I have an 89 GT. It has an F-cam, steel GT-40 heads that supposedly have been ported, a Cobra intake, the rest is pretty much stock as far as I know. I just bought the car. My problem is the engine surges real bad at idle at times. The guy I bought the car from said it was because the heads had been ported. Is this right or could it be something else?
Also, it has a vacumm leak that sounds like it is coming from under the upper intake manifold, but I haven't been able to find it. It doesn't leak when first cranked up, just after the engine has ran a couple of minutes. Any ideas where or what it could be. Thanks.

 

A vacuum leak would cause a surging idle, porting shouldn't... I'd get on finding that leak and see if it doesn't stop the wavering idle speed.

 

get some like DW40 and use the straw that it comes with, spray it and see when the idle changes once it changes you have found your leak. it may be a lose bolt, bad gasket or a vacuum line. mine was the vacuum hose that is on the back driver side. good luck.