160k miles, now best MPG is down to 19 mpg

J

jimmyducati

New member
I just turned 160k miles and my mpg is down to 19mpg max, down from 26 driving the same route for 2 years now. Car drives flawlessly, tires inflated appropriately.

I change the oil with 5w20 Pennzoil platinum synthetic using the oil life reading as an indicator. PTU oil is changed every other oil change with Amsiol sever gear 75/140. RDU oil is original. Air filter is clean, 3” downpipes, no cats and no CEL’s indicating an issue with the o2 sensors, witch are less than 2 years old.

I drive 80 miles round trip for word everyday, so this drop in MPG’s is a significant hit to the wallet!

Any known things that can cause this dip in mpgs other than winter fuel playing a factor?
 
RDU doesn't for Run Don't Undertake lol. Seriously though, why the neglect in the rdu and overkill with the ptu? Rdu is much easier to change imo, and throw Amsoil in that as well. Nice daily driver btw.

-Does the car use much oil?
-Did it all of a sudden drop or is this a gradual loss in mpg's?
 
Wow, that is a BIG drop.  x2 on what griggs95 said.  Change out the RDU fluid as soon as possible, hopefully it has little to no metal shavings in there.  If pasty/dirty looking, R&R repeatedly with "cheapo" fluid like Walmart SuperTech until it stays clean, then final fill with fluid brand of choice.

Check the intercooler for crud buildup, clean the MAF sensors.

Spark plugs might be getting long in the tooth, I assume you have changed them once already by now?  You can easily gain back 2-3 mpg if the plugs are worn enough.

Can you scope the valves to see if carbon buildup might be an issue here?  There isn't really an effective dropin FI cleaner that will help.  A GDI induction service would be my recommendation, it can also help gain back a good amount of mileage.

Has the coolant ever been serviced?  Should be on 3rd set of coolant by now.

Any issues with the AC compressor?  That can cause drag.  Brake calipers and slide pins too.

All the usual things with any car:  alignment, tire tread/pressure, removal of excess/unnecessary "cargo", ...

And of course pull codes, just in case.
 
The economy dropped off gradually over the last 10-15k miles, drivability however has been flawless as usual.
Second round for the coolant, due for a third now, was going to add some water wetter the next go round but it has never had an issue with heat. The plugs are about 20k old, properly gapped for the 4x+ 93 tune. Brakes are the 14.4” “top cop” z34 package, about 5k miles old and in perfect order.

Ill go through it this week after the coolant flush and change plugs, clean the MAF and double check the emissions equipment and toss in some fresh plugs for good measure. I didn’t think the RDU was a problem in these cars, guess that will get the cheapo flush with amsoil in the end. Flushing the PTU every other oil change may be overkill, but with a drain plug and fill tube installed, its so easy to do.

Does the intake valve cleaning cost a lot of $$? If its not to much, that will be the next step if they are bad.
 
Usually a GDI induction service is between 150-200$ depending on where you get it done.  Not sure if there is a DIY kit out there to do so.

Plugs are sp534 or something else?  Assuming you run 93 due to the tune and it is top tier.  Try cleaning the MAP sensors and IAT sensor with MAF cleaner first.  Check intercooler buildup.  If it has buildup, may need a new pcv valve and separator.

May be time for o2 sensors perhaps, per pmezzo's post ...
 
GDI induction service, is that the BG cleaning? I am going to run a scope into the intake tomorrow and look at the valves, if they have buildup I’m going to get a media blaster and compressor and pull the intake and clean them with walnuts. I don’t have much faith/confidence in the chemical cleaning and with the age of my turbos/engine I don’t want to risk deposits making their way through the system.

Plugs are sp534, and Shell 93 from the same gas station is all I purchase. I have a new set of O2 sensors in the box if i need them, but I’m not getting any codes for them currently.

Thanks for all the suggestions so far, great place this forum is!!
 
Yes, BG, but Valvoline also has a pro GDI system, I think ZSHO had his dealer use it last time ...  Of course if you can media blast them, so much the better!

Once you post fuel trims, we will have a better idea if we need to look at the O2s or not.  That's why I find the OBD devices VERY useful, saves a lot of parts swapping & headache.
 
SHOdded said:
Yes, BG, but Valvoline also has a pro GDI system, I think ZSHO had his dealer use it last time ...  Of course if you can media blast them, so much the better!

Once you post fuel trims, we will have a better idea if we need to look at the O2s or not.  That's why I find the OBD devices VERY useful, saves a lot of parts swapping & headache.
Click to expand...

Agreed.  Diagnosing and replacing is always less expensive than just throwing parts at a problem and hoping it fixes it.  The computer is there to help. 
 
I think you mentioned you installed new brakes! Double Check the Caliper slide pins which are common to cause the brakes to drag some.  Z
 
ZSHO said:
I think you mentioned you installed new brakes! Double Check the Caliper slide pins which are common to cause the brakes to drag some.  Z
Click to expand...

Brakes were One of the first things I checked, along with tire pressures, trans fill level and having the alignment checked. Alignment was slightly out and needed a tie rod end, fixed both same day.
 
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