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David H
David H, GM master certified
Category: Chevy
Satisfied Customers: 2675
Experience:  Cadillac Technician 18 years cadillac dealer.ase master certified.gm master certified
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Chevrolet Trailblazer LT 04 chevy trailblazer with code P0300.

Customer Question

04 chevy trailblazer with code P0300. I need help
Submitted: 3 years ago.
Category: Chevy
Expert:  David H replied 3 years ago.
Is the misfire only at idle?
Customer: replied 3 years ago.
No it does it through out the whole RPM range. I asked another mech and he wanted me to check the CAT, and it turned out good. The temp before the cat was about 500 and after was about 750-800 degrees F.
Expert:  David H replied 3 years ago.
Have you removed the coils and inspected for water intrusion?
Customer: replied 3 years ago.
They were all dry.
Expert:  David H replied 3 years ago.
Do you hear any unusual engine noise at all with the misfires?
Customer: replied 3 years ago.
If your meaning like a vaccum leak, no I don't hear any just the noise a engine makes when it has a bad misfire. Is there suppose to be any vaccum at the nipple where the small fuel pressure reg hose connects too?
Expert:  David H replied 3 years ago.
Yes the fuel pressure regulator should have full engine vacuum to it.Does it have vacuum supply?
Customer: replied 3 years ago.
There is no vaccum at that nipple on the resignator where the small hose from the regulator plugs into, there is plenty of vaccum from a nipple coming off of the intake manifold close to the fuel pressure regulator. I hooked the vaccum line to that nipple and engine still had a misfire.
Expert:  David H replied 3 years ago.
Is the misfire a sharp jerking type that gets worse on acceleration or more of a steady roughness?
Customer: replied 3 years ago.
It is pretty jerky at idle, it makes the engine, tranny, transfer case shack pretty good. With the throttle it will smooth out some but you can still hear and feel it missing.
Expert:  David H replied 3 years ago.
Do you happen to know how many and which cylinders are misfiring?
Customer: replied 3 years ago.
Well I did check the coils too by pulling the connection while it was running, I didn't get a change in idle when I pulled the number 4 and number 6 plug. So I switched those 2 coils out with 2 difference ones that I did get change when I unpluged them and got the same results. So the code of multiple random misfire is pretty spot on!
Expert:  David H replied 3 years ago.
Try running some sea foam through the fuel system to clean the valve and injectors first.Sometimes I get lucky and this helps.Reply back with the results.
Customer: replied 3 years ago.
I did about a month ago and it only had a slight miss at that time, I sucked about half the can into the vaccum hose coming from the brake booster into the intake manifold and the rest into the fuel tank. Thats when the miss firing got at its worse, I have gone through about a tank and a half of gas since then and all the other things that I have done.
Expert:  David H replied 3 years ago.
You are at the point where a fuel injector balance test will need to be done.This requires a TECH2 and a fuel pressure gauge.If that passes you will need to do a cylinder leak down test to check for leaking valve seats.This model has problems with the cylinder head valve seats leaking,this would still pass a compression test even with leaking valve seats.
Customer: replied 3 years ago.
So pretty much it needs to go to the dealership to have that done?
Expert:  David H replied 3 years ago.
No there are plenty of good techs that can do that on the side for way cheap,especially these days.You can usually find someone good on craigs list.
Customer: replied 3 years ago.
How do you do a cylinder leak down test?
Expert:  David H replied 3 years ago.
Here are the procedures for a leak down test.The tester can be purchased at most parts stores.It is important that both valves for the cylinder tested are completely closed and the piston is at top dead center.

Tools Required

Cylinder Head Leakdown Tester


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Important: A leakage test may be performed to measure cylinder/combustion chamber leakage. High cylinder leakage may indicate one or more of the following:

Worn or burnt valves
Broken valve springs
Stuck valve lash adjuster
Damaged piston
Worn piston rings
Worn or scored cylinder bore
Damaged cylinder head gasket
Cracked or damaged cylinder head
Cracked or damaged engine block

  1. Disconnect the battery ground negative cable.
  2. Remove the spark plugs.
  3. Rotate the crankshaft to place the piston in the cylinder being tested at Top Dead Center (TDC) of the compression stroke.
  4. Install leak tester
  5. Important: It may be necessary to hold the crankshaft balancer bolt to prevent the engine from rotating.

  6. Apply shop air pressure to the leak tester and adjust according to the manufacturers instructions.
  7. Important:

    Normal cylinder leakage is from 12-18 percent.
    Make a note of any cylinder with more leakage than the other cylinders.
    Any cylinder with 25 percent leakage or more requires service.

  8. Record the cylinder leakage readings for each cylinder. In excessive leakage situations, inspect for the following conditions:
  9. Air leakage sounds at the throttle control module or air inlet hose may indicate a worn or burnt intake valve or a broken valve spring.
    Air leakage sounds at the exhaust system tailpipe may indicate a worn or burnt exhaust valve or a broken valve spring.
    Air leakage sounds from the crankcase, oil level indicator tube, or oil fill tube may indicate worn piston rings, a damaged piston, a worn or scored cylinder bore, a damaged engine block or a damaged cylinder head.
    Air bubbles in the cooling system may indicate a damaged cylinder head or a damaged cylinder head gasket.
Customer: replied 3 years ago.
I will get back to you on some results.
Expert:  David H replied 3 years ago.
ok no problem

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David H
David H
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Cadillac Technician 18 years cadillac dealer.ase master certified.gm master certified