I know this isn`t Vette related but it may help some of us. I have a 4.3 V-6 in my daily driver and the ECI went bad so I changed it over to an Edelbrock carb and intake and HEI dist. The truck ran really fine but I noticed that sometimes it would smoke black at startup like it was a little rich. Well now it has started smoking blue (oil) and running really bad. Compression on all cylinders is 120 to 125 lbs. I have checked everything and I am beginning to think I may have washed out the rings. I rebuilt this engine about 30 thousand miles ago and it ran great untill this. Any ideas?

 

If it smokes when you take your foot off the gas pedal then odds have it to be valve guides or seals. Just my $0.02

 

that's what I was thinking but I had the heads done when I rebuilt the engine but before I bite the bullet and tear it down I am going to change seals this weekend

 

I have to ask what is the begining PSI per cylinder on a brand new engine for the engine you are asking the question about? 120 to 125 seems really low on a V6 most later model engines have gone back to high compression and should be over what you have stated but only a good manual will tell you the correct PSI per cylinder on a brand new engine.


I have a little 4 banger in my 1992 Miata where it is 192 PSI per cylinder brand new. At 156,000 miles I am down to 140 PSI per cylinder. It has another problem because it is an overhead valve engine and the spark plugs are directly down into the valve cover. I have the number 3 cylinder which leaks oil down into the spark plug and I have to retorque the valve cover again at 60 inch pounds. Different engine different issue.

How much of a rebuild did you have done? Top and bottom end? If the valves are not seated properly and the seals are bad you will see Blue Smoke because of valves will allow oil into the engine at the valve seals. One bent valve will blow smoke on the head and cause the seal to go bad.

Bottom end is always going to be the rings and if the were not staggered when installed it will also allow oil into the compression chamber. Not saying this happened but never align the rings they all should be offset in 1/3 increments around the piston when installed if a 3 ring piston.

 

Brand new this engine only runs about 140 at a 9.5 compression ratio. I did the heads when I rebuilt the engine, ground the seats, new guides and seals. I have never paid someone to build an engine for me I am 62 and have been building my own engines for over 40 years. Sprint cars, antique airplanes and all of my restorations. Cylinders were less than .0002 out so all I did was hone and re ring. Crank turned .010 and reinstalled. If the seals were bad it would smoke when you let off to shift and the vacuum rises and it sucks oil down past the seals this engine is smoking all the time. As for aligning the end gaps I have built race engines and staggered the end gaps only to pull it down to sweeten it up and find the end gaps all moved. Rings will turn a little not a lot but they will turn. I am afraid the fuel pressure was to high and it got a few dry starts and killed the oil rings. Remember this engine had a TBI and I changed it to a carb and used the in tank fuel pump with a 4 port bypass regulator. The regulator was factory set at 6 psi and I didn`t change it but it could have been high I didn`t put a FP gauge on the regulator.

 

http://forums.corvetteforum.com/c3-t...d-correct.html

The compression on most L48 engines should be between 160 and 172 PSI per cylinder on the L82 engines I think it will be slightly higher because they have a slightly higher compression ratio more like around 180 PSI per cylinder.

To test the rings and valves you can dump about 1 table spoon of oil 10w40 oil into each cylinder and see if the PSI increases. The oil will act like an additional seal on the pistion heads and assist in preventing blow by at the rings. If the PSI does not increase it is your valve seats. If it does increase it will indicate the rings. I have used this test before and it works. I used it on my little Miata engine and the PSI per cylinder went from 140 PSI per cylinder to close to 170 PSI per cylinder. So at that point I knew the cylinder walls are worn down and the rings are no longer preventing blow by into the bottom end of the engine.

Thinking now is a good time to test the PSI in my L48 engine since it is about the only thing I have not done to my Vette since buying it 4 months ago. I have not tested the Vacuum pressure or the Cylinder pressure yet.

Sorry I did not find the answer for you sooner.