Each valve cover has a vacumn hose that feeds back to the intake. On the driver's side it is right behind the filler cap. On the other side it is near the rear of the valve cover to a port on the side of the intake. On mine, both of these hoses were in bad shape, I first replaced them with what I had on hand which did not work well asw these are one size on oe end and a different size on the other end. I got new ones from AB.

I have recently learned that a lean condition or a sensor indicating lean will often cause the ECU to stay rich trying to compensate for a lean condition that does not truly exist. I think that is what I have dealt with for the past four years. P1316 continues to repeat sporadically.

 

I guess the purge valve and cylinder could be part of it as well.
Edit: I meant cannister>

 

Idunnoh Danny, I pulled my passenger side vac line and put the gauge there. I pulled the driver side, but it does not have vac at idle. I pulled the line (mechanical click to disconnect) that goes to the purge valve from the throttle body area of the intake, plenty of vac noise, but gauge only went to 13. If anything may have idled up a little. So I wasn't able to duplicate it easily. But if you pull off that line to the purge valve, you'll hear how loud a vac leak can be. Hard to not notice. But like Higgs said, start to remove and cap them, pretty soon you have eliminated all the easy spots.

I also don't think the return line to fuel tank is eliminated. When you turn on key, but don't crank truck, fuel pressure should not go to 50 PSI on a D1.

 

Line from the valve cover is pre throttle butterfly so not much there, I have the passenger side disconnected and plugged. THe Brake booster disconnected and vacuum guage connected. Measured low vacuum to Fuel pressure regulator. Tomorrow I'll comandeer 10' of hose and connect the Pressure regulator to a known source. I'm about 1200 feet altitude in the mountains of Western NC. THis problem developed slowly according to past owner who thought someone had mistakenly put about 5 gallons of diesel fuel in it because of the black smoke from the exhaust. I think its just rich mixture. Gradual onset points to wear or one of them blasted old lines. Can I check ignition timing and valve timing w/o pulling the timing cover?

 

I'm thinkin about running a vacuum line from a known source to the fuel pressure regulator and see if that changes the fuel line pressure and rich problem. If so...then pull the plenum and look around in there possibly a leak to the crankcase or somewhere. What do ya'll think.

 

I agree it should not pressurize like that. You could disconnect the return line at both ends and use an air compressor to see if it is obstructed.

Mine had severely corroded. I keep getting an occasional P1316 and today I crawled under it with the engine at idle. The passenger's side cat was rattling like a group of tin can bumping, it started doing that as it backs down from 2 or 3000 rpms. Guess I nned to find new cats and probably new O2's would help as well.

 

I could not duplicate with vac gauge attached to port on passenger side used by valve cover PCV. Lowest it got was 13. Don't know if vac to brake booster is same. But consider this, it may be more than vac.

I attached my fuel pressure test gauge. Turned key on, did not crank. No vacuum. Pump runs a few seconds them multi function elay clicks off. 34 PSI of fuel pressure. So how do you get 50?. And if you have 50, then same size injector pulse would spray more fuel, because source pressure is greater. The ECU can't tell fuel pressure, there is not fuel pressure sensor. Maybe a fuel pump was changed along the way, or a regulator for a D2 instead of a D1, etc.?

 

I think you would be better off buying a Mityvac hand pump so you can see how much vac you are applying and the result. Connecting to another vehicle may change things but you won't be any better off unless you intend to strap it to the side of the Disco like a side car.

I agree with the rich mixture. I don't know what you hope to find by tearing it apart, even if there was a missing end seal sucking vac and pouring oil out you would have had vac when you disconnected and plugged the breather port at the plenum.

You did plug the nipple AT THE PLENUM, right? Just want to be sure you didn't plug the hose off, while the nipple was sucking wind. If you disconnect ALL the vac lines going to the plenum and plug the PORTS off, and still see no rise in vac then recheck it for vac leaks.

If you cannot find a leak with carb cleaner anywhere along the manifold, at the injectors, or at the plenum seam I doubt you will see anything once it's apart. There is ALMOST nothing you can unbolt that you can't diagnose with carb cleaner while it is together.

Disconnect the fuel return line and blow through it back to the tank, it should be clear. If it isn't you need to find out why.

That said, I would disconnect the exhaust at the pipe/manifold joint and start it with a vac gauge on it.

An engine that can't develop vac either has a leak in the inlet system or it can't exhale efficiently.

 

Blew out the fuel return line 2 months ago. I thought about the cat stopped up but it pushes out the exhaust pretty well. I didn't really place a hand over the exhaust. Plugging 3 ports on the plenum either with a hose attched or a guage. Havent plugged the port for the Pressure regulator I'll try that tomorrow. Using a second car for vacuum would be to confirm that low vacuum is the entire problem, not all these other possibilities.
Thanks a ton for all the ideas, I gotta be closing in on it.
"You are never a failure til you quit trying"

 

Cats are welded on. No easy way to disconnect upstream. One looks normal, flat lookin mounted sorta vertical on the passenger side. Driver side is round looks like a smaller muffler on a 1950 Ford Truck. Might
be as easy to remove the O2 sensors and try to gain some ventilation there to test for clogged cats. No rattle when I tap it by hand.