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The 900 Mileage Writeup Posted by Saana88 [Email]  (#207)  [Profile/Gallery]
(more from Saana88) on Sat, 13 Oct 2007 09:51:33
In Reply to: fuel mileage, mac townsend, Fri, 12 Oct 2007 18:11:59 Members do not see ads below this line. - Help Keep This Site Online - Signup |
Mac,
Fuel mileage (thank you for taking the time to spell it right) depends on a number of factors. The amount of fuel being injected into your engine is governed by the throttle opening and what the computer does with regard to that information. In my '88 things are a little simpler, but the concept is similar. My '92 is virtually the same car as you drive, but with a little extra weight. By this time, I assume you've read the usual advice of not drag racing, avoiding cold starts/combining your trips, and all the usual information.
As you've guessed, the better an engine is tuned, the smoother things will run. Oxygen sensors DO get old. After around 60k, they tend to still generate information (this is why your Check Engine light is dark) but they don't react quickly enough to do any good. This is known as a "lazy" or "drunk" signal. By the time the sensor changes to generate the "rich" signal, that condition may not even exist anymore. Only as soon as your oxygen sensor will react can the computer change the mixture to give you optimum energy extraction from every bit of fuel. A sticky or hesitant AIC (idle control) valve will not help here, nor will an intake manifold, fuel injectors, throttle valve, or air filter in need of cleaning. These things can strangle your car. Since the air mass meter (which should never need cleaning since it self-cleans every time you switch the car off) is upstream of the majority of the intake, if these pipes constrict you end up with air restriction and a slightly rich mixture. Just get a suitable solvent and wipe out the throttle valve every spring and fall and spray it in your idle control valve a few times. A can of fuel injector cleaner once a season will keep your injectors clean. If it has been a while since your car has seen any fuel (cleaning) additives, get some serious stuff (I use BG 44k from dealers) and run that through, then use the opportunity to change your fuel filter, having disturbed and filtered out any deposits which may have been there. 900s are picky about their plugs, so use only NGK BCP5ES (EV for harder driving or longer life) at the correct gap in your B212 motor. The correct gap is important here. Working upstream, worn contacts in the distibutor cap and rotor will weaken your spark after a while. Use Bosch parts here because they are cheap, well-built and reliable (copper contacts instead of tin). A restricted fuel filter may also make the car labor more than it should and shorten the life of your fuel pump. A sticking thermostat (average service life: ten years) will either overheat your already head gasket-sensitive engine or keep it below normal operating temperature, which drinks fuel. While you're loving your car, check that the coolant expansion tank cap still holds pressure, just because an intact head gasket is a good thing to have. A leaking fuel pressure regulator will start out a little on the rich side and eventually fail outright, forcing the engine to full rich. Average age of an FPR for me: 150k. And as always, vacuum leaks will do all sorts of funny things with your engine mixture. I replace my vacuum lines every 2-3 years. Pay particular attention to the line from the valve cover to the intake, since oil vapors tend to liquefy this hose. If you have a California-model 900, it may have EGR, meaning that the EGR might not be working. Thrust: check.
Now that everything's running fine, move on to the other factors. Lift and drag are more important over 45 miles an hour (maybe 50 with a 900). Take off any wind-resisting accessories and get an air dam (front spoiler extension) from a turbo in the junkyard. I still don't see any significant difference whether my windows are up or down, so use them with discretion. And of course, make sure your brakes are not dragging. The rears especially tend to have this problem in '88 and later 900s. Be sure the piston and caliper still slide freely and that the parking brake releases all the way. I had mine too close for a year and it was 2-3 MPG. Lift and drag: check.
And now the most often overlooked component for better mileage: weight. New cars need to go on a diet. I have removed the (non-functional, too expensive to fix) AC systems on my 900s. One is a convertible (who needs this?) and the other has four gigantic windows. I live in northern New York and can put up with summer for a month or two. AC adds a significant amount of weight and drag to your motor. If the system is discharged, disconnect the power to the compressor and pull the belt. AC adds around 80 pounds to your car. (This is why AWD cars are not as efficient- it's a 350 pound surcharge.) Beyond that, there is a reason a base 900 went from 2600 pounds in 1983 to 2800 pounds in '93: fluff. Power windows, mirrors, seats and a sunroof all add weight to a car. Even the extra two doors and parcel shelf add a little weight for our four-doors over a hatchback.
If you keep the engine speed down and don't use the throttle any more than you have to (obeying the upshift light like I do in my '92 SC) you should be getting more than that. I write down my stats with every fuel filling and do the math and keep a running average on both of my 900s. Where you live is important. If you are in the middle of beautiful stucco sprawl and have tons of traffic lights and stop signs around, this will make the difference between 27 and 31 MPG as it did to my four-door when I was in college. Another fuel saver is to use your engine only when it's necessary. I'm not saying you should switch the engine off going downhill in neutral and then restart it with the clutch (the electronically-injected engines don't like this and they will idle high when you do). Instead, only use as much fuel to get around as is necessary. Try to slow down early for lights that are about to change so you're not starting from a dead stop. The less that you use your brakes and the more you keep moving between disruptions, the better your mileage will be. Coast whenever possible. And if you find you need to brake, mix it up keeping in mind that your engine computer switches the fuel flow off entirely (or nearly so) when you take your foot off the gas over 2000 RPM and will continue down to just above idle.
And yes, I get reformulated gas too (moreso in the summer, but also in the winter) and it stinks.
, Tue, 16 Oct 2007 19:42:56, Sat, 13 Oct 2007 09:51:33 <-- Viewing This Message, Sat, 13 Oct 2007 20:37:58
, Sat, 13 Oct 2007 09:43:33, Fri, 12 Oct 2007 18:45:05, Sat, 13 Oct 2007 00:43:52
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