start up engine speed

joedunlevy · October 8, 2005

I have a 1996 GMC Yukon with the 350 vortec. When I start the engine it runs up to about 1500 RPM for a couple of seconds then it calms down to 1000. After it warms up a bit it drops to 750. Is this normal? I thought that these newer computer controlled fuel injected engines wre supposed to maitain a fairly constant idle speed even when you first start them. I have a knock when the engine starts up and runs fast like that before the oil pressure has a a chance to build. Its seems to be worse when the engine is warm (wrist pin)?.

Mr. Smith · October 13, 2005

My off the cuff answer would be check the ISC on the throttle body and clean it. If that dont fix it then look into replacing after checking all the other possible things.

ISC=idle speed controller

I have a 1996 GMC Yukon with the 350 vortec. When I start the engine it runs up to about 1500 RPM for a couple of seconds then it calms down to 1000. After it warms up a bit it drops to 750. Is this normal? I thought that these newer computer controlled fuel injected engines wre supposed to maitain a fairly constant idle speed even when you first start them. I have a knock when the engine starts up and runs fast like that before the oil pressure has a a chance to build. Its seems to be worse when the engine is warm (wrist pin)?.

<{POST_SNAPBACK}>

GM-Tech · October 13, 2005

Even computer controlled vehicles start at a higher speed than just idle speed. The first thing a computer controlled vehicle does when you shut it down is to retract the ISC in preparation for the next start.

Sounds normal to me.

Mr. Smith · October 14, 2005

No not normal...

I have 2 GM and a Ford. They don't idle at 1500 and drop. When My 4.6 Bird did this the ISC was dirty. Cleaned it and it took a dump a few weeeks later. While I don't know the exact workings like you described I do understand thats not normal and should be looked at.

Even computer controlled vehicles start at a higher speed than just idle speed. The first thing a computer controlled vehicle does when you shut it down is to retract the ISC in preparation for the next start.

Sounds normal to me.

<{POST_SNAPBACK}>