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What happened to my mileage?

2001 Jetta, with 2.0 AZG engine. Until about five months ago, averaged about 27-28 mpg, combined city/highway. Over the summer, MAF sensor died, then coolant-temp sensor died, both replaced by me. Also replaced the plugs. Later, replaced the tires. That's it.

Car runs like a champ, good power, smooth idle. No sign at all that anything might be awry -- except that my gas mileage has dropped drastically. Last tank was 21 mpg. Mileage has decreased each tank full over the last couple of months -- 28, 24, 22, then 21. I used to go about 365 miles per tank, at fill-up yesterday the trip meter read 270.

I've lost 100 miles per tank, and there's no clue why.

I used an Actron Pocket-Scan to diagnose MAF and coolant sensor. Erased the codes, and the trouble light has not re-lit since then.

I'm wondering if I ruined the cat-con during the time the MAF and coolant sensors were bad. Would I get a trouble light and code if the cat-con were plugged? Would the rear oxygen sensor trigger it?

Or, does the engine-control module need to be re-set, beyond the capabilities of my little Pocket-Scan?

Can anyone shed some light on this? Thanks!

Reply to
Brian Running
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The only thing I can think of is the temperature when down and along with that the fuel supplied was likely reformulated for cold temps and that will reduce your mileage. Although it seems you have had a lot of reduction.

Reply to
Joseph Meehan

I thought of that, too -- but the reduction seems extreme. Another thought I had -- when I was researching OBD scanners, I saw a few references to people that had inadvertently scrambled their engine-control computer while doing DIY diagnostics. I wonder if I did that, leaving the car in its "full rich" mode.

Reply to
Brian Running

I had a MAF failure in my 99.5 Jetta followed shortly thereafter (6 months?) by a catalytic converter failure. The engine light came on and my mileage went down the toilet. It wasn't plugged, but there was a big chunk of material rattling around in there, and the rattling started long before it got bad enough to trigger the engine light. I think there's an extended warranty/reimbursement out right now on the catalytic converters (just like they did with the MAF). If it's bad, you should be able to get it fixed for free at the dealer.

...Sean.

Reply to
CheetoDust

Thanks for the reply, I think we might be on to something here. Apparently, this is not an unusual situation:

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I did have some rattling under the car, last summer, and it eventually stopped. I know that mechanical things never heal themselves, but I let it go because, well, it wasn't making noise and drawing attention to itself anymore. It was about the time that the rattling stopped that the mileage began to deteriorate. It doesn't seem far-fetched at all that the catcon monolith was working its way loose during the rattling period, and then finally lodged itself tightly in a position that caused a partial obstruction of exhaust flow, causing mileage to drop.

Now, I'll have to figure out how to get the dealer to diagnose this and replace the catcon, without charging me an exorbitant amount of money. Oy!

Reply to
Brian Running

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