, and if so, go from there.....EGR has been mentioned as possible culprit but I don't really understand WHAT that means....apologies for ignorance, I'm learning....& there's a buncha cool old trucks up on my blog auto literate.blogspot.com   Thanks guys---any more advice much appreciated.

[bd]

EGR stands for Exhaust Gas Recirculation.  It is a steel valve that bolts to the center right side of the intake manifold and allows hot exhaust gas to enter the intake stream through the floor of the intake manifold, diluting intake charge to decrease combustion temperature.  It is intended to decrease NOx exhaust emissions.

EGR concentrates heat along the right side of the intake manifold where the manifold joins the cylinder head.  The localized high heat accelerates gasket deterioration at the center of the manifold, allowing oil mist in the valley beneath the intake manifold to be drawn into the combustion chambers where it burns.  Evidence of EGR related manifold leakage is a heavy buildup of spongy appearing deposits on the insulators and electrodes of the #6 and #8 spark plugs.  If the problem has developed over an extended period, spark plug #4 will also have excessive deposits.  In such cases, replacing the intake gaskets is the only solution.

[bd]

Some additional information:

A few 2 bbl 350s had EGR in 1975.  In 1976, EGR applications were expanded to include 4 bbl 350s.  If you have EGR, the valve will appear similar to the image, below.

Although oil drawing past the intake gaskets is a common problem on EGR small blocks, it also occurs on non-EGR engines.

[peterb]

My mechanic said a cylinder leak down test would dwell us more than a compression test so we did it and the 350 checked out fine on all cylinders. I noticed some il on #1 & #3 plugs---but, as i say, the leakdonw test was fine and that would mean rings are not the problem, right? And we looked t valve seals that he replaced last year and they seemed fine. STILL get that blue puff on startup though. My mechanic guy, said to be brilliant, says that if it were an intake gasket leak, the truck would be smoking constantly and noticeably a lll the time not just the blue puff at startup. So I'm at a loss on my oil burning issue....still a quuasrt every 300 miles. only notice blue puff at startup, leak down test good, engine seems strong.....where to go from here?

[bd]

Does your mechanic have a diagnosis?

[peterb]

My mechanic is stumped. He said maybe we'll check exhaust valves but he doesn't really believe they could be the problem. I'm wondering about intake gasket---but he says you'd see smoke all the time---and I don't.  How to check intake gaskets?  Since we passed cylinder leak with flying colors is there any point in doing compression check?

[bake74]

My mechanic is stumped. He said maybe we'll check exhaust valves but he doesn't really believe they could be the problem. I'm wondering about intake gasket---but he says you'd see smoke all the time---and I don't.  How to check intake gaskets?  Since we passed cylinder leak with flying colors is there any point in doing compression check?

     Are you against a new set of eyes on the problem as in a different mechanic ?  Or are you set on only this mechanic, some times a second person who has not looked at the problem comes up with a different route or avenue of diagnosis.

[bd]

My mechanic is stumped. He said maybe we'll check exhaust valves but he doesn't really believe they could be the problem. I'm wondering about intake gasket---but he says you'd see smoke all the time---and I don't.  How to check intake gaskets?  Since we passed cylinder leak with flying colors is there any point in doing compression check?

You need to find a different mechanic.  All he's told you so far is what it couldn't be.

Check the following and answer every question...

Verify there is manifold vacuum through the PCV valve at idle and above.  Verify the "opposite valve cover" is vented through a breather filter, so the PCV draws clean air into and through the crankcase.

Retorque the intake manifold bolts to 30 lbs-ft, according to the tightening sequence in Figure 76, "Small V8," on page 184 (6-36) of the 1974 Unit Repair Manual.  Were the bolts loose?

Check the engine oil for fuel contamination by letting one drop of cold oil from the dipstick land on a clean fingertip.  Does the oil hold together in a tight circular puddle or does it wick outward along your fingerprint?

Run a dry/wet compression test and post the results by cylinder number.

What type of valve stem seals did your mechanic install?  Are they like the ones pictured, below, or are they the factory style umbrella seals?

Does your engine have an EGR valve?

One quart of oil in 300 miles is fairly high oil consumption.  If it's not leaking on the ground it's burning it.  There are a limited number of ways that can happen.  Valve stem seals/guides, intake gaskets, and rings are the most common causes. 

You won't necessarily "see" continuous blue smoke with high oil consumption, although it would probably show up on an infrared.