As part of my gauge project I installed an Autometer boost gauge, I tapped into the vacuum line going into the fuel pressure regulator, under the intercooler.

When I start the car, the engine idles up to about 2500 rpm, runs at a high idle for a 20-30 seconds, then settles down to around 1,000 rpm. But then the idle constantly fluctuates by about 200 rpm. I've not driven the car to see what would happen under load.
My guess is that I have a vacuum leak from the boost gauge install. My fingers are too fat to reach between the intake runners! But, before I go through the hassles of again taking off the intercooler, I thought I would ask the sages at NAM if I'm right, or if there is something else I should be looking at. And if I'm right, how does a vacuum leak cause the idle to fluctuate?
Thanks a lot,

Mark

 

You sir, have the classic symptoms of a vaccum leak. you are now getting more/less air than the motor thinks it is getting. Traditionally, spraying starter fluid on the offending area will cause the motor to change idle, however, starter fluid will go boom, so do it at your own risk.
Uber

 

Check all the fittings for any leaks, make sure the hose is not damaged in a way. Also make sure the gauge you have is not the problem. I have seen mechanical boost gauges that are defective, and they themselves the cause of the vac leak.

Cheers and good hunting

 

Most likely the end that connects to the fuel pressure regulator has come loose. When I did mine I had the intercooler off, tugged on the end that connected to the manifold port, and noticed that it seemed to pull out further than I expected for a hard plastic line. Turns out the other end, underneath the regulator, had come loose. Made triple sure it was in place before buttoning things up.

 

I have experienced small vacuum leaks from the intercooler gasket connection to the intercooler flanges not sealing correctly. The cure for me was to refit the intercooler after lightly lubricating the inside surface of each intercooler gasket with a thin coat of silicone grease.

Regards,
John Petrich in Seattle

 

Sounds like a vacuum leak to me too. Be sure to use teflon tape on any brass fittings (for the gauge), and be sure to use a rubber T fitting for the fuel pressure regulator vacuum line that is the correct size. Best of luck, and be patient!

 

I think the problem was that I was unable to locally find fittings that I really liked.
I went in search of a brass tee fitting - couldn't find.
Then I looked for a rubber tee, couldn't find one of those.
I ended up using some small vacuum hose to fit over the hard lines and a plastic tee.
Any sources of the brass fittings? The closest I came was at Ace Hardware, they had the appropriate sized straight fittings, but no tee's.

 

John,

Turns out you were right on the money! I took off the IC and the driver's side boot was not seated properly, allowing air in. Put it back together and all is now well. I'm glad I didn't have to deal with the small vaccum line by the FPR.

Thanks all!

Mark

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IB/W some mods

 

Mark,

Thanks for the feedback. Glad that your problem was so easy to repair. Those seals are NOT easy to get right every time. Goes to show you how careful and methodical we must be to successfully do mechanical work. Happy Motoring !

John Petrich in Seattle