Suffocating my Prius with grill-blocking

So, I've had my grill blocked for a couple months now with no problems until today. I figured most of my trips are short and it hasn't been above 40 degrees here since November. This weekend we needed to drive up to see the in-laws in North Dakota. Though we would normally take the Honda SUV on such a long trip (about 325 miles each way), we took the Prius this time. The Honda is newer and we haven't bought snow tires for it yet. The Prius has snow tires and I wanted to see how it handled a long trip with the grill block and tires, including mpg. The weather showed very cold and very windy conditions on our travel days so I didn't expect any overheating issues. In fact, I figured that, at freeway speeds, with the heater running around 70 and outside temps at 0 or below, I was in no danger of even reaching full operating temperature. So I modified my grill blocking technique to try to hold in as much heat as I could.

We left on Thursday evening. The main part of the trip was up I-94 between Minneapolis and Fargo heading generally NW, then north on I-29 for a while. The temp got progressively colder and headwind progressively stronger until it reached about -15 F and about 20 mph sustained wind from NW. Driving 70 mph on 94 and 75 on 29, I ended up getting 39 mpg for the whole trip. There were no operating problems on that trip.

We headed out about 1 PM today with a similar wind and ambient temp around or below 0 F, now going in the opposite direction. About 10 miles into the trip with the cruise set at 75, the car starts jerking. The MFD showed the power flow alternating between full engine power and battery power, and it can't keep speed. However, there are no warning lights. I take the cruise off and slow down. It's fine for a second, but when I try to speed up, it does it again. Then without having time to analyze the problem, I decide to pull over and remove a small piece of foam from the lower grill. I take off again, there is some hesitation, then it is fine the rest of the trip.

After thinking for a few minutes and remembering a thread I had just read this past weekend, I realized that I was suffocating the engine. My original blocking technique had left several spots open. I used 1" strips of foam compressed in the top. For the bottom, I used 3/4 in foam split in half lengthwise with each half folded over on itself compressed in the bottom slats. Depending on the lengths I had left, for a couple slats I just cut out a slot in the foam to line up with the vertical supports. For others, I put in separate pieces where I had short pieces of scrap. When I was done, I had two gaps; one in the middle about six inches across on one slat, and another between where the EBH cord comes out and where the plug cover is attached to a vertical slat.

Before leaving on Thursday, I filled both those areas as well, but figured I was OK because I had some areas at the edges of the insulation and the slats where the pieces get pushed in a bit from the wind. On the trip up, I had no problem, probably because I was driving into the wind and air was being forced at about 100 mph into those gaps. Heading back with the wind at my back, the air was only being forced through at ~50 mph and apparently not enough to keep from starving the engine at higher loads. After I pulled out the piece between the EBH cord and cover, there was more than enough air to get full power when I needed it, even at high speed.

So the moral of my story is: if you've done a good job blocking your grill, just make sure it's just right if you're headed out on the highway so that you don't choke your Prius to death.

 

I also figured the engine could still breathe, so I didn't worry about packing it as tight as I could. And because I had no problems on the way up, or even merging onto the freeway heading out, I didn't worry about it. Plus, I still had some small gaps in the grill despite my best efforts. Yet I know that's what the problem had to be. The thread I was referring to in the first post talked about someone driving an old car through heavy freezing rain. The poster said he/she had to get out every hour and break up the ice on the grill so the engine could breathe. I also remembered a time several years ago when I was driving home from work in a Saturn. Between the gate and the freeway, there was a major drainage problem. After a hard rainstorm, I ended up going through a couple large "puddles" during which the engine did something very similar in bogging down then racing to catch up.

In fact, I pulled out only a 3-4 inch long piece of foam in one slat from between the EBH cord and the vertical support where I have the EBH plug cover attached. After I did that, I got a little hesitation just as I tried to speed back up to 75, but not a moment of hesitation after that. I even went up several hills after crossing back into Minnesota and did a couple full power accelerations after getting caught between merging and passing traffic. I also know looking back it wasn't an overheating issue, though that was my first guess. I don't have a Scangauge, but I knew there was no possibility of overheating with the outside temperatures and topology I was dealing with when it happened (completely flat, less than 0 deg F). When I pulled off the road in Fargo to stop for lunch, the ICE turned on as soon as I touched the "gas" pedal at the stop light at the end of the exit ramp, despite a full battery. That means my water temp could not have been much higher than ~70 C even after a long period running at 75 mph.

The only differences between my outbound and inbound trips were a) ~10 degrees warmer, but still below 0 F, and b) a difference in wind speed relative to the car. In both cases, the wind was approximately NW or NNW at ~20 mph. By rough calculation, that means I was driving up to 95 mph relative to the air on the outbound trip and as low as 50 mph inbound.

And for a point of reference on mpg, I tried to match my route and speed as closely as possible in both directions, including pit-stops. While not as scientific as some others here, I can see the clear effect of wind resistance. On the way out I averaged about 38-39 mpg, and coming back about 45 mpg with Michelin X-Ice tires. I also noticed that when I had the higher mileage and a full battery, the HSD was engaging the electric motor for about 1 sec pulses approximately every 5 seconds, at least on flat ground giving me a boost in mpg.

 

Thanks for everybody chiming in on this, I really appreciate it. I'm as stumped as everybody else, but given the symptoms, I'm sticking with my diagnosis for now.

Two things about the EtOH. I think the point of it in small quantities is to help the engine run better in the cold (apart from point source emissions benefits). It hardly seems useful to add something that could cause the fuel lines to freeze, especially when they're still selling E85 this time of year. Second, I filled up on the ND side of the river the day before, then parked with a full tank about two miles later. I specifically bought the regular gas, even though the super was cheaper (go figure) and because the super had 10% ethanol and (by negative inference) the regular did not. I also think the bad gas hypothesis has merit, mainly because I had just filled up and the car had a day to settle, but then there's the coincidence problem again.

I do see how the inverter could also be the culprit. As I mentioned, the car was switching back and forth between charging and discharging every few seconds when the cruise was on, and that could result in higher inverter temps. After about 10 miles, it makes sense that everything would be warmed up by then. It was switching back and forth also on the way north for over 300 miles, so that really doesn't explain the difference except for the force that the wind was exerting on the front of the car. Maybe that was enough to keep the inverter cool.

But I heard no fans or any noise at the front of the car when I pulled out the foam, apart from the ICE running. I crouched down to pull it, and had my ear next to the fans. In fact, the foam pieces were still frozen to the front grill and the piece I pulled out was not warm at all. I read Evan's warnings about higher inverter temps after a long climb, but he never mentioned any odd behavior arising out of the high inverter temp, only the spike when he put it in reverse. Besides, according to his account, he stressed his car far more than I did. All that arctic air seeping into my compartment that would feed the engine would also act to cool the inverter.

I will keep an eye on it, and if it happens again, I will take it in and post the resolution.