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Old 11-17-2012, 09:07 AM   #1 (permalink)
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Exclamation 2003 impala 3800 misfire on cylinder 4

New to the forum so i would like to say thanks in advance for any and all help

my car is a 2003 impala with a 3800 . my cel is on with dtc p0300 i have a genesys scanner and watching the live data only cylinder 4 is missing .
now i have checked every thing i can think of and the miss is still there it only misses at idle if u bring the rpm upto roughly 1500 or more no miss.

things i have checked

1. new spark plugs (acdelco)
2. new wires (off name)
3. looked for vacume leaks with carb cleaner
4. swapped coils
5. swapped plugs
6.swapped wires
7. used a noid light to check injector circuit
8. swapped injectors
9.swapped ignition module with a known good one
10. car has a new cat. converter less tha month old
11. relearned crank position sensor

as u can see I could use a little help STUMPED
monday will be checking compression (although no abnormal engine noise can be heard) , also will recheck for a vacume leak with a smoke machine.

also as u can see i have access to most tools that could be needed

again any help would be greatly appreciated

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Old 11-19-2012, 07:09 AM   #2 (permalink)
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compression test

cyl. 2,5,6 175psi
cyl. 1,3 170psi
cyl. 4 145 psi

well figured that was enough compression being that it is suppose to not be more than 70 % less than the highest cylinder witch would have to been 122.5 or less

y lack of compression figured could have been caused by the bad Cat. causing a build up of carbon on the valves

i ran two bottles of top engine cleaner through the car and wouldn't u know it miss gone

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Old 11-19-2012, 08:08 AM   #3 (permalink)
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Good find glad you were able to get it fixed. I usually do top engine cleaner about every 60,000 miles but with the 2012 direct injection might change that 40,000.


2012 Impala LTZ 72,000 miles
2002 Buick Regal LS 405,000 miles daughter traded for a ford
2000 Buick Regal LS 205,000 miles
1998 Olds Silhouette 3.4V6 177,000 miles

Too many freaks, not enough circuses.
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Old 11-19-2012, 11:47 AM   #4 (permalink)
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145 is NOT ok when the others are 170-175, looks like carbon on the valve seat was the issue though. Glad it's fixed!

1978 Caprice Classic Landau
350 2-bolt, stock 4bbl
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