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Mileage Drop

I’m starting a new thread because the old one was entitled “Should Mechanic Refund My Money?” which is an issue that is long since dead. I was at fault. However, an interested reader might refer back to it for supplemental information.

My car is a 2003 Mazda 6. It is a 2.3L 4-cylinder/5-speed manual, with 61,000 miles on it.

My mileage decreased from to 25 MPG to 18.5 MPG. I started noticing this immediately after having my timing chain replaced. I had run my engine low on oil until the chain was audible, then drove it several more days (not realizing it was the timing chain causing the rattle), until finally the car lost power. I shut it down very soon after it lost power (as soon as I could get it to a safe place to park it) and had it towed to the garage.

Why would mileage decrease so suddenly? I have monitored mileage closely throughout the lifetime of the car, and it was almost exactly 25 MPG for years. Then suddenly 18.5. Looking for answers, I found the air pressure was down in the 20s. After inflating the tires to proper PSI, the mileage is now 20.5. But 25 MPG down to 20.5 MPG? I don’t get it.

(Please refer to our earlier discussion, “Should Mechanic Refund My Money?” for more information.)


Has anybody checked that the map sensor is accurate?

Some more obscure, and ominous, possibilities could be:
Lowered compression due to piston ring problems caused by lack of oil; meaning the engine is having to work harder. Lowered compression across the board may not even really be noticeable to the driver.

Timing chain off a tooth, or chains, depending upon the engine. The use of the word cam in singular is taken to mean a 4 cylinder engine.

The possibility if the engine is an interference fit variant that cylinder head valve to piston contact happened due to a jumped chain and led to slightly tweaked valves. This falls into the lowered compression scenario.

The one part I don’t understand is the bit “reclock the cam” which I take to mean the chain jumped and the shop reset it.
Given a jumped chain situation and lack of oil I would be very hesitant about “reclocking” anything and omitting a new chain/tensioner setup.

I still have the question. Does the engine idle glass smooth or is there any subtle stumble, even intermittent, at idle?

No stumble I can tell.

To DB4690: At one time I complained to my mechanic that my mileage was off. This was sometime not long after I moved to Prescott, maybe 2009 or 2010. Back in my home in Georgia, at 300 feet elevation and almost flat, I got 27.5 MPG. Here in Prescott, I was getting 25. At that time, they replaced the MAP, but it made no difference. I don’t know if this answers your question.

To OK4450: I’m not absolutely certain about the idle. I will be paying close attention to this. I have noticed, however, that the drivetrain seems less “tractable” than it was. In other words, it seems to want to “stutter” in the manner of someone who doesn’t know how to drive a manual and lets the clutch out clumsily and gives the wrong amount of throttle and the car jumps forward in spurts. Hopefully that description will convey what I am talking about.

oH…Prescott AZ is at 5300 feet elevation, and anywhere you go in Prescott, you’re on some sort of incline…just a matter of how steep the incline is.

300 feet elevation and flat = 27.5 MPG.
5300 feet and hilly = 25 MPG. Now, after the debacle with the low oil, 20.5 MPG.

OK4450, I am not sure about the chain/tensioner setup. I can ask the mechanic, or rather, tomorrow when I go back out, I will read the invoice/work summary to see if there is mention. Which reminds me, I need to get the owner’s manual out and read it over, as I said last night I was going to!!!

Hopefully you calculate your gas mileage using the odometer and the gallons pumped rather than going by vague things like the fuel gauge. Assuming that is so…

Given the recent “incident” I’m with OK4450 - either the timing is off by a tooth or the low oil period burned up the piston rings giving you low compression - or both. Checking compression is a very routine thing to do so you can have it checked if you want to know. Presumably you will be checking your oil regularly now (and tire pressure!), and you will now find the car burning oil if the rings are shot.

I’m also never surprised by coincidences. On a normal car that wasn’t nearly killed dead by its owner, sudden changes in fuel economy can be chalked up to things like thermostats failing open, coolant temperature sensor problems, and dragging / sticking brakes. Other issues that tend to appear more gradually include anything having to do with fuel or spark delivery and their management (plugs, wires, fuel filter, O2 sensors…)

PrescottAZ, you’re a good sport. Most people who post questions here and don’t get the answers they wanted (re: your original post about getting your money back) disappear, often after having a little tantrum about how rude everyone is. You kind of got shot down, took some abuse, and “[wo]manned up” about the whole thing. Good for you. Now start a new weekend (or equivalent) morning ritual. Grab your coffee (or equivalent) and spend a leisurely 30 mins once per week looking over basic stuff on the car - check all of your fluids, tire pressure, etc. AND, from now on if anything changes (new stuff you hear or feel or see or smell etc.) have it checked out. And you can always ask here for advice. Over the long run you’ll be a happier and safer car owner.

Thanks, CigRoller. Oh, and I’m a guy.

Bad rings = Uses oil and low compression.
Valve damage = Low compression.



You still didn’t tell me what “VDC” in VDCdriver stands for, and I cannot identify the blue vehicle on your avatar. There are so many SUV/CUV/stationwagon vehicles these days. The blue vehicle by your name appears new and trendy. Is it in fact a Volvo?

I’ve got nothing to add to OK4450 except the first thing I would check is the O2 sensors on a reduced mileage issue. With the timing issue, lack of oil, etc. I think you need to do the basics with the timing checked, compression test for the rings and valves, and a standard electronic diagnosis to check the sensors.

@PrescottAZ–VDC stands for Vehicle Dynamic Control, which is Subaru’s name for electronic stability control. After that feature saved my bacon while driving my '02 Outback 3.0 VDC, I decided that it was a decent moniker to use for internet purposes.

The vehicle in my avatar is my current vehicle, a 2011 Outback Limited 3.6R.
I don’t know how trendy it is, but it is an incredibly comfortable, reliable, and economical vehicle that I will probably keep for 8-10 years, just like my previous Subarus. The photo was taken at NJ’s High Point State Park a few years ago.

As to Volvos, I owned one…once.
Never again!

A little stumble off idle, time for a leak down test. I would skip the compression test as it probably will look good enough to pass, but a leak down test will determine if the valves did get slightly bent due to contact with the pistons. And no, it is not the mechanics fault for not checking this first. He might not have been able to check for this until he got the timing chain back in and timed correctly.

Leak down test?

Engine idle RPM varies from 730-760, according to my ScanGauge. And when I listen and feel for the stumble very closely, I think I can tell one.

It definitely does more of the lurching in low gears. As noted in an earlier post I made. Not as “tractable,” I said then.

And the ScanGauge’s mileage figure seems pretty accurate, as its measure of gallons used was only 0.1 gallons different than what the gas pump said (1% error) and its measure of distance only 0.2 miles different than the odometer (<1% error), last time I filled up. Plus, simple observation of the fuel gauge, which is very inaccurate in this car yet predictable, shows a great difference from what I had been used to for years.

VDCDriver: Interesting description of your handle. I have always heard good things about Subaru’s. I don’t think I have ever heard someone say, “You know, I had a Subaru one time and it was a piece of junk.” My friend, who is a maintenance man and is meticulous in his care of all things mechanical, swears by Volvo, yet I notice things like squeaky door hinges on his Volvo and other things that I would not expect from a quality car.

Various shops and mechanics have their own methodogy and one may do things differently than another.
The reason I asked about whether or not the idle was glass smooth is because one cylinder being down on compression as compared to the others (for whatever reason) or the chain being off a tooth can cause an idle issue that may be subtle or it may be severe; all depending of course.

Considering the oil loss, I would have run a compression test on it just to know where things stand internally. There’s a number of things that could cause a wandering idle or hesitation though.

As an ex-multiple Subaru dealer mechanic, I can say that I’ve heard a number of people curse Subaru to the heavens and will never buy another. Some have traded them off within a month or so of the purchase.
However, I consider them to be pretty decent cars and don’t have an issue with them myself. My last one went 300k rough miles (most after being heavily wrecked) and I kind of miss it.

Don’t read too much into the squeaky door hinges on the Volvos. That’s more than likely due to age, use, or even environmental conditions such as moisture, road salt, and so on.
Given the option of a Volvo or Subaru, my vote is hands down for the Subaru. :slight_smile:

I guess it’s settled then: Subaru wins over Volvo in our straw poll.

Not sure if they ran a compression test or not; however, it is not mentioned on their printed summary. They’re usually pretty thorough in their documentation, but that doesn’t mean the guy didn’t pull the plugs on all four and put the gauge on them…or however it is one does that.

@PrescottAZ–Trust me, I would have been delighted if squeaky door hinges were the most serious problem that I had with my Volvo! Despite buying it as a new car, and despite maintaining it better than the mfr specified, the car was a total disaster, from the paint, to the engine, to the transmission, to the fuel injection system, to the electrical system.

Here is a short list of the problems that I experienced:

As delivered, the heater didn’t work. After 2 non-productive visits to the dealer’s service dept, on the 3rd visit they finally discovered that the heat control knob on the dashboard was lacking the linkage to connect it to the valve in the heater core that controlled the flow of coolant into the heater core. Great assembly quality!

Despite changing the oil/oil filter every 3k miles, the engine began burning oil at ~50k miles. By the time that I finally dumped the car at ~75k miles, it was consuming oil to the tune of 1 qt every 500 miles

The transmission had a leak that nobody could seem to resolve. The transmission shop finally refunded my money for their failed repair attempt. However, this persistent trans fluid leak caused me to have to add 1 qt of fluid every 500 miles or so. Between the amount of trans fluid and the amount of oil that I had to carry for a long road trip, there wasn’t much room left in the trunk for luggage!

Due to bad grounds in the electrical system, in sub-freezing temperatures, all of the lights (including the headlights) operated with just a dim glow, and none of the gauges on the dashboard worked–until the engine reached full operating temperature.

The electric fuel pump burned out ~every 13 months. I got to be very skilled at replacing these fuel pumps, but I wish that it hadn’t been necessary to do it so many times, and that I hadn’t gotten stranded a number of times because of this problem.

The CIS fuel injection system was simply the work of the devil…

The paint was completely chalked w/in about 14 months of buying the car.

The car did have fantastic brakes, it had a good ride quality, and the seats were extremely comfortable. Those were the only good points regarding that car.

If I had more time, I would provide more details illustrating why I owned a Volvo…once.
By contrast, the total repairs on my 3 Subarus (representing 17 years and ~300k miles all told) were less than what I would do in the space of 1 year/10k miles on that accursed Volvo!

Ahhh…Come on, VDCDriver!

Such trivial things! You should really cut Volvo some slack, don’t you think?


Next week, I will be consulting my mechanic once again, and I will post a follow-up discussion then.

Thanks, guys, for all your counsel.


Let me add; I had a life time friend who owned nothing but Volvos for thirty years, till he bought a Croos country. After he had it for three years, I asked how many times he had to change the oil. Never he replied. It has been in so many times for warranty work, it always coincided with the oil change. The next year he traded in for a new Outback. He’s on his second Outback now. I am sure I mentioned this story to you…but I didn’t realize you too came from those dreaded Volvos as well.