I was wondering if these values are acceptable for a stock 1982 Corvette 350 Engine?

Cylinders 1, 3, 5,7, 2, 6, and 8 were in the 150 - 152 range

Cylinder 4 - 121

I've been fighting a slight miss and I have changed all the ignition components with new parts, checked it on a vacuum guage and it shows a slight "tick" at 18-19 inches @ idle but goes away on accelleration which leads me to assume a vacuum leak or ignition problem.

I was concerned about cylinder 4 being that far off from the others, I went ahead and squirted some oil in there and re-tested....nothing changed. That tells me it's the rings correct?

Any suggestions?

 

#4 - Exhaust valve seat clean?

 

Exhaust valve seat?

 

I agree with above that #4 is way too low, and it's probably not rings if the rest are fine. The only other plausible explanation is the valves, assuming the head gasket integrity is sound. The exhaust valve seat gets the most abuse and if the exhaust valve is not seating properly then compression can be lost.

 

If your engine miss happened to be on #4, for an extended period of time, you will end up with carbon deposits at that cylinder. That being the case you may also have a build-up through the exhaust for that cylinder, hence carbon trapped on the exhaust valve seat area. If this is the case, then while performing your compression test you may get slight blow-by at the exhaust valve because carbon buildup is preventing the exhaust valve from fully seating closed.

You can determine the condition of all cylinders by reading your spark plugs.

 

Thanks guys, I'm hoping it's something simple.

I don't want to sound dumb, but where do I need to look for "exhaust valve seat" area? Is this something I can clean or fix pretty easy?

 

The exhaust valve seats are in the heads. There are two valves per cylinder. An intake valve that opens to let air in and an exhaust valve to let exhaust gas out. The rockers under the valve covers open the valves.

 

Would this be a job that require's removing the head? Ouch, that TPI stuff is a pain to remove, not like pulling a carburator intake off....better than a blown engine though.

It seems like I might be able to do something under the valve cover on that cylinder?


Thanks again

 

The first step is to pull the plug from #4 and it's color will tell you a little bit about what's happening in that cylinder.

IF it looks like there is a lot of carbon then the fouling could have caused the buildup and the carbon could be cause the exhaust valve from seating properly when it's supposed to be sealed.

IF that's the case then yes the head will have to come off, and a machine shop will be in line to clean and rebuild the head if you're not up to the task.

It sucks but you cannot do valve work with the head on the engine.

 

What could have caused the fouling in that cylinder to begin with? To the best of my knowledge I did a compression test before putting the engine back in my car a few years ago and it read the same.....why I put it back in the car I don't know.....

I just installed new plugs so telling if it was fouled would next to impossible because I threw out my old plugs without paying attention to the orientation.....but I can tell you none of them was heavily built up or out of the ordinary otherwise it would have caught my attention.

But now the plugs that were on the car when I bought it might have been a different story, this is what I was dealing with.....I'm still using the same engine....just cleaned and repainted.





I've been debating on buying a set of rebuilt heads anyways and putting them on the car, it couldn't hurt because I'm pretty sure that engines never been rebuilt.....but it looked really good when I pulled the pan and valve covers after I bought the car.....no sludge or buildup....everything internally looked new

I would be able to tell for sure once I pull the head though, correct?

 

If you are concerned about carbon build up in that cylinder you may want to try dribbling some water into it while you keep it reved at say 2500 or so and at operating temp. This is an old school trick; the steam dislodges the carbon which is blown out the exhaust. My car spends a lot of time idling in the driveway (& rich), this past weekend I ran probably 12 oz. of water thru it. The idle smoothed out considerabley and the vacuum guage settled and registered 14" instead of 11". I still need to change the spark plugs but I am not near ready to get out on the road.

If that fuel inj. has a port for that one cylinder I would give the water a try or buy some of the Sea Foam and spray it in there. Some on here really like it for this purpose.

 

Do a cylinder leak-down test on that cylinder. This will tell you immediately what the problem is, and will determine if you have to pull the head. If it's a bad intake or exhaust valve, you'll hear the air hissing out the tailpipe or up through the intake. Very definitive test.

 

Sorry, steam does not clean the carbon off. Liquid that is not compressable is normally used for the removal of carbon, say GM's top end cleaner. This is poored slowly down the carb as it runs and breaks up the carbon build up and sends it (hopefully) all out the tailpipe. Most shops also used trans fluid back in the day. Both great for spraying for bugs and ticking off the neighbors with they're windows open! Your TPI you could also use a spray fogger (oil) or take it to a shop that can connect to the fuel rail and run an injector cleaner thru it. Similar effect plus you inj will like it.

Believe back in Lars day they used the water kits to cool the air/fuel mix, remember Holley (?) still had kits for sale, we had them at the parts store I worked at.

Your slightly lower compression is probably due to the #4 exh valve leaking and bleeding off some pressure. High heat from the EGR could be responsible for that over the life of the engine. Some other major factors such as bad cam lobe, rocker adjust - wear, worn lifter or even bad valve spring (very low spring force) can contribute.

Scam a leak down tester, loosen/remove the rockers, test with each cylinder at top dead - be careful of rotation as pressurized (even small amount of air will spin engine over) and measure/record percent of leakage. Again like comp test they should all be within 10% of each other. Listen as test is done, air out int to carb = int valve leak, air out exh pipe = exh valve leak, air to oil pan is ring/cylinder wear. Typical leak down values are between 10% - 20% depending on engine miles. New around 5% +\- 2% error.

Hope this helps,
Later

 

also you can try this, put about a teaspon of oil in the low cylender, if the compression jumps up its the valves.. if it doesnt chang it could be the rings


tim

 

Geez... have I dated myself as the ol' man here...?

Yes, I not only remember the water injection kits, but I actually installed a few of them. The primary purpose for the water injection was to suppress detonation. The kits were used in conjunction with a manifold pressure switch, so when manifold pressure approached atmospheric, a windshield washer pump motor would start up and spray water down the carb venturi through a pair of carb jets. For better effect, you could put some alcohol in the water. I thought everybody did this.... you could run regular gas with a high compression engine on the street and get away with it.

As far as dislodging carbon with water, you can do this, too. I know this, because I'm so old that I remember the trick... You take an empty Coke bottle (you gotta' use one of the real, glass Coke bottles to stay in style) and fill it with water. You take the air cleaner off the carb, start the engine, and rev it up. Then, you put your thumb over the bottle and dribble the water right down the carb as fast as you can without actually killing the engine. The engine will cough, sputter, puke, rattle, knock, blow all kinds of white smoke & crap out the tailpipe, and the carbon deposits in the chamber will get dislodged from a combination of the expanding steam and the near liquid-lock condition induced by the water pouring into the engine. Us old guys used to do it all the time as part of a tune up...

 

It's the other way around. If compression jumps up when you put oil in the cylinder, it's bad rings. If no change, it's valves.

 

Being the "carb guy" I figured you'd get a kick outta that and start up the "way back machine". I on the other hand have Etch-a-Sketch memmory, shake me up, and my mind goes blank! It's nice to be 40 and still have someone older around!

Always thought of the water trick as the poor boys method, it works and it's always around. Trans fluid, Seafoam, GM TEC, etc usually lubricated while using the "liquid lock" method of breaking up the carbon. We always seemed to have a qt of trans oil, with the tops punched or the spout still in it (can method for the younger crowd) left over from topping off a car's fluid before it went out the door.

Any of these do the trick, still not gonna fix this guys ride, seems like a different mechanical problem, that may require more money.

Happy 4th to all,
Later.

 

Sorry, but it does.

I use it because I don't want the neighbors to call the fire dept.