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Pete
Pete, Auto Mechanic
Category: Chevy
Satisfied Customers: 15191
Experience:  14 years GM dealer Technician.
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2003 Tahoe: Misfire..intake gaskets,plugs, wires coils & injectors

Customer Question

2003 Tahoe Z-71 5.3 Z. Misfire on cyls 4 & 5 at Idle. No misfire off Idle. Did intake gaskets,plugs, wires coils & injectors. Please help.
Submitted: 2 years ago.
Category: Chevy
Expert:  Pete replied 2 years ago.
Hello,I will be glad to try and assist you with this,Welcome to Just Answer.

Hi.

If you have replaced the spark plugs,plug wires and plug coils as well as #4 and #5 injectors and you still have this misfire then the next step is a compression test and a running compression test to see if the compression is lower on 4 and 5 compared to the rest of the cylinders.

I have seen broken valve springs cause this type of fault here and have had to replace them to correct this condition.

Usually it can be a valve issue or valve spring issue with misfire at idle and not off idle.

The compression test needs done next to see if you have any of these conditions.You can also remove the valve covers to inspect the valve springs.

Let me know if you need anything else,click accept if you feel satisfied with your answer as this is how I get credited for my time,Thanks Pete!
Customer: replied 2 years ago.
I should have stated that compression test has been done and they are all within specs.
Expert:  Pete replied 2 years ago.
Was a running compression test done as this is what usually will show up valve issues or even broken valve spring issues?
Customer: replied 2 years ago.
confused about a running compression test. Do you mean engine running?
Expert:  Pete replied 2 years ago.
Yes,remove one spark plug at a time and install the compression tester,then start the engine and run at idle,watch the gage it should pump up pressure and then stop,this is the running compression for this cylinder.

Repeat with all the other cylinders one at a time.Running compression should be approximately half of the normal compression.

If you have cylinders 4 and 5 lower then the rest at all then this can indicate a valve or possibly valve spring issue.

Let me know if you need anything else,click accept if you feel satisfied with your answer as this is how I get credited for my time,Thanks Pete!
Customer: replied 2 years ago.
I will get right back with you Pete aand then I will click accept.
Expert:  Pete replied 2 years ago.
Sounds good.

You may also want to spray some water or carb cleaner around the base of the intake to see if the engine runs rough or picks up speed as this usually indicates there is still a vacuum leak around the intake and since the gaskets were replaced it may be a warped intake.
Customer: replied 2 years ago.
checked intake when it was off. Not warped.
Expert:  Pete replied 2 years ago.
O.k.

Try the running compression test and see if anything is found.

Carbon on the valves can also cause this as well.
Pete, Auto Mechanic
Category: Chevy
Satisfied Customers: 15191
Experience: 14 years GM dealer Technician.
Pete and 8 other Chevy Specialists are ready to help you
Customer: replied 2 years ago.
Pete did you get my compression readings?
Expert:  Pete replied 2 years ago.
No I didn't.
Customer: replied 2 years ago.
I didn't do 7 or 8. cll 2-125, 4-110, 6-150,1-145, 3-95,5-90. Cyl does not have a misfire. Just 4 & 5.
Expert:  Pete replied 2 years ago.
Was this running compression as they all should have been close to the same?
Customer: replied 2 years ago.
running compression.
Expert:  Pete replied 2 years ago.
Usually with a running compression test you will get about half of the normal compression tests readings.When you do the running compression test the gage should rise up in about 3 to 4 strokes of the piston then top off,this is the reading I need for running and I apologize as I didn't explain it earlier.So if you waited until the gage was completely done going up this may be why these compression readings are all over like this.Usually you would see about 70 to 80 psi on each cylinder with a running test.

One other thing are there any other fault codes set besides the misfire fault codes?

Have you checked 4 and 5 injectors for an injector pulse?
Customer: replied 2 years ago.
no other codes. I am checking injector pulse now.
Expert:  Pete replied 2 years ago.
O.k.

Let me know what you have for injector pulses on cylinders 4 and 5.
Customer: replied 2 years ago.
have good injector pulse on 4 & 5.
Expert:  Pete replied 2 years ago.
O.k.

Good compression,good spark,new injector,wires,plugs and coils.This sounds like you are looking at a compression fault most likely from the valves unless there is still a vacuum leak at the intake or carbon build up on the valves possibly.

You can check for broken valve springs but the valve covers have to be removed then each intake and exhaust valve spring inspected to see if one is cracked or broken.

You can have a shop or dealer try and clean the engine of carbon with their machine.

But from what has been done here and what is described these sound like what are left that can be at fault causing this problem.
Customer: replied 2 years ago.
Thanks Pete. I will get back with youu.
Expert:  Pete replied 2 years ago.
Sounds good.

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