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p0300, ruh roh!

This is a discussion on p0300, ruh roh! within the External Engine forums, part of the LSx Technical Help Section category; The original MAF was a stock GM piece and not an aftermarket unit? Has the tune ever been messed with ...

  1. #21
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    The original MAF was a stock GM piece and not an aftermarket unit? Has the tune ever been messed with on your car? Have you checked for vacuum leaks? The PCV system is notorious for disinigrating on our cars. How about the connections between the MAF, the throttle body and the intake -- all tight and leak free?

    With the MAF disconnected, the PCM is estimating airflow rather than measuring it. If unmetered air is entering the combustion chamber from another source, such as a vacuum leak, you will get a lean condition. By chance, do you have an aftermarket cold air setup on the car, or is it coming through the factory airbox (stock lid or aftermarket will not matter)?

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    Quote Originally Posted by pajeff02 View Post
    The original MAF was a stock GM piece and not an aftermarket unit? Has the tune ever been messed with on your car? Have you checked for vacuum leaks? The PCV system is notorious for disinigrating on our cars. How about the connections between the MAF, the throttle body and the intake -- all tight and leak free?

    With the MAF disconnected, the PCM is estimating airflow rather than measuring it. If unmetered air is entering the combustion chamber from another source, such as a vacuum leak, you will get a lean condition. By chance, do you have an aftermarket cold air setup on the car, or is it coming through the factory airbox (stock lid or aftermarket will not matter)?
    The original was stock. I never messed with the tune, can't say for sure of the po did or not but my guess would be not. I did check the maf/tb/intake connections and they all look tight. I've also replaced all them with the new lid and smooth bellows (misfires were before). I check the pcv valve and it rattles. When disconnected at idle I have a vacuum on the tb. The pcv did come out from the hose on the tb easily so i put a small hose clamp on it but that didn't help. I don't see anything visually.

    I spent some time with laptop out and rev'ing the engine. The misfires appear to be happening right after 2500 rpm which iiirc is when the pcv should close, right? That would seem to indicate it's not closing properly, or maybe fast enough? Maybe it's dirty and needs cleaned? Or should just try replacing it?

    BTW - Thank you for the replies I really appreciate the help

  3. #23
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    The PCV hoses tend to deteriorate right by the throttle body and at the back corners of the engine. The entire hose assembly looks like this:




    Might need to get it on a scanner to see what is going on.

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    Yeah I saw in another thread that image when you replaced yours. I was thinking I should just get one to replace mine, as you said while they're still available. I'm reading up about catch cans now. Since I have the ls6 intake a pcv mod sounds to be easier for me. Maybe I'll do a mod instead...

    I have scanxl pro w/gm enhanced codes (and obd II connector). It has access to all the data available. I don't know what to look for. I just finished reading the book Chevy ls1/ls6 performance. There's 4 other books here to read. Maybe I'll read the GM Gen II ls-series Powertrain Control Systems next. Hopefully afterwards I'll be able to interpret the data to diagnose this issue more effectively.

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    That was back in 2010 when I purchased the replacement - hopefully they are still available. If not, you can construct one with some nylon "T's" and hose. I installed a catch can on our car years ago and it does work well. It's just odd that both MAFs demonstrate the same issue. You can watch airflow in grams/second (some scanners can scale for other readout parameters). Somewhere there is onfo on the approximate airflow at a given RPM - you can compare to see if your MAF is reporting in the ballpark. You can also look for spikes, drops, or any oddities in the rev range as you slowly open the throttle up to and past the point where it stumbles. I would carefully inspect the connectors for a bent pin, or one that has pushed back in the connector. The connections to the PCM can also be an issue, as can damaged wiring or corrosion on the connectors.

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    I did some more vaccuum testing with carb cleaner and couldn't affect idle, anywhere. Still stumped but haven't had any free time to investigate more into it...

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    We'll get this figured out...

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    Any one ever get this figured out have same problem same cylinders tryed all this need help

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    My problem seems to lie in the lower intake gaskets. The snap on scanner showed random misfires on all cylinder which would indicate vacum leaks around the base

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    Mine is only 2&3 changed intake gasket and intake altogether maf relearned crank senior changed senior had dealer upgrade computer cheeked grounds swapped coils swapped injectors swapped PVC cylinder balliance is good so don't think valve springs any other ideas

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    Welcome to the site!

    You checked fuel pressure, looked for vacuum leaks, inspected the PCV hoses, and unplugged the MAF? How about the plug wire installation -- you should hear/feel two "clicks" when installing the wire on the coil pack.

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    Thanks. Yes we have checked vacuum leaks none thought we had a little one on intake so we replaced it new felpro gaskets. Swapped out pvc glanced at the hose will inspect more tomorrow. With MAF unplugged miss goes away swapped out with known good one. Mis is still there when hooked up new one. Cylinder 2 and 3 are the worst. Checked fuel pressure its good checked fuel pressure regulator good. Checked the new plug wire many times. Swapped out coils swapped out injectors we are stumped. Also swapped our crank senior and had a relearn done at the local gm shop they also checked for any updates and installed them. Where a small town we all help each other out and where all out of ideas

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    Do you have a scanner on which you can see live sensor data? It's odd that even the replacement MAF resulted in the miss returning. Has anything been done to your airbox setup before the MAF that would cause its readings to skew? Is your idle ok?

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    Yes. We can see live data idle is pretty good only a few misfires it is the worst at 1500-2000 rpm not under load better under load but still not perfect nothing has been done to the airbox stock

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    Have also checked grounds twice and put on jumpers just to be sure.

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    What are you seeing for your short and long term fuel trims -- is it adding or subtracting fuel? Are the O2 senders rapidly cycling up and down as they should? Are all your engine and underhood grounds in place, secure and corrosion free?

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    Grounds good o2 responding like they should short term Avg bank 1 -4.7 bank 2 -1.7 but they fluctuate

    - - - Updated - - -

    And that's at idle

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    Ok ST FT at 1500 rpm Avg is 0 both banks live data is -3.1to +3.8 LT FT Avg is 0 both banks and LT FT bank 1 1.6 % live LT FT bank 2 is 2.3%

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    In the time it took me to look that up cyl 2. 5935 mis cylinder 3. 5955 and just a few on other cylinders

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    Hmmm. How many miles on the car? Did it ever sit for extended periods? Ever run a bottle of Chevron Techron Fuel System cleaner through it?

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