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> D_jet trouble shooting question - car is home, journey continues, June, update- no more problem-- the fix was simple....
DRPHIL914
post Dec 15 2014, 12:39 PM
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OK. car has been in shop for the past week, and today Bob spent several hours on it but what he found was when the MPS is disconnected from the vac hose it will idle and run, but when it gets connected it goes immediately lean and dies, and will not run with that hose on there- the MPS holds Vac no loss of pressure, but that doesnt mean that its not bad.
1- i thought that if an MPS went bad it defaulted rich? but when this is getting vac it goes lean right away and dies. but it will run and idle with out the vac , the AF meter then shows 9.8-10.2 or so which is of course rich but not flooding out.

so the question is why is it going lean when hooked up? when this issue started about 2 weeks ago, it had been running just fine and all of a sudden it quit running.

2. could there be another issue we dont see that would be making it way to lean other than the MPS?

might need to test another MPS, my 2nd one i just bought from someone here and it has never been opened but will not hold vac (slowly looses vac pressure) so it probably need to be opened and re-sealed. not a good candidate to trouble shoot with. ------

FYI- 1975 d-jet 2.0. newer wiring harness , CHT is newish(replaced 2 months before this issue started and was running perfect at that time)
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SLITS
post Dec 15 2014, 12:44 PM
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D-Jet Trouble Shooting

On the MPS check the ohms between:

Terminal 7 & 15 = 90 ohms
Termianl 8 & 10 = 350 ohms

Check each terminal to the MPS case for ground short.

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JeffBowlsby
post Dec 15 2014, 01:22 PM
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There might be a couple things going on here. If everything is correct the engine should not operate without the MPS. Something else could be fooling it to provide fuel to make it run. MPS do fail rich. Can you pull a vacuum with a vacuum pump on the MPS or not? Sucking on it with your mouth is no where near sufficient to test its ability to maintain a vacuum.

You said its new but just to be sure check the function (continuity and resistance of the TS2, if its out of spec, it could be dumping fuel that the MPS normally provides, enough to allow it to run.

Also, have you checked the injectors including the CSV? Leaking?
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DRPHIL914
post Dec 15 2014, 03:16 PM
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QUOTE(Jeff Bowlsby @ Dec 15 2014, 02:22 PM) *

There might be a couple things going on here. If everything is correct the engine should not operate without the MPS. Something else could be fooling it to provide fuel to make it run. MPS do fail rich. Can you pull a vacuum with a vacuum pump on the MPS or not? Sucking on it with your mouth is no where near sufficient to test its ability to maintain a vacuum.

You said its new but just to be sure check the function (continuity and resistance of the TS2, if its out of spec, it could be dumping fuel that the MPS normally provides, enough to allow it to run.

Also, have you checked the injectors including the CSV? Leaking?

The mps holds vac no loss or drop seen on vac pump gauge. The wires are connected to mps , it will not run when the vac is applied. .
I'm going to suggest to him that he recheck the cht again and check those values on the mps that slits suggested.
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76-914
post Dec 15 2014, 05:06 PM
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(IMG:style_emoticons/default/WTF.gif) That ain't right. (IMG:style_emoticons/default/screwy.gif) I've got a working MPS I'll loan you for the cost of postage. PM me if you need Phil.
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r_towle
post Dec 15 2014, 05:07 PM
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Flip the plug over on the MPS
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Dave_Darling
post Dec 15 2014, 05:59 PM
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The MPS plug is bi-directional; flipping it should have no effect.

Something is making the mixture very lean. Unplugging the MPS hose is making the mixture rich enough for the motor to run.

Last time I heard of something like this, the fuel pump had failed or very nearly so. There was just enough fuel making it through that the absurdly-large injector pulses from having the MPS unplugged allowed the car to idle, but only just.

How's the fuel pressure?

--DD
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DRPHIL914
post Dec 15 2014, 08:10 PM
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Dave, my fuel pressue is right at 30. I dont think its the fuel pump, but before I verified that I ordered a could a new one. Would a bad cht cause that super lean condition? It was cutting in and out before going completely bad. Has not come but I already towed the car to my mechanic.
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SLITS
post Dec 15 2014, 09:02 PM
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A bad CHT can cause bad problems as in the car not starting.

I bought one they couldn't get running. Upon examination, the CHT was not hooked up. Installed and connected one and the car fired right up.
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914bub
post Dec 15 2014, 09:13 PM
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Not trying to Hi-jack here, but one of the hoses to the MPS on my car is plugged off
. I heard that when the MPS diaphramn is bad, some people do that to "override" the problem?

Isn't there someone here that sells kits to rebuild an MPS's?
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BeatNavy
post Dec 15 2014, 09:23 PM
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QUOTE(914bub @ Dec 15 2014, 10:13 PM) *

Not trying to Hi-jack here, but one of the hoses to the MPS on my car is plugged off
. I heard that when the MPS diaphramn is bad, some people do that to "override" the problem?

Isn't there someone here that sells kits to rebuild an MPS's?

Tangerine. Check with Racer Chris or go their website: MPS Kit.
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r_towle
post Dec 15 2014, 09:35 PM
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QUOTE(Dave_Darling @ Dec 15 2014, 06:59 PM) *

The MPS plug is bi-directional; flipping it should have no effect.

Something is making the mixture very lean. Unplugging the MPS hose is making the mixture rich enough for the motor to run.

Last time I heard of something like this, the fuel pump had failed or very nearly so. There was just enough fuel making it through that the absurdly-large injector pulses from having the MPS unplugged allowed the car to idle, but only just.

How's the fuel pressure?

--DD

Given what is happening, a two minute test might help here.

Flip the plug, see if the condition changes.
If it does, you need to troubleshoot your wiring harness.
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DRPHIL914
post Dec 15 2014, 09:42 PM
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QUOTE(SLITS @ Dec 15 2014, 10:02 PM) *

A bad CHT can cause bad problems as in the car not starting.

I bought one they couldn't get running. Upon examination, the CHT was not hooked up. Installed and connected one and the car fired right up.

Ive seen that too.
When I bought the car it would not run. Pulled the mps and it was full of water . Got a rebuilt unit from automotion. Plugged it in and ran great for about a year. Then it gave out snd I sent it back for a refund. Found a working unit here and had been using it for past 3 or so years. Seems now it has tanked as well. But I want to get the car back , I dont think the shop really wants to try to figure it out . Once I have a good working mps should be able to figure out what else is going on.
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r_towle
post Dec 15 2014, 10:55 PM
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Get the car home.
you can do all the testing with an ohm meter, well most of it.

Rich
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DRPHIL914
post Dec 16 2014, 07:17 AM
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QUOTE(r_towle @ Dec 15 2014, 11:55 PM) *

Get the car home.
you can do all the testing with an ohm meter, well most of it.

Rich


Rich,

that is probably what i need to do. I was just at my wits end and really thought it was something non-djet related at the point where i gave up. i really did not suspect the MPS, but then i only tested the vac and that was perfect. I'd been thru everything else and was mad enough if someone had offered me $$ for what i have in it for parts(which is a lot) i would have just walked away. especially since i really needed to be driving the car this past 2 weeks. Ive been down a vehicl since i sold the boxster and regretted that, and this week an opportunity arose to get a high milage 2002 911 cabriolet for under 10K - needs a bit of work but could be a daily driver and that has taken my mind off the 914 for now- i've recommitted to getting back on the road just because im so stubburn and i cant sell it like this.....

btw another member here says that (scott NH) if the vac holds the diaphragm is fine, but that its the inner electronics that have failed . if that is the case then it might fail lean when hooked up to the vac hose, and when disconnected go rich.

Someone is sending me their MPS thats good to test on it.
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r_towle
post Dec 16 2014, 09:18 AM
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Get the car home.
I can send you down the official Bosch DJet tester ( to borrow)
Put that inline in between the plug and the ECu and run it through some tests.

But in the end, it can all be done with a multi meter.

First thing I would suggest is you do a simple ohm test on the whole fuel injection wiring harness.
When things get weird, that can be the culprit.
40 year old wires shorting out makes diagnosis a nightmare.
Then you can test out everything else.

You really need two things to make the car run.
The MPS and the cht.
So test those circuits, and unplug all others.

Rich
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Dave_Darling
post Dec 16 2014, 10:48 AM
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QUOTE(914bub @ Dec 15 2014, 07:13 PM) *

Not trying to Hi-jack here, but one of the hoses to the MPS on my car is plugged off
.


The MPS should only have one hose going to it. If the component in question has more than one, it isn't an MPS. Maybe the decel valve?

--DD
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r_towle
post Dec 16 2014, 11:08 AM
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Oh, an interesting test.
Have your mechanic unplug ALL other vacuum lines and plug them at the plenum.
The. Plug in the one line to the MPS, it's the only one required to run.

If your DECEL valve is stuck, added vacuum may make it change the timing.
If your AAR is stuck open, added vacuum may make it run lean.

When I diagnose these, I do unplug all other vacuum lines from the plenum.
I use rubber covers or electrical tape and close off all ports on the plenum.
You can then use just the MPS to have the car run fine.

If that works, you will then add ONLY one vacuum line at a time back into the system to see which one it doing it.

Rich
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Bleyseng
post Dec 16 2014, 08:13 PM
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QUOTE(r_towle @ Dec 16 2014, 09:08 AM) *

Oh, an interesting test.
Have your mechanic unplug ALL other vacuum lines and plug them at the plenum.
The. Plug in the one line to the MPS, it's the only one required to run.

If your DECEL valve is stuck, added vacuum may make it change the timing.
If your AAR is stuck open, added vacuum may make it run lean.

When I diagnose these, I do unplug all other vacuum lines from the plenum.
I use rubber covers or electrical tape and close off all ports on the plenum.
You can then use just the MPS to have the car run fine.

If that works, you will then add ONLY one vacuum line at a time back into the system to see which one it doing it.

Rich

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brant
post Dec 16 2014, 10:00 PM
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I've seen this once before. Ron's 74 LE did this. He bought an true NOS mps that was still in a bow he box brand new

Car would not run with the vacuum attached but ran well with the hose disconnected.
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