volunteerff21
8 Lug Hoarder
I had a very similar problem with my 96' with the 5.7L vortec. Would idle high sometimes when I'd start it up and other times it would start fine, I'd also have to stand on it for a seconds and the RPM's would come down to normal. My truck is automatic so I can't comment on what yours does when you push in the clutch. Mine turned out to be the upper intake manifold not seated correctly around the spider injector. I had replaced my stock spider with the MPFI upgrade which is a MUCH tighter fight into the manifold than the stock piece. Took a lot of persuading to finally get it to seat right. Even the stock piece can be stubborn sometimes.
It defiently sounds like you have a pirate air leak somewhere. Do exactly what Garrinchilders said and gently spray different places with starting fluid on the upper intake (where it meets the lower, around the spider, around the throttle body, etc.) Any change in idle will indicate that is where you have leak. If it is around the spider, try grabbing it gently with a pair of pliers and wiggling it while pushing down on the upper intake to get it to seat. If you need to, try putting some engine oil around the spider injector to help it seat (if a little bit gets into the intake it won't hurt anything). If that doesn't fix it or if the leak is elsewhere I'd suspect a bad gasket. Go down to your autoparts store and pick up an Upper Plenum Gasket Set. It has all the gaskets you'll need (upper to lower, TB, spider, fuel lines, etc.) I'd go through and replace them all since you have to buy the whole kit. If the crack is in the upper manifold either attempt to JB weld it or go to a pick and pull and get a new one (I think I paid $30 for one).
Hope this helps, my truck drove me nuts when it did this. I did everything you did, all new plugs, EGR, IAC, sensors, removed and replaced the upper intake IDK how many times. Turned out to be something as simple as seating the spider correctly into the upper intake. And mine never threw any vacuum leak codes either (although I wish it had!!!!!!)
It defiently sounds like you have a pirate air leak somewhere. Do exactly what Garrinchilders said and gently spray different places with starting fluid on the upper intake (where it meets the lower, around the spider, around the throttle body, etc.) Any change in idle will indicate that is where you have leak. If it is around the spider, try grabbing it gently with a pair of pliers and wiggling it while pushing down on the upper intake to get it to seat. If you need to, try putting some engine oil around the spider injector to help it seat (if a little bit gets into the intake it won't hurt anything). If that doesn't fix it or if the leak is elsewhere I'd suspect a bad gasket. Go down to your autoparts store and pick up an Upper Plenum Gasket Set. It has all the gaskets you'll need (upper to lower, TB, spider, fuel lines, etc.) I'd go through and replace them all since you have to buy the whole kit. If the crack is in the upper manifold either attempt to JB weld it or go to a pick and pull and get a new one (I think I paid $30 for one).
Hope this helps, my truck drove me nuts when it did this. I did everything you did, all new plugs, EGR, IAC, sensors, removed and replaced the upper intake IDK how many times. Turned out to be something as simple as seating the spider correctly into the upper intake. And mine never threw any vacuum leak codes either (although I wish it had!!!!!!)
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