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Hesitation

 
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moray-eel-bite
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PostPosted: Thu Sep 13, 2012 9:46 am    Post subject:  Hesitation Reply with quote

Hi all,

I've lurked here for a while but haven't posted. I have a 2003 impala 3.4. It has no MIL pending or stored, but acceleration is bad, the car 'bogs'. I just got an autoenginuity scan tool and am learning to use it. It looks like the oxygen sensor responds well enough (as in , cycles from ~0-0.9V in a relatively consistent pattern) but there are a couple values I find odd. First is that the ignition advance is at ~20 degrees... that seems excessive. Also, LTFT is up around -10 to-12, and manifold Absolute pressure is 11.5 mm HG which seems like it might be low, but I don't have the actual correct values to know for sure. If anyone is willing to assist, I do have an excel sheet with the autoenginuity values.
Anyways, hoping someone can shed some light on this for me.

Cheers.
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chibiblacksheep
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2000 Chevrolet Monte Carlo
2007 Chevrolet Impala
1968 Chevrolet Camaro
2013 Chevrolet Camaro


PostPosted: Thu Sep 13, 2012 11:25 am    Post subject:   Reply with quote

I would get the Cat checked out.

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Lingenfelter
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PostPosted: Thu Sep 13, 2012 11:34 am    Post subject:   Reply with quote

Check the cat before getting into anything else.

Have a backpressure test done at an exhaust shop or the dealership.

If the cat needs replacing, consider OEM even though it costs more than others, especially if you have to pass emissions testing where you live - aftermarket units sometimes throw codes which are an instant fail on emissions tests.

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moray-eel-bite
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PostPosted: Thu Sep 13, 2012 3:48 pm    Post subject:   Reply with quote

Hi Guys,

Thanks for the quick reply. I don't think it's likely to be the cat since the hesitation is really only bad under part throttle. When you mash it it seems fine. It seems to me if it was the cat then the higher the engine load the worse it would be? Does anyone know what 'normal' MAP and spark advance should be on these engines?

Cheers
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chibiblacksheep
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2000 Chevrolet Monte Carlo
2007 Chevrolet Impala
1968 Chevrolet Camaro
2013 Chevrolet Camaro


PostPosted: Thu Sep 13, 2012 4:07 pm    Post subject:   Reply with quote

Been too long since I had stock settings, so I can't tell you that information.

The cats are a known problem area though. Also it could also be a fueling issue if it works on WOT, at WOT the o2 sensor doesn't work, it goes back to open loop.

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Joe67
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PostPosted: Fri Sep 14, 2012 9:51 am    Post subject:   Reply with quote

Cat.
O2 sensors
Fuel pump?

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barnettimpala
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PostPosted: Sun Sep 16, 2012 4:14 pm    Post subject:   Reply with quote

Just the other day my car did something similar to what I think your describing. Started it up and was driving at a constant 20-25mph and it seemed like I could give it half throttle and it would just not go faster, felt like it was constantly bogging down, but when I'd give it more throttle it woule come to life. No codes as well and its only done it once.

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2002 Impala- SSC, AGX, 18" charger R/t rims w/8mm spacers, Magnaflow cat, U-bend delete, flowmaster 80 series crossflow, z28 tips, HAI, debadged, 35% tint, smoked headlights, full sound system
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legotech7
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PostPosted: Mon Sep 17, 2012 1:15 pm    Post subject:   Reply with quote

Had the same thing on my 2000 3.8 I had a cat installed a few years ago. Not OEM and have had nothing but code problems ( PO402 ) since. Was able to pass inspection up until recently. Going thru the forums, I replaced some items, EGR Valve, removed and cleaned the throttle body and replaced the o2 sensor after the Cat. Still had the hesitation thing going. But it would only happen on occasion on initial start up when cold for a second or two, than would run fine after that. Still going thru the forums, I thought it my be my Mass Air Flow sensor. So I got some cleaner specific for the MAF from CRC and followed their instructions for cleaning. Put the sensor back in the car, warmed it up a bit and WOW... what a difference. No hesitation and it seemed to have better pick up. I priced the MAF sensors, had no Idea they were so expensive, so the price for the cleaner was a deal by comparison. Hope to pass Inspection soon....
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