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My '04 OBW, 2.5, has an annoying 'stuttering' upon acceleration. Ruled out the EGR valve, felt like it was overactive, but, disconnected, it makes no difference. A Google search led to various Youtube videos, most pointing at Mass Airflow Sensor problems. But I've come to realize that my car has no MAF sensor, relying on a MAP sensor. Can this be the cause? Can it be cleaned?

The car shows no CEL codes, clean air filter, recent plugs, wires, dealer OEM coil, recent cats, car otherwise runs and shifts good, just the annoying stuttering/lack of normal feel. Still getting excellent MPGs, checked at every fill-up.

Any suggestions?

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That's a tough one.  How long has it been going on ?  Did you all the current work hoping to clear the stutter up ? 

Yes, a bad map sensor can cause a hesitation, Low manifold pressure makes the ecu think the engine is not under load  so it cuts the amount of fuel, causing a stutter or hesitation, this also makes for great fuel economy .  New map sensor runs 60-80 bucks or more , grab one from a wrecking yard or borrow one from a good running car and try.

Edited by montana tom

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The problem started over a year ago. It was my wife's daily driver since new, and when she got a '16 Leggy, I inherited it. She noticed it right away. Through the years, I've done all the maintenance and repairs, except the head gaskets. MLS Fel-Pros, hope they last. Just about all I've listed was done before I started chasing the problem, on an as-needed basis. The car just turned 165k miles, so I guess the MAP sensor could be tired.

I'm happy to grab a few from the junk yard, but how can I test them? There's plenty of this gen Outback/Leggys there, but they're all just as old, and with the digital odo readout, you can't tell the miles without a battery. I shot some MAF cleaner up the tube of my MAP sensor, so I'll try it out, but I may have to bite the bullet and get a new one.

Thanks for the quick reply!

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Don't bother testing a used one ... install it .  Get several , chances are if this is your problem one of the wrecking yard ones will change it. I wouldn't buy a new one until I was sure this is really the issue.  After you confirm, then worry about a new one. I see prices on them from 60-  80 bucks as long as its not calf. emission....  those are 180-280 bucks go figure ... 

EDIT)  By the way ,  165,000 is barely broken in.....  mine has 298,000 would drive it to key west today ! Well maybe wait till January.... 

Edited by montana tom

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Yeah, time to head to the yard. Eight bucks a pop tastes better than 60-80 bucks. Thanks...

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New user? I've been here since 2010. I needed to reset my password, and now I'm new? I wish it were that easy for everything else....

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if the problem is gone immediately after an ECU reset (battery disconnect of maybe scanner) it probably is an a:f - related sensor or wiring to one , if not, could be something more a physical nature.

 

Automatic? maybe try the FWD fuse. any torque bind when maneuvering tight - like parking?

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The battery had been disco'd recently, no change in problem. No noticeable driveline issues, strictly, as best I can figure out, an air/fuel issue. As was noted earlier in this thread, a lazy/malfunctioning MAP sensor could indeed result in better MPGs, as it trims fuel to match what it believes to be a lighter load. I'm leaning in that direction.....

Throttle body/IAC was cleaned not too long ago, and it was surprisingly clean. Idle is fine, good feel and speed. The problem was most noticeable in second and third gear, but recently I felt it in fourth/lockup on the interstate, at 70 MPH. As it turns out, no MAF, so, upon suggestion, I'm looking at the MAP sensor. It's probably due anyway.

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When the ECU is reset, the car is reverted back to a stock ROM 'map' of settings - ignoring the sensors until after a coupla 'drive cycles' and the montors are complete. That is, the computer has not learned anything from the sensors and starts with a 'locked in' A:F setting and 'typical' ignition advance.

 

If there's little/no difference in how the car runs after a reset, I doubt you have a bad sensor. I think you should perhaps look for vacuum leaks, cap/o-ring issue on fuel pump, dropped valve guide or burned valve, transmission problem ,etc.

 

 

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I removed the MAP sensor, and shot out the little probe tube with MAF sensor cleaner. I was on the way to the junkyard, to grab a few more MAPs, and the car reacted well. Well enough that I turned around, came home, put it into the garage, and gave it a real good soaking. I'm going to disco the battery, let it sit a while, and maybe it will relearn some new manners.

Thanks for the tip!

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I have the same engine in my 2002.  I changed everything over the last couple of years to fix an unsteady idle and hesitation during acceleration, especially after a stop.  The only two things that made a noticeable difference was changing the knock sensor and changing the upstream oxygen sensor.  Problem all fixed now.

Another thing to look at is oil on the ignition wires, although this causes a slightly different symptoms such is missing at high speeds.

 

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Knock sensor would be my primary suspect. They are difficult to diagnose but when over-active can cause the ECU to pull timing. The ECU will not code for this as it assumes a poor quality fuel situation is to blame. Live data with generic OBD-II gives nothing with respect to knock data on older models, and even in SSM modes only the newer 32 bit ECU's have per-cylinder knock counts so it's less than helpful. Best thing to do if you are seeing it pull timing or getting the typical hesitation symptoms is to just get a new JECS knock sensor. They are cheap and quick to replace. 

FWIW I've never had to replace a Subaru MAP sensor. And they are very easy to diagnose as the manifold pressure is in the live data streams. Should be about 19 to 22 in/hg. You may have to convert units and adjust for the output as it's usually an "absolute" pressure. 

GD 

Edited by GeneralDisorder

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That makes good sense. My '92 Cherokee 4.0 uses a MAP sensor, and I've never touched it. I certainly will look into the knock sensor.

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