Hey guys. Have a slight problem here. I replaced the intake manifold in my Dad's 5.4l V8 with the Dorman intake on Amazon, and it gives me some terrible vibration problems. When the car is idling, the rpms only shift by about 50 or 100, and it doesn't jump around a lot. You can barely feel the vibration. But, when I'm driving, it shakes when I give it low throttle at around 1500 rpms. The vibrations level out as I increase the throttle above 2000 rpms. There are no lean codes at the moment. I made sure to install the PCV elbow correctly with the O-ring, and I don't see a disconnected vacuum line anywhere. All the sensors are in their proper positions. The only noteworthy thing is that I can clearly hear the air hissing when the engine is running. It's not a quiet hiss either. It's easily noticeable. It seems to be coming from the center but I could be wrong.

I called Dorman and they told me to make sure that the fuel injectors are seated properly as that can cause an air leak in the system. They said it's been reported before this specific intake. I haven't attempted it yet as it's gotten late, but I would like to get the forum's perspective on the matter. What do you guys think this could be? A cracked intake manifold. Or was the tech right, and it's just that my fuel injectors aren't seated properly. I don't completely believe that it's the fuel injectors, because I'm quite certain you'd smell the gasoline. When the fuel injectors on my BMW e46 leaked, I could clearly smell gasoline and the rpms jumped around A lot.

 

Could be injectors.... could be a coil is not seated correctly... could be a leak you missed somewhere. I assume the correct torque sequence was used?

 

Dorman Intakes are terrible, they may look exactly like the original on the outside, but on the inside they are not even close. The radii on the runners are inconsistent as well as the diameter of the runners and flashing left behind from the molding process causing air flow problems such as excessive turbulence. They have also been known to have sealing problems as well. Aftermarket parts are not made to exact specs as OEM parts are, which is why aftermarket parts usually cost half the price of OEM. The only spec that Aftermarket parts are required to meet is that they fit the part being replaced, it does not even need to function exactly like the original.