Puzzeled; Drop in MPG

[whyfish]

I've been experiencing a gradual drop in MPG over the last 6 months. It's at a point where I think I need to take some action, but not sure what to do. I've seen about a 4 to 5 MPG drop over the last 6 months no matter the driving conditions; city, freeway, mixed and long road trips. My average full tank mpg's is now 33.5 as opposed to a steady 39 to 42 MPG up to 6 months ago. I knew I would take a hit with the 01M so don't even go there. I'm looking for rational explanations as to why I'm seeing a reduced fuel efficiency. Any input would be greatly appreciated. Here's my set-up:

03 TDI wagon, 01M (i know, I know), TB changed at 80,000 miles now at 123,000 miles. Always vent, so I'm getting an average 15.5 gallons with each fill-up. Newer tires (10K mile on them run @40 psi), regular scheduled maint performed at 120,000: OC w/ Mobil1 TDT 5w40, changed air filter, fuel filter, oil filter and cabin air filter. Recently upgraded suspension and brakes on all 4 corners, have a mild chip from VWMikel, running PP357 injector nozzels (85,000 miles one of my first mods) always use Power Service @ 4 0z. per tank. Calculate MPGs w/ paper and pen and don't drive like a maniac, but take advantage of extra power from time to time.

Discuss.

 

[Lightflyer1]

What tires before and now? They can make a huge difference.

 

[UFO]

Thermostat. That's what happened with mine last fall through the winter.

 

[whyfish]

Tires

Tires are Michelin Primacy 195-65-15; factory specs. running @ 40 psi.

 

[jettawreck]

Has your driving routine changed much??
Tires really can make a difference. New ones almost always return lower mileage than worn ones-providing they are same type/size/etc.
Can you tell if the tranny is going into high gear and torque converter lock-up mode as it should? If you are running in high gear w/o lock up mode your rpms will be higher than "normal" by a couple hundred (?) rpms. I've never drove a TDI automatic, but on most autos, if you watch the tach at slow, steady acceleration you can plainly see the shifts (although with the newer 5 and 6 speeds its more difficult) and after yours gets into 4th it should drop a few more rpms when the lockup occurs.
I actually get better mileage in non-freezing temps w/o additives. Not sure why, but I do.

 

[Conan]

Dragging brakes? Parking brake? Might try getting up to speed on a safe, low-traffic road, coast to a stop, and see if any of the brakes feel warm. (I just made up that experiment, but it might work.) Obviously, you could jack it up and see if the wheels spin freely with the brakes off, but I think a small parasitic drag might be hard to spot that way.

 

[whyfish]

Jettawreck; Transmission is serviced every 40k miles. I neglected to mention that was part of my 20k service/maint. I performed at 120k miles. The transmission is tight and shifts at appropriate RPMs. very smooth, acts like its new, really. And no, my driving habits/routine have not changed.

Conan; I'll have to check the brakes for drag.

UFO: I'm also going to look into changing out the thermostat. That sounds like it mught be the culprit. I've heard aging thermostats can cause lower fuel economy.

Apprciate all of the input.

 

[krmason]

Definitely change the Thermostat. I just did that on my 2003 Golf. my mileage went back up to where it should be. to check it I drove 59 mph on cruise over a two week time. I got 62.8 mpg. (of course I have mods to give me better mpg). Before the thermostat changed I was getting 47 MPG. After the change my normal driving gets me 52 MPG.

Also check to see if the injection pump is leaking. I fixed that problem on my car also. Take your hand and check the bottom of the injection pump for diesel.

 

[jettawreck]

Jettawreck; Transmission is serviced every 40k miles. I neglected to mention that was part of my 20k service/maint. I performed at 120k miles. The transmission is tight and shifts at appropriate RPMs. very smooth, acts like its new, really. And no, my driving habits/routine have not changed.

Conan; I'll have to check the brakes for drag.

UFO: I'm also going to look into changing out the thermostat. That sounds like it mught be the culprit. I've heard aging thermostats can cause lower fuel economy.

Apprciate all of the input.

Sounds like you're taking good care of the 01M.
If you have VCDS or Scan Gauge, of course, you can check the actual operating temp. Or a cooking thermometer in the coolant bottle. My '01 and the '03 both got new 'stats this year. The '01 this winter when it was cold, mostly for better cabin heat. It wouldn't get over 170F, even with the radiator fully covered, but it was winter. Improved the cabin heat and warm-up time a fair amount, it gets to and maintains 190F most of the time except coasts. The average mpg not much improvement but some. The '03 would get up to 180F on a nice day. Put a new 'stat in that one last week so its a bit early to tell. Runs a nice 190-192F. Filled up this morning (824 miles/14.92 gal=55.2 mpg) and it was about the same as last years average. Next tank maybe a bit better.
Buy either an OEM 'stat, or I've had success with the Wahler's, but even those don't fit the coolant flange w/tabs. Aftermarket/part store 'stats I did not have success with. Two different brands didn't get as warm as the failed OEM.

 

[NewTdi]

Thermostat. That's what happened with mine last fall through the winter.

X 2 do a test

 

[K5ING]

Has your driving routine changed much??
Tires really can make a difference. New ones almost always return lower mileage than worn ones-providing they are same type/size/etc.
Can you tell if the tranny is going into high gear and torque converter lock-up mode as it should? If you are running in high gear w/o lock up mode your rpms will be higher than "normal" by a couple hundred (?) rpms. I've never drove a TDI automatic, but on most autos, if you watch the tach at slow, steady acceleration you can plainly see the shifts (although with the newer 5 and 6 speeds its more difficult) and after yours gets into 4th it should drop a few more rpms when the lockup occurs.
I actually get better mileage in non-freezing temps w/o additives. Not sure why, but I do.

I'm in the same boat as the OP, only my mileage hit happened almost overnight. I Just got new tires for my Golf. Same brand/size/model as I had before (Yoko YK520's). My mileage has gone from 50 down to around 43.

I always ran 50psi in my old tires (max sidewall pressure is 51psi) but had the new ones aired up to only 45 (although my personal tire gauge says 43). I'm going to get them up to around 48psi and see what happens. 50 was just too hard of a ride, and it was causing the middle to wear more.

 

[jettawreck]

I'm in the same boat as the OP, only my mileage hit happened almost overnight. I Just got new tires for my Golf. Same brand/size/model as I had before (Yoko YK520's). My mileage has gone from 50 down to around 43.

I always ran 50psi in my old tires (max sidewall pressure is 51psi) but had the new ones aired up to only 45 (although my personal tire gauge says 43). I'm going to get them up to around 48psi and see what happens. 50 was just too hard of a ride, and it was causing the middle to wear more.

I couldn't get away with that much air press. The roads are too rough around here. I keep about 38/40# and a bit less in the winter when its even rougher and the suspension is stiff. -30F will do that. Last year was one of the worst for pothole and frost heave damage.
I need new tires on the front of the '03. The old Perrelli's are about worn out, but I hate to go get new ones. Last tank was 55.2 mpg and I know I'll take a bit of a hit with new thicker rubber. Has to be done fairly soon.
Pretty impressive mileage K5ING! 435K is 2.5 times of where I'm at. Must keep up on your maintainance!

 

[K5ING]

I couldn't get away with that much air press. The roads are too rough around here. I keep about 38/40# and a bit less in the winter when its even rougher and the suspension is stiff. -30F will do that. Last year was one of the worst for pothole and frost heave damage.
I need new tires on the front of the '03. The old Perrelli's are about worn out, but I hate to go get new ones. Last tank was 55.2 mpg and I know I'll take a bit of a hit with new thicker rubber. Has to be done fairly soon.
Pretty impressive mileage K5ING! 435K is 2.5 times of where I'm at. Must keep up on your maintainance!

Thanks for the kudos, and yes, I try to keep up on the maintenance. I'm pretty easy on the car however, and that helps.

Keeping that much air in the tires makes for a rough ride for sure, but the roads around here are pretty good. The only thing I'm worried about is that I've taken nearly a 10mpg hit on the mileage. Today, I drove to the store. 10 miles each way (8 of it at 65mph with no stops), and I was only doing 40mpg. Even with the A/C running, it's usually at least 47. That's a hell of a mileage hit.

 

[jettawreck]

Thanks for the kudos, and yes, I try to keep up on the maintenance. I'm pretty easy on the car however, and that helps.

Keeping that much air in the tires makes for a rough ride for sure, but the roads around here are pretty good. The only thing I'm worried about is that I've taken nearly a 10mpg hit on the mileage. Today, I drove to the store. 10 miles each way (8 of it at 65mph with no stops), and I was only doing 40mpg. Even with the A/C running, it's usually at least 47. That's a hell of a mileage hit.

Yep, that's too much to explain off on new tires. Keep looking/thinking and you'll figure it out. Given the short trip and your mpgs listed, I'm assuming you're getting these numbers off your SG or MFA display. Have you changed any adaptations w/VCDS lately?? I know when I changed the EGR settings on the '01 it threw my SG numbers way off, although the actual numbers not really.

 

[dieselfuel]

Check my sig for an easy way to check your thermostat.

Good luck!

 

[K5ING]

I know that I have a bad thermostat. It's been that way for a few months now. I can drive at highway speeds with the A/C on in 95 degree heat and never make it out of the 170's according to my ScanGauge. However, I've still been getting mileage in the high 40's to low 50's even with the A/C running. My mileage hit came literally overnight after getting my new tires (same brand/model as I had before).

One thing I noticed, however. With the temps in the low 80's, and running without the A/C on, my coolant temps would be in the 180's. Hotter than with the A/C on in higher temps.

 

[Conan]

Your A/C makes both your fans run all the time. You may have a bad coolant temp sensor or something else that's keeping the fans from coming on when they should.

 

[K5ING]

Your A/C makes both your fans run all the time. You may have a bad coolant temp sensor or something else that's keeping the fans from coming on when they should.

I think the coolant temp sensor is all right. It only got up to 184 degrees with the A/C off, and I don't think that is high enough to turn on the fans. I just need to get the money together to fix the thermostat. It still doesn't explain the overnight drop in mpgs after the tires though.

 

[whyfish]

Dieselfuel,

I followed the link in your signature. Good tip.

Question: Where is the thermostat housing located in the engine bay?

Don't pile on, at least I know where my timing belt cover is located. And the EGR valve, too!!

 

[jettawreck]

Dieselfuel,

I followed the link in your signature. Good tip.

Question: Where is the thermostat housing located in the engine bay?

Don't pile on, at least I know where my timing belt cover is located. And the EGR valve, too!!

"Question: Where is the thermostat housing located in the engine bay?"

Follow bottom radiator hose to where it is clamped onto the plastic coolant flange which is held onto engine block with two 10mm bolts w/allen head (5mm??) indents in them.