Ok I know that I could probably search and read 50 threads about specific problems people are having but I thought why not just make a thread (maybe have the mods sticky) that gives a checklist of things to go through if you experience a big drop in fuel economy. Make it simple for us non engine savy people. My specific problem is that I have noticed a 30% drop in mileage in the last 2 months and the only thing different is that is getting warmer and I changed my oil 2k miles ago. What are the first 5 or 10 things I should look at before I take it in or try something more drastic?
Recent drop in fuel economy
- Thread starter Nurumkin
- Start date
Joe_Meehan
Top Post Dawg
* Dragging brake
* Low tyre pressure
* Change in driving (more city driving?)
* Low or bad transmission fluid, or slipping transmission
* Any CELs
* MAF
* Clogged air filter or intake
* Blocked exhaust (damaged)
However first you want to make sure all scheduled maintenance is up to date.
* Low tyre pressure
* Change in driving (more city driving?)
* Low or bad transmission fluid, or slipping transmission
* Any CELs
* MAF
* Clogged air filter or intake
* Blocked exhaust (damaged)
However first you want to make sure all scheduled maintenance is up to date.
Last edited:
Bkcorso
Veteran Member
Checking the thermostat is pretty simple, and has caused serious mpg hits when it won't allow the engine to operate at normal temps.
re
The guy at the auto parts store said that I should do my fuel filter, glow plugs/wires, and air filter. I have not done any of those since I got the car 36k miles ago (I'm not a very engine savy person so I don't know to do these things)
The guy at the auto parts store said that I should do my fuel filter, glow plugs/wires, and air filter. I have not done any of those since I got the car 36k miles ago (I'm not a very engine savy person so I don't know to do these things)
Joe_Meehan
Top Post Dawg
The guy at the auto parts store does not know TDI's. The only thing on that list that might be important would be the air filter. He is thinking gasoline cars. I suspect he does not know the difference in between a glow plug and spark plug and why they are so different. If you were having a glow plug or wire issue it should be giving you a CEL (check engine light).
Bkcorso is correct, the thermostats is suspect, I should have had it close to the top of my list.
Bkcorso is correct, the thermostats is suspect, I should have had it close to the top of my list.
ZipiTDi
Veteran Member
Yes, check the thermostat.
Change the MAF = mass air filter
What kind of diesel are you running? If it's more than B5 (5% biodiesel), such as B20 or greater, and you recently switched over to this, you may have a clogged fuel filter. BioD is a really good solvent that will loosen deposits in your fuel system and clog the fuel filter.
Change the MAF = mass air filter
What kind of diesel are you running? If it's more than B5 (5% biodiesel), such as B20 or greater, and you recently switched over to this, you may have a clogged fuel filter. BioD is a really good solvent that will loosen deposits in your fuel system and clog the fuel filter.
dieselfuel
Veteran Member
Check thermostat first, it's the easiest thing to check.
Drive the car til the engine reaches max. op temp. (20-30 min.) Park the car and turn of engine. Remove the coolant tank cap SLOWLY to remove any built up pressure (usually there isn't much built up pressure, but be careful non the less).
Take a temp reading by placing a stainless steel meat thermometer into the coolant. You should get a temp reading of at least 180F. Anything less, and you need a new thermostat. I'd pay the big bucks ($35-50) for a genuine VW part. Aftermarket thermos for these TDI's have been known not to work well. Also, you may want to spring for a new thermostat housing since they are plastic and may get damaged when removed. THEY ARE USUALLY LESS THAN $10.
If your thermo checks out o.k., then I'd look at the fuel filter next. But, I bet you have a faulty thermostat.
Please keep us informed.
Good luck!
Drive the car til the engine reaches max. op temp. (20-30 min.) Park the car and turn of engine. Remove the coolant tank cap SLOWLY to remove any built up pressure (usually there isn't much built up pressure, but be careful non the less).
Take a temp reading by placing a stainless steel meat thermometer into the coolant. You should get a temp reading of at least 180F. Anything less, and you need a new thermostat. I'd pay the big bucks ($35-50) for a genuine VW part. Aftermarket thermos for these TDI's have been known not to work well. Also, you may want to spring for a new thermostat housing since they are plastic and may get damaged when removed. THEY ARE USUALLY LESS THAN $10.
If your thermo checks out o.k., then I'd look at the fuel filter next. But, I bet you have a faulty thermostat.
Please keep us informed.
Good luck!
milehighassassin
Top Post Dawg
Per scanguage my car runs around 170-178 range. Usually 175. In the winter it runs lower when VERY cold. I am talking -30 or so plus I'm doing 65 MPH+.
Edit: I meant 180-187.
MikeMars
Veteran Member
- Joined
- Mar 6, 2008
- Location
- UK
- TDI
- Vento 1.9 TDi (retired), A4 1.9 TDi (rear end collision), VW Passat 1.9 TDi (retired), Audi A2 1.4 TDi
If you read the temp via scangauge you are also relying on the temperature sender to give you an accurate reading - the temp senders often go wrong (I've had temp sender failures on two out of my last 3 cars).
josh8loop
Veteran Member
- Joined
- Sep 15, 2009
- Location
- Vero Beach, Fl.
- TDI
- 2002 VW Jetta TDI Manual(performed 01M to 5-speed swap) - 183,000 miles and climbing!
MikeMars,
I agree-my 2 yr old OE replacement coolant temp sensor is about 10 deg F off. It is reading too high as compared to my new in-line digital temp probe. Don't necessarily trust the CTS you have unless you reconfirm calibration by some means.
I agree-my 2 yr old OE replacement coolant temp sensor is about 10 deg F off. It is reading too high as compared to my new in-line digital temp probe. Don't necessarily trust the CTS you have unless you reconfirm calibration by some means.
dieselfuel
Veteran Member
milehigh,
Have you used the meat thermometer to check coolant temp at the coolant tank? That seems like a pretty low temp.
Have you used the meat thermometer to check coolant temp at the coolant tank? That seems like a pretty low temp.
milehighassassin
Top Post Dawg
Not yet. I'll check tomorrow. I watched it today and it was probably an average of 185
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dieselfuel
Veteran Member
185 sounds about right.