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Chris (aka- Moose)
Chris (aka- Moose), Technician
Category: Mazda
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Experience:  16 years of experience
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2003 MPV. Bucks and misses terribly under load. Recent

Customer Question

2003 MPV. Bucks and misses terribly under load. Recent development. Idles OK, runs well when cold and under light load, but try to accelerate and it feels like super lean miss or ignition failure. Scan shows misfire on cyl 2, but that goes on and off. A single cyl miss is not bad enough to cause the terrible missing under load. Checked MAF sensor, which comes out low on specs 1.2v at 1500 and 1.5v at 2500, versus specs of 1.4 and 1.8 respectively. Not great, but not bad either. MAF voltmeter jumps up sharply on accel, does not move slowly up. Most everything else checks out. Yes, plugs are old, car has 92K, but the MPV club forum is full of guys changing plugs and coils and not getting a fix, or long term fix, from this kind of missing under load. I am looking for ideas before buying a new MAF or testing something else.
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Submitted: 7 months ago.
Category: Mazda
Expert:  Chris (aka- Moose) replied 7 months ago.

Welcome, I'm Chris (aka Moose). Experts aren't employees / agents of Just Answer. We are independent service providers using the Site to sell their Expert knowledge to Customers. I strive to provide EXCELLENT 5 STAR service to you, let me know what's needed from me so your pleased with my assistance and will rate me Good or Excellent.


You must first go after and address the codes you have. That means a new plug and wire on cylinder 2. If needed then a compression test on 2 and surrounding cylinders. If needed then a injector flow and scope test. You should be getting other misfire codes if the MAF sensor was to blame.


Thanks for asking your question and letting me try to help you.

Please make sure to complete the rating on your screen when your pleased.

Follow up/Reply as many times as needed before or after rating.

Expert:  Chris (aka- Moose) replied 7 months ago.

Hello again Chris here,

1) When customers come to Just Answer the website and the experts want them to have a great 5 star experience.

2) We also want them to be so pleased that they come back for future needs.

3) I have the feeling you were not pleased with my help and or need me to help more. If so please do reply.

4) I have an option to upload diagrams, repair guides, find a repair facility, or call you if that would help.

Thanks Chris

Customer: replied 7 months ago.
Chris -Yes, Item #3 meets my response: " I have the feeling you were not pleased with my help and or need me to help more. If so please do reply."The car, as noted, runs terribly under any load. The issue is load related, because under light throttle, as in cruise or going straight or downhill with vacuum high, there is no misfire. The misfire under load is so severe that even one disconnected plug could not account for the poor engine response, especially since there is no miss under light load. As noted, this JUST started. The bucking and misfire happened to my wife on the freeway on a rainy day. Later, I could not duplicate the problem, the engine ran just fine. About a month later, the problem became permanent. To all my experience, this indicates something going steadily bad, like a sensor or a poor connection causing a missed signal/ground, etc. All connections were cleaned and checked. From research, such poor response under load points to things like a bad MAF or throttle position sensor (not just a misfire from one coil, plug or injector, as the mileage is unaffected) is as likely cause. The misfire feels like a fuel delivery problem, but it goes away if the vacuum is kept high or RPM exceeds 3K, wherever the load lets vacuum rise. My inspection of the plugs on bank 2 indicated a regular mix - nice tan color. Car does not burn oil. There is a tendency for this condition to disappear when cold, pointing to possible connection or sensor failure when all signals come up upon warming. I think other factors, beside a misfire on cyl 2, are much more likely to be the cause than a bad plug or coil/injector. I was going to eventually replace plugs and bank 1 coils anyway, but, as the car was running very strong and averaging some 16 mpg before this became a regular problem last month, the plugs and coils were far from my suspicion as a likely cause. Perhaps now you could understand my lack of pleasure with your reply?In any case, I have accelerated my plan to replace all plugs and get new coils for bank 1, although I doubt this will solve the problem. I also doubt any mechanical issues, but will test compression anyway. I fully expect this problem to persist, after replacement of plugs and potentially bad coils, without the system throwing any codes to indicate a problem. We will see.
Expert:  Chris (aka- Moose) replied 7 months ago.

So do you want me to keep helping or do you want to close the post or try another tech?

Customer: replied 7 months ago.
Well, it would not have hurt if you had some ideas other than the plug/coil/injector on cyl #2. In any case, I would appreciate any further help, after I button up the coil and plug change and test it out. Compression is fine on #2. We will see what happens.
Expert:  Chris (aka- Moose) replied 7 months ago.

Thanks for checking the compression. Do you have a scan tool that can monitor the TP reading and MAF sensor?

Customer: replied 7 months ago.
No codes remain. Acceleration weak but steady, like a lean condition. Will review fuel trims.
Expert:  Chris (aka- Moose) replied 7 months ago.

Keep in touch if needed.

Customer: replied 7 months ago.
Fuel trims are fine on Bank 2, with total (STFT+LTFT) typically under less than +/-3 except for occasional spikes to -6 or -7. Bank 1 has higher negative STFT's and much higher negative LTFT's. Total trim Bank 1 goes double digit negative frequently, only 2 positive totals (28 samples).High negative total trims on bank 1 are most common at idle, deceleration and low throttle. If the PCV basically vents to Bank 1, perhaps that might explain the fuel trim difference? I do not see anything in the data that would help explain the car's poor performance under load.
Expert:  Chris (aka- Moose) replied 7 months ago.

The PCV dumps into the intake so both banks get equal PCV flow and excessive PCV flow would indicate an internal engine issue.

The trims are effected by the readings from the O2 sensors. Since your getting negative trims over -10 that means the O2 sensor is seeing not enough oxygen left over after combustion. Which means the computer observes this as rich, to much fuel getting out the exhaust. Things to cause a rich issue are a misfire, stuck open injector, stuck open exhaust valve.

Customer: replied 7 months ago.
Mechanical problems unlikely, no other codes. Leaves injector or O2 sensor, right?
Expert:  Chris (aka- Moose) replied 7 months ago.

Thats correct, but I would expect a O2 code if the O2 were bad. You can swap the O2 sensors to see if the concern moves to the other bank.

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