I'd try to be concise, but I think it's intellectually impossible for me.

I've driven a stock (except clutch) '86 S4 NA Sport for ages and the around town MPG has always been a respectable 23 to 24. Over the summer I repaired or simply replaced due to age the following:

- alternator (re-mfg)
- water pump (re-mfg)
- added a fuel check valve just past the fuel filter
- replaced every piece of vacuum line I could find and followed a marvelous color-coded diagram that is posted somewhere on this site in order to reconnect them properly
- had all injectors cleaned/tested at Injector Rehab
- new gasket when reassembling the intake after injector work, of course
- cleaned every ground to fix a starting problem (thanks guys)
- performed the 4 tests of Deceleration Control System per FSM (fuel cut, TPS = 1 volt, anti-afterburn, dashpot). More on this later.
- timing check
- only 20K on air filter and it looks pristine
- tire pressure good and brakes not dragging

For many, many years the car has always been a lengthy crank, hot start yet turns on a dime when cold. No big deal as it ALWAYS starts without the need to have a manual fuel cut switch. Since my "repairs", when I step on the clutch to shift during acceleration, the deceleration of the engine has developed that bub, bub, bub growl that was never there in the past. In addition, my mileage has dropped to 21 MPG. I have run several tanks at this consistent rate.

I know I'm acting more than a little nit-picky in asking for help with such a minor problem on a fine running car, but I've run out of ideas and this still bugs me. If I can find the color-coded vacuum diagram that I used I'll post it to see if you guys agree that it is correct.

Thanks for any leads you can offer.

 

I still don't understand how people say they're getting 20+mpg in town. I'm getting 10-11mpg in town and 20 on the highway on an otherwise stock s5 N/A (Exhaust + intake). I'm getting 187 miles/300kms on a tank...

 

I'd like to get that gas mileage, too.

 

OP>> Not sure what to say.. Double check for any possible vacuum leaks?

Something is wrong with your car... EDIT: I just read your other post: https://www.rx7club.com/2nd-generati.../#post11253480
~_~ Stop going WOT between lights and you'll "fix" your problem :P Drive it like a normal car and you'll get EPA ratings pretty easily. Save the fun driving for fun times!

 

What's your TPS resistance reading at idle, with engine/thermowax fully warmed up? Should be right at/as close to: 1000 ohms (1k ohm). Start there, if it think's the throttle's more open than it really is, I could definitely see it having some decel combustion going on, & not hit fuel cut.

Also, post some pictures of it in my sport model owners thread, in the non-tech 2nd gen forum

 

I know I'm being really picky, Ryan and Tom. After all, we're not driving hybrids for mileage. But let me say there are times I really drive my seven and I don't expect that 23/24 mpg and then there's the more typical times I'm rather gentle. I don't want to hijack my own thread with another discussion, but 10/11 mpg unless you're on a track is indicative of an issue. I hope to god you didn't mean 10/11 kpg!!!!

A new vacuum leak? Maybe, but I can't find one.

The TPS adjustment to 1K ohms is a whole 'nother topic. I've followed another documented procedure that deviates from the FSM where one sets the running TPS voltage to 1 volt. Your comment motivated me to read some more posts and I'm not sure what to make of them. It seems like there are committed believers in both camps (1K ohms vs. 1 volt). I grabbed a procedure from the following link and I will see where my car sits with ohms.

.... picture on the way, Landon

https://www.rx7club.com/2nd-generati...-101-a-235007/

Thanks for the ideas, guys.
Pam

 

23-24MPG in town?

i'm calling bullshit. must be too much plastic surgery is affecting the brain. even the beamer doesn't get that and it's a sluggish 4 banger.

only way i can envision it is if everyone gets out of your way and you do not have to wait for any stop signs or lights. but you are Pamela after all so maybe they do give you those priveleges. even so, city mileage is erratic, even a single stop light out of order will **** your mileage. that's why i don't EVER bother figuring out mileage in the city, get figures on the highway which can easily be repeated.

i think you're worried for nothing. if you're really concerned then double check for vacuum leaks, making sure you didn't miss a chunk of old gasket material and ditch the fuel pump check valve, not necessary.

as everyone else will tell you these cars are unforgiving with economy, if it runs better than it did then that is a pro and not a con. it is still a sports car after all.

 

I have fun when I drive ALL the time. Why would people own these cars if they don't rev it up to 5-8k rpm every chance they get, aha.

Also, Pamela, if that's your real name. I'm only jealous is all.

 

Ryan>> lol, Touche! I'm just a bit cheap, so a $500 gas bill every month would be too much. I drive around the city all day at work [7 places average] and only rev up at random times [few times/work day], my average is at 20.34. But... A tank that's 100% city and stop/go I got 16.32

 

I may not be very concise, but I am rather good at math. It is reasonable to be suspicious of 23/24 city mpg, though. What I call "city" is just my driving around the suburbs where I live and it is typically a mix of city/highway. Still, I know I'm getting that mpg or else every station pump I've refilled at has been gracious in giving me more fuel than the meter charged me for (that will be the day). In the past I would refill at about 330 miles and now its 300. That's more info than even I find interesting, but the most popular comment seems to be a polite "live with it, I'm lucky as it is".

Regarding TPS adjustments, it's such an easy thing to do I'm going to experiment for a tank or two with the TPS set by resistance instead of volts as I discussed above. I was curious as to whether they would produce different settings and they are vastly different. Right now my re-tuned 1k ohm adjusted TPS is registering .74 volts with the initial set coupler jumped and .73 volts without the jumper.

Thanks for the idea.

 

Strange I have my tps set to 1k ohm and at volts reads almost the same as what u said .74v. Im Looking forward to your results.

 

I wouldnt say bs to the mpg. My s4 has 214k on a stock setup and no engine rebuild ever and i was getting about 24 city. Atm my car is down due to the omp lines. When i took the intake off to replace em my lower had a 1/4" blowout and my plenum gaskets had to be grinded off they were so hard. Ontop of that my vacuum lines had turned to pcv plastic. They all had to be shattered off. My injector orings were no different. Now with all this i got 24mpg city ontop of these obscene vacuum leaks. Try to explain that

 

Wouldn't getting ~330 miles on a tank be something like 19mpg? That's assuming you weren't bone dry. 330 miles/ 17 gallons = 19.4 mpg. Unless you filled up when you still had 1/4 tank left.

 

I do get 15 mpg on my 87 NA. Like I said in my other thread, I drive it like I stole it. I cannot do a lot about the gas mileage its always 15 mpg, no difference if I am accelerating easy or just revving it up on every occasion and winding out every gear. Today we did a few 120 mph runs where the engine stayed a really good amount of time over 7krpm. I am pretty sure the gas mileage will stay at 15mpg. Thats 350 km on one tank +- 55 liters.

But one single time I managed to get 21 mpg, that was a late night drive back from a meeting I did 503 km in total with one tank , It blew a coolant seal and I had to stay under 4k rpm and I was low on coolant too. The seal is still blown but after some changes to the cooling system I can drive it like usual.
Edit: Oh, the 21mpg was 100% highway driving at like 3AM .

 

This is EXACTLY how I drive. I got 20mpg over a 600km drive 100% highway and two pee breaks/tim horton breaks. Then I drive like I stole it in the city and get 10-11mpg. I got 10.3mpg on my first fill up for 55 litres and drove 225kms and then 47 litres for 230kms.

 

point is, it seems most cars that are getting better than average mileage in fact are not running properly.

 

Maybe, I havent tampered with it. I however do regular maintenance. What are you suspecting in particular thats making it better gas mileage than average ?

I do know that the stockers do run pigrich and I already had the idea to lower the fuel pressure a bit to avoid running rich for no reason. But how to reduce it, if its even possible, No idea.

 

I love a good debate, but this isn't really very productive even though I enjoyed the stories. Thanks to those of you that backed-up my tale with similar mileage, and yes we have concluded that rotary MPG is highly susceptible to how you drive and how your car is tuned. I was simply looking for where a couple of miles-per-gallon went after I tried to improve things.

I have gained so much knowledge from you fellow RX-7 fans and your comments spread throughout this site, I'll be happy to chime back in after I drive a while with my reset TPS. Maybe it will be of use to someone in the future. For now, I'm headed off to Landon's state for a break with all of the other tourists (but at least I'm flying 1st class).

And just to tie up the loose ends, Ryan, I've got a 16-gallon tank, my low-fuel light hasn't worked this century and I just fill 'er up when the needle nears empty with a bit over 14 gallons. I'm sure my simple math still holds up. And I just love the conclusion that my better than average mpg's are due to my car not running properly. Priceless.

I'd certainly welcome a discussion on where our mpg's can disappear too. We're all paying $4/gallon (Canada, not sure) and you really lucky CA folks. Is that $5.25 now?

Pam

 

That makes more sense because I assumed all FC's had the 18.5 gallon tank. I didn't realize the S4's had a 16 gallon. Gas where I live, a walk away from the Michigan border, is at 1.379 a litre, so about 5.22 a gallon.

 

vacuum leaks, improper timing, semi fouled injectors and misadjusted TPS will actually increase mileage but performance will suffer. as will stuck 5/6th port actuators(above 4k).

removing the catalyst and leaning out the mixture(vacuum leaks will also net the same result) will get you the best economy, but i'm not advocating that so take it for what it's worth. this is the only scenario that actually gives a performance increase as well as better mileage.

most of this mainly applies to series 4 which are a bit richer mapped than the series 4 which is what it is right out of the box. if you got a magical one that gets excellent all around mileage you should never complain.

 

I had originally set my TPS per the 1V method, but changed to the 1kohm, just liked it better haha. I hope this is fixes it!!!

Have fun on your vacation, definitely keep us posted on the out come

 

The difference in mileage could be attributed to the different gas in the hotter months. I believe they put more ethanol in the gas in the summer. This can and WILL lead to less mpg.

Rob R.

 

Maybe running lean. Poor performance though in sure. Change the fuel filter maybe

 

Just to close this discussion out from my view, I ran a couple of tanks with the TPS set to 1k ohms instead of 1 volt. I'm sold on the resistance test. The bub, bub, bub on deceleration is almost completely gone. It makes me wonder, though, if a factory setting would have been even higher than 1k ohms. It's just a symptom and not a real problem, but would the little remaining deceleration combustion go away with an even higher setting than 1k?

In addition, my mileage hit the 22 mpg's for the first time since I did the work mentioned at the start of this diatribe. Colder weather in MN always costs me a little mileage and the car is now in storage until spring. Yet, I can't help but ponder some of these things. If anyone has actually built or bought the Mazda TPS tester with the two lights, I'd love to hear what the ohm's are on the TPS after using the "approved" test device. That number could change the tuning specs for all of the RX7Club contributers (or maybe they wouldn't believe us).

 

I always use the resistance method, much more precise imho Engine warm, set it to 1kOhm and call it a day. Maybe clean the contacts if I feel like doing it