Question about 2002 GMC Envoy

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Rough idle only when engine is cold

When the engine is warmed up the idle is fine. Gas mileage is OK.
The rough idle isn't an intermittent misfire type, but a vibrating high frequency type. 
2002 Envoy 4.2L with new plugs and K&N air filter. No check engine light has come on. 
Any help would be appreciated!

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  • Arty45 Nov 04, 2008


    2002 Envoy SLT 4.2L. Rough idle close to stalling when engine is cold. Put in new plugs. Thought it would resolve problem. No luck Any input would be appreciated!

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Where is the egr valve located? i haven't seen one on mine...??

Posted on Nov 17, 2009

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Check to see if the vehicle has EGR valve most of them are either elecrtronic or vacum controlled disconnect it and start it check and see if this smoothes out cause it sounds like the valve is staying open while the engine is cold if thsi solves the problem replace the valve.

secondly take a small screw driver aor a tiny hammer and tap the valve while the engine is running if it smoothes out replace the valve the valve is carbon filled and needs to be changed .

Posted on Nov 19, 2008

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I have a 92 yj 2.5l 5 speed. it has only after warming up a misfire/rough idle. its fine when its cold and once it warms up and idle comes down it misfires/idles really rough. may be lacking power also. i...


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The ECT has two wires going to it and is dedicated to sending information to the engine computer. It has nothing to do with your dash gauge that shows coolant temp. That sender has only one wire and is dedicated to the coolant temp gauge.

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I am having ongoing problems with the truck miss firing and rough running. Timing is ok, replaced cap and rotor,coil. It still starts rough,idles rough and misses when at cruising speed,although it seems...


Misfire is a common driveability problem that may or may not be easy to diagnose, depending on the cause. A misfiring cylinder in a four-cylinder engine is, pardon the pun, hard to miss. The loss of 25% of the engine's power output is the equivalent of a horse trying to run on three legs. The engine may shake so badly at idle that it causes vibrations that can be felt in the steering wheel and throughout the vehicle. The engine also may be hard to start and may even stall at idle, depending on the accessory load (air conditioning, headlights and electric rear defroster, for example).

When misfire occurs, performance suffers along with fuel economy, emissions and idle quality. And, when a misfiring vehicle is subjected to an emissions test, it will usually fail because of the unusually high levels of hydrocarbons (HC) in the exhaust.

What causes a cylinder to misfire? Basically, it's one of three things: loss of spark; the air/fuel mixture is too far out of balance to ignite; or loss of compression. Loss of spark includes anything that prevents coil voltage from jumping the electrode gap at the end of the spark plug. Causes include worn, fouled or damaged spark plugs, bad spark plug wires or even a cracked distributor cap. A weak coil or excessive rotor gas inside a distributor would affect all cylinders, not just a single cylinder.

"Lean misfire" can occur when the air/fuel mixture is too lean (not enough gasoline in the mixture) to burn. This can be caused by a dirty, clogged or inoperative fuel injector; air leaks; or low fuel pressure because of a weak pump, restricted filter or leaky pressure regulator. Low fuel pressure would affect all cylinders rather than an individual cylinder, as would most air leaks. A leaky EGR valve can also have the same effect as an air leak.

Loss of compression means the cylinder loses most of its air/fuel mixture before it can be ignited. The most likely causes here are a leaky (burned) exhaust valve or a blown head gasket. If two adjacent cylinders are misfiring, it's likely the head gasket between them has failed. Also, if an engine is overheating or losing coolant, it's likely the head gasket is the culprit.

Intermittent misfires are the worst kind to diagnose because the misfire comes and goes depending on engine load or operating conditions. They seem to occur for no apparent reason. The engine may only misfire and run rough when cold but then smooth out as it warms up. Or, it may start and idle fine but then misfire or hesitate when it comes under load. Also, it may run fine most of the time but suddenly misfire or cut out for no apparent reason. Intermittent misfires can be a real challenge to diagnose, so let's start with a steady misfire in one cylinder before moving on to intermittent misfires.

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