Sudden drop in fuel efficiency

[mochanges]

I recently had my car serviced and had a lot of work done (rebuilt axles, new ball joints, timing belt, water pump, ATI crank pulley, new O2 sensors, fixed and replaced some worn parts in the transmission, all new coolant hoses, oil change). Since I got the car back, I've observed the mileage through 2 tanks of gas and it's quite a bit lower than before the service. I am about 20-25% worse than before. Any ideas what might be behind it? New O2 sensors?

For full disclosure I did try RS-R Ran Up and saw a boost in mileage, not sure if that really matters but figured I would mention it.

 

[JD Cross]

How accurate are your pre-service fuel mileage numbers?
Do you have good data on, say, 10 tankful's in a row?

It's not clear when you used the RS-R.
Was it before or after the service?

 

[liftnot]

Who did the work? Shad/Don or Jim R.? they are very competent w/ the NSX in No. Cal. Anybody else is a wild guess. 20-25% is a lot. The TB could be off, all the news parts(gearbox, axle) needs to break in(not likely the reason), who did the alignment?
I also use RS-R and only notice a decrease in engine noise/vibration and longer warm up time. No noticeable increase in mileage.
The weather has been pretty cold as well.
I drove mine to Louisiana for Thxgiving and it was freezing/cold temp. most of the way. My mileage went from 30mpg(summer temp) to 25-26mpg over the trip.
Other than that, how does the engine run compare to b 4?

 

[davidf]

Wow, 30 mpg. Impressive.

 

[JD Cross]

My mileage went from 30mpg(summer temp) to 25-26mpg over the trip.

A 30 mpg summer average is very impressive indeed!

My stock 91 averaged 26.4 mpg driving from Vancouver to NSXPO and back.
The best tankful was 29.7 mpg, all highway miles too.

You must have a special driving technique to get that kind of average

 

[BATMANs]

I'm guessing the OP is running winter gas.... which is known for less MPGs

 

[Honcho]

I'm guessing the OP is running winter gas.... which is known for less MPGs

+1

Also can be driving habits- you might have a heavier foot now that you know your NSX is properly sorted.

Cooler temps also mean more time in open loop and running rich.

Only other thing I can think of is your timing belt is off by a tooth or two, meaning the engine is not as efficient and has to work harder for the same load conditions.

 

[JD Cross]

A 20-25 % mileage drop is suggesting something major.

Winter gas might explain a 2-5 % drop.
Honcho's point about the timing belt is worth a look.

We need info from the OP about the quality of his pre-service and post-service mileage data.
If he has a record by tankful of his mileage before and after then you can draw some conclusions
However if his data is "notional" then there maybe nothing wrong at all.

 

[davidf]

What the hell is Winter gas?

 

[BATMANs]

http://www.howstuffworks.com/fuel-efficiency/fuel-consumption/summer-fuel.htm

 

[mochanges]

Didn't even think of winter gas, definitely could be it. The car was away for about 3 weeks, maybe the stations/refineries made the switch while it was gone?

Here are answers to the other questions:

Pre-service mileage is pretty accurate, I actually measure every tank and have been doing so for the past year or so.

I used the RS-R Ran Up 2 oil changes ago, so I had one oil change in between adding the Ran Up and the most recent oil change done as part of the service.

Shad did the work and I've found him and his shop to be awesome, super competent and very honest. I asked Shad what he thought, I haven't heard back yet but I know he's a super busy guy. I need to get the front realigned, just got new front tires yesterday so hope to do that soon. A couple knowledgeable friends also suggested the cold weather has something to do with it. The engine runs great, seems to feel a little more "free" and eager to rev though that could be totally in my mind. Forgot to mention I also got rid of the K&N intake I had in favor of a stock airbox/scoop/Unifilter combo.

I may have a slightly more heavy foot given I have more confidence in the car post service, though the last tank I was pretty easy on it. Lots of highway driving, as is usual for me. I will have to investigate the timing belt being off by a tooth. I really hope that's not it and it's the other factors suggested.

The car definitely smells rich, more so than before.

Thanks everyone!

 

[JD Cross]

Pre-service mileage is pretty accurate, I actually measure every tank and have been doing so for the past year or so.
Thanks everyone!

I measure my mileage on every tank too.
What is your actual pre-service mileage over the previous 10 tankful's?

 

[docjohn]

have you ever had your fuel injectors cleaned/replaced? I sent 96 injectors to RC and 3 were badly clogged the others crappy but still squirting...my gas mileage was not great as well......things have improved since getting these serviced.

 

[cmthomson]

"Smells rich". You do have cats, right? :wink:
Could be the intake air temperature sensor being gummed up. When mine was, the exhaust was smelly even after warmup. Cleaning it fixed various problems, not least the cold idle speed, and the exhaust smell when warm.

 

[EAC]

I get 230 miles per tank in my element (summer) and 180 miles per tank in the winter..

 

[mochanges]

Here's the next round of answers:

Previous 10 tankfuls averaged around 25-26 mpg, again almost all highway.

I haven't ever had the injectors cleaned, I should look into that. Was strongly considering doing the RDX injector swap but the ECU issues people had scared me off. I'll have to look into the cleaning.

I do have cats in place! I noticed the 1 time I had the car on a dyno it was rich at low RPM and relatively lean at the high end, was hoping the RDX injectors could help there. I'll have to look at the IAT sensor, though that would we weird to all of a sudden affect it right? Shad did mention my intake manifold was a mess and needed a good cleaning.

Pre-service I was getting at least to 350 per tank, and when I was willing to risk running out of gas I actually topped 400 on a tank.

 

[JD Cross]

Here's the next round of answers:

Previous 10 tankfuls averaged around 25-26 mpg, again almost all highway.
I haven't ever had the injectors cleaned, I should look into that. Was strongly considering doing the RDX injector swap but the ECU issues people had scared me off. I'll have to look into the cleaning.
Pre-service I was getting at least to 350 per tank, and when I was willing to risk running out of gas I actually topped 400 on a tank.

25-26 mpg sounds right for normal use.
You haven't stated your mileage on the last two measured tanks.
Miles per tank is a useless measurement as you don't know how many gallons are left in the tank when you fill it.
I sent my injectors to RC earlier in the year.
A very good value maintenance item

 

[Honcho]

I sent my injectors to RC earlier in the year.
A very good value maintenance item

Totally agree. I run Techron religiously at each oil change and my injectors were still in bad shape. Based on my data from RC, I would suggest a 60,000 mile cleaning interval for a daily driven NSX and a much shorter interval for a car that sits all winter.

 

[JD Cross]

Looks like the OP bailed on his thread as no new info forthcoming

 

[BATMANs]

Perhaps he bailed for the holidays......

 

[liftnot]

You must have a special driving technique to get that kind of average

Ninja tips for the Holidays:
1st - you must go for the Kung Fu Panda look.....Type R is not gonna do it.
2nd - going thru TX/LA using summer 93 and no hills:wink: got me 33 mpg. CA summer gets me 30.
3rd - RDX mod.
4th - lost my left kidney to have that x-tra 6 gear

 

[JD Cross]

Ninja tips for the Holidays:
1st - you must go for the Kung Fu Panda look.....Type R is not gonna do it.
2nd - going thru TX/LA using summer 93 and no hills:wink: got me 33 mpg. CA summer gets me 30.
3rd - RDX mod.
4th - lost my left kidney to have that x-tra 6 gear

I'm definitely getting old cause I don't understand the any of the above.

I can understand the gas mileage numbers however.
I can see for one tankful, on flat/downhill roads, on a cool day, with a tailwind, driving at 60 mph all highway, you could get 33 mpg.
Averaging 33 mpg over the thousand miles it would take to drive across TX and LA I wouldn't have thought possible.

Leaving Canada and switching to 10% ethanol gasoline dropped my mileage by 2.2 %
Driving in TX/LA/CA summer heat with 10 % ethanol won't help mileage either.

Octane rating has nothing to do with gas mileage so not sure why you would mention that.

Still puzzled how you can get such terrific mileage unless you drive about 60 mph (2500-2600 rpm in 5 th) or your tire size is overstating your mileage.
What is your tire size?

 

[liftnot]

1st just how my car looks like a Panda not a R. Nothing to do with mileage but less ricey.
Tires are 215/265-17/18
IMO, 6 spd is the key - barely turning 3k at 80mph on cruise. I have yet drove 60-65mph on long distance.
RDX injector mod gives better fuel atomization - does it help? WDIK
my car weights 2640 lb. w/o me(140lbs Ninja) - WDIK, I have another 150lb yet to remove.
Higher Octane 93 was available in TX/LA but not in CA so just curious to try - WDIK
Now going up to Edmonton from San Diego is a different result but not much lower than 30mpg.
This is all b 4 I use RS-R Ran Up and the only difference is longer warm up time & the engine feel more smooth. MPG did not change.

Its ok. you're not the only one who doesn't believe me.

 

[docjohn]

..........I don't think it was an issue of belief.....more of understanding......:wink:...........for me it is tough to follow you folks who are savvy texters ,with all that abbreviated jargon.

 

[mochanges]

I'm still here! Just really busy with work.

Last 2 tanks got me about 21-22. I always measure based on my fill up so I know exactly how many gallons I used on the last fill up. Total miles/amount of gas put in to top it off gives me effective gas mileage. I did Laguna Seca yesterday so I'll have to throw out the next fill up as I was using just under a quarter tank per session.

Liftnot, the RDX mod actually improved your gas mileage? Interesting...