Originally Posted by qha_vn
my 90 ls400 has below fuel consumption, normal or not? how about you?
City: 12 mpg (20 liters per 100km)
Highway: 17 mpg (14 liters per 100km)
Note: city average speed = 20km/h and "highway" (its what they call) = 50km/h
the consumption numbers base on my light foot driving style consistently in 8 years.
That's way too low. On my 1990 LS400 I get 17mpg in city, and around 24mpg on highway. I always gas up at the same gas station and using the exact same pump. That way I have reliable numbers and I always write them down on the receipt to be able to track how my MPG changes as I work on the car.
Things to check:
- Vacuum lines, especially the ones going from the P/S vacuum valve at the bottom of the P/S pump to the intake. In my case, the little plastic part where the vacuum hose plugs in was cracked, and that ruined my MPG on a cold day. I fixed it by buying new vacuum hose in a dealership (only a couple of bucks), and repairing that part of the intake where the vacuum hose plugs in. Even if those two lines look perfect, they have probably been soaked in PS fluid for many years, and at the very bottom, they may be leaking a bit. Best to change them; not terribly hard, I did it myself.
- New wires and spark plugs. I bought NGK (3764) BKR6EIX-11 Iridium IX spark plugs on amazon.com and they are absolutely fantastic, I dare say, they are part of the reason for my terrific MPG. I also changed wires at the same time: NGK (6403) TE132 Spark Plug Wire Set. Also available on Amazon and the price there was the best at the time. They are OEM, so they are properly numbered, proper length, they make changing wires job a breeze.
- New rotor and distributor. Didn't have the time to wait for OEM, so I replaced them with Bosch, which works just fine (and is cheaper). Also from Amazon.com
- Coolant temperature sensor, the one that sits on passenger side of the engine, closer to the front. Its been mentioned many times around here and costs peanuts.
- Run something like Chevron Techron injector cleaner (on special at Costco very often), a few times
- Very important, at least in my case, was to flush the cooling system, using Prestone cooling system super cleaner which can be left in the system for few days. I did just that, and my car was running 100 times better! Before that, it never overheated, never had any visible problems, but my transmission was not running at optimal temperature due to cooling system not giving me the best performance. Once I flushed it, it was like a new car.
- I replaced all my o2 sensors with Denso (OEM), and at first I did not notice any difference, but a few days later, it all seemed to have come together and all at once there was a lot more pep to my car (this was done around the same time when I did the cooling system flush and thermostat).
- And obvious things: use only premium gas, use ONLY OEM air filter and remember to clean it every month or two, change oil regularly using synthetic Mobile 1 and OEM filter, make sure your ECU capacitors have been changed...
Basically I now only have to touch the gas pedal it is that amazing; the only time I step on it a little harder is on a very steep uphill when I am passing someone, or when merging and have to get from 20mph to 65mph in an instant.