MPS issue, car only runs with hose disconnected

[loux951]

Here's the situation...my 1970 1.7 with FI will only start if I disconnect the MPS vacuum hose. The engine was running rich, I checked the old MPS (didn't hold a vacuum) so I replaced it.

The only way I can start the car now is to disconnect the vacuum hose from the MPS (puts the car in the same scenario as when the broken MPS was on) and it runs rich. As the car is running, I reconnect hose to the new MPS and the car dies out. There is definately vacuum pulling from the hose. I'm guessing that the MPS is sensing the vacuum, adjusting (leaning) the mixture appropriately, and the car stalls.

I've check and adjusted the fuel pressure.

Any ideas?

[Jared at Pelican Parts]

does it have the mixture screw on the ECU?

[Dave at Pelican Parts]

So what you're saying is that the basic mixture is very very lean, it sounds like. Let's see, what can cause that?

Air leaks can cause some mixture changes, even in a D-jet system. Probably not to the extent you're experiencing, though. Low fuel pressure could do it, too. Wires that are not connected correctly, or are broken, can do all kinds of strange things. The head temp sensor could be shorted to ground, but again I don't know that the change would be enough to require an unplugged MPS to run.

I'd definitely check the head temp sensor and all of the wires you can find.

--DD

[pbanders]

Perhaps the reason the car is running when you disconnect the hose is that the hose is acting as a bypass air source? Does it only run when you have the hose off the MPS and pulling in air? Or are you plugging the hose? If you're letting it pull in air, try leaving on the MPS hose, and pulling the hose that goes to the auxiliary air regulator. If the car runs, then you've got a different problem than the MPS. It could be a combination of improper setting of the air bleed screw on the throttle body, basic engine timing, and/or an idle mixture problem.

[Brant]

Brad,

2 cars out here have similar type of issues.
I have not been able to figure out why and was curious if you had ideas.

I'll be specific. There is a 1974LE with correct number matching parts. Its MPS holds no vacuum and is bad. But when I put a good MPS onto the car it dies. It runs pretty good with a blown MPS and run basically the same if the vacuum line is disconnected (and open) as it does with the vacuum line connected onto the bad MPS.

the other components test good.
the CHT tests good.
the owner even bought a correct spare brain and changed it with no effect.

brant

[hardflex]

I've never had a Djet car run too lean, so this seems like a tough one, but I'll suggest a couple of theoretical possiblities.

resistance in the ground connections

trigger points worn to the point they just break instantaneously.

Since there are 2 different circuits to drive the injectors, even these would be predicated on both being equally faulty and not likely. It's got my curiousity up.

[pbanders]

When all else fails, rent the VW1218 tester from Pelican:

http://www.pelicanparts.com/

It electrically checks out everything in the FI system, including the MPS and the ECU, all major circuit blocks, as well as the harness. It does NOT test if the hoses are connected correctly, fuel pressure is right, ignition timing and dwell are correct, engine has sufficient compression, etc.

[pbanders]

Originally posted by Brant
Brad,

2 cars out here have similar type of issues.
I have not been able to figure out why and was curious if you had ideas.

I'll be specific. There is a 1974LE with correct number matching parts. Its MPS holds no vacuum and is bad. But when I put a good MPS onto the car it dies. It runs pretty good with a blown MPS and run basically the same if the vacuum line is disconnected (and open) as it does with the vacuum line connected onto the bad MPS.

the other components test good.
the CHT tests good.
the owner even bought a correct spare brain and changed it with no effect.

brant

Is the idle bleed screw completely screwed into the throttle body? Is the fuel pressure correct?

I'd have to see the car myself to figure out what's wrong. If you rent the VW1218 tester and it checks out OK, then I'd look to other areas as to the cause of the problem.

[hardflex]

if the bleed screw is completely screwed in, it could be dependent on the blown MPS diaphram to bleed air thru for idling. Thus full throttle gas from the brain and air from the MPS hose would make it run, but rich. Connect the good MPS and it would die from lack of air.

Just a thought