Sudden drop in fuel efficiency

[JD Cross]

If you keep track of mileage by the tankful you don't get "about" mileage, you get calculated mileage i.e. 24.6 mpg.
You don't state what the calculated before and after mileage numbers are, which suggests you are using estimates.

If you normally got an estimated 25 mpg and now an estimated 22 you've had a 12 % mileage drop not 20-25 % as you started this thread with.
12 % is significant but not alarming like 25%.
Your drop could be attributed to winter gas (up to 5 %) more idling time (winter) change in driving pattern (more city miles), change in driving style (faster?).

This is way out there but by any chance did you change the speed sensor during your transmission repairs?

 

[mochanges]

My numbers are approximate since I don't write them down, I do the calculation when I fill up and remember where they ended up. Anything that's anomalous sticks out. I use the mileage on the trip odometer and divide by the gallons used to fill up.

My speed sensor is due to be replaced! I know because 2 track days ago, on the hottest track day I have ever done (it was 115 degrees ambient) my speedo started jumping up and down on the way home. I ordered a new sensor and it's sitting there waiting for me to install it. That would definitely explain a lot if that's the issue and it was either overstating how far I had gone before, or it's understating it now. I'll have to fix it and see what happens. Great call!

 

[mochanges]

Figured I would update the thread. Speed sensor was replaced with no improvement. The car is now with Acura of Fremont where I've been getting great support. In hindsight I should have taken it to them right away but I was waiting for Shad to get back to me since the problem started after I got the back back from Driving Ambition.

This week the car started bogging and hesitating in part throttle, then in steady state throttle and then started backfiring while idling. My exhaust sounds very different from normal, the smell of the exhaust is very different from normal and the MPGs never went back up. Acura of Fremont is taking a deeper look.

I emailed and called Shad 4-5 times over a 2.5 month period, beginning when I got the car back. I finally got a return call but couldn't answer when he called (driving home from work). I called back 2 days later and followed up with an email detailing what I needed his help with and it's back to radio silence, so another 3 weeks or so. I'm really disappointed as I feel communication was great up to, and when my car was there, but no response at all after I got it back. I'm finally doing what I should have done: accept that I am not going to get help from Shad and find help locally.

 

[Yes!NSX]

So, any updates? Did they manage to find the problem?

 

[mochanges]

Yes, looked like the culprit behind the bogging and backfiring was old spark plugs with the one on cylinder #1 (i think) being especially bad. Why those were not changed during the service I cannot answer. After changing those, the car fine. Mileage, however, still hasn't really gone back up. I'm starting to think that my really high mileage tankfuls were a fluke, maybe caused by the bad speed sensor? I'll probably never know, but my MPG is in the 21-22 range. I even threw in RS-R Ran Up which previously had given me great results and no improvement at all this time around.

I would love to get my injectors cleaned, I might try that if I can find some solid downtime with the car.

 

[cmthomson]

Why those [spark plugs] were not changed during the service I cannot answer.

I can offer a suggestion: PRICE. When I had my 90K done, the guy looked at the $60 per plug and balked. He put in new (off the shelf) plugs because I insisted, but they caused so much EMI that my radio and garage door opener barely worked. Eventually this was diagnosed and OEM platinum resistor plugs were installed (yes, at about $60 per plug...)

 

[Rope]

I feel that if your mpg changed that drastically it is something major like your cam gear being off by a tooth. Not saying that the service was bad but we are all human and errors happen. As far as the RDX injector mod goes there is an issue with the idle unless you reset the ECU when the engine is warm and then let it idle and figure itself out for the next ten minutes. After that shut the car down and turn it to the on position with out cranking the engine followed by turning it off again to save the initial idle trim. The RDX injectors gave me 27 mpg with the stock 5 speed at 80 on the highway and since swapping out the tranny to a 4.55 final I get 21-22 mpg. You should get better mpg then me. Do you have the 6 speed or 5 speed? Have you tried the Chevron Techron concentrate fuel injector cleaner? I used it for two tank fills prior to an oil change and raised my mpg's by 3-4 and throw a bottle in on the last tank prior to an oil change and have kept the same efficiency.

 

[mochanges]

Cmthompson: Wouldn't surprise me if that was what happened. I'm scared to find out what other corners were cut during my service.

Rope: This is great advice, thanks! I believe the dealership that eventually got things fixed looked at where the belt was and the gears were where they are supposed to be. I did just put in a bottle of injector cleaner that I read about (I can't remember the name), but I'll give the Chevron stuff a try next. I'm running the stock 5 speed. That is killer mpg you were getting, I think I need to take the plunge soon.

 

[docjohn]

Larry B is a fan of the techron ,,,,,so you would be in good company

 

[dablackgoku1234]

Did you get a valve adjustment recently?

 

[mochanges]

Did you get a valve adjustment recently?

I did, one of the things Driving Ambition was supposed to do.