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Old 07-23-2008, 10:11 AM   #1
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Check engine light comes on

I own an old Mazda 626 (1995 ? 130,000 miles ? 4 cylinder ? automatic) and used to get 22-25 mpg for a mixed city and hwy drive.

Few days ago I started to put in practice few easy gas saving techniques that make perfect sense, but never thought they might matter a lot. Basically what I do is I avoid hard accelerations, drive at speed limit or lower and coast as much as I can. So I can?t wait to see the result when I will fill the tank next week.

However the thing that concerns me most is that the check engine light started to turn on within 5 minutes from a fresh start in the morning. Does it have anything to do with the fact that my RPM rarely gets above 2000? In the past the oxygen sensor used to cause the red light to come on and I had to replace it almost every other year. When it was causing the problem it was mainly during hot days and usually while climbing a hill with high RPM or during hard acceleration, but the light would stay on for only 20-30 seconds and then shut off. Well, now it?s happening at very low RPM and during rather cold conditions and the light stays on for minutes.

Many decades ago my father took his car to a mechanic to have the valve seats re-shaped. They were all burned up and compression was poor. Also the top of the cylinders were all covered with stuff that looked like burned saw dust. The mechanic said all this is caused by running the engine at too low RPM?s. My father was indeed avoiding high RPM?s because he thought it will cause the engine to wear out sooner. I wonder how new engines handle combustion at low RPM?s.

Thanks,
Dual Trace
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Old 07-23-2008, 10:19 AM   #2
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Low RPM driving shouldn't throw a code. Then again, your car shouldn't need a new O2 sensor every two years.

Be sure to pull the code that set your Check Engine Light (CEL). Unfortunately, 1995 is one year before the OBDII standard was required in all US market cars, so it probably doesn't have OBDII. You may need a less common specialty code scanner to get the code (or a friendly mechanic).

Carbon build-up can be avoided easily. I like to just rev it up once in awhile while making a difficult left turn or while in a hurry; that should blow out any carbon buildup without requiring me to rev high on a daily basis. There are, of course, other methods for avoiding it...and maybe it's not much of a problem with modern engines and unleaded fuel, anyway.
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Old 07-23-2008, 10:37 AM   #3
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I would say if you are going thre O2 sensors that quick you should look at having the catalytic converter replaced, I believe that is your culprit...

Also stop by an auto parts store, at least around here they will read check engine lights for free, tell you what the code is and rest it...

My normal method is to check the light, write down the code, rest it, then drive normally...if the same code comes back then start looking at what might be causing it...a blinking CEL is usually more important and critical then a solid one, usually the owners manuals tell you to pull over and not drive the vehicle if the light is blinking...

As for carbon build up, I use a product called Seafoam, love the stuff works great...personally never had any problems with it, used it in my Mazda, my Hondas and some friends cars...but I did have a friend who had some weird issues after using, not sure if it was directly related or not but just a word of caution...read the label and follow the directions if you decide to go that route GL HTH
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Old 07-23-2008, 10:42 AM   #4
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My Mazda was made for Canada and not any shop here in US can read the code. I always have to go to a Mazda dealer/shop to have the code read and three times they found a faulty oxygen sensor and once it was the EGR valve. Lots of money…

It may not be the oxygen sensor this time and it might be that whatever it is, it coincides with my gas saving experiments.

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Old 07-23-2008, 07:20 PM   #5
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Clean the MAF*, check for oil in the intake, if found replace PCV breather filter and PCV valve, run a bottle of 70% or greater concentration isopropyl alcohol through a tankful to clean everything up. (Major constituent of many flue system cleaners, emissions cleaners etc, and also sold as gas line antifreeze)

(*Hosing it off with brake cleaner usually works, carefully though, make no mechanical contact with the wire element, it will break. Alternatively the element assembly if you pull it out will usually jam nicely into the mouth of a gatorade bottle, a clean one, into which you can add an inch of the isopropyl alcohol in the bottom, and shake hell out of it)
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Old 07-24-2008, 10:07 AM   #6
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Road Warrior,

Now this is beyond my knowledge and most likely skills. Things I can do are just change bulbs, spark plug cables and other easy stuff.

What are MAF, PCV breather filter, PCV valve?

Also what is that supposed to help for? Check engine light? Or is it just good practice?

Thank you,
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Old 07-24-2008, 10:38 AM   #7
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On Mazda's and Fords a frequent problem is that the Mass Air Flow sensor, which is a hot wire sensor, it's degree of cooling and hence resistance is used to derive airflow for the ECUs fuelling calculations often becomes dirty, either from city fumes, or from oil spray in the intake due to a clogged PCV (Postive Crankcase Ventilation) valve. This will cause the fuelling to be off and the check engine light to come on, typically will give a code about injector pulsewidth or something, meaning it can't run as rich as it thinks it needs to.

The MAF is typically in a plastic housing immediatly after or integrated into the air filter box, it typically has a flat 4 wire connector plugged into it on top. IF you have the tools (often a security torx or TX star bit) to remove it from the top side you can slide the sensor element out, it will be like a round plug with a pen diameter tube mounted under it, inside which you can see a couple of should be shiny wires, which may look dull or black. That's the bit to hose off with cleaner, but don't touch the wires. IF you can't get into it from the top, then removing the housing, or peaking into the top of the air filter box, (Undoing a couple of hose type clamps necessary) you should be able to see the tube assembly at the top, and be able to squirt it with something from there.

The PCV breather will be in the airbox under the filter and will probably be black, oily and disintegrating, you can either buy a replacement or make one from plastic scouring pads. There may be just a rectangular mounting with a hole in one side to which a small hose connects, meaning a previous owner removed it altogether...

The PCV valve is typically on top of the valve cover with a hose coming off it and connecting to the throttle body (The big metallic mechanism on the air hose with all the throttle cables) These are just pushed or screwed in.

Typical costs, new PCV valve, $5, new breather filter $5, spray can of brake cleaner or carb cleaner $5.

Manuals, or dealers will tell you that the whole MAF assembly must be replaced at a cost of $80 upward...

So that may clear the check engine light after a couple of runs, and the isopropyl in the tank, I would recommend for gently degunking the engine after it's been running rich with PCV splatter spraying in there for a while.

If you need additional help with any of that, look at the autozone.com website and on the left panel there's a repair guides link, entering your make and model should give you an index of repair procedures, and you should be able to find pics of where the PCV valve is and how to change it, and where the MAF is (but they probably don't mention cleaning it)

Total time to do all of this is half an hour, but if you're double checking and second guessing the first time allow an hour. Also might take longer if you discover an oily mess all over your airbox and intake and decide to clean it up. Might be a good time to pick up a new airfilter also if you have not checked or changed it recently.
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Old 07-24-2008, 01:51 PM   #8
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Thanks a lot for taking the time to walk me through all this. Will also have a look at autozone.

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Old 07-27-2008, 12:04 AM   #9
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Yeh you want to get the codes from it.
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Old 08-13-2008, 08:56 AM   #10
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Well, a mechanic recommended by a friend had a look at it and said the code points to the oxygen sensor. He also looked at the MAF and said it?s clean, so he changed the oxygen sensor. Check engine light is OK now for more than two weeks.

Here is my mileage chart. The blue trace is since I started to put in practice some simple fuel saving techniques. My highway and city miles keep changing, so I am not sure yet how much it helps.
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