After repairs Oil Pressure drops to 0 at idle

There's a number of variables, but it is entirely possible the coolant was leaking into the engine oil. Antifreeze will melt the soft outer layer of engine bearings which will cause low oil pressure, particularly after it warms up. Warm oil is thinner and runs out of the normally tight clearances between the bearings and the parts they support. That can be overcome temporarily by increasing engine speed so the oil pump pushes a greater volume of oil. That will make the pressure go up. Two different mechanical pressure gauges can give two different readings at different times and with the oil at two different temperatures so it's not a matter of who to believe. The common place to start is by removing the oil pan and crankshaft and connecting rod bearing caps to inspect the bearings and measure the clearances. If the clearance is excessive, a new set of bearings can be installed before any damage is done to those journals that ride on them. I did that to my old '88 Grand Caravan at 325,000 miles and it's still going strong today.

Could be a coincidence. There is a sensor on the EGR valve to tell if it moves to the right positions. Usually that sensor causes more problems than the valve itself but a lot of sensors can't be purchased without the valve. Ford trucks have another somewhat common problem with misfires caused by plugged tubes leading from the EGR valve. What happens is five or seven passages become plugged with carbon so those cylinders run fine. All of the exhaust gas goes through the one passage that is still clear so that one cylinder misfires because it can't run on exhaust that displaces fresh air. I'm getting off topic because I don't think that is the problem. I think your mechanic replaced the EGR valve / sensor assembly based on the Check Engine light. When that light turns on there will be a diagnostic fault code stored in the Engine Computer that leads to the circuit or system with the problem, not necessarily the defective part. There are two ways to approach the Check Engine light. Most mechanics think they are being conscientious by replacing the most common, most likely part based on that fault code, and 99 percent of the time they turn out to be right. A different way to approach the problem is by testing that part first and the related parts to be sure they're changing the proper one. That takes time and shops have to charge for their time just like every other service profession does. Shop owners hate having to charge their customers more for labor than their competitors charge. They want that car in and out as quickly as possible. To avoid the higher labor costs for more testing is why mechanics replace the logical part and hope that takes care of it. They have the percentages on their side. The one time out of a hundred they are wrong is when they have to do more in-depth testing, sometimes for free. Some people accuse them of being a bad mechanic or of just trying to bleed more money out of them. That is silly. No mechanic I know would intentionally not replace the part he thought was bad in hopes you will come back with the same problem and think he's a good mechanic. They want your car fixed the first time just like you do. The way some of them overcome both problems is by selling you more parts than you really need, thinking "one of them will surely solve the problem". We had one fellow in town who enjoyed a fantastic reputation as the guy who cost a little more but you knew your car would be fixed the first time. What most people didn't know was he never attended any of the high level classes put on by Carquest and NAPA. He was one of the arrogant few who think they know all the answers already and they can't learn anything new. In reality he was covering up the fact that he didn't understand the newer cars so he just replaced lots of parts. Down the road were a bunch of shops with "inferior" reputations because they sold fewer parts and took longer to repair the cars because they did more testing. They were really the best trained and most conscientious and they had a lesser reputation because they tried to put the customer's wallet first. Sorry for getting so far off the topic. I don't know how your mechanic diagnosed the problems but sometimes it helps when you understand when they are trying to save you money and why it can sometimes backfire.

I saw Doc mentioned a mechanical gauge. I did not see any mention of one actually being used? Just so that you can understand the concept, I have a pic of the one I use for testing (Imma Plumber, gauges are a everyday item) Here's what I recommend, find you a "Nice/ Pretty Gauge that if you "wanted to"--you could mount it under your dash permanently ($20 or so at auto parts store) But as for testing purposes, take out the sending unit--screw the fitting in----without crimping or kinking it, Bring it out of your hood and in thru the window. Now you can Observe it while you drive---If all is good! (BIG TIME PEACE OF MIND!)--It's the darn sender or the gauge in your dash. Fix them--OR nice and pretty, bring the tube thru the fire-wall and mount it permanently (I have mine inside rubber fuel line to protect it)----Thought this might help--The Medic