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#3011 - 04/24/09 12:11 PM PE Oxygen sensors reading  

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This is a short PE scan I made of my 98 Trans Am (stock LS1 engine).
Notice the mV differece between the banks.
The light blue line is the AFR measured with a WB installed just before the passanger side catalyst.

In CL both banks have +/- 0.5% the same fuel trims so now I'm surprised to see that in PE the sensors show different values.

Should I care about that? I know the stock sensors are fine just for CL operation, but I was thinking they would show more or less the same values in PE.

Notice also the smaller difference at lower RPM (around 3000 rpm). Is it possible that a mechanical reason causes the difference? Like a damaged lifter or pushrod? A weak spring? Something that isn't working fine only at high RPM?

Thanks for any comment

Stefano

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#3012 - 04/24/09 05:36 PM Re: PE Oxygen sensors reading [Re: ]  
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The difference of 812 mVolts and 890 is only a AFR difference of about 12.7 to 12.5 AFR so not that far off.
Both sides of engine exhaust makeup rarely are equal

Add long tube headers and many times one O2 is at a different distance then the other sides O2 thus one reason for reported difference.

Cannot speak on that one wideband O2 as what it reports but again kinda half an answer since other sides exhaust has no wideband and what it reports.

PCM of course ignores the O2s for WOT but also the PCM only reacts to O2 values once a minute so a captured scanner recording of 45 seconds might not give a true AFR rate from 02s.

It is hard to see the lines in graph due to line colors and gray background but it seems once above 4,000 RPMs which is where PCM stops looking at MAF and more to VE that imbalance seen clearly.

It seems both have about the same switching rates, sometimes one head has better flow and thus imbalance seen as the exhaust rate speed up.


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#3014 - 04/24/09 11:28 PM Re: PE Oxygen sensors reading [Re: teamzr1]  

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The common internet opinion is that the PCM stops using the VE above 4000 rpm and only goes by the MAF crazy
I personnally don't know: I never had a prouve of when the PCM uses MAF or VE.

#3015 - 04/25/09 10:52 AM Re: PE Oxygen sensors reading [Re: ]  
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Sorry I stated that differently then what I wanted.

Yes below 4,000 RPMs the PCM can use predicted airflow.
VE is NOT normally used, it is a backup if MAF is seen as faulty or some decel conditions


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#3018 - 04/27/09 01:16 AM Re: PE Oxygen sensors reading [Re: teamzr1]  

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There is a noise coming from one head, like a bent pushrod or a damaged lifter... maybe a weak spring.
I was thinking at a relationship between the mV difference and some mechanical issue because the problem starts only after a certain RPM.
OK I know: I should check a lot of other things. This engine has now 100'000 miles, been supercharged during 30'000 and I can't expect everything is still perfact.

What would you do with this engine? Just let it work as long nothing bad happens or consider a rebuild? Or buy a low milage rebuilt engine? I've never rebuilt an engine and I should buy a bunch of tools. Chances are I spend a lot of money and mess up the engine cry

#3020 - 04/27/09 07:53 AM Re: PE Oxygen sensors reading [Re: ]  
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That is a hard call to answer as we do not know how hard it is to gets parts for the LS1 engine in your country and what you could spend.

Maybe do a compression and leakdown cylinder tests and see how the rings are doing. If still fine then maybe at the least pull the heads and redo the valves and springs.

At the least even with not taking heads off is replace springs, check pushrods are true and same for rockers.

If needed replace what you can without even taking heads off.
I'd really test the fuel injectors, sparkplugs and plug wires.

If you yank engine, then flip a coin as to a total rebuild but best is swap to a LS3, stronger engine and suited if going boost later on.

Here is a 2008 C6 of Melvin's who is at the USA base in Germany I just tuned this weekend

He was able to pull 192 MPH on the highway and gained about 50 HP with just a tune so LS3 is a strong engine


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#3023 - 04/27/09 01:00 PM Re: PE Oxygen sensors reading [Re: teamzr1]  

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Parts over here cost about 3x - 4x more than in the US, that's why I import almost everything (not a joke! I had to pay $50 for a AC belt + PCV valve)

Compression is fine for all 8 cylinders, I didn't had the chance for a leakdown test.

I'd like a new engine like the LS3 but it's not legal to install more powerful engines so in case of an accident I'm in troubles with police + insurance (not that having a blower like the last few years is better, but at least I can remove it and go back to stock in few hours).

It would be fine a rebuilt 98 engine that would fit the existing PCM + harness. Are you guys even rebuild those old engines? Or swap them wir newer ones?

#3024 - 04/27/09 02:19 PM Re: PE Oxygen sensors reading [Re: ]  
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Not sure if you were using water/meth or not and if so if you stopped as it helps keep the valves and piston tops clean.

If leakdown is fine then rings and ring lands are good so I would not worry about the bottom end and spent the money on cleaning up heads as with 100,000 miles I would at the least put new valve springs in, check all parts for valves that are in the top of head and check that pushrods are true.

We have teammates that have 140,000 miles on 97/98 C5s and have boost on them.

Issue is that the early LS1s had oil fume/pressure buildup issues by crank so at the least if you went with a 2001 and newer LS1 or LS6 block if doing a engine swap is stronger.

We tend here to re-pin the 80 pin PCM connectors and use the 99-03 PCM and flash as a 2003 and whichever trannie is used when reworking 97/98 cars. The PCM is faster and has more and better tuning tables.



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#3033 - 04/30/09 01:28 AM Re: PE Oxygen sensors reading [Re: teamzr1]  

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Yes I was using wather/meth injection but since I removed the blower (6 months ago) and engine is otherwise stock I don't use it anymore.

Bottom end: is there a way to check main and rods bearings without removing the oil pan? Would they be really noisy if they aren't OK?

Removing the heads on a F-Body is such a pain that I'll probably prefer to drop engine + tranny. So checking for other things will be much easier.


#3034 - 04/30/09 02:04 PM Re: PE Oxygen sensors reading [Re: ]  
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Not sure if you really could tell if noise is coming from bottom end or not but being the engine used to have boost and the amount of miles on it I would first suspect the valvetrain including weak springs

You could still use the water/methanol even with engine NA, I have on my vette for years now.
At least it would clean and then keep cylinder/valves clean as carbon buildup will induce more noise along with knock as carbon gets thicker.

At least pull the valve covers would allow you to check at least for maybe weak or broken springs, esp if there are dual springs as the thin inner ones tend to break.

I would think the compression and leakdown tests would give you a good idea are parts you cannot see how well or poor the internals are.

Too bad it is tough getting parts in your country as a LS2 or even LS3 would work and even stock they put out more HP/Tq then your LS1.


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#3035 - 05/01/09 01:21 AM Re: PE Oxygen sensors reading [Re: teamzr1]  

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Yep, at least a check and then decide if take the rest apart too.
Sell the car and import a low mileage Corvette from the States is an option too (we discussed this already).

Thanks for now, I'll post more about the topic as soon I'll find out how healthy is the engine.

Stefano


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