peterls

So, I bought my 1990 ls400 about six months ago. I live in an area full of steep hills and I take too many short rides to have a clue about real MPG of my LS400... nevertheless, I tried to improve it, starting with most obvious things: ICV, Synthetic oil, cooling system, o2 sensors etc.

Funny thing is, I did notice engine working better and better as I changed those things, but never immediately.

Finally, I had to take a trip this past few days, and did not even have a chance to change my Timing belt or wires and spark plugs as I planned, so they are still waiting to be done.

But, during the trip, on hwy5, doing around 75mph using cruise control, I got to almost 24mpg (23.75 to be exact). Not to mention that power and pickup from this 23 yr old car is just incredible. The funniest part is - it improved slowly over the past few weeks, mostly after I changed everything that had to do with cooling: thermostat, temperature sensor (no immediate difference though, only now, weeks later), O2s were changed (all 4, and no immediate difference either), and transmission shift solenoid, fluid and filter were also changed. (Using red line AT fluid right now, but will change to OEM at some point in the future).

My 1990 LS400 has a bit over 150k miles, and gets almost 24mpg, weeks after anything was done to it. I could tell that during that period of time, it has been driving better and better. The engine is just incredibly smooth, powerful, and just goes - both from stop or at hwy speeds. Bascially, something has happened when I changed all these things, and I don't know what - but it is great, just would like to understand it a bit better... I am thinking that perhaps synthetic oil in combo with coolant flush that I did, cleared things out and now allows for engine to work at optimal temperature in all areas.

IF you have not done a coolant flush yet, do it, don't delay. I did mine over a period of few days - from Monday to Friday, I drove with the prestone coolant flush and water only. Then I flushed that, and replaced it with Toyota red coolant at 50-50 and that was that.

Maybe the ECU took a while to adjust to all those changes - 02, cooling, sensors etc and now I am reaping the benefits?

LScowboyLS

yes, the ECU remaps its fuel trim and other factors in response to your driving style and the new improved information from your O2 sensors, etc.

Lighter weight oil helps mileage as well.

If you haven't replaced your Engine Coolant Temperature Sensor with the real Toyota part yet, do that and increase mileage even further! (around $50)

671HOT92

Haven't done those yet but planning to do plugs, rotor, cap, coolant, wires, and the exhauast is plugged, any ideas on exhaust not to loud, thinking about simple turbo, any thoughts. I was thinking it might be ecu, after a couple miles It starts to shake like its not engaging but after it cools down it will be fine for the next few miles till I get home. Gnna do exhaust first then see whats next, Any thoughts lmk, tks. By the way had it for about 4 years at 80000 mls, now has 90000.

LScowboyLS

it is rare for an LS400 exhaust to become plugged, so don't assume it is until you have done a proper back pressure test - what year is your LS?

671HOT92

Haven't done those yet but planning to do plugs, rotor, cap, coolant, wires, and the exhauast is plugged, any ideas on exhaust not to loud, thinking about simple turbo, any thoughts. I was thinking it might be ecu, after a couple miles It starts to shake like its not engaging but after it cools down it will be fine for the next few miles till I get home. Gnna do exhaust first then see whats next, Any thoughts lmk, tks. By the way had it for about 4 years at 80000 mls, now has 90000. It gets about 20-22mpg. Only done regular oil change royal purple and trans fluid. Great car R.I.P. Mr.Toyoda ,.Daddy Lexus Peace.

671HOT92

Its a March 92 prod. date. I put my hand at the tailpipe and very little pressure coming out. Feels like person blowing.

LScowboyLS

that doesn't tell you much, LS400 doesn't put out a lot of air pressure out the exhaust in the first place, and an exhaust leak, for example, could reduce that even further - do the back pressure test at the pre-cat O2 sensor port on both sides before deciding your exhaust system is plugged, they rarely ever are.

peterls

I did the temperature sensor months ago... followed by O2 sensors (all OEM), and then coolant flush, and so on... its just funny that I first thought there was no benefit, as I could not really see any, but now, months later, the car is driving better and better, even though I did nothing lately!

As you said, it could only be that ECU is remapping everything... not that I am complaining, just really pleasantly surprised at the change, and it should serve as encouragement for everyone else - DO change your parts, even if they don't seem to give you trouble, you will most likely experience BIG improvement, over time that is.

If I were to pinpoint where the change started being obvious, I would say - it was after the coolant flush. Don't know if it was related to that, but somehow I think it is. When I say coolant flush, I don't mean just - dump and replace - but dump old one, fill with prestone flush (I got mine at Walmart and it said on the bottle something like "dual action" or whatever, and it said you could keep it in the system for a few days) and water and run it like that for about 100 miles or so. Then, dump that, flush once or twice with distilled water, and then fill with Toyota coolant.
Even as I was driving with the prestone flush in there, I was noticing some improvements, although not as drastic as now.

Oh, one question for LScowboy: I measured the coolant temperature just as I shut of the engine on a relatively hot day, and it was 188F, right at the thermostat. Is that the right temperature? I thought it should be around 185F?