Help with lower intake manifold/plenum

[TXRusty]

Just removed the intake manifold on my f-250 SD 5.4 Triton. I was surprised to find a plastic insulator shield around the lower plenum that attaches to the bottom of the aluminum intake manifold. The plastic is brittle and some of it has cracked off. The truck has 200,000 miles. What is the purpose of this plastic part and do I need to replace it? It's still intact for the most part. The aluminum box (lower intake) looks okay.

[Ekkehard]

Hi,

I had exactly the same question (see my post "engine misfires, cuts out" further down the list), the insulator shield in my van (2001 4.6) had gone into "a thousand pieces". I talked to an FMC engine engineer, he thought that the cover is there for NVH-reasons (Noise, Vibration, Harshness), but wanted to verify the information. As soon as I have received his answer I will post it here.

Regards
Ekkehard

[LxMan1]

I believe you are correct, it is to help mask the intake moaning under heavy throttle.

[TXRusty]

Thanks for the input - I agree. Most of the plastic was intact and I reinstalled it as it was.