I Both recently a 1990 3.8L Thunderbird SC 5 speed and replaced a head gasket and all the top seals?

Question

When i start the car the rpm are at 1500 but after the car heats up the car sttarts acting up the rpm drop to 500. When the vacum drps the car sounds like is goint to turn off and the rpm drop to 200 then the vacum increases and the rpm rise agin to 500 and it keeps doing the same thing does anyone know what is the cause of this problem?

Answer 1

When you bolted the intake manifold on to the heads, you bolted down one first, then you bolted on the other one.  

That's bad.  Now only one side has a good flat fit.  The other one has vacuum leaks all over.

You need to remove the manifold, set it on to both heads and snug all the bolts evenly in sequence so both sides get equal tension as you snug them more and more firmly until you get to the proper torque.

I bought a car that had been running on only one bank of cylinders.  It did just what you describe.

If you're a cheapskate (like me) and you intend to use your old gaskets, use red silicone to assure an airtight fit.  Don't use too much.  It can migrate and block the oil pickup screen.

Answer 2

It sounds like you have a large vacuum leak or some vacuum lines mixed up.  Check your work over including all vacuum lines and connections.  

Answer 3

This is a common mistake the heads themselves are the problem. The combustion chambers on those and the 4.0's get eaten away from running lean. The running lean is caused by slight back pressure in the exhaust injectors not working at peak efficiency and a few other little things.  I see cars all the time for sale where they say just needs head gaskets. WROOOONG...