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Head gasket check / TDC location
Posted to Technical Tips Forum on 8/18/2012 39 Replies

I helped a buddy who was struggling with a Cadillac for days with a running hot issue . I also used this a day later with our other shop and a very random P0300 , 01,02,03 code on a Honda.

The head gasket issue seems to be a thorn in the side of many people and has burned me in the past several times. It may be as subtle as intermittent misfire or as serious as a split radiator tank. You may even get a vehicle with an air pocket in the cooling system that once you fix may run at a good temperature for days and then show up again. The point im trying to make here is when faced with these type of complaints investigate some before you do say a radiator, a tune up, a water pump, or the famous a thermostat !

You may say what about the blue block test ? Its the ohm meter of tests IMO if it tests bad its probably bad , if it tests good it still may be bad.

Now air in the cylinder is the best way and is nothing new to most guys. The problem is the pressure and finding TDC with most engines. The Cadillac I literally did in 20 minutes at 5:30 last friday and ruined some ones weekend but sold no more parts or wasted no more time. Granted I used misfire history to find # 3 had 25 counts to help speed this up.

I use a Vacuum gauge slid on the compression hose with the schrader removed. If you slowly rotate the engine by hand you will easily see the compression stroke over 10 PSI and the point past TDC start pulling vacuum. The nice thing is you can go back and forth as many times as needed to find that 0 psi sweet spot, remember backwards mean backwards readings. When I get there sometimes I take a long screwdriver with a dab of paint and make my own reference mark on the crank pulley. You can also use it for the companion cylinder if needed to speed things up on that cylinder. Mainly I use it so when slowly adding air If the engine starts to rotate clockwise I know I need to slightly turn the engine past my mark counter clockwise and vice versa.

I use straight shop air with an adapter which is 135 PSI at our place. Once its on TDC its not moving unless bumped PS : take socket and ratchet off crank =). I crimp the overflow hose and fill the bottle or tank about 1/8 below . If you have a cracked tank it can be tough but its probably going to be obvious.

This Cadillac raised the level to overflowing in about 30 seconds so be patient and leave it on there if needed. One more PS do not put your face near the fill hole at first , you maybe be surprised and soaked at the same time if bad enough.

While most of this is not new to many people I continue to see parts thrown at vehicles when maybe this test could be sold first in some cases . We get .75 hours to check most vehicles figuring a 4 cylinder makes up for Van. Its very easy to master so play around with it a few times even if you have timing marks. Its only a few seconds to slip a gauge on it.

Once again another not so technical , not so smart tip that I hope saves your butt before its too late.

Mike from Ohio

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