Troubleshooting Cause of Sudden Drop in Gas Mileage

No clue how, but if it just started happening after the PCV valve replacement, maybe there's a clue. Forgot to reconnect something, introduced a vacuum leak??

 

Be sure all the advanced tech in your Prius doesn't distract you from remembering all the usual suspects. Has your car become harder to roll? I've seen Bob Wilson advocate doing controlled downhill roll tests: you need to find a downhill where the traffic is light enough you can start at the top and roll down in neutral, trying to match all your test variables (temperature, wind, how you start off, etc.) as closely as possible, and you see what speed you've reached when you cross a known point at the bottom. Of course this is most useful if you did it the first time when your car was in top shape, to get a baseline figure. Then later on you can tell if something got worse.

My latest Prius adventure was a dragging brake. Doesn't matter how advanced the hybrid powertrain is, asbestos pads grabbing the wheel rotor will make a slug out of it just as well as any other car.

I schemed up a test that worked pretty well for me to tell when the dragging was fixed. There's a town about a 15-minute drive from here on a very little-traveled road, so I can cruise the full 15 minutes without touching the brake. Arriving in town, there's a parking lot big enough that I can turn right off the road into it and roll to a stop, again all without touching the brake (shifting into B first to slow down, and finally R below about 7 mph, which isn't any problem for a Prius, to slow the rest of the way to stop). The idea is to come to a stop after 15 solid minutes of driving but with no use of the brake, then get out and stick a hand through the wheel spokes to feel each brake rotor.

Before I did any work, my left front rotor would be somewhat warm and the right front would be uncomfortable-to-touch hot. Same test after changing the right front caliper, left front still warm, right front stone cold. Encouraging result. So I went ahead and did the left front too, and now both rotors are not at all warm to the touch when I run that test. That, clearly, is what the baseline should be for the brakes in good condition.

The key to the test is to have a safe place to make that final stop without any use of the brakes. If you end up having to use them at the last minute, the rotors will be hot from that, and not tell you anything about whether the brakes were dragging before.

I only mention brakes because they were my own most recent issue. The main point is, even in a Prius, don't get fooled into only thinking about fancy Prius stuff ... all the usual suspects can still kill your mileage the same way they would in a Corolla. How's the tire pressure? Hearing any new rolling noises? If you have a ScanGauge, Torque app or the like, anything changed in your fuel trim reading? Have you ever taken compression or leakdown readings? ... etc.

-Chap

 

"The idea is to come to a stop after 15 solid minutes of driving but with no use of the brake, then get out and stick a hand through the wheel spokes to feel each brake rotor."

Excellent advice, but please be very careful touching that rotor :>) If something is dragging, it could be quite hot.

 

I'd go back to square one and re-do the entire process of changing the PCV valve. It sounds like an air line or even some wires might have come off somewhere. As mentioned above, a vacuum leak can be most noticeable at low rpms, so double check any you can find for proper connection, especially around the coil/plug wire you might have removed.

 

no point to disconnecting your 12v, but you might want to test it.