Cleaned MAF and IAT - motor still bucks/hesitates 2500-3000 RPM, no CEL

Just what the subject says. It's noticeable mainly when accelerating slowly through that RPM range. Hard acceleration is unaffected.

Cruising is mostly unaffected, but... on a recent roadtrip, lots of cruising took place right around 3000 RPM (75 MPH, 6th gear); with no changes in speed, wind, or slope, the instantaneous MPGs kept see-sawing between high 30s and low 20s.

What's this unmodified, unraced, dealer-serviced POS doing to me now that I'm out of warranty?

[ForestGrump]

how many miles?

I'm at 42,900 now. I would say the trouble started at about 41,500. I just put 900 miles on it during my road trip.

I forgot to mention another, similar symptom: While cruising in this RPM range, regardless of speed/gear, that same bucking and/or hesitation occurs when I accelerate.

In all cases, the bucking/hesitation is very mild and just barely noticeable.

[ForestGrump]

I'm going to say clean the EGR valve. It is sticking and not opening/closing properly. search the forums for p0401. Others have reported the car no longer bucks/hesitates after cleaning.

I have 70k on mine, and it threw the p0401 at around 66k. I have to say it drives better now that I cleaned it, but I also did spark plugs at the same time.

[kingkam]

[quote author=Farnsworth link=topic=192842.msg4060718#msg4060718 date=1309370356]
Just what the subject says. It's noticeable mainly when accelerating slowly through that RPM range. Hard acceleration is unaffected.

Cruising is mostly unaffected, but... on a recent roadtrip, lots of cruising took place right around 3000 RPM (75 MPH, 6th gear); with no changes in speed, wind, or slope, the instantaneous MPGs kept see-sawing between high 30s and low 20s.

What's this unmodified, unraced, dealer-serviced POS doing to me now that I'm out of warranty?
[/quote]

could be one of your oxygen sensors or hpfp

Goddam what a PITA. I think I'll just live with it until I can dump this POS on a bigger sucker than me.

http://www.themazdaforum.com/showthr...zdaspeed-3-EGR

Fuel pump? Really? My worthless 1996 Oldsmobile at last waited until 60k miles before eating its fuel pump. My MS3 is a bigger pile than a 90s GM product... awesome.

[ForestGrump]

It isn't all that hard. My EGR had buildup, but the pipe and intake tract were clean. If I were to do it again just the EGR valve. If you want to dump the car, I'll be your sucker (if the price is right)

[t147]

it is not a fuel pump and it's not a O2 sensor. O2 sensors do not cause driveability issues like this. Don't say it's a wideband... the computer will compensate for a burned out O2 with the second's readings and preset values then it illuminates the MIL. A fuel pump wouldn't cause an issue at just one RPM. His problems would be all over the place they wouldn't be RPM specific. I'm not saying it's not fuel related, just not a fuel pump. Too many uneducated people on this site go ohh it's a fuel pump replace it and then they're like om nom it's not the fuel pump.

How is your idle? You call your car a POS, if you had some sort of monitoring device, even a boost gauge that read vacuum in this case, diagnosis would be a little more simple.

here are a few things can cause what you described, water in fuel, oil soaked air filter, cracked spark plug, arching plug or coil pack, dirty fuel filter, and a multitude of other things.

If this were my car, I would get some dry fuel from the gas station or auto store (.98) , replace fuel filter because it needs done anyway and supposed to be before 40K. Fill up my car (91+) at a shell or somewhere reputatble not cirlce K or Sheetz or loaf and jug. If none of this solve the problem, then I would use a Multimeter and monitor slowly my throttle from closed to WOT. With a slow sweep you should see a smoothish curve from bottom to top without dropouts. If there's a drop out, and is consistent if you do the test a few more times, you need a throttle position sensor. If none of this works, you need to take the car to a sevice station you trust or a stealership.