Saturn bucks mid-range

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  #1  
Old 06-03-14, 03:56 PM
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Saturn bucks mid-range

1997 Saturn SC2, 146500 miles with California emissions package. Idles nicely, does okay if I go lightly on the throttle, and is willing to accelerate hard if I give it enough gas (and there's a pretty distinct jump in power when it decides to go). In between, though, there's a range where it sort of bogs down, sputters, or starts bucking hard. Bucking requires some load (it won't do it in neutral and I haven't seen it in first or second) and around 20003000 RPM.

The bucking started with very specific parameters (2500-2600 RPM, right about 60 MPH, just the right grade uphill) but the range where it does it is gradually expanding, so the behavior is getting more consistent. It's now doing it when trying to maintain a speed. However, I'm able to "limp" the car by alternating between gunning it to get up some speed, then letting up and coasting a bit, then gunning it again.

The car does not buck if the EGR is unplugged (although it throws a P0405 about the EGR position sensor), but power feels lower than usual. Plugging it back in (even without erasing codes) brings back the bucking.

Bucking does *not* occur while engine is still cold.

Sensor data:

I i've been running an engine monitor, hoping there would be some good data when the symptoms appeared, but there doesn't seem to be anything unusual or different when it's bucking.

* The upstream O2 sensor shows normal oscillation between maybe about 0.4 and 0.9, although it'll drop to 0 on a long downhill or stay high (~0.90V) if I'm at WOT going uphill. Normal oscillation resumes at the bottom of the hill, or maybe 10-20 seconds after cresting the top.

* Timing advance hangs around 40 at cruise, and declines toward 0 (or even small negative values) when accelerating, especially at high RPMs.

* LTFT (Long term fuel trim) seems to be steady around -20%.

History:

* Just cleaned out the EGR valve today; it felt "crunchy" beforehand, but no change.

* Did a bottle of fuel injector cleaner two months ago

* Fuel filter replaced 6 months ago

* Fuel economy prior to the gun it/coast solution was decent, ~32ish MPG on trips, ~upper 20s around town. (Doesn't even seem bad now.)

* Downstream O2 was replaced 6 months ago because of a recurring P0138 (Downstream O2 sensor high too long) every time I visited New England (hillier than home); code has not reappeared since and sensor typically hangs around 0.7.1V, although on a long downhill it sometimes abruptly drops to 0. Noted there was some carbon on the old one, I don't know what is expected though; that sensor was only about 2 years old, having been replaced along with the front exhaust/downstream catalytic converter.

* Spark tests okay on all 4 cylinders, however, spark plug #2 shows a little more browning on the ceramic than the other 3. I believe this was true on the prior set too. The electrodes shown normal wear though.

* Plugs and wires were replaced about 18 months (7500 miles) ago.

* Car consumes about 0.5L ( qt) of oil per tank, but my impression is this is normal for Saturn DOHC models of this age/mileage.

Thanks in advance for help and guidance!
 
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  #2  
Old 06-03-14, 08:46 PM
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ok the -20% ltft indicates its running rich. and bucking while egr plugged up. try unhooking the exhaust before the converter. and driving around the block. if the converter is plugged you will get more pressure when the egr opens , choking out the engine(less clean air) which also means less O2 so a richer condition.
 
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Old 06-04-14, 03:21 PM
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Aaahhh... How about cleaning EGR? EGR does some mean things to RPMs, if dirty.
 
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Old 06-05-14, 06:51 AM
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EGR was cleaned just prior to original post, no change in behavior.

I had dismissed cat clog because of the pep it has when given enough gas... but taking out the upstream O2 does fix it.

So assuming it is a cat clog, is it likely to be the upstream or downstream cat? The upstream one is original, the downstream was replaced autumn 2011; the upstream O2 sensor shows just mild plating (similar to the spark plug electrodes) while the downstream had carbon deposits when I replaced it 6 months ago.
 
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Old 06-05-14, 12:45 PM
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Do you have access to acetylene torch? You can punch 2 holes, before and right after the cat, to
1. check on even air flow
2. see if it changes behavior, as it releases cat backup, if there is one.
And then easily weld them back over.
Some say a can of lacquer thinner via gas tank and 2 hrs of 70mph on FWY right after, burns the crud out of cat.
 
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Old 06-09-14, 06:54 PM
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Had the same problem on a 99. Got an EGR blanking plate used on cars without EGR, GM part number 24575592 for under $6 at a Chevy dealer. Mounted it between the manifold and the EGR valve. Might have had to use longer bolts or studs but no more bucking.
 
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Old 06-10-14, 04:35 AM
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Just a thought out of left field, but check the engine temp reading on the scanner to make sure you're getting a proper "hot" reading when the engine is at operating temperature.
 
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Old 06-10-14, 08:23 PM
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Confirmed this as a plugged catalytic converter. I'm not cool enough to do acetylene, thanks for the inspiratio, I hooked up a shop vac to the exhaust and pulled the upstream O2 sensor, and see how the flow was. Virtually nonexistent.

Which leads to the question why a cat would plug in 2.5 years? Something's wrong elsewhere. But ignoring that for now, fixing the cat plug...

With a friend, we pulled the front exhaust off. At some point, the upstream catalyst shattered and we found catalyst rocks in the pipe between cats. They'd apparently been in there a while, because they rounded off like beach stones; they look remarkably neat with the roundness creating patterns in the honeycomb. I'm guessing part of my cat-clog problem is shrapnel from this.

Pressure from the shopvac didn't do anything for the clog, but forcing some water through did a little. Modifying from on-line suggestions about adding lacquer thinner to the fuel, we just poured some directly in there and let it soak a bit, then hosed it out and got some airflow through it. Repeated a few times. Not like it should be, but might be good enough.

Thanks to all for suggestions and ideas. I think I'm good for now.
 
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