Dropping MPG; Need Suggestions/Confirmation

[whyfish]

Been a while since I've posted or been active here. Reason for that is "Life" and no issues with my sweet ride (03 TDI Jetta Wagon).

Lately, I've been experiencing a drop in fuel economy. In the past I was averaging 37 mpg in the city and 44 mpg on the highway which would average out to 42 mpg per tank; average tank being 15 gallons. I do vent when filling up. Yes, it's equipped with an 01M transmission.

So now I'm averaging 37 mpg per tank with 60% surface streets and 40% freeway driving. Same as previous driving habits. I changed headlamps a couple of weeks ago and noticed strong smell of diesel fuel in the engine bay. Poked around but didn't see anything that suggested fuel leaks. I'm very diligent about changing all filters on a regular basis; fuel filter every 20,000 mi. Oil Filter every 10,000 mi. Cabin and air intake filters every 20,000 mi. So I'm not inclined to think it's the fuel filter gumming up.

I'm wondering if anyone might have a suggestion about what to focus in on to reverse my dropping MPG? Leaking fuel pump seals? But wouldn't I see some fuel pooling up in the belly pan? (have evolution skid plate installed).

Leaking fuel injectors? What would I look for if that were the case?

Please help. Thanks, all.

 

[Dave_D]

Did you check the return hoses from the injectors to the fuel injection pump? They tend to deteriorate over time and will leak diesel onto the injector bodies, resulting in a strong diesel smell and a loss of mileage.

Dave

 

[1.9glstdi]

VCDS you coolant temps bet you have a dead thermostat should read in the 97.C range and you're probably in the 80.1C range. Check your cooling fans while your there.

 

[annieneff]

Something is leaking in your engine bay if you can smell diesel in there. Leaks in there can be hard to track down, especially if they are small and/or only happen under certain circumstances (like if it only happens when it's hot, or when you trounce on the accelerator, or etc.). Open the hood and check under varying conditions; ie when it's fully warm end up and running and when it's fully warmed up and parked for 1 minute, 5 minutes and 1/2 an hour, after it's been idling just awhile but hasn't warmed up, after a sharp turn, after it's been sitting awhile (a day or two or longer), and so forth. It's possible/probable you don't see where it's leaking because it's dripping on something hot and evaporating quickly. When the engine is cool sniff around in there with the engine cover off (if you have one) and try to isolate the general area the smell is coming from. Then look at that area with special attention when you're checking under different conditions. It's also easy to pressure test this system. If you can't find the leak yourself get it tested and you'll find it no problem.
Return hoses are a common culprit, especially with the age of this jetta. Fuel pump is another, especially if you used any amount of biodiesel, but biodiesel or not those seals and lines all wear out eventaully. Look those over closely under different conditions, and smell them the best you can. Injectors don't usually leak from my experience and what I've read. Usually it's the hard lines that connect to them that do. If the injectors have been monkied with (taken out or loosened) anytime recently it could be the metal washer seals under them. But those only leak if they're not seated properly.
Every time you change your fuel filter you need to bleed the system to get the air out. That makes me suspect the hard lines to the injectors. Look at those closely first. Btw, why are you changing your fuel filter so often? Diesel is pretty clean these days. Even if you use biofuel you don't red to change it that often. I'm sure others will lambast me for suggesting you change it less often, but 20k seems excessive and unnecessary. Changing it that often invites things like your experiencing. The injector lines can be improperly bled, the injectors can be dorked or otherwise damaged, the return hoses on the injectors get diesel on them and wear out faster, same with the radiator hoses, someone could inadvertently introduce dirt or some other unsavory thing into the system. You can, of course, minimize all these risks, but doing it more often than you need to ups the risks for no reason.
Anyway, there's my lecture on that.
Finally, the change in mileage seems to be more than what you would expect from a fuel leak, especially if it's small which it sounds like it is. Does the transmission feel like it's functioning properly? When you hit it on hills or otherwise does it shift okay; any jerking or slippage? I ask because of the obvious issues with that slushbox. That would easily account for the loss in mileage.
Figure out the leaking issue first and if that doesn't improve the mileage check other things, starting with the tranny. And stop changing your fuel filter so often .
Let us know what you find out in fixing this. Help others that have this same problem in the future.

Andrew

 

[1.9glstdi]

Fuel filter drain at 10k miles and replace 20k miles. Also cold have cracked hoses and sucking air.

 

[josh8loop]

Certainly find that fuel leak. Culprits are many but can include fuel injector return lines, leaking crush washers underneath injectors, leaking IP head seal, and perhaps leaking water drain underneath fuel filter. You have an auto so you don't likely have a clear fuel line between the fuel filter and the IP so you can't easily check for air in the fuel lines.

Leaking fuel filter water drain, and leaking fuel injector return lines are usually accompanied by air intrusion into fuel system and hard starts. The IP can have other areas that it can leak. There are various covered ports on it that can develop leaks too. Some are underneath and on the side of the IP. If you don't find the fuel leaks there, then remove the TB cover and check for fuel leaks from the pulley area of the IP.

 

[whyfish]

Great Responses

Hey everyone, thanks for all of your answers. And especially to annieneff. Your post took some time and effort to put together. I really appreciate the comments.

Looks like I have a decent week-end project on my hands. Luckily, it's supposed to be warm and sunny this weekend. Can work on the car outside!!
When I've narrowed it down to one or two culprits, I re-post.

Regarding the comment on the transmission. I service it every 40,000 mi with new fluids and filter. The transmission is in surprisingly good working order. No slips, no hard shifts. Holds a steady rpm on the freeway. No issues with the 01M like so many other auto-box owners have posted about. "KOW"

 

[Helo_mech]

I'm pretty sure Oregon mandates B5 biodiesel, so you might get a little less mileage from that, probably no more than 1% though. They also winterized the fuel from Oct-March, this can lower economy by up to 10%

 

[FlyTDI Guy]

I just had to reseal my pump because of leaks. Check it out carefully. Fuel will pool lower on the pump casting and also possibly stain your thermostat housing and/or alternator wiring. Even my small leak stole a few mpg and was noticeable. I was surprised but the reseal has restored my former mileage. If you smell raw diesel, then diesel is present and needs to be found. Good luck!

 

[josh8loop]

Hey everyone, thanks for all of your answers. And especially to annieneff. Your post took some time and effort to put together. I really appreciate the comments.

Looks like I have a decent week-end project on my hands. Luckily, it's supposed to be warm and sunny this weekend. Can work on the car outside!!
When I've narrowed it down to one or two culprits, I re-post.

Regarding the comment on the transmission. I service it every 40,000 mi with new fluids and filter. The transmission is in surprisingly good working order. No slips, no hard shifts. Holds a steady rpm on the freeway. No issues with the 01M like so many other auto-box owners have posted about. "KOW"

Sounds like you have been superbly maintaining your 01M. Kudos!
Just wanted to verify you have flushed/refilled your differential with gear oil right?

Another thing to check for poor mileage may be sticking brakes. The E brake cables have been known to get wonky on these vehicles, and need replaced now and again-especially in the salt belt!