Hi all,

I have a 2002 Trailblazer and lately my engine starting pulsating. If sitting in park or drive it the revs will fall to nearly zero, the lights will dim and it will almost die. Then the revs spike back up to around 1500 and it starts all over again. Any ideas?

 

Sounds like your alternator is going bad. I would suggest having it checked at a local parts store like Advance Auto and they should be able to tell you if it's bad. Another check is to pop the hood and see if there is a white powder residue on the alternator or around the alternator. This tends to happen when they go up. It's about $120 part and if you're mechanically inclined, you should be able to change it by yourself in about an hour.

I would definitely suggest having it checked before it dies and leaves you stranded.

EDIT:
Anybody else have a suggestion?

 

will do that today, thanks

 

Since you have the new Trailblazer, I'm going to move this into the GMT360 Tech section.

I am going to assume you have the I6 4200 Vortec motor and not the v8.

I'm not sure that an alternator would have that much effect on the idle quality.

The first thing that I would suggest you do is to pull the throttle body off and give it a thorough cleaning. The throttle bodies on these engines are electronic and if they get gummed up by carbon gunk, the idle can suffer.

Go grab a can of sensor safe carb cleaner, pull the throttle body and clean it. You'll have to remove the intake resonator housing (on top of the engine). Next, you can pull the electrical connection at the throttle body as well as the vacuum connection. Then just unbolt the throttle body. Clean it and reassemble reverse of disassembly.

Once you have that done, unplug your battery for 15-20 minutes (just do it before you start pulling everything else apart). This will cause the PCM to relearn the throttle position to idle speed values.

 

very good, gives me something to try this weekend. Thanks a bunch

 

Throttle body is another good one! I forgot I had to do that awhile back too.....around the same time the alternator went up.

 

The I6 4.2L engine is very much different from the venerable 4.3L and specifically in how it controls idle speed. The old IAC valve on the 4.3L throttle body is much less affected by buildup. The electronic throttle on the I6 4.2L engine however, is significantly affected by this buildup and can become erratic if resistance to opening/closing the throttle blade is high.

 

Huh.....Learn something new every day!