After rebuilding the head on my friend’s Miata, I was extremely disappointed when the car started right up, but wouldn’t idle or run below a few thousand RPMs. It certainly indicated a vacuum leak, but where?
Boy, at times like this it sure would be nice to own a smoke detector, but have you seen how much they cost?
After a few searches and viewing some video clips
http://www.youtube.com/watch?v=-V_W_SFxYoI&feature=related, I decided to explore the basic premise behind a smoke detector by using common tools, soapy water, a garden pressure sprayer, a leak down tester and...a little Yankee ingenuity. No offense to those living below the Mason/Dixon Line.
Before you read any farther realize you will be using pressurized air which may present personal hazards if you don't do it correctly. Wear eye protection and
never, never run with scissors:eek:.
You get my point, right?
First order of business was to isolate the intake manifold so I could pressurize it. I removed the throttle body and capped off the intake with a latex glove and some tape. Then I installed my leak down tester to one of the cylinders and set the cams up so the intake valves were open and exhaust valves were closed. I checked the other cylinder’s valves to ensure they weren’t in a position to allow air to escape into the exhaust system.
http://www.m42club.com/forums/album.php?albumid=67&pictureid=366Next, I slowly applied air pressure to the leak down tester. At 25psi the latex glove was fully inflated and I could hear air escaping, but couldn’t see where. Using a mixture of dishwashing soap and water in a garden pressure sprayer
http://www.m42club.com/forums/album.php?albumid=67&pictureid=369, I started from the top and sprayed the surfaces where the intake manifold mated to the cylinder head. This picture shows how easy you can get into small areas with pinpoint precision. Here I am about to spray around the fuel injectors to see if a seal was damaged during the installation.
http://www.m42club.com/forums/picture.php?albumid=67&pictureid=367 For reference, here’s what you will see when the soap is applied to an area where air is leaking out
http://www.m42club.com/forums/album.php?albumid=67&pictureid=368; Hint: The bigger the leak, the bigger the bubbles. A very slight pinhole may take longer to show up and it will have a frothy appearance.
In this particular case I found I had installed the intake gasket from an NA (First Generation) Miata vice an NB. After changing out gaskets I set everything up again and I barely got to 3 psi before the glove blew off like on over-inflated balloon!
So in summary this method may be a bit crude and it wasn't on an M42, however, there are a few advantages to doing it this way vs. spraying a hot, running engine down with flammable aerosol liquids while listening to a change in engine rpms. I don't know about you, but even when my car is running right the rpms fluctuate just a bit:D
