Help point me in the right direction for my poor MPG
 

Hey guys- 87 NA, sport model. On the past 3 fill ups of mixed driving, I've gotten: 15.1, 15.2, 14.8 mpg (fill up, reset trip, drive tank down, fill up, divide miles driven/gallons to fill). I'm doing an easy 20 at least in my 88.

So- It has:

-New tires, and allignment
-New brake master & properly bled brakes
-Injectors were removed & cleaned, then re-installed
-Engine hasn't been in the car more than 500 miles. It's a used S4 engine, but was rebuild 2 years back. Fires right up everytime, but do not know the compression figures.
-New O2 sensor, confirmed operation of closed loop with checker light
-Not pulling ANY trouble codes w/ code checker (was pulling an O2 sensor, but it has been replaced & no longer comes up)
-New thermostat & radiator installed with engine, and full shrouding. Temps are normal
-Rats nest removal, vac routing is identical to the 88
-Main cat is replaced with a straight pipe, 2 pre-cats still there, however I'm quite positive they're just the casings
-Properly timed, and idle properly set, as well as TPS, to FSM spec
-No exhaust leaks, no fuel leaks
-Good condition used spark plugs. I disassemble lots of these engines in the junk yard (cordless impact FTW), and always take the plugs if they're near new.
-New E8023 Airtex brand fuel pump and fuel filter. Looking at factory specs for this pump, it's min pressure out put is 55, & max is 95 psi. Factory pump specs are 64-85, from FSM. I have NOT checked fuel pressure @ the rail yet, but this is one of the thing's I'm speculating could be causing the problem. Will post results once I know
-Water temp sensor: NOT checked yet either. It's a possibility the wiring has a short between the sensor and ECU, as the harness was "repaired" by previous owner w/ new injector pig tails connected in with butt connectors. I removed them, and soldered in GM style injector connectors. BAC was also not working, I had to replace Transistor T801 in the ECU, as it was fried- probably from a wiring short, and finally, now this is a weird one.. With both oil press & water temp sensor wires hooked up, temp would stay at C, & oil pressure would slowly climb to operating. With only water temp hooked up, neither would budge. With only oil pressure hooked, it would read proper pressure. I re-ran the coolant temp wire straight from the wire going to the gauge, through the speedo cable hole, so that problem is solved, but it proves the wiring could be faulty on the Water temp sensor.

So- How do I check for bad wiring leading up to the water temp/thermo sensor, (back of waterpump housing), and what else could be causing this drop in MPG?

Thanks guys!

Check the voltage for that sensor at the ECU. Pin 2I to 2C, which is green/white and a black wire. I'm not certain of exact values, but I know it shouldn't read 0 or 5v, somewhere in the middle.

Checked. Between the Green/white wire & black wire (black wire isn't where it's said to be in FSM, however) I'm getting 0.07V DC. FSM calls for .4-1.8V.

To confirm- I'm doing this with the engine running, probing the wire as it enters the yellow pig tail, w/ the Multimeter set to "DCV 20". I'm seeing variations of 0.05 to 0.12 in 20 DCV setting. Pretty near zero to me

One of the reasons that we see the demise of the rotary is because of the poor fuel mileage. Back in '87 the EPA rating was 15/22 and that was certainly before they changed the testing to reflect driving conditions that are considered the norm, rather than the inflated figures that were supplied by the EPA.

Our '89 TII sees about 19/20, but the driving environment is far from what can be considered city driving. As long as the car is operating properly, It is what it is.

Yea they're not great, but with how I drive my 88 I get at least 20 every fill up. I can not get this 87 above 15 for the life of me, so there's got to be something wrong mechanically.

& yeah, it stinks about the poor fuel economy. I just look at it as- fuels still low enough in cost to not persuade me to drive anything else, and the NA's are just so damn cheap to buy that it'd take years driving an econo box (& a less fun car at that) to break even w/ the poorer economy of the rotary. I'm a die hard fan here :)

Well, let's just make sure you're probing the right wires here. When you're looking at the FSM pinout diagram, it's pictured as if you're looking at the back of the connector, or straight at the ECU without the connector plugged in. If you're still getting a very low voltage, there's 1 or 2 potential issues here. Either your sensor is shorted and sending a very high temperature reading to the ECU, or the wires are shorted together. Unplug the ECT sensor and recheck voltage. While you're at it, check the plug to see if the little pins inside have pulled back out of the connector. I had that happen to mine once.

Wow I feel dumb-- I was probing pin 2D as it's a solid black wire. Through doing some searching, on the 86-87's pin 2C (ground) is brown/black. Now add in the fact that I'm red-green color blind, & that wire originally looked like it was Red/black, so I was totally probing wrong.

Bringing the car up to temp now, with the leads of the DMM confirmed in the right pins, 2I (green/white) & 2C (brown/black on S4.0, black on S4.5), I'm getting a .49V reading. Seems like that's perfectly normal. So I'd have to say the water thermo sensor is in good shape, as is it's wiring. Cross that off the list.

Next I need to go out & buy/borrow a fuel pressure tester, as it looks like that pump out puts 10psi over what factory spec is. If that is the case, will the fix be installing a new FPR, adding a resistor to the power wire going to the pump, etc?

Thanks for the help, glad it's getting some where!

I've battled poor economy issues as well, but that was after being more retarded than you think YOU are... I installed a Walbro255 in an NA :lol: Horrible idea! If you're going to use a resistor to drop voltage to the fuel pump, you'd need a higher power rating resistor, similar to the resistors you'd put in series with injectors.

my vert kind of does the same thing, mileage was bad this winter, but it did something weird and mileage has come way up as its gotten warmer, so this is having my suspect the air temp sensors. its gone from 14mpg to 18mpg... which is weird.

in theory though if its in closed loop, this pretty much eliminates the engine and control system, and you're down to mechanical things like tires and wheel bearings.

I wish I could complain about 15 mpg's haha.

I rejoiced the other day. I got 13 mpg!!! A full 3mpg up from 10! :lol:

I'm still working the tune. Slowly, but surely, it's getting there.

Haha, well you guys are all a LOT faster than my S4 NA, lol.

OK, so I bought a fuel pressure tester- Definitely finding some issues here!

I've got the gauge between the primary rail & fuel filter. I see 40 psi w/ the IGN ON, fuel pump jumper jumped. With the engine running, I'm seeing 53 psi, UNCHANGED if I pull the FPR vac line or not. This has GOT to be the issue, right???

Another problem: The fuel pressure drops to 0 psi within a few seconds of shutting power OFF. This sounds like the check valve inside the fuel pump itself is bad, from what I've read/searched up on. Should I replace/warranty out the pump, or would it be easier/acceptable to do a check valve in-line. This definitely is the reason it takes forever to start up when hot, lol.

That's all for now-- Does it sound like my pump is in normal condition other than the check valve, being @ 40 psi at idle, and it's just a bad FPR?

Thanks for all the help!!

yeah lol... if its 40psi with the engine off, assuming -18psi in the intake at idle the fuel pressure should be 40-18= 22, give or take

Dang, imagine if you had just gotten that perfect condition stock pump from the yard instead of spending extra money on an inferior part....

As fast as fuel pressure bleed off, its supposed to stay up around 20 psi for around 15 min. I have an fd pump w/ aftermarket regulator set for 43.5psi base. Idling it drops to around 38-40. After shut down it holds 22psi for 10-15min.

Unfortunately there is more to fuel economy than fuel pressure, I have 250 miles on my new engine(s5 t2 w/ 9.4 rotors) [200 miles being highway] and I'm just below the 1/4 line. Doubt I'll make it to 300. For reference my old engine (87 t2 140k) got 17/19.

Yep, a bad FPR will definitely cause a significant decrease in fuel mileage, as well as difficult hot starts. Keep in mind, you *generally* shut the engine off at idle (unless you're some kind of weirdo that shuts it off while cruising and then coasts in to your parking spot). So if the fuel pressure is jacked up when you shut it off, you probably come close to flooding it. I wouldn't chase the "fuel pressure dropping off" goose until you've replaced the FPR. I think your issues will go away once you do that.

I wish. I drive an 84 F150 with a 351W. I get 8 mpg's at the very best.

But yes, with a bad FPR, you are killing it hahaha.

Yeah, just a little. You saw my car at the meet in Jacksonville last year. I had the v-mounted convertible with the megasquirt ecu. It's running better now :)

umm ,, fresh stock motors idle around 18 inch Hg
,, which is -9 psi , which puts most cars with 40-42 psi base pressure to 31-33 psi at idle

Swapped the rail, haven't had a chance to fill up & calculate milage, but I will say this.

From full to 3/4 I've gone 35 miles, with the old FPR that'd failed, I got here in under 15! It's definitely better, & it's a bit faster too being leaner, though all the back fires have gone away, haha.

So thanks a bunch for the help! Hopefully someone else can learn from this thread-

AGreen, ya I remember you ;). Sounds good I hope to see the car at another meet some day! Glad it's running good too, can't beat that

After reading, I'm really confused at this point. If you get 15 MPG and previously the 15 miles was full to 3/4, very odd, now using the same calculation, looks like it's now 35 miles full to 3/4, this would indicate that you're now getting 35MPG. Interesting, what am I missing?

The gas tank is not a perfect shape, it's tapered. So the first 1/4 and last 1/4 will go by faster.

Certainly not tapered that much, and the full to 3/4 is the same in both scenarios, but one <15 miles, the other 35 miles.

I don't find this to be so, here's my theory why:
With a full tank the float for the sending unit is at it's max height, submerged in fuel.
Even as fuel level lowers, the float won't move till the gas is below the bottom of the float itself...I don't know how much gas this is (easily computed by the more adventurous) but it does explain why almost every gas gauge I've ever seen sits at FULL and then seems to precipitously drop to around three quarters.

Like most automotive gauges, the fuel level is only accurate at a specific point and is increasingly worse above and below that spot.
OEM speedos are traditionally made to read correctly @60 mph and are off everywhere else.
I'd guess that the fuel gauge is most accurate at 1/2 tank.
FULL is almost irrelevant but depending on the contents of your wallet, how it reads at E will be worth learning.

actually i played with a sender unit once and the sender is actually pretty accurate, when you have it outside the car, half way on the sender is actually half way on the gauge, and etc etc.

i had tweaked the float on this car, hence the messing about, but i notice that the gauge isn't linear partly because the float hits the top of the tank before its 100%, so it doesn't get its full travel. (it may have been the other way around, it was a long time ago)

i rebent it, however use with caution! i actually got each quarter of the S4 gauge to be about 4 gallons, which is fine, but when its on E its out! the stock way gives you 3-4 gallons on E, and we're used to that.

Ok, maybe tapered wasn't the right word. It's rounded at the top, rounded at the bottom. That's what I've always chalked it up to at least, and people believe me for the most part. :lol: My s-10, however, takes forever to go from full to 3/4. After that, it starts to drop off really quick. That one, I believe, is just due to the non-linearity of the fuel sending unit. Or maybe its gas tank is actually tapered pretty bad. My Cx-7 is pretty steady from full to 1/4. After that, you'd better be looking for a gas station quick, because you'll only have about 10 miles of fuel left once it reaches 1/4.

Who knows (or cares), it just does, and we all just deal with it. :)