I have a 2004 Chevy Trailblazer 4.2 engine, It runs fine at Idol, but on the road it misses real bad, the Check Engine light dosent come on, I changed the Plugs but it didnt help, I thought it might be the EGR valve, but that year and the 4.2 dosent have a egr valve, does anyone know what the problem may be???

Most likely solution is to replace the s[ark plugs and check the ignition coils boots. The #4 cylinder plug has probably has a carbon track down the side of the porcelain part of the plug which will cause the cylinder to misfire.

When the Check Engine light flashes it means there is a significant emissions problem detected and it needs to be fixed very soon. Continuing to drive with this condition will destroy the catalytic converter. You may have gotten debris in one or more of the injectors when the gas got low.
The first thing to do is get the codes read and find out what the computer is reporting bad.

A jet engine type of noise from the engine area usually means the fan clutch is going bad. I don't think the tach is related to this, that could be the stepper motor for the tach going bad.

For engine to run it has to have:
Fuel, air and spark.
So if it doesn't start, something's missing.
Is your fuel pressure OK? Is your spark present? Is ECM firing injectors?
Check all those basics and I'm sure you'll find what's wrong.

could be a sensor...have it scanned...sounds like the problem....and good luck.

More than likely, you have a failed fan clutch. That is a frequent problem on the 4.3 I6 engine. It will cause a roaring noise and "vibration" and allow the engine to overheat at low road speeds. As far as your check engine light and other driveablity problems, it may or may not be associated. Have someone with a code reader check it. The TrailBlazers were infamous for electrical gremlins.

On top of engine to the back, and down a little bit by firewall, will be some vacuum line's, the one with the elbow will be leaking, to check start engine and feel back there, when you fine the vacuum leak you'll know, replace that elbow, and engine will run fine, save the new part you replaced more then likely it's good.

clean your throttle body....same thing happened to me and my truck runs great now.

there is a TSB (technical service bulletin) on this issue , with directions to install foam weatherstrip to the AIP.(air intake panel) . TSB #06-06-04-048b
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