About 2 weeks ago my Service Engine Light came on while driving to work in my 2001 F-150 4.6 4x4. On my way home from work I stopped at the local Auto Zone to have the code read and it was P0300 random multiple misfire and P0304 cylinder #4 misfire. After a few more days of the truck hesitating and running rough I searched this forum and concluded the P0300 code was most likely from having the origional spark plugs with 108,000 miles on them and it would not hurt to change them and the P0304 was most likely the cylinder #4 coil on plug problem. I have a million projects going on at home and decided to have the spark plugs changed at the local ford dealership. The plugs were changed and when I picked up the truck it ran 100% better than it did when I brought it in. It ran great for the next 2 days or 200 miles or so and the Service Engine Light came back on. Figuering the new code was the P0304 because nothing was done to the cylinder #4 COP I stopped back at the local Auto Zone and had the codes read again and they were P0304, P0174, and P0171. The truck is once again running horrible just like before the new plugs. This weekend I cleaned the MAF sensor with electrical contact cleaner, installed a new air filter, checked the entire engine compartment for vacume leaks and found none. What I did notice was what looked like black plastic shroud that was broken under the intake manifold? I do not know what this is but I was able to get my hand between the aluminum intake runners and get most of it out. It is a black plastic material that is very brittle. Any ideas on what this is? I also sprayed around/under the intake with starting fluid while the engine was running to see if there was a vacume leak and I heard no increase in idle speed. Any help would be greatly appreciated regarding this matter. Thanks.

 

Well, you still have the #4 misfire (probably the COP).

You also now have the infamous vacuum leak. Since you didn't have it before, blame it on the dealership and get them to fix it. Otherwise, fix your PCV hose, particularly the elbow where the hose connects to the intake at the base of the throttle body.

Steve