I have a 94 chevy s-10 pickup with a 4.3 vortec and ever since I swapped motors cause the old one was blown Ive had this problem. Everytime I start my truck it will Idle up to around 1800 rpm then drop down to 600 then back up and back down. This will last for a good minute, to a good 5. When it is warm it runs fine and after that runs like a champ. I’ve had several mechanics look at it and they can’t diagnose the problem. Ive replaced a lot of sensors. Someone please help me out!!
Sounds like there’s a vacuum leak somewhere on the engine. Since you did an engine swap, the first thing to check for are any disconnected/misrouted vacuum hoses. The emission sticker under the hood shows all the vacuum hoses connections/routings.
Another thing to check for is a leaking intake manifold gasket on the replacement engine. If that’s the problem, it would explain why the idle is erratic when the engine is cold, but settles down once the engine is at operating temp.
Tester
Ok i’ll check that out and then i’ll get back to you if the problem still consist. Its just weird of why some days the problem will only last 30 seconds to 5 minutes sometimes. It used to get so bad it would kill my motor. But when its done doing that it idles pretty good so we shall see. Thanks!