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Tool Hut
Tool Hut, ASE Certified Technician
Category: Chevy
Satisfied Customers: 550
Experience:  Owner of a mobile diagnostics company assisting auto technicians with difficult vehicles
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Pete...2004 GMC Envoy XL 4WD at Autozone I

Customer Question

Hey Pete...2004 GMC Envoy XL 4WD at Autozone I think (couple of months ago can't remember the exact codes) said misfire on cylinder 2 or 3....I have lost the readout from Autozone.(REPLACED ALL 6 PLUGS AND THE COIL OVER THE APPROPRIATE CYLINDER) Waste of time. Codes also said vacuum leak (should have known something else was up besides a bad sparkplug)....vehicle has low horsepower, trouble going up hill, poor acceleration, etc, etc.... Most importantly an occasional LOUD tapping or knocking, whatever you want to call it. It COMES AND GOES, sometimes at idle (embarrassing at a stoplight)...sometimes while just I say it comes and goes. From my online research ( I am not a mechanic), I have diagnosed a burned valve due to the LOUD knocking and vacuum well as finding out that burned valves are a common problem in the 4.2l. Today I was removing the intake manifold. I found that the bolts were way to easy to remove as if they had not been torqued down...and more importantly the 2 bolts between cylinder 5 and 6 were not screwed in...ya know ...still sticking attempt to tighten them down. The last 3 bolts in the manifold were tightened down but I don't know if they were torqued down. I don't want to remove the injectors or the head if I don't have to (I am prepared to do a complete valve job if I removed the head regardless). Any insight...could this simply be a vacuum leak at the intake manifold. Doesn't make sense to me that a vacuum leak could cause this loud and intermittent tapping...but as I have said I am not a trained mechanic like your self. thank you for any insight!
Submitted: 1 year ago.
Category: Chevy
Expert:  Tool Hut replied 1 year ago.

Hi, My Name is ***** ***** for posting your question to just answer. I am a ASE Certified Master Auto Technician with 20 years experience. I am confident we can resolve your problem together.Please remember I don't know your skill level. I can't see, smell, hear, or touch the vehicle. It may take a few back and forth replies to get to a satisfactory resolution.I also try to give all of the information you may need expecting a positive rating.

How did you determine it has a burnt valve? did you use a vacuum gauge?

The lean condition typically cannot cause the knocking sound. Because your vehicle has Variable valve timing it is not unheard of for the lean condition to make a knocking noise. Likely the cam actuators knocking. I would reseal the intake and try again before going in to far. Problem is the lean condition may have contributed to a burnt valve.

Customer: replied 1 year ago.
Thanks Sam! I will reseal the intake and see what happens. I did not use a vacuum gauge, only the codes, so I am assuming some things. I know a vacuum leak can cause a misfire. I will reseal before going to far as you have advised. No attempt was made to even begin to screw in 2 of the bolts to the manifold (possibly an innocent oversight by the person who last removed the manifold) . I will get a new gasket and reseal tomorrow and drive the Envoy and see what happens. Let you know
Customer: replied 1 year ago.
I tried using carb cleaner to spray around the seam of the intake and engine and felt there was no leak but when I started removing things I realized I had not been very thorough and likely missed the leak due to all of the electrical tubing next to the engine that makes it so difficult to get to that area the manifold
Expert:  Tool Hut replied 1 year ago.

Get a vacuum gauge and check your vacuum while idling. If the needle is nice and steady there is no reason to tear apart. If it fluctuates then you will need to tear apart. You should have about 18 in/hg at idle. let me know your results. a vacuum gauge is inexpensive and available at most parts stores. (less that 20 dollars) and will verify if teardown is required.

Expert:  Tool Hut replied 1 year ago.

It has been a couple of days since I have heard from you. Do you still need assistance? I have also noticed that I have not received a rating from you. If for some reason you question did not get answered please reply. I am only compensated for my time when you leave a positive rating. Your understanding is appreciated.