I've been struggling to set the idle on my stock 2.0 DJet with no success.

With the idle fuel mixture set to max, the idle screw on the throttle body turned all the way in and the engine warm, it idles at about 1500. If I reduce the idle fuel mixture to get the idle down to 900, the idle becomes unstable, seeking between very low RPMs and 900.

Everything is stock and otherwise runs absolutely great.

I've check for the obvious vacuum leaks, plugging every vacuum line except MPS with no effect. This should have eliminated the lines, aux air valve and decel valve.

Everything is adjusted to stock:
Valves - .006 and .008
Fuel pressure - 29psi
Timing - 27 degrees
TPS set properly and circuit board looks to be in good shape

Any thoughts are appreciated!

Did you check the breather gasket, plenum, injector seals? If so, I’d suspect the MPS. I had one drive me now nuts before. Got a spare you can throw on it.

Possibly injector seals?
Just a thought.

Injector seals are new and all gaskets look good.

I've sprayed carb cleaner on areas that could leak. I've found that if there's a leak this will cause the engine to stumble a bit. There were no signs of leaks.

Not sure how to test the MPS. Can't block the line without affecting engine running. I've sucked on the port and the diaphragm seems to be good. No sign of a leak. Also, it runs great other than at idle.

Good battery,grounds?
Things seem to be pointing back to the TPS

Battery ground cable is solid, and transmission ground strap is new.

I,ve tried two throttle bodies, both of which seem to have a good TPS. As I understand it, the throttle mixture adjustment would not have any affect if the TPS was not properly indicating that throttle was at idle position.

Your getting close.I would get a hand operated vacuum pump and check the MPS just for general principles.Its a good old system it just has old age problems like me,Ha!

Have you cleaned then dizzy advance plate?

Assuming you have a stock distrubutor.

Also check the two advance and retard cans for leaking diaphrams.

Not sure what year D-jet....you didn.t say.....all D-jet compments should match for that year.......as they are different.

Distributor is stock, and advance plate moves nicely.

I've plugged both vacuum lines so that should eliminate a diaphragm problem, correct?

I had a similar problem. Turned out to be a cracked phenolic intake gasket.

Carlos

This site lists bench testing procedures for the MPS anlong with tons of other helpful info on D-jet:

https://members.rennlist.com/pbanders/DJetParts.htm

I chased D-Jet problems for two seasons before I finally capitulated and let Chris Foley rebuild my MPS with a new diaphragm and modify it to be tunable.

I was a religious experience. All my D-Jet problems have magically gone away*. I have gone from "I hate this friggin thing I'm installing carbs" to "this thing drives great."

The MPS diaphragms are mechanical plates with a limited lifetime. It's worth the expense to have them rebuilt.

*I do have one annoying issue, but I think it's the nature of the beast. Car starts and runs fine cold but then as the heads get hot/warm, but before the engine is fully warmed up, the computer dials down the fuel and it runs too lean and won't idle smoothly. But once I get 5 minutes on the engine and it's fully hot the engine idles like a charm. I've given consideration to putting in a switchable or time-relayed resistor on the CHT, but haven't garnered the motivation yet...

Another place to look is at the plenum. I discovered a leak in one after chasing a high idle. the support tubes between the top and bottom leaked. I only found it by dunking it in a sink full of water and putting a couple pounds of air to it.

Click to view attachment

JB Weld cured it.

Two most likely suspects on D-Jet cars are timing over-advanced or vacuum leak somewhere. From Anders' site, here's a list of possible vacuum leak problem areas. The plenum, in particular, is a hard one to detect. They are prone to cracks at the support posts, which are hard to see.

1. Intake runner-to-head gasket/spacer
2. Injector seals
3. Intake runner boots
4. Air plenum (e.g. cracks or rust holes)
5. Throttle body gasket
6. Throttle body shaft (worn body or shaft)
7. Distributor vacuum adv/ret cell
8. Auxiliary air regulator (stuck open or leaky)
9. Deceleration valve
10. Manifold pressure sensor
11. Vacuum hose cracks
12. Cold-start valve gasket to plenum

Go through these one by one as best you can and check 'em off.

Edit: Olympic just beat me to it

Once, I had new injector seals installed therefor I assumed that the seals couldn't be the problem. Then one day I noticed #2 injector was at a bit of an angle. I loosed the clamp, straightened the injector, retightened and that was it. I too had sprayed starter fluid in that area but missed it. Just saying...........Also, I forgot to mention the throttle body shaft. Well known culprit, too!

I plan to get a vacuum pump over the weekend to test the MPS.

I'll also inspect the plenum carefully, and may pull it out, pressurize and dunk it.

I've tried two throttle bodies, one of which looks new and has virtually no tracks on the TPS board.

Thanks for all the suggestions!

Bingo - a crack around the support tube right where you showed in your picture. I guess I need to pull the plenum to check the undersides as well.

Thank you very much! I would not have considered a crack in that area since there appears to be no stress, but then again, these things are 45 years old!