Let me first start off by telling you that this post is about something I wasn’t even aware could happen. Most vehicles used to have steel fuel tanks that would rust over time and leak. Manufacturers noted this and found a way to make plastic (flexible) fuel tanks a practical replacement.

gas cap

Have you ever heard a hissing sound when you removed your fuel cap which you could only conclude to be a lot of pressure escaping, specifically when it was hot out? In a newer car, you shouldn’t. This is basic physics at work. You get pressure buildup in your fuel tank from expansion and contraction of vapors.

evap diagram

Modern vehicles have sensitive and complex vapor (evap, or evaporative emissions) release systems in place to relieve the pressure buildup in the tank and recycle these vapors into the throttle body to be used in the combustion process, helping economy. A trouble code will be thrown in the event of a failure here or a leak in the system. Your local repair shop will fill the evap system with a smoke that they can watch come out of any holes in the system.

What if the system does the opposite of leaking? What if it won’t release this pressure buildup, or actually puts a vacuum on the fuel system? Here’s what happens:

vacuum fuel trank

That is an older Mercedes-Benz diesel. Clearly they hadn’t thought the idea of having the fuel tank in the trunk through completely. Now, diesels build up far more fuel pressure than gasoline engines, but this simply to show you what happens. Here’s another extreme case of vacuum on a fuel tank:

The case that brought this to light for me, was a 2011 Ford F-150 that had the following symptoms:

  • Erratic fuel gauge readings
  • Check engine soon light on
  • Intermittent misfires

The technician scanned the fault codes and came up with a P2196 – O2 Sensor Signal Biased/Stuck Rich Bank 1 Sensor 1, and a P2198 – O2 Sensor Signal Biased/Stuck Rich Bank 2 Sensor 1. The culprit?

evap purge valve

A stuck closed purge valve. This put a vacuum on the fuel tank, warping it, which was messing with the float level throwing off the fuel gauge, and creating a rich fuel condition (more fuel than air in the fuel mixture), which fouled out the spark plugs. After replacing the purge valve and the spark plugs, the truck ran mint.

This case shows you that sometimes something seemingly so minor, can cause bigger problems down the line. I can attest to this, but I’ll save that for my next post.