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Missfire and ticking P0300


04compg

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Ok so I work at a parts store and a lady brings her Grand Prix I’m with a P0300 and I notice it has original wires and assuming plugs so I send her on her way with plugs and wires. Motor was quiet at this time. Then a week later she comes in and says her son changed them and it’s still missing and told me I could have it for $100 so I picked it up and drove it 15 miles home. Get home and it’s ticking bad. I find that 2 plug wires were crossed and fired it up. Still misses and still ticks. Changed the oil thinking it was washed out with fuel from her driving it so much missing and no change. I’m having a hard time tracking down the miss as it doesn’t sound like a rocker, the sound is best heard from the top of intake. Car has the 3100 any suggestions?

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I'd start with a compression test. This will fairly reliably tell you if the miss is due to worn piston rings, valve issues, etc. Then if you can do a wet compression test to rule out rings, then you can do a leak down test to isolate the issue. Course, if all tests come back good, I'd change the lower intake manifold gaskets and rebuild/ check the lifters as this sounds kind of like lifter tick.

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I'd start with a compression test. This will fairly reliably tell you if the miss is due to worn piston rings, valve issues, etc. Then if you can do a wet compression test to rule out rings, then you can do a leak down test to isolate the issue. Course, if all tests come back good, I'd change the lower intake manifold gaskets and rebuild/ check the lifters as this sounds kind of like lifter tick.

should I replace the rockers and pushrods too? Done a few lim gaskets before but never have replaced lifters in a engine before
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As long as they look good I keep them, but I would verify good compression before you go tearing into them. Also, the orientation of the push rods and the order of both rockers and rods are EXTREMELY important. You can't switch them up because of how they wear. I usually cut holes in a closed small cardboard box and label it with each cylinder and place them as I go. There's a few videos on inspecting and rebuilding gm lifters on YouTube, but if you have any questions, let me know. However, with it ticking like this, I'd prepare bare minimum to replace at least one, and at worst the cam since the roller can damage the lobe. You can inspect the cam by manually turning the engine and looking down each lifter hole. Good luck!

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Well, that sounds healthy.

 

It's a GM V6.

 

You can toss junkyard parts on any of these cars and they'll work just fine.

 

However, if it's misfiring like that and all you're getting is a P0300, I'd venture to think something in the valvetrain let go.  Sounds like a broken valvespring or failed/stuck lifter.  Time to rip into it.  Plus, the 3100/3400's suck because of the goofy offset rockers/pushrods.

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I’m having a hard time tracking down the miss as it doesn’t sound like a rocker, the sound is best heard from the top of intake. Car has the 3100 any suggestions?

Pick yourself up one of these........and start listening

 

Automotive-Mechanic-Stethoscope-Engine-D

 

seeing as you are employed by an auto parts store there should be one hanging on the wall

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What year? My mother’s ‘03 Grand Am has had THREE rocker arm studs pull the threads completely out of the heads, because GM wanted to save 2 cents by using smaller diameter bolts, which puts greater stress on the threads.

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Pick yourself up one of these........and start listening

 

Automotive-Mechanic-Stethoscope-Engine-D

 

seeing as you are employed by an auto parts store there should be one hanging on the wall

I have one and can’t seem to track it down other than the top of the intake, which leads me to think lifters, maybe from it missfiring for so long it washed out a lifter with fuel? Is that possible?
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I have one and can’t seem to track it down other than the top of the intake, which leads me to think lifters, maybe from it missfiring for so long it washed out a lifter with fuel? Is that possible?

I'd be surprised, but I have dealt with these lifters collapsing and not wanting to refill. Course, that was because of the dexkill and the lower intake gasket leak gummed things up. I was able to just take apart the lifter, soak it in acetone for a day, inspect the surfaces, and re-assemble and it's been fine since.

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I'd be surprised, but I have dealt with these lifters collapsing and not wanting to refill. Course, that was because of the dexkill and the lower intake gasket leak gummed things up. I was able to just take apart the lifter, soak it in acetone for a day, inspect the surfaces, and re-assemble and it's been fine since.

do the lifters have to go back to the same spot they came from or can they get mixed up? Was going to just pull all of them and let them sit in parts cleaner then oil for a day
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Lim gasket is probably sucking in coolant and lifters might be full of gunk from it all mixing, It is a 3100 after all. Unless its been maintained right which most are not, the LIMG has probably been leaking awhile. If it looks wet behind/below the P/S pump or TB its a good chance the gasket has failed. They also like to go bad internally on the other side of the coolant passage where the plastic part separates from the sealing part. Happens just as bad on the 3400 too but since using a copper based RTV on the valley seals I have had better luck. Defanitly do not use the crap that comes in the gasket set, get Permatex everytime.

 

Once you get the pushrods out a magnetic tool will pull the lifters right out. After you take the plastic pushrod guide off Im recollecting, its easy.

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This one is a 03 as well, I’ll probably just drive the car up to the crusher if it is more than a rocker stripped out

aaaah.....it's an LG8 engine, somewhat more stout than the older L82.

 

do the lifters have to go back to the same spot they came from or can they get mixed up? Was going to just pull all of them and let them sit in parts cleaner then oil for a day

preferably.....even tho one is not dealing with a flat tappet cam try to return the lifters from where you found them.

 

Have you narrowed down the location of the clattering to either the front bank or rear bank?

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do the lifters have to go back to the same spot they came from or can they get mixed up? Was going to just pull all of them and let them sit in parts cleaner then oil for a day

 

I put fuel injectors in my ultrasonic tank, to tell them apart I use colored zip ties. If I mix them up I won't know how much improvement was made on them individually.

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aaaah.....it's an LG8 engine, somewhat more stout than the older L82.

 

preferably.....even tho one is not dealing with a flat tappet cam try to return the lifters from where you found them.

 

Have you narrowed down the location of the clattering to either the front bank or rear bank?

I can’t tell at all, the noise is loudest on top of intake
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with the stethoscope one should be able to distinguish at what point the clatter is the loudest.....put the end of the scope on various points of each valve cover.....listen.......which cover produces that loudest sound?

 

the clatter will resonate up the pushrod to the rocker arm.

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Changed the oil and added a quart of marvel. Quieted it down a lot, now only makes the noise when revving down, to me this confirms lifters since I wouldn’t think any oul additive would quiet a rocker noise

You're mistaking the sound you're hearing at the rocker for being a faulty rocker. That's not the case, when the lifter collapses you won't hear a clattering at the lifter, you'll hear the clattering at the rocker arm where the arm contacts the valve stem. Since there is a lack of pressure there to keep the clearances tight the end of the rocker now has a gap between it & the valve stem & that's the clattering one hears.

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