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Discussion Starter #1
Sup guys!

I've been a bit concerned for my gas milage for quite a while now, last fall i got ~295-335 miles/galon. Then when winter came around i got 235miles/galon. I thought that this had to do with winter tires and shitty weather but after the winter it didnt get any better, did some mods during the spring:

Aem short ram
Skunk2 exhaust
Serge bo reflash
Serge bo headers with test pipe

But these did not affect the milage at all, made the car quicker tho but not as much as i where expecting.

1st gear seems fine, but in 2nd and upwards the car seems to struggle a bit. No engine lights or anything like that.

Any ideas? Car has 145 000miles on it. Might have to do some 2nd and 3rd gear pulls to get some actual numbers.
 

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Discussion Starter #2
90% of my my driving is highway ~70mph. Idk where to even begin since i know it isnt tires or brakes. 25% increase in fuel consumption is quite a bit. We only have 95-98 octane fuel here in Sweden so it isnt the fuel either. Seems like I have the ability to just change the fuel filter instead of the whole pump assembly, can anyone confirm? Seems like a good place to start since it shouldnt be that expensive.
 

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Discussion Starter #3
Did some pulls just now. This is with 98 octane fuel, empty gas tank but with subs, sound dampening, etc. weighing probably ~90pounds. The weather was 22C/72F.

Speeds are wonky because I converted them from KM/H

40mph - 62 3sec 2nd gear
0-62 8sec
0-62 8sec
50-87 8sec 3rd gear
62-100 9sec 3rd gear
62-93 10sec 4th gear

"Original numbers"

65kmh - 100 3sec 2nd gear
0-100 8sec
0-100 8sec
80-140 8sec 3rd gear
100-160 9sec 3rd gear
100-150 10sec 4th gear

Now I'm not a "good" driver in the sense of 0-60 but I think I should be able to go below 8sec with my current setup. Unfortunately, I was not able to get footage of the speedometer but as soon as I hit 60 I let go of the gas.

1st 0-60 pull: https://streamable.com/n2dwx
2nd 0-60 pull: https://streamable.com/gr1xk

Any ideas what I can do to try and find any faults?
 

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Discussion Starter #4
I still haven't figured it out, starts quickly, runs good, just not good enough... Gonna change sparkplugs on Thursday since I don't know when they were changed before.
 

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Discussion Starter #5
The old sparkplugs were worn out but I did not feel any change in performance after getting them changed. The mechanic also said that I need to change both rear wheel bearings. I guess that could also affect the MPG.

Anyone got any ideas?
 

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Discussion Starter #7
Are your wheel bearings humming? Is there alot of run out in them? Curious what his diagnosis was.
Im not noticing anything while driving. When the mechanic was going to change my brakes he noticed that especially the rear left was in really really bad shape appearently. When he spun the bearing it made all kind of noises.

Ordered a obd2 scanner to help me diagnose the car but it will take weeks for it to get here.
 

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Discussion Starter #8
Changed both wheel bearings and got the car "smoke tested" ! No leaks sadly enough :( Since now im pretty much out of ideas... All i can think of is that the map sensor is feeding the ECU faulty information. I feel like the fuel consumption has gone up even more ~1,1-1,2L/10km.

Screenshot of car in idle just after i drove it home so the engine was warm and the ac was off.

46483


Any ideas bois? 🤔
 

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Discussion Starter #9
Long term fuel trim on the highway 110kmh/65mph is ~8,5-10,5%. Would this be considered high?
 

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I haven't watched my fuel trims on this car before but those numbers don't sound too high... Maybe I'll watch mine and see what I get.
Did you have the car aligned after changing wheel bearings? If the bearings were really bad, it may have had an effect on the alignment.
Have you had your injectors cleaned?
Check for exhaust leaks upstream or around the primary o2 sensor.
Make sure your PCV system is functioning properly.
 

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Discussion Starter #11
I haven't watched my fuel trims on this car before but those numbers don't sound too high... Maybe I'll watch mine and see what I get.
Did you have the car aligned after changing wheel bearings? If the bearings were really bad, it may have had an effect on the alignment.
Have you had your injectors cleaned?
Check for exhaust leaks upstream or around the primary o2 sensor.
Make sure your PCV system is functioning properly.
Thanks for the advice!

Definately not anything to do with the wheel bearings, or exhaust leaks. I guess the actual sensor could be faulty. Never had my injectors cleaned. Will have to do some research on the pcv system cause i have no idea how to make sure that its working, going back to the mechanic on thursday. Really bothering me that the car isnt throwing a code :( since the fuel consumption is so high and the car is actually quite a lot slower now..
 

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Discussion Starter #12
Im about to give up and just try and sell the car if someone even would want to buy this piece of 🤬.. Very unfortunate since ive spent so much money on it to keep it alive and running. There is nothing wrong on the hose that goes down to the engine mount. The Ltft is 10% with and without the hose being blocked. The mechanic and i lifted up the car just to check on that hose and it looked like it was in great shape. There are no intake leaks. Car runs good on idle and when driving but uses up a ton of fuel and is slow. The PCV valve is working. Fresh sparkplugs, no engine lights :(
 

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My LTFTs at highway cruising speeds ranges from 2.7-6.7, so yours is higher but not terribly so. Watching LT and ST the composite was generally around +11. Looking at your screenshot above your STFTs are consistently in the negative... Is that still the case? Go for a drive and watch your o2 sensor readings (AFR) alongside your fuel trims. Watch for running rich at cold idle, very negative fuel trims at idle, correlate AFRs against long and short fuel trims in as many conditions as you can.
The next thing to do is have the injectors professionally cleaned/refurbished, or buy another refurbished set that is ready to swap in. We'll see if the next set of data supports this.
 

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Discussion Starter #14 (Edited)
Cold idle
46513

Driving
46514

Warm idle
46515


Seems like the o2 sensor voltage is bouncing all the time, doesnt look very normal..

Also obv I ****** up the MPG conversation before... Just to clarify, im around 20MPG commuting 25min to work ~70mph
 

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I know this is pretty self explanatory but your short term fuel trim shows that you're running a little lean and your long term fuel trim shows that it has been running lean for a while. Which is why you're consuming more fuel than you should be... overall, it's using about 13% more fuel on average.
If it were me, I would look into vacuum leaks (like a bad intake manifold gasket or cracked hoses), dirty/clogged injectors, or maybe even a failing fuel pump. Basically anything that could cause it to be getting more air or not enough fuel. And depending on what the problem is, could also be the reason it's a little sluggish, like if it's a faulty fuel pump or something. 🤷‍♂️
Since its not bad enough to throw a code yet, it's just going to be trial and error so I would suggest just starting with the cheapest fixes and work your way up until those fuel trims even back out to 0% on both short term and long term.
Just my two cents.
I hope you get it figured out though.
And if you do, please update the thread... it could help save some people a lot of headache one day, especially if it's an odd or uncommon problem.
 

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Discussion Starter #16
I know this is pretty self explanatory but your short term fuel trim shows that you're running a little lean and your long term fuel trim shows that it has been running lean for a while. Which is why you're consuming more fuel than you should be... overall, it's using about 13% more fuel on average.
If it were me, I would look into vacuum leaks (like a bad intake manifold gasket or cracked hoses), dirty/clogged injectors, or maybe even a failing fuel pump. Basically anything that could cause it to be getting more air or not enough fuel. And depending on what the problem is, could also be the reason it's a little sluggish, like if it's a faulty fuel pump or something. 🤷‍♂️
Since its not bad enough to throw a code yet, it's just going to be trial and error so I would suggest just starting with the cheapest fixes and work your way up until those fuel trims even back out to 0% on both short term and long term.
Just my two cents.
I hope you get it figured out though.
And if you do, please update the thread... it could help save some people a lot of headache one day, especially if it's an odd or uncommon problem.
According to the mechanic, there are no intake leaks since he did a smoke test. Fuel consumption is up 50% from where it should be. He also doesn't believe there is anything with the injectors since the car is running good on idle. Do you think the reflash could make it not throw a code?
 

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A code will still be thrown with reflashed ECU. A car with moderate injector issues can still run fine at idle... Especially if the issue is only with 1 or 2 injectors. I just dealt with this on another car. Fuel trims were perfect everywhere while driving, wideband showed good AFRs, but when I first started the car it would show running rich until the ECU leaned out the mixture. It would run -20 at idle, exhaust would smell rich, and my fuel economy dropped about 30%... but all else appeared fine. I had a leaking injector which was making 1 cylinder run ridiculously rich but the ECU was leaning out the other 3 making the measured mixture stoich. Installed a different set of injectors and now I run -6 at idle. Haven't had the repair long enough to judge fuel economy yet but exhaust smells normal now so I expect the issue is fully resolved.
 

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Discussion Starter #18 (Edited)
Thanks for the responses guys so we are thinking that there is something wrong with the injectors? Cause i literally have no idea at all. And im really running out of patience and money for that matter on this car :( I still cant believe how a 50% increase in fuel consumption cant throw a code..

WOT
46539

Interesting that the stft droos to 0 when WOT
 

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STFT is at 0 during open loop, the system only operates during closed loop to maintain a stoich fuel mixture. At WOT that goes out the window as the ECU adds fuel to give you more power.
 

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Are you 100% sure you have no exhaust leaks? You can do a smoke test on the exhaust system too just to be sure.
Pull your plugs out and inspect the ends. The carbon on them should be tan or whitish. If they are black, then the whole fuel system is too rich. This could mean issues with the fuel pump/pressure regulation, or injectors. If one or more are black, but not all, it's most likely injectors. See if a local shop can clean/rebuild them for any cheaper than ordering a rebuilt set.

Part # for injectors is 16450-RBB-003, which also fits the 05-06 RSX-S and 2006-2011 Civic SI. They shouldn't bee too hard to find.
 
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