I get 19 MPG in the city on my 92 SC400 and 21 MPG in the city on my 95 SC400. Highway is a little better. I would suspect o2 sensors. MPG issues can be difficult to track.

 

Upstream O2 sensors are brand new. I haven't looked at the downstream ones, but they don't really do anything other than tell the ECU they're connected, do they?

I'd be stoked if I could get 19-21 mpg city.

 

Every o2 sensor in the SC does the same thing, just in different areas. An o2 sensor communicates with the ECU and from there the ECU can calculate the proper air to fuel ratio mixture in all circumstances, whether it be in cold weather, or at 14,000 feet above sea level. If you have a faulty o2 sensor, chances are your gas mileage and performance will decrease, and I know for a fact that all Lexus vehicles eat o2 sensors like candy. I'm not sure if 1 o2 sensor could create such awful gas mileage but IMAO I think it's worth a shot.

 

Perhaps, but it seems like a long shot and likely waste of $100+.

1) O2 sensors tend to go bad gradually, not suddenly like this mpg drop.

2) Why would highway mpg be right at spec and city mpg be 25% below it?

3) Everything else I've read says that the downstream sensors do little more than tell the ECU the catalytic converters are working. And whether or not the cats are working doesn't/shouldn't affect the air-fuel ratio. It's the job of the upstream sensors to report back to the ECU for that.

I guess the downstream sensors would be a lot easier to access for an experimental swap, but I'd have to look up some pics to see if they use the same plug as my old upstreamers. I know there's also some spark plug trick, but I'm not sure what's involved in that. Is that just to plug the hole instead of having a bad sensor in there? What does the plug connect to then, and why couldn't it just be left unplugged with the bad O2 sensor? I'm thinking there must be another step I'm missing.

 

Well, I didn't get the mpg I'm getting now without replacing the o2 sensors. GL anyhow.

 

But did you replace all of them simultaneously or experiment with just the downstream ones to see which set affected mpg? Because like I said previously, the downstream ones shouldn't have much effect, if any (AFAIK).

I think it's most likely that the low city mpg has something to do with my long-crank start-up issue and workaround. As in, that is dumping fuel somehow, and it's not so much that city driving is lower mpg as it is that city driving entails more starts per mile driven, so more fuel being dumped.

 

Ali SC3 has spoken:

https://www.clublexus.com/forums/per...ml#post8714444

1) The rear O2 sensors are unlikely to be a factor here.

2) My ~14 mpg city is actually exactly what one would expect out of the V8. It's the 18 mpg rating that is off.

3) The highway rating is fairly accurate, thus my confusion over why I'm hitting that and not the city rating.

My around-town drives are usually short and the car is just warming up by the time I reach my destination, so I could see being at the low end of typical.

 

yeah the rear o2's do not affect the tune at all, its only for emissions.
14mpg average is actually a good average for a sc400 in the city. I am not sure what kind of city you can get 19mpg in and then only get 2mpg more on the highway, unless the city is like a nascar track with all roundabouts or something.

from post 1 though it does sound like you have a potential leaky injector issue. the v8 should start right up no issues every time. Not sure what the proper test is I think maybe a fuel pressure tester will show if you have a leak. most of the times when it happens on toyota's it will run fine, but when you shut the car down it will leak into the cylinder making startup a pain.

its also probably not a coincidence that your highest mpg's correlate to your highest amount of gallons used, because likely you get a better average of your longer and shorter trips. when calculating over a small amount of fill up, shorter trips can make your MPG look horrible, well because it generally is on shorter trips, and you don't have enough longer ones to average it out.

these cars run rich until the o2 sensor kicks in, which can be several minutes or longer on most 92-95 models, even then after the o2 is on and working it can still run rich when you get on it.
so if someones average drives during their tank is 30 minutes, and someone elses is 10 minutes, I can promise you that person who drives for 10 minutes is going to get much much worse MPG, on a 400 14 would be great at that point.
The person driving longer is actually going to get closer and closer to the highway rating, but I think 16-17 is pretty good for a sc400 in the city. more than what my turbo sc300 gets for sure (stopped calculating it was so bad).

and you are probably right in that you can maybe get better gas mileage by getting on it when its cold, cause its going to run rich anyways, so the more air you can suck in such as BFI exhaust etc.. should help with that until it warms up then once o2 is on you want to not step on it anymore.. I know its confusing stuff.

 

Thanks, that makes me feel better about the mpg I've been seeing, as well as helps me understand the system a bit clearer. Now I can turn my attention back to other issues like the potential leaky injectors. I was thinking about just replacing them and hoping that solves it, but it's a fairly time intensive task to embark on without having first run whatever tests are necessary. The one lead I got for a shop in town that has the right diagnostic equipment charges $100/hr and didn't sound too hopeful they would be able to solve it in a small number of hours. First words out of the guy's mouth: "Those are tough cars to work on." Uh-oh.

My mechanic tried getting a fuel pressure reading just before the fuel rail, but the fitting in the engine bay (alongside the engine oil and transmission oil dip sticks) couldn't be undone to tap in. It held on so tight, it snapped his wrench in half. He had tried gradually working it back and forth to loosen it, but there was a point it wouldn't go past. It's the same problem I had with the fuel filter, where the threads weren't cross threaded or stripped (confirmed by multiple people and multiple fittings) but it was one of the most difficult things I've ever unscrewed, and completely impossible to screw the new one back on. The old line eventually had to be cut out and replaced. It seems the factory sealant goop has hardened into concrete. Maybe I need to bite the bullet and have another section of fuel line cut out in the engine bay so a test port can be tied in.

 

that pretty tough if the cannot undo that bolt. I would have tried a lager wrench. not sure how you would go about it then other then pulling off the old ones and sending them out for testing. if you need it to happen much faster you can pick up a spare set on the cheap online or in the classifieds here and send them out for a clean and flow test and then you know they are ready to drop in. I have heard of the issue before on random toyota's including the sc400, but its not the most common one by far. other things could cause it to start up slow unfortunately its hard to eliminate without taking them off or swapping for another set. other things that affect start are like vaccum problems.
the startup routine is pretty simple though the ecu does not look up too many things but it does assume some amount of pressure in the rail as when you first crank it does a "start injection" or crank pulse of fuel where it injects more than the normal amount to get the engine to fire off. If there is no pressure in he rail for whatever reason the engine probably doesn't get the crank pulse of fuel, and you have to wait for the normal amount of fuel it injects after that to build up and warm the engine until it fires off, which can take a while you know if you have ever hard a carb car that most engines need more than the normal amount of fuel to start when cold.
because you say that turning it once and then doing it again makes it fire off, you are getting double the fuel pulse and likely by the second time the pressure has built up.

maybe you can jump in the diagnostic connector +B and the FP for fuel pump, which makes the pump run continuously, give it 3-5 seconds and then see if it starts up right on the first crank.
with the pump running before you start it you should have full fuel pressure regardless of a small leak. that 1 leaky injector cylinder maybe wont fire off but the other 7 will get it there.

 

I've got a knack for finding the uncommon issues. Yeah, I think I'll order a new set and have them ready to drop in once the intake manifold is removed. When I installed the supercharger on my other car, I had everything apart for a few weeks while working on it and was able to send the injectors off for cleaning without it holding anything up. That would be more problematic here.

The crank pulse explanation makes sense. If the pressure in the rail is dropping due to injector leakage, then that initial blip of the ignition is getting it back to normal "operating" condition. What I can't figure out is why the fuel pump ECU jumper bypass, (+b and Fp) and other methods such as turning the ignition to "On" and back off several times, or giving it gas with the ignition "On" haven't worked reliably. They're like 50/50, while the quick bumping of the starter method has worked every single time. It's like the starter has to do its job in order for the fuel pressure to build.

 

are you cranking the car for a second or just moving it to on? you have to actually turn it to where it starts cranking to get the fuel pulse. the click before that doesn't do anything. the +b and Fp will pressurize the rail if it isn't already. giving it gas with the ignition on but engine not running or cranking does nothing but move the flap in the throttle body some, remember its electronic fuel injection it will not inject till its cranking and then it will inject as long as its still running.

moving the throttle around may seem to help or change how it starts but really what you are changing when you have your foot on the throttle and starting it is how much air is entering the engine, not really the fuel, this is for starting purposes.

 

Hmm, interesting question. The stuff I found on various ways of building fuel pressure weren't SC-specific, and may not have been referring to electronic fuel injection. I thought the idea was that turning the key to "On" activates the pump, but again, if that wasn't SC-specific, it wouldn't take into account the 2-speed pump.

But even with +b and Fp jumpered, it seemed hit or miss if that built enough pressure to make it start up right away. Doesn't that just bypass the pump ECU and make it run full-speed? As in, it wouldn't change the start-up behavior, would it? Maybe it would pressurize the rail a bit faster if the normal mode is slow, but I thought the pump ECU only goes to slow speed when less fuel is needed, i.e. cruising?

Does the +b / Fp usage require waiting a certain period of time with the key in the "On" position, maybe? I don't remember if I experimented with that, specifically.

 

lets clarify, the fuel pump does not turn on normally till the car is cranking, with the key to on, the pump is not on. the factory fuel ecu is designed to not run the pump with the engine off as a safety factor. it assumes you have fuel pressure from the last shut down. sometimes cars "prime" the pump by turning it on for a second or 2 wheny ou key on, our car's really do not do this on the factory ecu much if at all.

so by jumpering those 2, you are telling the pump I want you to turn on and push fuel through the rail, even though I am not starting or have the engine running. the pump will turn on and you will hear the fuel wooshing around with the engine off, this "primes" the system so even if you had a leaky injector you should have full fuel pressure when you fire it off now. so while the pump is going you start it and it fires right off, then its likely a fuel pressure or pump problem or even fuel ecu problem you are encountering.

also to note if you have a bad enough leak you wouldn;t want to run the pump on its own for too long, you could flood it out eventually.

 

Ok, so I was able to test your theory out a couple times today. I jumpered Fp and +b, turned the key to "On", waited a few seconds, and let 'er crank. Both times, it started quickly. Very comparable to the second crank on the "quick bump" method. Assuming that holds true for a few days, it would seem to confirm that the issue is fuel pressure dropping. I can't remember if I specifically tested that sequence before or if I was just trying an immediate cranking with the pins jumpered.

I could hear the pump running when doing so, but couldn't hear the fuel whooshing around the front half of the car. Then again, the car is so heavily muffled, I could barely tell when it was running with the stock mufflers!

So, I guess the big question now is, does that confirm it as being injector leakage or does it open up the possibility of it being a fuel pump ecu issue? I'm thinking no to the latter, because a properly running pump ecu wouldn't affect anything happening prior to cranking.