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99 Legacy flat below 2000 RPM


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7 replies to this topic

#1 dmplatt

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Posted 02 August 2004 - 09:39 AM

2.2, Auto

I hate to say "dead," but that is how it feels below 2k - there has always been a slight hesitation but it has gotten pretty bad lately.

Step on the gas, count to 3 while you wait for the needle to pass from 1500 to 1900, and then GO!

Any ideas what might cause this? Plugs, wires, and a coil pack all replaced in March...

Thanks,
David

#2 blitz

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Posted 02 August 2004 - 06:04 PM

Does the acceleration delay happen at full throttle, part throttle, or both?

Did the problem suddenly get worse, or did it develop in a gradual fashion?

Are there any driving techniques you've found that minimize the flat spot?

Also, is there anything else that changed about the performance of the vehicle? Noise? Vibration? Smells? Fuel mileage, etc? Any CEL codes?

#3 dmplatt

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Posted 02 August 2004 - 06:49 PM

Some clarification - power is normal from idle (700) to 1500, then flat until 2000, then away we go...

Happens at both full and part throttle, no technique to minimize, and no CEL. I'm am not sure that it was "suddenly" but it is definitely worse than usual.

Fuel mileage has been lower for about 3 weeks, but it has been HOT and I always use the A/C - not to mention my driving has been all around town which is unusual for me, so I think I can attribute it to that.

No other noises or smells that weren't there before.

#4 blitz

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Posted 02 August 2004 - 07:35 PM

Only a guess, but the symptom as you describe it, has the hallmark classic knock-sensor false-triggering thing going on.

Subaru's flintstone-era knock sensor has a tendency to pick up mechanical resonances from the crankcase (to which it's mounted) and mistakenly interpret it as spark knock. The ECU then rapidy removes about 15 degrees of spark lead (which is allowed to return to normal in the absence of further triggering) causing the sudden "air pocket" in the acceleration.

If you'd changed to a different oil recently, that could be the difference in resonances which the knock sensor is picking up.

#5 blitz

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Posted 02 August 2004 - 08:32 PM

HESITATION ON ACCELERATION - 11-53-98 (2/99)

Applicability: 97-98 Legacy, Impreza, and Forester Manual Transmission Vehicles with 2.5l and 2.2l engines.

In the event you encounter a customer complaint of a slight engine hesitation between 1500-2500 rpm's when the engine is cold or hot, perform the following

Eliminate all external influences, such as an incorrect or dirty air filter, loose or tracked intake duct, dirty fuel filter, low fuel pressure, PVC system, or low engine vacuum that would indicate an external leak or an internal engine component.

If all external components are confirmed to be operating within vehicle specifications, the hesitation may be caused by the ignition control logic in the ECM. Under certain low rpm driving patterns, the ignition control system can pick up engine vibrations through the knock sensor and may retard the ignition timing. This ignition timing is learned by the ECM and placed in memory. NOTE: This area of memory can not be viewed by using the Select Monitor. When the vehicle is driven uinder these conditions, the timing may be retarded and could cause the engine to hesitate on acceleration. To confirm this condition, road test the vehicle while viewing the Knock Sensor Signal on the Select Monitor. If you duplicate the hesitation, and the reading on the monitor is around -10 degrees, you will need to change the ECM to correct the concern. [my note: this is an enhanced ECM, not a replacement with the same component]

#6 blitz

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Posted 02 August 2004 - 08:47 PM

OK, one more well-seasoned quote:

"The hesitation experienced in many Subarus during low speed acceleration is due to the engine management computer's overreaction to noise that is sensed in the knock detector, which is essentially a microphone on the engine.

The knock sensor is a piezoelectric microphone that is situated on the top of the engine block, approximately seven inches below the bare throttle cable near the back of the intake manifold. It is black in color and shaped like a stack of eight, 3/8 inch size, flat washers. That is to say that it is a doughnut shaped piece, the diameter of a quarter. It is bolted to the engine with one, vertically situated, eight millimeter bolt that is about 40 millimeters long which accepts a 12 mm socket wrench. It has one wire that plugs into a wiring harness just below the throttle cable.

To save the computer from sensing ambient engine sounds that may be mistaken for engine knocking, the sensitivity of the microphone needs to be reduced. The easiest way to accomplish this is to insert a layer of vibration dampening material between the knock sensor and the engine block. This may result in the need to replace the 40 mm bolt with one 50 or 60 mm in length.

The prototype that has cured the hesitation problem is a teflon spacer beneath the sensor with a second layer of urethane beneath that. The final solution may be a less sensitive knock sensor or an adjustment to the engine management computer."

#7 dmplatt

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Posted 03 August 2004 - 10:50 PM

Blitz-

You mention different oil could cause the issue - could "bad" gas be the culprit? I ran the tank pretty low and filled it up last night, slight hesitation this morning - completely gone this evening (I've driven about 30 miles today).

#8 blitz

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Posted 04 August 2004 - 11:10 AM

Well, it could be this, that, and the other. It's impossible to diagnose an unfamiliar vehicle remotely, but if filling up with a fresh tank of gas fixes the problem, then assume it was bad gas.

The big secret to problem solving is actually allowing for verifiable problem resolution as, opposed to jumping to premature conclusions or any involment re: wishful thinking. :)

Always make one change at a time, then measure the result.




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