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The way to fix it is to get a good code reader and post the results here, then follow directions from these guys. If nothing else, if you can't work on it yourself, you'll at least have a real good idea of what's wrong with it so that you can sound like you know what you're talking about to the next tech you find. Literally, I won't take my truck to a tech if I don't know what's wrong with it. You might as well just sign a blank check and hand it to them. I have a truck sitting in my driveway for 3 years now for that reason, lol.
My guess, and it's only a guess, it that they pulled your IPR valve, left the screen in, and then stuck in new IPR valve in on top of it. Your old IPR was likely good, but just dirty - some little dirt got into the valve and won't let it close, so you can't build oil pressure. Not sure that would cause a stutter though.
I'm hoping the other guys chime in, because I've never seen an IPR like that. If he told you he pulled that out of your engine, I think he's lying unless someone knows something I don't. This is what all the ones I've ever seen look like:
I guess that might be some kind of cover that comes on a new valve to protect in during shipping. Does it just pull right off? I guess the guy stuck it on your old one, who knows why.
Still, a new IPR from the dealer is less than $400, and that's a rip. And no one it takes a trained rep more than an hour to swap in. Shouldn't have cost more than $500 total parts and labor at the most. And the valve likely wasn't even bad.
Can you tell us where the ICP sensor is located on your excursion (behind the turbo or on the passenger valve cover)? That way we know if your engine is an 03 or an 04 engine. That is necessary to know, for buying parts and for some repairs.
As IHateCommieCars posted, the cost to replace that should not be over $500, and that is assessing the part cost at full OEM retail.
The screen could be stuck in the HPOP opening, but the IPR is a "dump" valve. The oil goes through it on its way to be returned to the crankcase. The screen parts SHOULD not be in the oil rail system or injectors. The screen should NOT easily fall off.
I suspect you got a cheap aftermarket IPR valve installed for that crazy cost of $900. You also might have the screen still in the IPR port. We can help you get to that IPR port if you want to try doing it yourself. Otherwise you are stuck with fighting the shop that you went to to fix it, or eating your costs and going to a more reputable shop. Either way, you want to be 100% sure that your IPR valve is an OEM valve.
The ICP is either on the passenger valve cover, under/behind the alternator (late 04) or under/behind the turbo (early 04) next to the IPR valve. You need a special socket to remove the IPR.