UPDATE
I had a loose hose on the passenger side valve cover going to the brass nipple. Put a new clamp on it and ran the truck for 30 minutes and no codes. Will take the truck out for a shake down later today.

PENDING
Find exhaust leak

 

The FV308 won’t cause any issues. I’ve installed a ton of them now onto customers LR’s (D2/P38) without a single code popping up.

However a vacuum leak can cause issues be it from a loose hose clamp, air getting past the threads, upper intake manifold gasket, brake booster line, SAI lines or non SAI nipple not sealed well, IAC hoses, bad or failing connection for the IAC hose on the intake hose, bad or missing O-Ring at the air box lid.

Time for a true smoke test to rule out any leaks.

 

UPDATE

I used a shop vac and reversed the air so it was pushing up the muffler and sprayed the exhaust and SAI with soapy water and sure enough found the issue
SAI had a massive leak on the drivers side at the union bolts, tried to tighten down but, still getting some bubbles behind the bolt
SAI on passenger side has a minor leak on both manifold bolts
Flange to exhaust manifold has leak

I can live with this for now, however, I want to try to resolve.

Question - How do I get a better seal on the union bolt and the manifold bolts?
My hack approach, get some high temp liquid copper gasket and spray around all (3) bolts, while truck is running at 3k rpm, so that the liquid copper gasket is sucked into the threads and creates a seal.

 

I like to use never seize on the threads of the nuts. Put a film of red silicone on the end of the fitting that goes in to the cylinder head, then let it cure before assembling. Snug it down after it is cured. Also good to put red silicone on the threads of the cylinder head fitting - do not let it cure before installing. I also like to coat the exhaust gaskets in a film of silicone and let them skin before installing, then I snug down the bolts until they just touch both surfaces - torque 24hrs later.

 

If my hack approach does not work, I will pull the SAI off and follow your advice

 

UPDATE
Ran the truck today and still getting P0101 and P0505
After clearing codes, starting the engine no ups and downs in the idle Picture 1. Took the truck up to 3.6k RMPS and held for a minute Picture 2 and let the truck return to idle Picture . Codes reappeared and idle was going up and down.

At start up


At 3.6k RPMs for 1 minute


Back at idle and codes are back




I did a smoke test and could not find anything & I ran some carb cleaner around all vacuum and upper plenum with no increase in RPM.

I did some more reading and I am wondering...is the PCV308 mod causing the issue (see below)

 

P0505 is an Idle Air Control Valve fault code P0101 is an MAF out of range fault code. Have you taken the IAC off, cleaned it out with carb/brake cleaner and made sure it rotates smoothly?

I've installed so many FV308's onto 99-04 D2's and even on 99-02 P38's and no one has come back with a single P0101 or P0505 fault code. Others can report back with their experiences with the FV308, but this would be the first case of an issue I am aware of.

When you used the shop vac I saw where you put it up the exhaust. Did you take it and go from the intake tube into the engine to test for an upper vacuum leak? That's how you'd find a vacuum leak on the intake. It could be the nipple on the intake tube going to the IAC (very common spot to break), the hose clamps on the IAC to the intake manifold, upper intake gasket, sticking IAC, or the threads on the FV308 (I've never had to use Teflon tape on the threads, but I know some people that have), or the threads on your 1/4 Nipple at the valve cover (I've never installed a 1/4 Nipple there as I've been able to remove/re-install the OEM nipple when doing my FV308 mod).

I've only experienced one bad IAC in my entire career with messing with D2/P38's and it popped the P0505 fault code. I cleaned it, and it still tossed a code. Slapped in a good known replacement and boom problem went away. I don't think it's bad in your case though as you've got a MAF fault code along with it. Heck some D2's I've messed with didn't even have the plastic baffle in the valve cover anymore. It fell totally apart and was just sitting inside the valve cover. That meant it had 100% flow thru to the intake and it wasn't tossing any codes. I was simply replacing valve cover gaskets and noticed the internal baffle was gone completely.

 

1. I have not cleaned the IAC, never even thought about it to be honest and can quickly do that next
2. I was tracking towards the FV308 as that is the only "NEW" thing I did after the head gaskets
3. I used the vacuum to find the exhaust leaks by reversing the air and soapy water, however, I did sprayed the whole engine down and could only find leaks at the SAI connections and of course the flanges but, I would not think that would cause P0101 & P0505.
4. I can take the vacuum and try to get it to push air up the intake prior to the MAF and use soap to see if there are any bubbles since the smoke test and carb cleaner did not net any results

Out side of the rough idle after the code appears, the truck is running like a champ which again is baffling to me.

 

UPDATE
Cleaned the IAC, no noticeable change and codes came back
Pushed air into the intake post MAF and found two leaks
1. Leaking around the brake booster on the upper plenum. Not the line but, the fitting that line goes into. Not sure how to tackle that yet...
2. Leaking at the brass nipple on the passenger side valve cover, I can pull it out and try to put some kind of high temp RTV on it and screw it back in.

 

Try wrapping the threads on the nipple with some teflon tape or yes some RTV.