Hello 914World, While I answer a lot of questions, I rarely post, BUT I am not well versed in D-Jet. I am having an issue that seems like it should have a simple answer, but so far, I am missing it.
So I am putting back together a 2.0L recently rebuilt for a friend and once I got it all back together, and once it started, the engine won't come down below 3K RPM.

I am assuming this is some kind of Vacuum leak, maybe? It runs strong and smooth at 3K but doesn't go down with closed throttle.

SO, the engine is all new and I need to break in the cam, which, while 3k is probably ok, I'd like to get it sorted before I do that 15 min of 2k RPM.

I've checked all the vacuum lines and connections and see nothing wrong. I am thinking to get a smoke machine, or similar to try to see what I may have missed.

I am an L-Jet Guy with little experience with D-Jet and would LOVE some pointers or suggestions.

Thank you in advance and I did do some searches and haven't yet found anything like this.

CupO'Meat!

(the car is a 74 2.0L and I used Jeff Bowlsby's vacuum line diagram to double check)

Auxiliary air regulator wide open is the most likely culprit. Plug the hoses and see what happens

Leak at throttle body? Timing?

Advanced ignition timing will raise the idle RPM, but 3K is probably more than that could account for. You have a vacuum leak somewhere--and very likely, it's a hose that is not connected to anything or a hose fitting that doesn't have anything attached to it.

Carefully look at the plenum. (The "intake air distributor" as it is called in the manuals, or the manifold if you're old fashioned.) Look for holes or fittings. There should be none that are open. Also look at the top and bottom of the plenum--there are two posts inside it to help support the top of it, and cracks can develop around where those posts attach. We have also heard of cracks forming in the flange where the top and bottom half are joined.

Make sure you don't have any hoses that are not connected to anything. (Possible exception: If your throttle body only has one small fitting on it, the vacuum advance fitting on the distributor dashpot does not get connected to anything.)

You have a vacuum leak. Find it and the idle should come down.

--DD

I would
1) double check timing
2) remove all vacuum lines from the plenum and put tape over them
3) remove vacuum lines from throttle body and put tape over them

You must leave the MPS unit connected to the plenum.
Pull the hose off the plenum and do the suck test (like when we were kids trying to get a straw to stick to your tongue)
If it stick, it holds a vacuum, plug it back into the plenum.

If that worked, methodically connect one hose at a time, start motor each time.

Yeah 3k is about 3/8 inch dia size leak if it is vacuum. Could be timing and a leak combined. If it is running good I don't think it's going to be 3k advance in the timing though. Sounds like vacuum.

Does it have the passive pcv? 74 is an odd year if I remember correctly, early was pcv valve and late was passive?

Are you using the same plenum and throttle body as before the rebuild? Was the idle ok then? If so, I'd look for other sources of vacuum leaks. A sneaky one is where the intake runners meet the heads. Sometimes the spacer/gasket gets hung up on the engine tin, instead of sittling flush on the head.

These are all great, THANK YOU!

I'll be going through all these suggestions today and answer/try to see what I can find.

SO, consensus is likely Vacuum leak, which is what I suspected.

OFF TO FIND IT!!!!

I had this problem after re-installing my 2.0 a few months back. Turned out I had plugged the MPS wire (male spade) into the oil temp wire (fe spade).

The MPS has a four-wire connector. The oil temp has a one-wire connector. I'm pretty sure that wasn't what you got swapped around...

--DD

You're right Dave, I need to go out and look again. It definitely was that fe spade with the clear plastic cover that connects head temp sensor to the ECU. Maybe it was the oil pressure wire that I plugged in. Went w/o any sleep for 34 hrs combined w/jet lag, a miserable cold & a 72 yo brain.