My last time at the track showed that I had lost 4.5 mph. I think I may have traced it to a lean condition. I had the valvesprings changed at first thinking that may have been the problem, really because I did find one bad out of 5 I had tested. I don't have a fuel pressure guage mounted inside the car but after making a couple trial runs on the street, noticed the scanmaster was showing 650 mv from the o2s. When I got home I found that the car seemed to be idling rougher than normal. It seemed to be a dead miss. After spraying water onto each header tube, to see if they were all firing, the problem seemed to be coming from the #5 cylinder. Yesterday I did a compression check and got a reading of only 125 psi compared to the 190 psi reading I got a couple weeks ago. I'm not too sure now how to determine if the problem is coming from rings or possibly burnt valves. I did pump air into that cylinder while the valves were closed (I also removed the rockers) and found that when I stuck my finger into the exhaust port, could feel air coming from around the exhaust valve. What I'm not too sure about is, could it be that I just forced too much pressure into the cylinder forcing air to come out from around the valve? It was a shop air pump that was set to deliver 180 psi. What other tests can I do to narrow down the problem area? Please note also that there is no abnormal smoke coming from the exhaust pipe. Thanks for any help you guys can give.

 

Perform a leak down test. 180psi should not affect the leakage you found however there should be NO or minimal air coming out of the valves(intake ports and exhaust ports). The O2's will read a lean condition with a misfire.

My guess would be either a bent valve, or worn valve guide.

Good Luck,
Sean(Pig)

 

Thanks for the reply Sean. I don't know if it makes much of a difference but I just found out that the pressure I was forcing into the cylinder was actually only 130 psi not 180. The amount of air I felt coming from around the valve was pretty substantial, enough for me to actually feel it when placing my finger close to it. What would a leakdown test show that a compression test won't?

 

Id say thats gotta be a bent valve dave. Could you possibly do a leakdown test with the exhaust port sealed off with a gasket, flat peice of metalbolted to tthe head, if the pressure stays up higher, then I would think that it would be bleeding off in the exhaust port meaning bent valve.

 

A leak down test will show in percent of how much air is leaking. For instance if you have 100psi (shop air or supplied air) plugged into a cylinder and the leak down gauge (0%-100%) shows 20%, that means 20% of that 100psi is leaking out somewhere. The three common places are crankcase (most popular), exhaust and intake (on that selected cylinder). Sometimes on a blown head gasket car it will leak into the cooling system.

Hope you have a leak down tester, they can tell you some good information.

Good Luck!
Sean

 

Thanks again for the reply. So unless I'm getting this wrong, the only difference between the compression check and the leakdown test is the capability of the leakdown tester to show percentages of compression loss where the compression test just shows the maximum compression that cylinder is able produce. But neither neither will show exactly where the leak is coming from. Because I'd done a compression test not long ago I do have definite numbers to compare my new tests to. With this info I would think it wouldn't necessary to perform a leakdown test...or am I wrong?