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Please try this simple test of your exhaust


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Ok, my 97 Legacy Outback wagon has a really funky "flat spot/hesitation/miss at between about 800 and 1200 rpm. Engine cold or hot doesn't matter. With the AC on and idle at about 900 or so, I'm sitting in a paint shaker at the traffic light and it is driving me nuts!--oh, manual transmission so the driveshafts aren't an issue.

 

I think at least one of my catalytic converters may be stopped up or failing, and here is a test one of you guys can try and reply back to the thread...please!

 

Ok, start car with cold engine without touching the accelerator. Starts right up and is running at about 1500 rpm.

 

Test 1: At the initial starting RPM, check the amount of exhaust pressure you feel on your hand by holding your hand close to the tailpipe. Make mental note. I'm not feeling much at all at the initial idle of 1500 RPM.

 

As the engine warms up, the RPMs decrease to 800 or so RPM, with a few slight up/down episodes. Now that it is fully warm and idling at 800 RPM....

 

Test 2: Again, how much pressure do you feel at the tailpipe with your hand? I'm feeling MUCH more exhaust flow against my hand at 800 rpm and warmed up than at 1500 rpm with a cold engine.

 

Give it a try for me, would ya? I'm thinking my cats are toast and the 800-1200 rpm funkiness may be the engine trying to overcome the obstruction.

 

Thanks!

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Ok, my 97 Legacy Outback wagon has a really funky "flat spot/hesitation/miss at between about 800 and 1200 rpm. Engine cold or hot doesn't matter. With the AC on and idle at about 900 or so, I'm sitting in a paint shaker at the traffic light and it is driving me nuts!--oh, manual transmission so the driveshafts aren't an issue.

 

I think at least one of my catalytic converters may be stopped up or failing[...]

Think of your engine as a "pump". Problems at either the intake or output (exhaust) can have similar results, to say nothing of the "pump" itself. If you own a vacuum gauge (if you don't, and want to do engine diagnosis, I strongly suggest that you get one), you can check for exhaust restrictions and a whole lot more. Rather than my just explaining how to check for restrictions, here are links to interpreting gauge readings (search the Web for others):

http://www.classictruckshop.com/clubs/earlyburbs/projects/vac/uum.htm

http://www.obd-codes.com/howto/diagnoseengine.php

 

A vacuum gauge can be purchased for under $20, unless you buy a fancy one with a chrome bezel meant for dash mounting; in that case, some can cost more like $50.

 

Of course, due to management systems, a poorly running engine may have electronic problems in addition to or instead of mechanical ones. Fortunately, a properly-used vacuum gauge can help to determine which is more likely.

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What? I got no takers on the test!? IF you do it like I described it, I promise you won't slip with a wrench and bust your knuckles!:lol:

 

The vacuum gauge test I agree would be a good one, but for a "quick and dirty" comparison to start with....:brow:

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Ok, I just tried it, and I added one thing to your test. My son helped out, by adding an extra bit of pressure to the accelerator pedal once the engine was warm, to get the RPMs back up to around 1500. Here's what I observed -- engine cold and idle at 1500, there was a light exhaust pressure on my hand. Engine warm and idle at around 800-900, apparently stronger "puffs," but they came more much more spaced out than when the idle was at 1500 (slower puffs, no surprise). Then we tried taking the RPMs up to 1500 while the engine was hot, and what do you know? The pressure felt the same as with the 1500 RPM idle cold, although the exhaust was definitely hotter! Don't keep your hand near the exhaust pipe too long if you try this once the engine is hot. :D

 

My conclusion - this test doesn't tell you definitively if the cats are plugged, or not. Sorry!

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The vacuum gauge test I agree would be a good one, but for a "quick and dirty" comparison to start with....:brow:

If nothing else, a vacuum gauge won't leave your hand smelling like burnt dinosaur :D . And yes, I have in the past stuck my hand in car exhaust streams.

 

Although it would be a lot easier to see on a gauge, does the "power by palpation" test lead you to believe that the cylinders are firing evenly, or do you notice a periodic drop in exhaust pressure? If it's uneven, I'd consider a single-cylinder "misfire" scenario, which could have several different causes. If it's relatively constant (considering that with a 4-banger, the power cycle doesn't come around very often at idle ;)), I'd start with the simple. Maybe all you've got is a vacuum leak; check for disconnected or split vacuum hoses, or a bad power brake booster.

 

I still think the vacuum gauge is an under-appreciated and under-utilized tool. :)

 

EDIT: A couple of more links to interpreting gauge readings:

http://www.ifsja.org/tech/motors/vacuum.shtml

http://www.v8sho.com/SHO/IntakeManifoldVacuumTest.htm

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Ok, my 97 Legacy Outback wagon has a really funky "flat spot/hesitation/miss at between about 800 and 1200 rpm. Engine cold or hot doesn't matter. With the AC on and idle at about 900 or so, I'm sitting in a paint shaker at the traffic light and it is driving me nuts!--oh, manual transmission so the driveshafts aren't an issue.

 

I think at least one of my catalytic converters may be stopped up or failing, and here is a test one of you guys can try and reply back to the thread...please!

 

Ok, start car with cold engine without touching the accelerator. Starts right up and is running at about 1500 rpm.

 

Test 1: At the initial starting RPM, check the amount of exhaust pressure you feel on your hand by holding your hand close to the tailpipe. Make mental note. I'm not feeling much at all at the initial idle of 1500 RPM.

 

As the engine warms up, the RPMs decrease to 800 or so RPM, with a few slight up/down episodes. Now that it is fully warm and idling at 800 RPM....

 

Test 2: Again, how much pressure do you feel at the tailpipe with your hand? I'm feeling MUCH more exhaust flow against my hand at 800 rpm and warmed up than at 1500 rpm with a cold engine.

 

Give it a try for me, would ya? I'm thinking my cats are toast and the 800-1200 rpm funkiness may be the engine trying to overcome the obstruction.

 

Thanks!

 

a cloged converter would act as a choke or govenor on the engine. Two things will happen. The top end and acceleration will suffer, you wont have a problem at low RPM. Secondly the cat is going to be running really hot, possibly red hot.

ALso there is the exhaust pipes and muffler that can collapse internally that can give you the same symptons.

I dont remeber if there are two O2 sensors on this car. If there is you would get a cel on a catlyst effecieny code. If not take it to a shop where they can properly diagnose it. This is too expense a thing to guess at. but i dont think its the cat.

 

nipper

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I like the vacuum gauge method.

 

Hands in exhaust streams are a not really precise tests. By the way, and engine is a little more than an air pump, since the fuel explosion creates a much larger volume exitting than entering the engine. Those gauge reading tips have taken this into account - they note ignition timing as causing some problems.

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Ok, my 97 Legacy Outback wagon has a really funky "flat spot/hesitation/miss at between about 800 and 1200 rpm. Engine cold or hot doesn't matter. With the AC on and idle at about 900 or so, I'm sitting in a paint shaker at the traffic light and it is driving me nuts!--oh, manual transmission so the driveshafts aren't an issue.

 

I think at least one of my catalytic converters may be stopped up or failing, and here is a test one of you guys can try and reply back to the thread...please!

 

Ok, start car with cold engine without touching the accelerator. Starts right up and is running at about 1500 rpm.

 

Test 1: At the initial starting RPM, check the amount of exhaust pressure you feel on your hand by holding your hand close to the tailpipe. Make mental note. I'm not feeling much at all at the initial idle of 1500 RPM.

 

As the engine warms up, the RPMs decrease to 800 or so RPM, with a few slight up/down episodes. Now that it is fully warm and idling at 800 RPM....

 

Test 2: Again, how much pressure do you feel at the tailpipe with your hand? I'm feeling MUCH more exhaust flow against my hand at 800 rpm and warmed up than at 1500 rpm with a cold engine.

 

Give it a try for me, would ya? I'm thinking my cats are toast and the 800-1200 rpm funkiness may be the engine trying to overcome the obstruction.

 

Thanks!

 

 

Have you check the operation of the EGR valve and/or transducer? A slightly stuck open EGR valve, even due to carbon buildup on it's needle port might cause the hesitation/bogging that you describe.

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