Where to look?

885.0conv · Feb 22, 2011

I'm pretty sure that there is a vacuum leak. I installed some gauges to watch what was going on when driving the car. I have problems when a I punch it at a dead stop the car bogs down. The fuel pressure gauge stay constant at 40-41 psi through normal driving and WOT. The vacuum gauge reads about 9-10 at idle and goes to 0 when I push it when I let off the pedal the gauge goes to 30-35 and there is after fire out of the tail pipes. I get 11's on codes and good with cylinder balance test. Just wondering if this vacuum is good with the mods I have done. Thanks

stykthyn · Feb 22, 2011

Vacuum should read ~18 with a stockish valvetrain. Depending on your cam I would definitely say you have an issue.

885.0conv · Feb 22, 2011

I have a stock cam so 18 should probably be the reading.

Wildstringer · Feb 23, 2011

If you take a small propane tank. the one you would use for soldering plumbing together. they make a small torch kit for these. hook up only the propane and use the small torch head as a sniffer. turn on the propane just a little (of coarse do not light it). move it around the intake and vacuum ports. You will hear a change in idle if you find the vacuum leak. the problem could also be internal. How many miles are on the engine?

jrichker · Feb 23, 2011

Wildstringer said:
If you take a small propane tank. the one you would use for soldering plumbing together. they make a small torch kit for these. hook up only the propane and use the small torch head as a sniffer. turn on the propane just a little (of coarse do not light it). move it around the intake and vacuum ports. You will hear a change in idle if you find the vacuum leak. the problem could also be internal. How many miles are on the engine?

Very bad idea. Hot propane gas + a spark + fuel lines = very bad fire. I would hate to read that you just got out of the hospital after your Mustang burned up while testing for vaccum leaks

There is no easy way to find vacuum leaks. It is a time consuming job that requires close inspection of each and very hose and connection.

Small vacuum leaks may not show much change using a vacuum gauge. The range of "good readings" varies so much from engine to engine that it may be difficult to detect small leaks. The engine in my first Mustang pulled about 16.5" of vacuum at 650-7250 RPM, which I consider rather low. It was a mass market remanufactured rebuild, so no telling what kind of camshaft it had. Average readings seem to run 16"-18" inches at idle and 18"-21" at 1000 RPM. The only sure comparison is a reading taken when your car was performing at its best through all the RPM ranges and what it is doing now. Use one of the spare ports on the vacuum tree that is mounted on the firewall near the windshield wiper motor.

Use a squirt can of motor oil to squirt around the mating surfaces of the manifold & TB. The oil will be sucked into the leaking area and the engine will change speed. Avoid using flammable substitutes for the oil such as propane or throttle body cleaner. Fire is an excellent hair removal agent, and no eyebrows is not cool...

The vacuum line plumbing is old and brittle on many of these cars, so replacing the lines with new hose is a good plan. The common 1/8” and ¼” vacuum hose works well and isn’t expensive.

Diagram courtesy of Tmoss & Stang&2birds

Vacuum leak due to slipped lower intake manifold gasket...

Ask Nicoleb3x3 about the intake gasket that slipped out of place and caused idle and vacuum leak problems that could not be seen or found by external examination. Spay everything with anything you have, and you won't find the leak...

885.0conv · Feb 23, 2011

Thanks j for the advice. I have changed about 90% of the lines over the last two years. The engine has about 80k on it. I put heads and intake on about 2 years ago. I did notice a better accleration when I retorqued the lower intake. So this sounds like a leak and nothing wrong with internals such as valves or pistons rings.

Where to look?

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I want to measure mine. It doesn't look that tall.

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