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2005 Chevy Tahoe 5.3L (T) 120345 miles...have a miss on

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cylinders 4 and 5, what...
2005 Chevy Tahoe 5.3L (T) 120345 miles...have a miss on cylinders 4 and 5, what has been done to repair this is, replace plugs and wires, clean the throttle body, fuel inj cleaning, also replaced the 4th and 5th injectors, compressing on those cylinders are 175, checked ground G103, G104 and another ground on the front frame sprayed around the intake to check for vacuum leaks, none found and even smoke tested...LTFT on bank to is higher on that side 7.0 than bank 1 around 3.2, STFT looks good, o2 sensors were lagging but were also replaced....what else is there???
Submitted: 2 years ago.Category: Chevy
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Answered in 23 minutes by:
2/11/2016
Chevy Mechanic: Gary, ASE Certified Technician replied 2 years ago
Gary
Gary, ASE Certified Technician
Category: Chevy
Satisfied Customers: 1,826
Experience: Senior Technician
Verified

Hi there,

Does your engine have AFM or DOD?

AFM engines are known for some valves not operating once the engine has oil pressure.

Are your sure it is cylinder 4 and 5?

AFM cylinders are 1,4,6,7.

Are the coils firing?

What do the spark plugs look like?

Gary

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Customer reply replied 2 years ago
its not AFM... yes live misfire data states serious misfire activity on those cylinders, coils firing, even swapped coils with another cylinder to verify coil issue, plugs looks normal, they just thought they needed a tuneup but looks more serious...
Customer reply replied 2 years ago
swapped wires and plugs per there request, prior...
Customer reply replied 2 years ago
oh also says P0300, its a pending code but flashes the CEL regardless no, hard code, then goes away once you start driving, runs great besides that...
Chevy Mechanic: Gary, ASE Certified Technician replied 2 years ago

Ok, you have swapped the injectors, does it have injector pulse on those 2 cylinders?

Tested with a noid light?

Compression could still test ok, have you run the engine with valve covers off?

Could have bad cam, lifters or valve of its seat once the engine is running?

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Customer reply replied 2 years ago
it has pulse, verified again, no haven't run with valve covers off, but on both sides? it runs strong wouldn't you feel two cylinders failing?, problem just at idle
Chevy Mechanic: Gary, ASE Certified Technician replied 2 years ago

That's certainly a strange issue you have there.

Even though you have done a compression test, how about conducting a leakdown test on those two cylinders and a pair of good cylinders for comparison.

Sounds odd to pull hard on 8 cylinders, but lean misfire at idle.

Have you checked the inlet manifold bolt torque?

Gary

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Chevy Mechanic: Gary, ASE Certified Technician replied 2 years ago

We have one in our range that produces P0300 misfires.

Early versions were unable to locate fault. It then became a new ECM program.

Does your misfire data count up on cylinders 4 & 5?

Gary

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Customer reply replied 2 years ago
it does
Chevy Mechanic: Gary, ASE Certified Technician replied 2 years ago

Ok.

I've seen many engines with high carbon on the valves that cause rough idle, misfire codes etc.

I would conduct a leakdown test on those 2.

Then I would pull the intake manifold, and clean the back of the intake valves with an upper cylinder cleaner.

Subaru use this on their engines at every service.

I have used this product on many cars.

Gary

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