Restore lost MPG

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2008 Mazda5 GT
I've been getting unusually low MPG the last few fill ups, avg. 15MPG city. I don't recall it being this low but shrugged it off. Replaced the fuel injectors a short while back with reman. ones (also rough idle) to see if it might kill two birds with one stone - not really any different. Then I looked into components affecting the fuel system and next in line to tested was the purge valve. Took it out and test it (blow through straw) to see if it open and closed with current - open/close seems fine but I didn't think about volume of air passing through. I bit the bullet and bought a purge value solenoid ($45 shipped OEM on ebay) and figure it is worth replacing considering the age of the car. Bingo! MPG back to normal. I'm not saying this will boost your MPG as many things affect it. Just wanted to share b/c the difference is HUGE and worth $45 gamble.

Super easy to replace. Easy access right under the hood with engine plastic cover off, top right corner. All you need is a 8mm and 10mm socket with an extension and a pair of plyers to release two hose clamps. Anyone can do it.
https://www.jimellismazdaparts.com/...odel=14357&modelYear=2008&ukey_category=24716
Item# 18-741C
Part Number: L51818741



This further confirms my thought that the EGR is likely clogged and causing my idle...
 
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I've been getting unusually low MPG the last few fill ups, avg. 15MPG city. I don't recall it being this low but shrugged it off. Replaced the fuel injectors a short while back with reman. ones (also rough idle) to see if it might kill two birds with one stone - not really any different. Then I looked into components affecting the fuel system and next in line to tested was the purge valve. Took it out and test it (blow through straw) to see if it open and closed with current - open/close seems fine but I didn't think about volume of air passing through. I bit the bullet and bought a purge value solenoid ($45 shipped OEM on ebay) and figure it is worth replacing considering the age of the car. Bingo! MPG back to normal. I'm not saying this will boost your MPG as many things affect it. Just wanted to share b/c the difference is HUGE and worth $45 gamble.

Super easy to replace. Easy access right under the hood with engine plastic cover off, top right corner. All you need is a 8mm and 10mm socket with an extension and a pair of plyers to release two hose clamps. Anyone can do it.
https://www.jimellismazdaparts.com/...odel=14357&modelYear=2008&ukey_category=24716
Item# 18-741C
Part Number: L51818741



This further confirms my thought that the EGR is likely clogged and causing my idle...
You say increase in the Subject line but actually a decrease?
Mileage?
Is it still doing it?
 
Changed heading from MPG Increase to clarify

Nothing increases mfg MPG without trade offs, such as durability or loss of something else.

I had mechanical issue which this did fix. I’m back to 19-20 city avg. and my EGR is not clogged up, works freely. Car still has rough idle when in gear. Seems to be a problem with Mazdas and no one has resolved.
 
If you have an automatic...and while idling, you shift to neutral and it is very smooth....and you shift to Drive and there is a vibration...that's pointing to your Torque Converter being worn.

My 2008 acts this way.
 
while idling, you shift to neutral and it is very smooth....and you shift to Drive and there is a vibration...that's pointing to your Torque Converter being worn.
uhhh, that's a sign engine mount/tranny mounts are worn.
 
True but not always.

I had a Honda...same issue, we replaced all the motor and transmission mounts and while we fixed the clunk the engine was making, the vibration remained. Ran smooth like butter in neutral. A minor vibration in Park or Drive. At that point it's the Torque converter.

I sort of assumed he was looking for an answer that wasn't obvious.
 
True but not always.

I had a Honda...same issue, we replaced all the motor and transmission mounts and while we fixed the clunk the engine was making, the vibration remained. Ran smooth like butter in neutral. A minor vibration in Park or Drive. At that point it's the Torque converter.

I sort of assumed he was looking for an answer that wasn't obvious.
Did you take apart the TC and identify where exactly it is failing? Agree it is most likely TC but no one has been able to pinpoint the exact source (which component within the TC?) to confidently state so; I'd like to know why.

In theory, it 'could' be:
*Vacuum leak intake manifold- gasket letting in (ever so slight) air once the engine is at temp. considering all of the other seals and gaskets that as failed and replace, this is a possibility. STFT runs rich when the car is cold and may mask the problem.
*Vacuum leak exhaust manifold or piping, pre upstream O2 sensor - once the car is warm, there's a slight leak, which in turn is affecting the O2's reading and therefore impacting air/fuel on the intake side. possible and I know there's an exhaust leak somewhere down there.
*Vacuum leak brake booster baffle- a slight leak here would impact idle when brakes are applied. Low possibility b/c pumping brakes (D or N) does not have an impact.
*Fuel pump - clogged fuel filter where when under load, it is not supplying enough flow. Not likely IMO b/c when under load, when the engine is cold (TC lockup), it idles high during warm-up.
*Torque converter - internal leak/worn seal where it is not able to maintain fluid pressure/flow. Reaching out to a Youtuber that tore down an FNR5 and he suspects a leak in the strator/reactor. It does feel as though the TC has trouble unlocking. Worm clutch is a system to the problem.


More details.


Considering the car's age, I plan on running Seafoam on intake + seafoam in crank + Lucas transfix. After that, may throw in some Motorcraft friction modifier just b/c... old car, why not (leftover bottle).? 😅
uhhh, that's a sign engine mount/tranny mounts are worn.
PMM, TMM, RMM all new (was new).
 
I'm certain, it's not a vacuum leak. It's the same type of vibration in my 2008 Mazda 3, 2002 4Runner and long dead 2003 Honda Odyssey. They all had that minor vibration in Park/Drive once their mileage got over 75k miles or so.

I just redid the entire top of my 4Runner that I've owned since new, and replaced literally every hose and piece of rubber including a new Throttle Body and while other driveability issues were resolved... there is still that slight vibration while in Park or Drive.

At this point, I'm pretty sure it's just how Torque Converters age.
 

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