I have a 88 Mustang 5.0 and Ive had this problem ever since Ive dropped my new motor (not really new, just a motor from a 91 w/ 32,000 miles). When I first start my car its fine, but than after 25-35 seconds the motor starts whistling REALLY loud. It sounds like its comming from the back right side of my manifold (Edelbrock RPM). I just cant seem to figure out what this is. :dunno:

Also, I have a major idling problem. When I start the car I idle very clean and sturdy (1,000- give or take a hundred), but than after a minute or so it boggs BIG TIME, it will drop all the way down to 400-500 and than jump to 1,300. THAN, it will go back to normal :scratch: . Ive tried alot of different things to fix this but I just cant seem to nail it. Any suggestions?

Also, does anyone know what I should run my timing at?

Also, I just baught a BBK adj. fuel regulator :D Any suggestions w/ what I should be running that at?

If anyone has any suggestions or has went through any of these same problems and knows how to resolve it I would greatly appreciate your input. Thanks everyone!

I'd check the vacuum lines and the map sensor.

You have a vacuum leak more than likely....

Timing stock is 10 degrees....depending on if you run premium or not you could go to 14 Degrees or some cars even take more but that all depends on the tune and other variables

Fuel pressure....Do you have a gauge? if so put the gauge on first and see where your stock setting is before starting. As a base point depending on mods you can start between 38-42psi....but you really need to get it on a dyno to find your optimal settings and set timing and fuel pressure.

I too would think it a vacuum leak
You can check this with a small propaine torch type of bottle.
about 10 bucks at Walmart

don't light it but simply spray the propine in suspected areas while the engine is running. Than listen for an engine spike in RPM.
This should help in finding the problem.

Since Rubber tends to get flexible when heated and metal tends to expand? Well, this might explain the in consistancy.

i broke the vaccuum line that runs under the from of the upper intake plenum in my 92 and had the same problems. replaces the hose and was back to normal. i think there is a line to the back of the intake also

I got my fuel reg hooked up yesterday and I am running it at 40. The only time that I get the car to hold a steady idle is when I have lines open when they are supposed to be capped. Im not sure if I want to leave it like that though. Also, I am going to set my timing very shortly. I guess I will start out at 14 and see how everything goes. Thanks a million guys. Anymore suggestions?