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2008 2.4L code P0301

4968 Views 26 Replies 6 Participants Last post by  Mechromancer
I got the car a few weeks ago. the previous owner drove it as is for 15k miles. when i acquired the car it had barely any power through 1st and 2nd with an obvious misfire that would clear out once up over 40mph. always throwing ESC and Service TPS. and at redlights would bounce around and misfire bad enough to where id have to start shifting into N while pulling up to stop signs and redlights just to avoid the situation entirely. the misfire is still there in neutral just not nearly as bad. you can smell fuel like its running lean when outside the car. no smells inside really besides the must A/C but im working on the cabin airfilter and a/c cleanout as soon as i get her running right.

For what ive done to the car, after getting the codes ran and recieving diagnostic of P0301, I will place in a list below,

New Battery
air filter
spark plugs
MAF sensor
throttle body
o2 sensors
new pcv hose that goes to intake
purge valve solenoid
sea foam in gastank

after doing all that^ drove for 100 miles or so and there was still a misfire and rough idle.

so i removed the header and catalytic converter as previous posters seemed to have similar issues. the header seemed to have alight crack but it was hard to tell if its just on the surface or not.

no light would shine through the 1st catalytic converter, so i punched a hole through the middle with a piece of fence pipe. (i know its frowned upon but before buying a new header/cat assembly i wanted to try to remove it from the list of problems or diagnose it as the problem.)
put it all back together and
started it up and the misfire and rough idle was still there only now it sounded like a ricer from me mangling the catalytic converter. needless to say the malibu isnt leaving anytime before the new header gets here.

ive since then drained and replaced the oil, new filter and o-ring.
replaced the VVT solenoids, intake and exhaust.
and performed a pressure test.
first thing i'll note is when i removed the plugs cyl1 was wet with fuel, black and cracked on the upper porcelein. the other three looked rough for 100 miles but nothing compared to cyl1. ive read in other posts that these le5's just dont like NGK so i bought AC delco to replace these after the pressure test.

1/55 2/140 3/135 4/135

I closed the engine back up with new AC/delco sparkplugs and havent started it as I am waiting on the new header and catalytic converter.

when i was unplugging the injectors to perform the pressure test i noticed that the boots which hold the injectors inplace were kind of misaligned and when i tried to straighten them out i felt like there was way to much wiggle room. I wish i noticed this when replacing the injectors but hindsight is 2020 LOL, i will be replacing the Injector boots and orings as well.

and deleting the OEM engine cover air box thing just to remove it as a culprit for any vacuum leaks.

before installing the new header and cat i was planning to use the AC delco Engine top cleaner through the spark plug holes and through the throttle body just to remove any carbon build up as im praying that is the cause for the cyl1 low compression.

That's where i am at currently. waiting on parts reading a decade of 2.4 Le5 posts.
any tips advice experience or help would be greatly appreciated. I feel like im on the right track now? but maybe not. im open for discussion
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55 lbs compression in #1 is your problem. All 4 cyl. should be within 10% at the most of each other.
It needs a teardown, head taken off, to inspect valves and piston. Or as least valve covered pulled to check cam at the minimum.
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thats what ive feared all along. you dont think low compression could be caused by carbon build up? I just got my hands on some AC delco engine top cleaner and planned on using it tomorrow hoping it might be a less intrusive fix to the problem
Carbon would increase cylinder pressure not drop it. An only a very small amount of increase well within a few percent.
Low compression / dead cylinder = misfire and carboned up sparkplug and cat failure due to misfiring cylinder.
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so replacing the cat and fixing any vacuum leak issues is just wishful thinking at this point? i read somewhere that carbon build up could lead to a stuck ring resulting in low compression but that seems less likely at this point then an actual valve issue. however, my $50 can of top engine cleaner must be used before i'll submit and open the valve cover up. ive read in a GM service bulletin that the P0301 could be fixed with a De-Carbon process, fuel treatment, and top tier fuel. 16-NA-383 1..6 (

i want to add, as i shouldve stated in the first post. the car is under 100k miles, but doesnt appear to have been to regular on fluid changes as the oil i took out was very dark. i also would bet that top tier fuel was never used as in my area the stations that are on the approved list from GM arent the "new" cumberland farms or wawa's but the 30 year old sunocos mobils and bp's. i see the reason of wanting to go to the new stations but now i know the difference in fuel quality i'll be sticking to the old stations from now on.
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Trying to de-carbon can't hurt. Carbon messing with valves can happen with port injection but more so on direct injection motors. It is a cheap process with mild-moderate risk. Physical cleaning is more effective than chemical but I've never read a case of long term resolution from either on this forum.

Misfire from carbon buildup is usually from excess carbon on valves causing reduced airflow or sticking altogether. The low compression and port injection pretty much rules that out. Low compression is sometimes a stuck valve but most often bad rings.
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lets say i put on the new catalytic converter and header, air box delete, new injector boots, and use the top engine cleaner. and the Malibu still is misfiring. the next step is to remove the valve cover and inspect cam and valves? is that a job a somewhat mechanically inclined fella can accomplish at home or is it always better to just leave that to someone with experience?
You have a '08. That's not a DI motor but port injected so carbon on the intakes is not a issue like in later Direct Injection motors.
Repairing the low compression in #1 is the priority and your immediate problem, cracked exhaust manifold can be addressed at the same time if head has to come off motor.
Other than solving the bad cylinder issue any other money spent is a waste of time and money as you do not know if your motor is even repairable or economical to repair.
Wish I’d of started out here instead of assuming I’d be able to throw some sensors or plugs in and call it a new car. what would you say the next step would be repairman54? is there a way to self diagnose the issue and fix it at home?
Low compression in #1
I myself would get the air adapter to put compressed air into the cylinder thru the spark plug hole, moderate 50 psi pressure, and listen for air leak in the throttle body, oil fill cap and exhaust pipe. That is the quickest way to roughly check if valves or piston has issues.
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On saturday I went ahead and ran 16oz of AC Delco top engine cleaner through the throttle body, following the GM service bulletin.16-NA-383 1..6 ( Ran alot better, not perfect but better. planning on doing a second run in a few days, just wanted to give it time to see if it clears up anymore. New header and catalytic converter came in yesterday, got them both installed. No more ricey sound and no more cracked header/bad gaskets.

HOWEVER, the hard shifting is much worse now that I have an almost fully functioning le5. I plan on doing a fluid check today or tomorrow, and also while working on the cat i noticed there was a sensor or solenoid going into the transmission, the wiring is frayed slightly and im sure it gets wet when driving in rain puddles etc.

anyone know what sensor that is? i would think replacing it is ideal and the wiring aswell.

and if anyone has experience with solving the intermittent hard shift issue pls share as im not sure the transmission can take many more clunks.
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Low compression in #1
I myself would get the air adapter to put compressed air into the cylinder thru the spark plug hole, moderate 50 psi pressure, and listen for air leak in the throttle body, oil fill cap and exhaust pipe. That is the quickest way to roughly check if valves or piston has issues.
after i run the next 16oz of top engine cleaner i will be checking/replacing the sparkplugs which will be the perfect time to do that test.
wow car has had the parts cannon fired at it many times. Piston and/or piston rings on cyl 1 are damaged
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parts cannon running overtime, sourcing all the best discounts and pricepoints though. ive found most acdelco parts can be found on amazon for half the price of the parts warehouse. so far the top engine cleaner seems to have fixed the cyl1 misfire. however my trans shifting issue is more of the problem now. going to check fluid here after lunch time and go from there. i think the sensor i located with the exposed wiring is the trans/speed sensor? ive ordered the connector to get that wiring fixed asap.
Trans fluid is pretty thin and very dark. new jugs came into today so that'll be getting done this weekend. going to order the new wiring for the trans speed solenoid and get that here for sometime next week. hopefully monday or tuesday. sometimes the malibu shifts and runs smooth as a sewing mahine but then others it hard shifts clunking from 1-2 and 2-3. just trying to cover the obvious bases before i start thinking major trans issue. i figure old fluid and a frayed wire could cause all kinds of shifting issues or is that just wishful thinking?

sidenote: at 400 miles i'm going to do another run of top engine cleaner as GM rec sometimes 2 doses is needed.
Would low compression in one cyl cause intermittent issues or would it be pretty constant?
Low compression is usually considered one or both of two things: more than 10% below the lowest of the other cylinders, or lower than min. spec.

If it is constantly low on compression, it'll be a constant issue.
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When I reset the battery, and wiggle around the connectors onwhat I believe to be the BCM all issues go away momentarily then they come right back. Back to the drawing board

three large connectors on the driver side one behind the battery where the brake lines attach 2 infront of the battery. Removed and replaced several times. Going to add some die electric grease as they seemed completely dry.
fixed the frayed wire to the transmission, and changed the fluid. hard shift is still present. however, im starting to think this is all more of an electrical issue. as when I reset the battery and do the previously mentioned connector moving it all goes away for a few miles. anyone have any ideas? dash is flashing the ESC OFF service traction service tire monitor, but those go away when i do the reset. then when they come back the problem is back.
1/55 2/140 3/135 4/135
About 3 weeks ago the compression in cyl 1 was bad. What is the current standing of the compression in 1?
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