I’m working on bringing a car that hasn’t run in 10 years back to life. I recently posted about finding rust and corrosion in the cooling system which initially led me to thinking there may be an issue with the head gasket. Some input from members helped me realize the buildup was likely caused by tap water sediments.
Regardless, I decided to run a leak down test to ensure there weren’t any compression leaks. The engine doesn’t run, so the leak down was done on a cold engine. I pumped 100 psi into each cylinder, cylinders 1-5 lost less than 10% compression. Cylinder 6 could only build up to about 40 psi and only held 15-20 psi. I used a rubber glove over the throttle body and determined the leak was coming from the intake valves.
I pulled the throttle bodies to inspect the intake valves; all the intake valves were cleaner than I expected and had similar levels of buildup. When rotating the crank, all the valves seem to be moving smoothly and closing without any obvious gaps. The valve clearances have been adjusted and are within spec. With the cylinder losing 50% compression instead of 100%, I hoped the cause was just a dirty valve, carbon deposits or something like that, but that doesn’t seem to be the case. I cleaned up the valves and retested, the figures improved slightly. Compression built up to about 50 psi and held around 30 psi. I checked the intake port and couldn’t detect air escaping… that’s odd.
Next, I checked the exhaust side, no air leak there. The cooling system is still disassembled, but I did my best to check the water pump location and coolant pipes for leaks, couldn’t find any. So, I pulled the dipstick, put my ear to the tube and heard a faint whistle… damn.
Edit: I thought there could be a stuck piston ring, but I ran the leak down again on cylinders 1-5, they held compression and also had air escaping from the dipstick tube. This makes sense, especially on a cold engine. So, I checked cylinder 6 again, the intake valve leak reduced after the valves were cleaned, but not entirely. So, I'm back to thinking the valve, valve seal, or valve seat is failing.
Edit: Cylinder 6 Endoscope photos added.
Regardless, I decided to run a leak down test to ensure there weren’t any compression leaks. The engine doesn’t run, so the leak down was done on a cold engine. I pumped 100 psi into each cylinder, cylinders 1-5 lost less than 10% compression. Cylinder 6 could only build up to about 40 psi and only held 15-20 psi. I used a rubber glove over the throttle body and determined the leak was coming from the intake valves.
I pulled the throttle bodies to inspect the intake valves; all the intake valves were cleaner than I expected and had similar levels of buildup. When rotating the crank, all the valves seem to be moving smoothly and closing without any obvious gaps. The valve clearances have been adjusted and are within spec. With the cylinder losing 50% compression instead of 100%, I hoped the cause was just a dirty valve, carbon deposits or something like that, but that doesn’t seem to be the case. I cleaned up the valves and retested, the figures improved slightly. Compression built up to about 50 psi and held around 30 psi. I checked the intake port and couldn’t detect air escaping… that’s odd.
Next, I checked the exhaust side, no air leak there. The cooling system is still disassembled, but I did my best to check the water pump location and coolant pipes for leaks, couldn’t find any. So, I pulled the dipstick, put my ear to the tube and heard a faint whistle… damn.
Edit: I thought there could be a stuck piston ring, but I ran the leak down again on cylinders 1-5, they held compression and also had air escaping from the dipstick tube. This makes sense, especially on a cold engine. So, I checked cylinder 6 again, the intake valve leak reduced after the valves were cleaned, but not entirely. So, I'm back to thinking the valve, valve seal, or valve seat is failing.
Edit: Cylinder 6 Endoscope photos added.
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