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Sudden MPG drop

860 Views 30 Replies 7 Participants Last post by  ganda1f
Hey everyone,

I recently changed my CVT fluid (using the proper Honda fluid) and serpentine belt and afterwards I’m seeing a 4 - 5 MPG drop (from 48 to 43/44). This is over the course of two tanks.

I made a mistake refilling the CVT, underfilling it about a quart. When I started the car to drive it and warm it up, I could hear some not too loud screeching from the engine bay, so I stopped right away. All in all I drove maybe 500m. I realized and corrected my mistake, which resolved the sound and driving feels normal. It’s slow to creep in D from standstill, but maybe it was like that before, I’m not sure.

I’m a little worried about the CVT though. Did I damage it causing the loss in MPG? Could it be caused by the new serpentine? Could removing the wheel and reattaching it cause the wheel to drag? Would love to hear what you all think.
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I indeed think the 33 might be a bit under, but 45 to me feels like it would really be over. I know some others on the forums here run it, but I’m running 40 now. It already feels like having to do an evasive maneuver in the rain might be risky, so I’m actually considering lowering it a bit and seeing the impact on my mpg. Not sure how the weather is in Poland, but I wouldn’t like 45 psi on frozen winter roads myself 😬.
I landed at 40 psi by experimenting a bit and making sure that the entire surface of the tire made even contact with the road. After a longer ride (say ~30kms) on a clean road, typically a highway, the tires will have a pretty clear color difference where they touch the road and where they do not. I started looking into this when I discovered increased shoulder wear on the tires already after about a thousand kms with 33psi. So I really have no idea how the h3ll Honda came up with this 33psi cr4p.
I once ended up with 45psi accidentally because of a heat wave in the summer and already then on dry road the handling went bananas. Maybe during "normal" summer weather I'll try 45psi but I don't think running it in the rain or winter would be safe enough.
 

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Regarding MPG/KPL, my 01 CVT used to clock in over 50 most of the time with: A) careful but not pedantically so driving working for me; B) the topography of my landscape is pretty hilly which is a mixed bag; C) my IMA was weak, I had no belly pan, and didn't monitor my tire pressures carefully.

When I had 2 different people use the car for many months at a time, driving it without any significant understanding or concern for the details of how to maximize efficiency, the MPG tended to range from about 43 to 48 per tank. I personally occasionally slipped down into that range, usually when hurrying (maintaining high average speeds) on long trips that included some significant hill-climbing. This

While I understand the principle of higher tire pressure being good for MPG, my own experience has been that above some reasonable number (depends on the design of the tires and the weight of the car), the returns are diminishing. In my insight, I tended to run in the high 30s and didn't see any measureable increase when I pushed it over 40. When I might have let 1 or more tires drop down into the low 20s, I do believe I noticed.

I live in the mountains of the southern end of the Rockies (NM) where snow and ice have become fairly intermittent over the last two decades, but am still fairly aware of the need for decent traction when those conditions emerge and do find that higher pressures (over mid-30s) on that care yielded some noticeable loss of traction. I have a particularly icy, shaded, windey hill which on occasion becomes wicked-treacherous and the smallest difference can matter... I have deliberately dropped the pressure in my insight to about 20 to be able to navigate those conditions with a little more control and it the extra softness and traction area definitely helped!

My Chevy Volt is quite a bit heavier and I find the difference both in MPG and in traction to be less dependent on tire inflation, but then I am running Bridgestone Run-Flats which may not suffer is much with lower pressure given their very stiff sidewalls? The Volt has much more precise instantaneous instrumentation... especially with My Green Volt monitoring the current draw and speed (and therefore miles/kWh) and I find that under similar conditions/terrain that driving style significantly dominates mpg, including acceleration/deceleration and speeds. Some of my best efficiency actually comes from climbing a fairly steep 9 mile hill from my home at 40-50 mph, driving a few miles around a fairly flat mesa-top townscape at under 25 mph and then back home (9 more miles with a 2000 ft drop) at 40-60mph.

When I commuted the same trip in my Insight, I didn't do quite as well, probably because the downhill run was pretty much 15 minutes of "idle" without a big enough battery to return more than a little of the energy from the climb through regenerative braking. Occasionally I would "hypermile" this route, but the power-steering/brakes without engine were problematic for comfort/safety. I suspect I could have done a great deal better with an MT on these routes.

Previous to my Insight I hypermiled/coasted my 84 CRX-HF the same commute up to 70mpg per tank. I don't think I ever broke 65mpg on a tank with the insight, but beat the CRX on all other routes.
 
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I don't record these (outside my faulty anecdotal memory) but a reasonable estimate is that I probably arrive home with about 5kWh/10kWh useable from a nominal 25mile RT (including some "around town". Under the best conditions, the climb consumes more than that, I regen about 1kWh on the return... so about .5kWh/mile...

The run from my home to my (former) workplace and/or the downtown location of the small mountain town is nominally about 11 miles with the first and last mile being only slightly uphill. I hit the top of the hill with more than the 40% retained by mountain mode, showing 4 or 5 (of 10) bars on the dash... depending on circumstances, I may or may not trigger mountain mode before heading back downhill...

That first/last mile on the mesa-top is too flat to coast the whole way back down, but the consumption is very limited for that short run... sometimes that is when mountain mode kicks in if I allow it and then the minimum .10 gallon gets burned... but I avoid that naturally.

Your question has me wanting to track those three (six) legs more specifically. Unfortunately here in mid-winter I never (as of this year) get a clean run without triggering the ICE in some combination of 1) almost immediately; 2) early in the climb; 3) the duration of the climb; If I (or the cold-mode) run the ICE the whole climb, it still slips behind with some high RPMs on the steep parts.

I'm looking into the battery-heating system to see if that is failing (intermittently) because the cell-by-cell performance monitored during and after use/charge doesn't show any obvious issues.
 

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Discussion Starter · #25 ·
Man, I really like this car. I was finally able to take it on a longer drive, by myself, going to work and back. All in all adds up to a little over 190 km and, this time, included going into the city to pick something up (which took half an hour or so) and some highway traffic jams. It did the entire thing in 4.3 L / 100 km (54-55 MPG) which is the best trip yet. We’ll have to see if the MID is vastly underestimating, but do far it seems pretty accurate.

Given that this was all through 0 - 1 degrees C, it’s absolutely amazing to me. I’m curious how much a (partial) grill block would help, because I had the feeling fuel efficiency kept going up even 30 - 40 minutes into the trip. I can’t wait until it’s nice summer weather again to see how it behaves then.

Speedometer Odometer Trip computer Gauge Car
 

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Discussion Starter · #26 ·
Alright, final update and confirmation. I filled up the tank today (halfway fill) and with the distance driven and the liters of gas pumped it came down to 4.77 L / 100 km. The MID clocked it in at 4.7, so pretty accurate 👌🏼
 

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My experience for over a decade of checking this nearly every fill up was that it was very accurate, I chalked up discrepancies mostly to differences on just how/when I cut off the pump...

Even though it has been decades since it made sense (to me) to "top up the tank to the next round number" as my father always did "back in the day" I still find myself dropping in another fraction of a liter (.07?) now and then. I'd guess that the fill-tube is likely to hold a larger fraction of a liter if you push it.

The pump auto-cutoff sensors are likely to vary from pump to pump, even time to time?

I'm very impressed with the ability of modern computer-controlled vehicles to measure fuel consumption on the fly so accurately... it makes sense when you think of how precise the fuel-delivery via computer-injection needs to be.

Glad you are enjoying your Insight. I'm a little jealous... having flashbacks to my first year of ownership!
 

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I’m curious how much a (partial) grill block would help
Most likely nothing, or cause overheating when there is a need for good airflow through the radiator, unless your thermostat is malfunctioning and opening too early or stuck open. The cooling system is designed to keep the coolant at the optimal operating temperature regardless of weather. If it is cold, the thermostat won't open and there won't be any coolant circulating through the rad anyway.
 

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Alright, final update and confirmation. I filled up the tank today (halfway fill) and with the distance driven and the liters of gas pumped it came down to 4.77 L / 100 km. The MID clocked it in at 4.7, so pretty accurate 👌🏼
Damn, mine is almost always about 10% too optimistic. Even when I consistently filled up to the brim until I actually saw fuel in the inlet to rule out any error due to pump cut-off.
Same thing when I ran the distance over 3 or 4 tanks (some 1500kms) to average out pump cut-off.
 

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Discussion Starter · #30 ·
Most likely nothing, or cause overheating when there is a need for good airflow through the radiator, unless your thermostat is malfunctioning and opening too early or stuck open. The cooling system is designed to keep the coolant at the optimal operating temperature regardless of weather. If it is cold, the thermostat won't open and there won't be any coolant circulating through the rad anyway.
I don’t thinks this is true. Quite some people have reported improved mileage from a grill block with temperatures as high as 10 degrees C (although I have read about this mostly for Gen 1). A grill block would also reduce the amount of cold air coming into the engine bay, which can also cool down the engine, even without coolant being pumped through the rad. There’s nothing the cooking system can do about the cold air in the engine bay, it can really only start to cool when the temp seems to go too high. I’d say an ideal system would try to trap as much heat as possible (or even actively generate heat, but that would cost energy) until the motor is at perfect operating temp and then try to keep it there.
 

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If you have an engine that is designed to be water-cooled, no amount of cold air around it will have any significant impact on its operating temperature. Higher under-hood temps on the other hand, will cause all sorts of mayhem especially to all plastic and rubber elements which will deteriorate much quicker, become brittle and crack. Typically connectors, cable insulation, rubber vacuum lines. The exhaust manifold has a heat-shield in order to prevent frying everything around it. Higher intake air temp will rob the power. Yes, less fuel can be burnt in hotter air because the air is less dense, but that robs the available power. Any additional mileage gains from taping up the grill, and other openings for that matter, if at all come from reduced aerodynamic drag.
If one wants to reduce cold starts to reduce engine wear, then there are dedicated engine heating systems, such as webasto that accomplish that. However, that is more relevant during really extreme temps like -20C or so. My dad once owned a car equipped with an engine heater (that warms up the coolant and oil) that was powered from the standard 240V power grid but in the end, the cost of increased electrical power consumption outweighed any savings in fuel consumption. Tested over a period of two months with temps oscilating around +/-3 degrees C.
 
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