2004 GMC Envoy vibration while idling

[RKereliuk]

I have a 2004 GMC envoy 4.2 L with 80,000 miles, that vibrates in all gears while idling. However, the RPMs stay steady at around 600. I took it to a local GM dealership and they said it needed repair or replacement of the following:
-new Spark Plugs
-new #1 ignition coil
-clean throttle body
-water pump
-new thermostat as the temp doesn't get above 160 degrees
I have done all of the above except the thermostat which I plan to do this weekend. Also, I removed the throttle body to clean both sides of the throttle plate and disconnected the neg battery cable to allow the computer to reset.
I don't thinks its the motor mounts because the vibration is constant in all gears. Any ideas what to check next would be greatly appreciated. Thx!

[ProTech]

Unfortunately, if the other repairs you did failed to fix your problem this is going to end up being a very expensive repair.

The problem with the 'vibration' or more properly called a 'misfire' is due to an internal cylinder head problem. These 4.2 engines have had some problem with the valves inside the cylinder head which usually requires the entire cylinder head to be replaced.

You might want to bring it back to the dealer so they can concentrate some diagnosis in that particular area. If you've got a miss on the #1 cylinder, they can do a compression check and/or take a look inside the combustion chamber with a boroscope.

If cylinder head replacement is needed, expect to pay around $1500-$2000 to fix this problem.

[RKereliuk]

If it was a misfire due to the internal cylinder head problem, wouldn't it misfire at other times? Like at take off or even accelerating at different speeds? The vehicle runs very smooth once I hit the gas.

[ProTech]

Nope, the tell-tale sign that it's a head problem is that it happens only at idle. Once the vehicle gets going, the misfire goes away. You definitely have a cylinder head/valve problem. You don't want to keep driving it, or it will just be more and more expensive to fix.

[Greasemonkey]

Hello and thank you in advance for any help or advice you are able to give.

I own a 2005 GMC Envoy SLT with the 4.2L I6 engine at 145k miles and have been experiencing a very slight roughness at idle for the last six months to a year, especially when the engine has been running a while and is at nominal running temp. Not a major mechanical problem such as a definitive cylinder misfire and resultant power loss, but merely a little bump and skip when at a stop that belies the engine's otherwise great overall health. Here's a list of things I have done/replaced on my own time to help alleviate this issue thus far, none bearing a positive result where the slight rough idle is concerned:

-- Thoroughly cleaned the throttle body
-- Replaced the spark plugs (with AC Delco 41-103 Iridium-tipped)
-- Replaced all 6 coil packs (Dephi brand coil packs)
-- Replaced the throttle body gasket
-- Replaced the intake manifold gasket
-- Spray cleaned the IAT sensor
-- Spray cleaned the MAP sensor
-- Replaced the variable timing solenoid (Camshaft Position Actuator Solenoid)
-- Replaced the air filter
-- Changed the oil

And the issue remains, a very *slight* miss at idle causing a slightly perceptible roughness especially when the engine is warm and most noticeably when the transmission is in "Neutral". I am currently seeing no SES light and no codes to speak of, nothing to help me further diagnose this issue. Curiously, I notice that the roughness seems to fade to almost nothing when I use a fuel system cleaner (like Chevron's Techron) with a full tank of gas. Scratching my head over that one.

Worthy of note is that after all the services I performed myself on this truck (which together amount to a major tune-up) the vehicle is running beautifully and getting the best fuel mileage of its life with great pickup, power and acceleration. While the engine does have 145k miles on it, I do believe that as well as it is running, sitting at idle should not be any kind of issue whatsoever. Please help, thank you.

[ProTech]

Seems like you've done just about everything. That pretty much only leaves a fuel injector issue, or a compression problem due to a valve issue in the cylinder head. I would probably bet on the latter, as it's pretty common on those engines with a lot of miles, no matter how well maintained.

[Greasemonkey]

Thanks so much. I was afraid of that, but confronting bad news is the best way to get over it. I think.

Any chance that a underperforming fuel pump, clogged fuel filter or possibly a vacuum leak could be a possible culprit before I commit myself to having the cylinder head pulled? <fingers crossed>

Any possible further or long-term damage at risk if I just continue to drive the truck and live with the issue? If so, what sorts of mayhem am I risking?

Thank you again, I appreciate your expertise!

[ProTech]

You seemed to already have fixed anything that might be causing a vacuum leak, such as the intake gasket and throttle body gasket. It can't hurt to check around, though. You might want to check the PCV hose, the brake booster hose, etc. I really doubt you'll find any issues there. An underperforming fuel pump or restriction would have worse driveability issues than you are currently experiencing, but a fuel pressure check can never hurt anything. You should have at least 50psi with the key on. If you continue to drive it, it will eventually get worse and you'll end up having a dead cylinder. This can take months to years depending on how you drive it, so it's totally up to you. It might be ok until you're ready to get rid of it, if you don't want to spend a bunch of money right now.

[Greasemonkey]

Re: The possibility of an injector issue, would the engine not throw a trouble code of some sort indicating injector trouble?

Checking the brake booster hose and pcv hose are good quick tips (I do believe I read elsewhere that it's possible that the brake booster and/or its hose are possible vacuum leak points). I'll check them out tomorrow when I get the chance.

Thanks so much again. Hoping against hope for the cheap and clean fix!

[ProTech]

Usually, yes it will set a code. I'm really doubting it's an injector, though.