I have a 2003 Ford Mustang. I just got the intake manifold fixed. My boyfriend drove it today and said it shakes when you pick up speed. Also, the engine light came on. Why?

Do a Web search under the terms (Your make, model, year vehicle) intake manifold runner. Or intake runner control solenoids, rods, etc. You may find Youtube videos.

The runners are sandwiched under or inside the intake manifold. Some had plastic clips which crumbled and the operating rods became disconnected. Some runners stuck too bad for the rods to move them.

Check for a possible cold start injector problem or a possible motor or transmission mount that is loose or broken.

You can get a free scan from Autozone, Oreilys, or Advance auto parts. This should indicate a problem. If the standard test scanner does not verify a problem, the Dealers have more advanced scanners which can capture intermittent problems.

There are so many possibilities that you need more tests to find the problem. Offhand, if this is a V6 engine, there were problems with intake Manifold runners and sandwich gaskets for 2 piece intake manifolds.

Stuck manifold runners could provide lousy low rpm performance and perfect high rpm performance just like you describe whether a V6 or V8. The 2 piece intakes were used on V6 engines and once you get above midrange in Rpm, it could run acceptably at higher Rpm.

I'll explain the part. The manifold runner is like a venetian blind. It is suppose to move to change drawing air into different cavities under the Intake manifold. Sometimes the system works with Vacuum and sometimes with Electric solenoids.
If you see small pods or solenoids going through the Intake, the external solenoid or pod may be your problem. The plastic clip on these parts can break.
Also since your stereo was changed, and solenoids can be Electric powered, someone may have cut the power to the Bank 1 intake runner solenoid when they installed the radio.

its your intake manifold runner. its a silver box. they all go bad. ether a corroded chip bord or the bar falls off that controls it. If you hear a rattle sound coming from the dash its a barr that fell off if no sound then corrosion. ford wants 600 to fix this but you can do it yourself without bying a new one. ill try to post a link.

http://passionford.com/forum/ford-focus-st/400597-st170-fault-code-p1518.html

could from catalytic con. or manifold when is broken.

I'd start by having the tires balanced and remounted. Rotating them wouldn't hurt either. Start there.

P2004---Intake Manifold Runner
Control Stuck Open (Bank 1)

These torque specs are from the Haynes manual (Ford pick-ups 97' - 03') This is a 2- step proccess. step-1 44-in-lbs step-2 89-in-lbs. There are 14 bolts that hold the manifold in place, tightening sequence goes as follows: start from the center( WORK IN A "X" PATTERN)

Check for vacumn leaks. Replace coil. Check plug boots for oil or water on them. PCV hose from intake manifold will get soft and collapse. Replace motor mounts. The passanger side mount is a fluid filled mount.