Why am I only getting 10 MPG in my SE?
 

Ok, I bought this car last week and after filling up a whole tank and only getting 150 miles on it I was like dude, wtf? I tried running some fuel injector cleaner through it (against my better judgement, I dont like putting crap like that in my gas) and it didnt do anything.

My friend thinks its my fuel injectors, he says they need to be cleaned or replaced (thats freakin expensive)

anyway, im clueless on the whole EGI thing, I was just getting the hang of carbs when I bought the SE. Whats wrong?

P.S. Im not driving it hard, I have been shifting at 4-5k and not flooring it all over the place.

http://www.rceng.com/service.htm#INJECTOR

RC engineering isn't too expensive, and they're supposed to be pretty damn good!

It could also be something else...

I would say that it needs new spark plugs, wires, rotor button and distributor cap.

Your spark plugs would be the first thing I would check. If they are really worn, it may still run however cause your gas milage to fall badly.

Good luck

I replaced all that stuff the day after I bought the car.

4-5K rpm shifting all the time is not good on gas mileage. when i had my SE engine i could get up to 320 miles in one tank of fuel because i was only shifting betwen 2.5 and 3.5K rpm. if i shifted higher than that my fuel mileage will dropped. another thing that could give you shitting gas mileage is dirty fuel filter and old air filter. if you replaced those well is the injectors.

EDIT: have you check that the 5&6 ports open freely? that copuld cause alot of fuel mileage problems too

are your sa (fb) rx7's Fuel injected there all carby here in aus :( :(

Yah your best bet would be to change air filter if you havent already (if it was the fuel filter you would have lack of fuel so that wouldnt make you waste anymore, but its good to change anyways if you havent) Next would be cleaning the injectors, shouldnt be expensive at all. If those dont change anything it could also be your 02 sensor.

If you just bought the car a full tune up is in order. Also, my GS was getting 10 MPG or so around town when I was new to it and running in 3rd @ ~3000rpm all the time. If you're just cruising, even in town get it into 5th gear, drops the revs to around 1500 or 2000.

If you shift after 3500 rpm, you are allowing the 5th and 6th ports to open, therefore taking in more air and fuel. This combined with various other reasons already mentioned could be the cause of your bad fuel mileage.

I don't know what to make of all these different answers :eek: but I can tell you my experience with my SE that's a daily driver (in the Summer).

My SE gets almost 25mpg on the Hwy.

My fuel gauge reads low, so I let it ride on E for quite a while before filling, I just have to make sure I fill up at about 350 miles or so. I usually put 15-16 gallons in it at a fill-up, so the lowest mileage I see is about 20-21.

If it's strictly in-town driving (hard) it fluctuates quite a bit more, but nowhere near as bad as what you're getting.

Are you going to the meet at Lake Nokomis on the 24th? There will be some guys there that can help you for sure, I'm pretty sure I'm driving my SE up for it.

Everybody has so far made suggestions about the obvious fuel and air inlet stuff, but you might also want to consider that you have a fuel leak somewhere that's just dripping fuel out under pump pressure - this would lead to poor fuel economy, but may be harder to find because you're looking almost entirely under the hood.

The fuel pump on the EFI cars is putting out about 35-45psi (variable based on engine RPM and power), and will quickly recycle your entire fuel supply in only a few minutes of driving if you were to remove the return line and dump it overboard. If you have a pinhole leak somewhere that only shows up when the FPR bumps up the pressure, you could be squirting fuel somewhere that it's not obvious that you have a leak.

Some places to look are at the fuel rail and injector seals - peer in between the plenum tubes with a flashlight and look for pooled fuel on top of the lower intake manifold. If you see any discoloration here, you have a rail leak or injector seal leak that NEEDS to get fixed. Leaving this area to leak allows fuel to overflow the manifold inlets an drip down onto the exhaust system - very dangerous. Also check your Pulsation Dampner at the front end of the rail and remove the plastic cap to see if there is fuel leaking here - this was a known issue with Gen 2 cars, but deserves a check to be sure it's not leaking.

From there, check both your feed and return lines going to the rail and make sure that the tubing says "Fuel Injection Hose" and not "Fuel Hose" - standard fuel hose (for carbs) will quickly breakdown and cause pinhole leaks throughout their length that leads to soaking of the hose and pressure loss. If someone replaced these hoses with something other than INJECTION system hose, you could have another fire hazard waiting to happen.

Trace the fuel lines back to the fuel pump and filter and look for any obvious signs of leakage - any sand and/or grit that's adhered to the lines, hoses, or parts will be an indicator. Also, you might try putting down some clean cardboard under your car overnight and let it sit. Any leaks will be wet spots on the cardboard to help you identify where it may be dripping out. If sitting without running doesn't show any drips, let it idle and then rev it up a few times to see if you can get some idea of where the fuel may be going.

After you've done these checks on the outside, if you find nothing, at least you know that the fuel is truly going down the intake. From there, it's a 1 hour process to remove the upper intake manifold and DEI chamber and take out the injectors - most likely, you have a 'leak-down' problem that's not allowing the injector to close fully, resulting in constant flow and rich mixture all the time. Do you get any black or grey smoke when you step on it?

I get about 17-20mpg mixed city/highway - I think that's about normal for my mods, and I tend to drive pretty fast. At only 10mph, you definitely have a problem that needs to be fixed. HTH,

did that with the plugs, wires and dizzy cap/rotor

I read that you should keep RPMs above 2k whenever possible to keep carbon build up to a minimum. In my 82 GS I got around 18 MPG no matter where I shifted (I redlined that sucker a lot, too)

I dont think that my 6-ports are working properly, I am taking off the intire intake sometime next week and cleaning the whole thing

I wasnt aware of any meet, I guess I should visit the northwest forum. Ill try and make it there.

LongDuck: Ill check for leaks in the fuel system monday, I work at a quick lube so I can check out the underside of the car really easily. Although I am going to check for under the hood leaks tomorrow before I drive the car again, I dont want to blow up :eek:

Thanks for all the help, everyone. Hopefully I can get this sucker back up to 18 MPG soon, gas is getting unreasonable ($1.79 :eek: )

I haven't been getting great gas mileage in my se either, lately. I have been averaging around 14-15mpg on total in town driving. I used to get about 17-18. I do need to change my plugs, but I was wondering if the O2 sensors would mess up the gas mileage. I know they can on other cars.

I suppose they can. There was a thread around here somewhere that told how to make an error-code checker for the ECU, It would probably tell you if your O2 sensor is bad... Ill check that out too

if the O2 sensor is bad it will throw your gas mileage out. because is not reading the air/fuel mixture and the ecu will throw alot of gas to the engine.

O2 sensor signal is only used during steady state light throttle settings and idle. The ECU takes the O2 signal and uses this in a 'closed-loop' calculation to mildly adjust the frequency and duration of injector pulse. What this means is that if your engine is idling, or your cruising down the freeway at a steady speed, then the computer is making minor adjustments on-the-fly to the injectors to put in exactly the amount of fuel that you need to burn efficiently. This is so that you get maximum mpg on the highway, and to provide for a smooth (low emissions) idle setting.

The O2 sensor is only about $27 at my local BAP, so if you're in doubt, this is something that's both easy to replace, and will make a noticable difference in your mpg and idle quality - it's worth the trouble and can be changed out in 10 minutes with the right wrench (O2 sensor socket attachment). Buy the tool, save some cursing...

At all other driving conditions, the O2 sensor signal is not used, since the ECU can't deal well with constantly variable throttle inputs and rapidly rising engine speeds and loads - these are accounted for through the TPS, AFM, and Inlet Air Temp senders to help the ECU determine roughly how much fuel should be going in to accelerate the car at maximum rate - this is not the most efficient running state, and is managed through 'open-loop' fuel mapping stored in the chips on the ECU.

Open-loop follows a pre-determined map (determined by Mazda way back when), while closed-loop uses O2 signal to make minor corrections. If your O2 sensor is bad, the engine will always be running in open-loop mode, which will decrease fuel mileage and performance, but not so much that you'd only be getting 10mpg. I think it's a combination of factors, but the O2 sensor could be contributing.

Simple enough?

well ill probably replace the O2 sensor too, since its so cheap. it has been idling a little low and inconsistant (500 RPM, then up to 800 every once in a while)

i had to replace soo many sensors in order for my gsl-se to run properly. i said fawk it n slapped on a dellorto instead. so much easier to tune n faster. eats alot of gas though...

Not to topic shift, which I am, but... DAMN I can only WISH that gas was $1.79. Its been $2.20 plus for months where I am.

Most of the Rx-7 owners in MN seem to own FC's, and cruise the FC3S forum...

Look in the regional thread over there if you would like to check it out before deciding to go. I'm definitely driving my 85 up to do a little partying in the TCs!

Hey, I called NAPA today and asked about an O2 sensor. they had one specifically for my car ($40 something) and a universal one for $21. Is the universal one ok?

gas prices are a bitch. 1.85 for 87 octane........2.05 for 93......

At least you can get 93! We can only get 91 and we're up to about $2.30+ per gallon!

Back to the guys question . . . if there is no fuel leak (but that was a good point someone made) then the engine is obviously running way too rich. Therefore anything that would cause it to run rich would be a prime candidate - a fuel filter will NOT make it run rich - high fuel pressure, a bad coolant temp sensor, or a miscalibrated air flow meter COULD cause it to run rich, etc.

The first thing you should do is get it on an emissions analyzer and check for high CO (preferably under load, like on a dyno) to confirm that it is running rich before throwing a bunch of parts at it.

Well, I filled up with gas again yesterday and I drove 70 miles. I used 1/4 tank (so 4 gallons) so I am getting about 17.5 MPG now. I know that using the gauge for gas mileage is a crappy way to do it, but it couldnt be that far off.

I didnt do anything to it except check for fuel leaks (none) and run some fuel injector cleaner through it 2 tanks of gas ago...

Ill wait and see if the problem comes back.

Note: I wasnt driving any slower or anything, I even raced one of my friends twice...