I've got a 1998 E150 with a 4.2L V6 that's giving me fits. I had a new engine put in about 3 years ago, a Jasper installed by a local shop. It's done good til now. It's giving a P306 code usually, though has been a 305 and 306. It has no power in general and in the upper rpm range starts bogging down and ticking, almost knocking. That's when it lights the engine light and the code. I can reset the code but it'll come back . It also has a hissing kind of sound like a vacuum leak. I've done plugs, wires, fuel filter, air filter, coil pack, fuel cleaner/additive and it still does it. I checked the intake runner controls and they both seem fine, tried checking for vacuum leak. What else??? Thanks for any help.

 

You have a misfire on #6 and or #5.

This is where I'd start.

It sounds like you may have a bad intake gasket. Spray some carb cleaner around the base of the intake and see if the idle changes. If it does, you found a leak.

Also a bad injector can cause a misfire. Clear the code, swap #1 injector with #6 and see if the code changes.

 

I took injectors out of 4, 5 and 6 and cleaned with carb cleaner. I had already put some Lucas cleaner in the gas but I took them out and cleaned them also tonight. Swapped them around and it made no difference. I took out inj. 6 again and swapped it with #1 and still no difference. What else besides intake and/or gasket is there? I did spray starting fluid around and found no leak.

 

Is a leak more likely to be between upper and lower manifold or between lower manifold and block?

 

Check for coil and plug and wires issues as well.

 

Lower mani and head.