A few years ago, a colleague of mine . . . not a mechanic . . . asked me to check out his truck, because it sounded funny, and it wasn’t running right
Anyways, it was quite obvious that he had a gigantic vacuum leak, because it made a whooshing sound. I hooked up my scanner and retrieved a bunch of misfire codes, as well as lean codes. But there was also a MAF code, saying it was out of range, or something like that. I quickly verified my suspicion that the intake gaskets were flat. No coolant running through the plastic manifold, by the way
We agreed that I would fix his truck in my garage, at my house, on the weekend. The real problem was the flat intake gaskets. It was obviously causing the lean codes. But it was also causing misfires with a cold engine, before they expanded somewhat when hot. But never enough to fully seal. The out of range MAF code was also caused by the intake leak.
After fixing that, and verifying the repair, I threw a new set of plugs at it, because it was due by mileage.
The truck ran great afterwards
The funniest part, was that when I removed the intake, a bunch of acorns fell on top of the valves. I used a vacuum cleaner and needle nose pliers to retrieve them. I spent quite a bit of time, with a flashlight and mirror, to make sure I got all of them. No matter how well you vacuum an engine before removing an intake, something always seems to fall in. There is always those few seconds when you remove the manifold, before you have a chance to cover up the ports
One more thing . . . I decided to be proactive. While I had the intake off, I replaced both knock sensors and the subharness. It seemed like a good time to do it. Not to mention that the sensors looked pretty crusty, and the harness had gotten brittle over the years