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Guest Message by DevFuse
 

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Engine stumble on slight acceleration


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8 replies to this topic

#1 Guest_PAezb_*

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Posted 04 August 2003 - 08:17 AM

OK, need some opinions:

96 Outback wagon, 2.5 with 116K.

The car run great generally, good acceleration, cruises fine, good gas mileage. But I've got an annoying problem when after reaching normal engine temperture (running it at highway speed for several minutes), when I come to a stop, and then ever so lightly press (1 to 1.5k rpms)on the gas pedal, the engine runs rough and stumbles. This is not noticeable when I accelerate a little harder from a stop position.

Under the hood when I move the throttle a little by hand, I notice the EGR valve is opening, what appears to me, prematurely on the slight acceleration. It also opens momentarily on abrupt accelerations too (2-3k rpm), but it doesn't stumble, I think that is normal - correct? If I disconnect the vacuum hose on back pressure transducer, effectively disabling the EGR valve operation, the engine will stumble on slight accelerations.

I've checked all the vacuum hoses for cracks/leaks. The EGR valve movement by hand appears to be ok. So, opinions whether it may be EGR valve, EGR selanoid, back pressure transducer, or any combination thereof?

BTW, after a road trip yesterday, the ECL came on. Engine runs fine, except for the slight acceleration issue. It may be related but I wont know until I get the code(s) pulled at AutoZone. A side note, I called my dealer where I purchased the car, Morries Subaru in Minnesota, to find out what they charge just to pull the codes - $89.95 to pull the codes and reset the ECU !!

Dealers!!


Thanks, Paul

#2 Guest_PAezb_*

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Posted 04 August 2003 - 08:22 AM

Sorry, a sentence in my post above should read:

"If I disconnect the vacuum hose on the back pressure transducer, effectively disabling the EGR valve operation, the engine will NOT stumble on slight accelerations...."


I need my morning coffee....


Paul

#3 Guest_1 Lucky Texan_*

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Posted 04 August 2003 - 08:23 AM

Although I'm a new Subaru owner and am less familiar with them specifically, I've had a coupla' other vehicles with the same symptom you're experiencing and it was the TPS in both instances.
fyi
Carl
1 Lucky Texan

#4 Guest_97SubaruGT_*

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Posted 04 August 2003 - 01:15 PM

My 97 Legacy does exactly the same thing, similar milage on mine.
Sometimes it does stumble and cough for a second then once the RPMs go up, it picks up and it's fine.
I had my EGR valve/pathways cleaned several months ago since the CEL came on with EGR Pathway Malfunction code. The local shop cleaned it and re-set the codes and it did not come back since. Don't remember if I had this problem before the EGR service.
I think it happens more often when I change gears out of P or N, like at a light. Sometime at a long red, I shift the auto into N and then when I shift back, it takes a second to engage. Could it be the tranny?

Any help would be appreciated.

#5 Guest_PAezb_*

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Posted 04 August 2003 - 02:22 PM

I just had the car at AutoZone, one of the counter guys brought out their OBDII scanner. Forgot the actual code but the display read "EGR Flow Malfunction". So, price on a new EGR valve is approx $160, the EGR Solenoid around $80 (AutoZone and Dealer are comparable). I have no way of telling which it is, or both, or a block/clogged vacuum line. So, I ordered an EGR valve gasket ($10 at AutoZone), when it comes in I'll pick up some new vacuum hose too. I'll try cleaning the old valve, and replace new vacuum hose on the EGR system.

Interestingly, the AutoZone guy was pretty sure it is the EGR valve, the dealer parts person I spoke to says that the it's very rare that there is a problem with the valve, and more likely the solenoid. So, who to believe?

Hopefully, this just need a good cleaning. I'll let you know how it comes out...:rolleyes:

Paul

:rolleyes:

#6 Guest_Commuter_*

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Posted 04 August 2003 - 06:32 PM

Do you have an auto or a manual transmission?

I have a 97 OB, automatic. I got the "EGR Malfunction" code several months back (PO400 IIRC). The dealer told me that there was nothing wrong with the EGR. The fault stemmed from a little pressure sensor (more like vacuum sensor) that mounts on the passenger strut tower, toward the front of the car. It is black, about 3/4" cube, has 3 wires going into it and a vacuum hose connection. And it is expensive! :mad:

The vacuum sensor is one of the inputs to the automatic's shifting algorythm I was told. As a secondary function, it "checks" the EGR. That is, it looks for a pressure (vacuum) change when the EGR is triggered. Since my sensor failed (or rather was dying), it erroneous threw a CEL for the EGR.

One thing I did notice was slightly sluggish and long shifts on occasion. Not bad, but noticable. Once the sensor was changed, this disappeared. I too have some stumbling upon slight acceleration, most noticable in the low revs. However, this did NOT change. I think I have a carbon issue again... like 2 or 3 years ago. Changing my O2 sensor has helped, but I think I'm going to have to do one of those MotorVac things again.

So... just a word of caution. It very well could be the EGR itself. But it might be the little pressure sensor too. I think these cars are just reaching the age where this sensor is starting to fail. It was somthing that the dealer had just learned about recently as a possible source of the problem with an EGR CEL code. I'm not entirely sure, but I don't think the manuals have this sensor.

Something else to check. I had an email with one person (96 OB) who said he'd check this sensor against the Haynes (or Chiltons) data and it did not meet spec. He then tested his wife's car (a 97 I believe) and it tested out of spec too. *shrug* |I I have no idea what this means. Perhaps they were both fine... perhaps they were both failing. Just a word of caution, since that little black cube is darn expensive.

Commuter

#7 Guest_PAezb_*

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Posted 04 August 2003 - 08:20 PM

I remember seeing you post about that awhile back.
The auto tranny seems to shift fine though, crisp but not jerky, and no hesitation. I did removed and cleaned the EGR valve and gasket (guess I didn't need to order a new gasket after all - it's metal, careful cleaning and you can reuse it). Wasn't too bad, carbon/soot wise, and the valve seem to work ok, diaphram looks good. Replaced a couple of vacuum lines too. Reinstalled, but no difference. I beginning to suspect the EGR solenoid now.

Thing is, when the engine is warmed up to full temp, and if I gentle nodge the throttle ahead by hand on the throttle body, I can see the EGR valve open - thats when the engine bogs down. Sometimes it opens then quickly pops closed, with no impact on the engine. Other times it opens and stays open, bogging the engine, until you let off the throttle a little, then it pops closed...hmmmm.

At this point, I may let the dealer take over and tack me up by my wallet. I think I also have a belt tensioner thats beginning to knock. And replacing the tensioner, if it is the tensioner, may be a bit beyond my skill set, and tools available.

Anyway, I think I've done well to diagnose and save some cash by fixing the last few months of problems myself - thanks to all of you in this forum, and my local library where I am able to check out the Hayne's Repair Manual for this car. Just seems that a lot of sensors and other things are failing at once. I guess it's probably to be expected at 117K miles. Still like my Subie though - getting 27-28mpg on the highway, and I love that!

I'll let you know what I find out.

Paul

#8 DAB

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Posted 15 June 2006 - 08:28 AM

OK, need some opinions:

96 Outback wagon, 2.5 with 116K.

The car run great generally, good acceleration, cruises fine, good gas mileage. But I've got an annoying problem when after reaching normal engine temperture (running it at highway speed for several minutes), when I come to a stop, and then ever so lightly press (1 to 1.5k rpms)on the gas pedal, the engine runs rough and stumbles. This is not noticeable when I accelerate a little harder from a stop position.

Under the hood when I move the throttle a little by hand, I notice the EGR valve is opening, what appears to me, prematurely on the slight acceleration. It also opens momentarily on abrupt accelerations too (2-3k rpm), but it doesn't stumble, I think that is normal - correct? If I disconnect the vacuum hose on back pressure transducer, effectively disabling the EGR valve operation, the engine will stumble on slight accelerations.

I've checked all the vacuum hoses for cracks/leaks. The EGR valve movement by hand appears to be ok. So, opinions whether it may be EGR valve, EGR selanoid, back pressure transducer, or any combination thereof?

BTW, after a road trip yesterday, the ECL came on. Engine runs fine, except for the slight acceleration issue. It may be related but I wont know until I get the code(s) pulled at AutoZone. A side note, I called my dealer where I purchased the car, Morries Subaru in Minnesota, to find out what they charge just to pull the codes - $89.95 to pull the codes and reset the ECU !!

Dealers!!


Thanks, Paul

I went to our subaru dealer after listening to Car Talk and bought a coil pack and for ten bucks installed it, and no other problems to date. THe dealer even let me have the pack at wholesale price because they had not been able to find the problem...they charged $45 for a computer check...but we are in the South and maybe some things are cheaper. DAB

#9 blitz

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Posted 15 June 2006 - 10:16 AM

This may or may not be applicable for your specific problem:
-----------------------------------------------------------------------------

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 cannot be viewed by using the Select Monitor. When the vehicle is driven under 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.]

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