my truck has only been idling at aboutr 200 rmps. normally it idle at 500- 700 rpms. what causes this and is it a problem. i think it mite be the throttle position sensor boyt i hear that it is expensive to replace. is it worth it?

might be something wrong with your tach,. i think the starter spins it that fast,..

Nick

I would check the tach too, it doesn't seem like it would even run at 200 rpm....seems like it would just shut off when you don't have your foot on the pedal. Are you saying the engine really does idle slower than it did before or are you saying that the actual engine idles ok but the tach just shows 200 RPM now??

The tach runs off the alternator and depends on the alt pulley to be the correct size for the tach to be accurate. If you have recently changed your alternator, that may be where your problem came from.

At that RPM look at your oil pressure, if it sounds like it is truly running that slow, you might now want to idle it at all.

it seems like it idles slower. but i dnt think its idling 200 rpm. we took my alternator off awhile back. mite be the problem. truck seems to have no problems while idling

Alternator is bad or on its way out. Check your alt, the tach is signaled from the alternator.

negative thats why i took it off to test it. alternator passed.

most shops do not check the tach signal only if it charges enough

After reading here and there and realizing that the tach is off on my Burb, I believe that my alt. pulley is too small. Yes the alt. has been changed. I have a 1994 2500 GMC Suburban with an "F" 6.5td. The local alt. rebuild place says they have a 56mm "OD" listed but I have read that maybe it should have a 62mm pulley. I measured across the face on my pulley and it measured 2 5/16 inches which is about 59 mm. My question is, what is the correct size pulley for my alt. and how is that measurement taken.