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Ken, ASE Certified Technician
Category: Chevy
Satisfied Customers: 754
Experience:  GM Certified Technician, 25 Years Automotive Experience
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Chevrolet trailblazer LT I have a cylinder 6 misfire on a

Customer Question

I have a cylinder 6 misfire on a 03 Chevy trailblazer . I have changed all 6 coils spark plugs and # XXXXX injector check engine light still on . Computer was reset and came back on what do I go ?
Submitted: 5 years ago.
Category: Chevy
Expert:  Ken replied 5 years ago.
Now you need to do a cylinder leak down test to see if you have a valve problem. This motor is known for valve seat problems. Most of the time it is a miss fire at idle only at first, if driven on too long it becomes a miss fire all the time. Ken
Customer: replied 5 years ago.
Had diagnostic from dealer compression is good in # XXXXX and 6 did node test no open or shorts found
Expert:  Ken replied 5 years ago.

Subject:P0300 DTC and Engine Misfire at Idle - Possible Leaking Intake Valves

Models:2004 - 2006 Buick Rainier
2002 - 2006 Chevrolet Trail Blazer
2002 - 2006 GMC Envoy
2002 - 2004 Oldsmobile Bravada
2005 - 2006 Saab 97x
with 4.2L Engine (VIN S - RPO LL8)



On rare occasions, a SES light and P0300 DTC may be encountered as a result of single cylinder misfires only at idle. These misfires may or may not be felt and they will disappear off of idle. If the engine misfires do not occur at idle, this PI does not apply.


If the SI diagnostics do not isolate a cause for this concern, perform a cylinder leakage test as outlined in the "Cylinder Leakage Test" procedure in SI and document all leakage rates on the repair order. When performing the cylinder leakage test, it is very important to hold the crankshaft with the related piston at top dead center to ensure that the valves are fully closed or inaccurate results may be obtained. If an aftermarket leakdown tester is used, the instructions of the aftermarket tester should also be referenced because with some aftermarket testers, it may be necessary to limit the air pressure to 50 PSI in order to obtain accurate test results.

If a leaking intake valve is found, replace the cylinder head and valves.

Expert:  Ken replied 5 years ago.
If the miss fire is only at idle you will only find it with a leak down test. Compression test will pass. Ken
Customer: replied 5 years ago.
It's not a misfire during idle
Expert:  Ken replied 5 years ago.
when do you feel it then?
Customer: replied 5 years ago.
Sluggish going pass 40mph check engine light starts to flash when above 50 it 60 mph
Expert:  Ken replied 5 years ago.
Its only #6 what fault code is setting?
Customer: replied 5 years ago.
Expert:  Ken replied 5 years ago.

does it just lack power or do you feel a bucking miss fire


Customer: replied 5 years ago.
Lacks power . Stepping on gas pedal harder to gain speed hills are worse
Expert:  Ken replied 5 years ago.
Do a back pressure test on the cat. If the cat is partly plugged it will cause a lack of power and can pick on one cylinder. Check the fuel pressure. Low fuel pressure can cause problems and #6 is the last one on that fuel rail. that is what i would check next. Ken
Customer: replied 5 years ago.
Could a fuel filter cause low fuel pressure ? Or is that just from fuel pump ?
Expert:  Ken replied 5 years ago.
Yes !! It acts just like a bad fuel pump or a plugged cat !! All 3 feel the same when driving down the road. Ken
Customer: replied 5 years ago.
So a cloged fuel filter can cause a #6 misfire ?
Expert:  Ken replied 5 years ago.
Yes, on that motor more than others because #6 is the last on on the fuel rail. This would cause a lean miss fire and most likely under a load when more fuel is needed. If you can see the mis fire counter on a scan tool. you will see a lot on miss fire on #6 and some on others. #6 is missing the most. Ken

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