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)

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.

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.

That ain't right. I've got a working MPS I'll loan you for the cost of postage. PM me if you need Phil.

Flip the plug over on the MPS

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

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.

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.

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.

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.

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.

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.

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

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

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

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.

i went over and tested the MPS at lunch time- holds vac, and T7-15 was 92ohms, and T 8-10 was 340. so that means its good, right?

i also have 2 other s and one does not hold vac perfectly but the values were the same on it, 92 and 335.

i think from here i can still test a known working MPS but i need to go thru the harness like Rich suggested since it seems like it may be something else making it super lean and taking the hose off the mps makes it default rich so it will run -? i kind of suspect either the new CHT went bad or that circuit did, either way this is what thact could cause. hm....

The only thing you didn't state was whether you tested each of the terminals to the MPS case (short to ground). The ohms are in specification so the coils inside are good.

i did but did not know what i should be reading on this. i tested all 4 and the meter stayed at 1, ( sorry for my ignorance, i forget which is good - but it showed 1)

If you have a digital multimeter and it read "1", you have no continuity hence no short to ground.

Sound like the MPS is good, unless someone putzed with the adjustment screws. If the epoxy is still covering the screw then I have no more ideas.

Phil, you've got a bizarre one going there! I found this thread on PP....relates to some of what you're seeing. Don't know if it'll help but who knows....
http://forums.pelicanparts.com/porsche-914...sconnected.html

Did you ever try out that spare ECU I sent down? I don't know if it could be related but try it for the hell of it if you haven't already. Jon

I think what you've determined is there is no open in the electrical circuitry.

The real key is the electrical inductance of the 2 sections.

The purpose of the MPS is to sense the pressure and change the inductance accordingly. The inductances are used by the ECU in a monostable multivibrator to control the width of the injector pulses.

Thus if the inductance is off, the engine will run lean or rich.

When I got my 1.8L running with DJET the MPS was way off in its setting for the ECU as the engine was super lean. It wouldn't idle and ran like shit. I had to dig out the epoxy on the MPS and adjust it to richen up the mixture.

By the engine running when the MPS disconnected from the intake vacuum, this is telling the MPS to apply maximum pulse width. So it sounds like your engine like mine is running really lean with the MPS.

Initially I fooled the ECU by adding resistance in series with the CHT sender. This fooled the ECU into thinking the engine was cold and the ECU needed to widen the injector pulses making the engine run a bit richer. I solved this issue by adjusting the MPS.

BTW, what resistance is the CHT sender reading when the engine is cold and hot? If the CHT is shorted or very low resistance, this could also cause the engine to run really lean.

Usually a really lean condition will still run but have really high idle and hunt like a vac leak. In that case I did adjust the mps until it brought it back into the 12 range on the AF meter. Right now I have not gone back to test the cht but I am suspecting the cht or cht circuit. I just think that unplugging the mps making it go rich is a compensation for the lean r inning due to cht not being right somehow. I Called AND Left Him A Message To Try To Go Over This With him but he did not return my call. so I will try again tomorrow.

Oh, you have already adjusted the MPS from the stock setup?

I got one ive been using that was already adjusted. the i had set to factory specs by Scott in NH but it was way too lean. so i tuned it with a/f meter, about 2 years ago, but since this new issue began i had not messed with it.

I sent him a variable resistor so he could check that side of the puzzle, also.

going to re-test that CHT, and see wheere the values are before i us the variable resistor, then use that to see what's required to bring it into spec if its out- but on the advice of another member i went ahead and orederd another new CHT as well because i figured it could not hurt to have an extra one on hand

alright a bit of an update: just spoke with Bob Woodman, my mechanic. I pulled the ECU and went thru the harness and tested all circuits - no issue there. Pulled the Cht, tested hot and cold, all good right on spec as well. Next went back to the MPS. with car on and running but that vac hose off it starts and runs, although very rich at @8-9. next he hooked up the vac meter to test it and once he had it ot just 5mmhg it killed the car- and the AF went crazy lean! - at this point we both think that its that stupid MPS so we should have a good test mule coming from kent iin a couple days, and see if that is it for sure. Now i also have a spare ECU that was sent to me to test and that really is the only other thing it could be, and i just dont think it is the ECU.

Rich,
Why flip the plug? The inner prongs go to one coil and outer to the other, so either way you will get the same inductance signal. Which may not be correct as we have heard. Just my $.02

Picked up the car tonight and drove it home.
A/F meter reading high I think. Idle at warm is 13, under part load is between 14.5- 15; or so. That seems too lean but it did go well . Now.it will be interesting to see how the other one that is being sent to me reads.

So what did you do in order to drive the car home? Sounds like you either fixed the problem or at least got it partly solved.

--DD

Switched out the mps with another unit that I had. It's been nested with and has a ver slight leak down on the vac but good enough to start and rum with out killing dying or stalling. Just have to source a good mps for keeps. Going to be able to test one that Kent is sending over. But it seems like the at was the major issue.

Under light load you should be leaner than that, but when you tip into the throttle it should go rich - 13.2 to 12.5.

Was that mps a brand new one?
Sounds jet exactly like the scenario I've seen previously
Put Kent's mps in and verify your numbers get better

Chris is right that your afr is still wrong. I'm betting you have adjusted the one in the car? Way off.

If Kent's mps truly proves to the you good numbers.
Then take the diaphragm from your good mps and rebuild your old one.
I think there a something wrong with your new mps. I've seen this only once before. Carefully compare the two diaphragms and use nothing else.
Maybe have Geoff calibrate it if you want it perfect

That's where I was before it went bad. I'm going to try to get this one adjusted to a better mix . I have 2 of the rebuild kits from the group buy so I'm going to go ahead and find out if I can make one of the other ones work. I will do some comparison with the one from kent, Nad another one that I will be testing and purchase it it tests out good - that one is an unmolested one so I what to see where the numbers on these are.

Phil

The one that got me home was one i picked up a few years ago that was opened up and revealed by someone else at some point. It holds vac good. Epoxy plug was taken out so at some pint I had it sent to Scott to set to factory settings. Had not use it until now and it appears that the factory settings are lean on my car. Scott used a LCR set it up.

The only published MPS calibrations are on Brad Anders page, which are a collection of measured calibrations of existing known good and NOS MPS. I have found that they are also lean for unknown reasons. The Wavetek meter (I have one) can be useful to get the calibration close and that is all. The only way to get the A:F ratio exact is to use an exhaust gas analyzer or WB O2 to calibrate the MPS. Each engine will be different based on its condition, wear, differential flow rates of injectors, fuel pump settings and performance, sensor variations, etc and the EGA/WBO2 is the only way to get it optimized.

Convincing evidence that the factory fine tuned each car individually is indicated by the mark on the ECU idle knob ring.

I agree
the factory setting are far from ideal
remember how different gasoline formula's were back then!

todays gas is a lot different
perhaps the factory settings were better? for gasoline of the day
or perhaps the factory settings were too lean even back then for fuel mileage and simplistic testing equiptment.


as you rebuild one of your cores, make sure that its a 2.0 housing...
the 1.7 housing have a WOT stop in the casting that you would need to grind out if you are using one of them on a bigger motor.

I still can't wait to hear what happens when you plug a different, known good MPS into the car. I think that your entire problem was caused by this MPS. Hell if I know why, but i've seen it once before.

you never replied.... was this MPS a NOS one?

No, as stated in my previous post, it was rebuilt at some point but too lean. I spent a hour on it tonight adjusting slowly the mps trying to bring the mix closer to 12.7 that anders says is the target for part load mix. It's still hitting about 14 on acceleration up a slight grade. That was after adjusting the center screw about 3/4 turn. At.least it starts and runs. Tomorrow I'll work a bit more on it but will take it a quarter turn at a time , test drive, and I will keep working it down closer to the 12-13 under part to full load. I've been going back over all of anders data etc. For some.reason the idle is a bit high but that could be the lean setting as well. I have to recheck the tps setting. It seems like it is not set exactly where I set it after it was in the shop. I think there a are is fine. It was fully closed tonight after the engine warned up.

FWIW Phil, that one is for a 75-76 so may run a bit lean. But it will sure as Hell work for your needs.

Any news?

Spent part of one day messing with it , reading up on the mps tuning, it did start right up Friday night, and took it for a drive but have not done anything with it the past 2 days. Yesterday I spent the day inspecting and test driving the Audi tt, - bought it and brought it home. Big reason was really all day rain yesterday and today.

So I have not been able to try out the mps that you sent to test until today.

The car started but was a bit rough at first. I let it warm up and see where the idle would settle to and see what the A.F. meter was reading then took it for a drive around the neighborhood.
Upon starting the a.f. reading was 10, after warming up went to 12-12.5 during drive on acceleration it was reading about 15-16 ,y cruise was about 13.5-14. . This is a never opened or adjusted one from Kevin (rhoadguy)
Buy I think I can say that this thing is still not running right. There is something else causing problems. It just does not run smooth, feels like it misses here and there. Yes my other MPS was a major cause of it not starting but there is something else going on here too that I have not figured out yet. It will be running fairly well and idling and suddenly the idle drops l I me it's going to die. Also feels like it's misses or skips here a ND there. Almost like a short.

Now that I have an mps that is known good, I may have to go back thru and try a couple things like plug wires, points etc.
I should get a chance to test a few things tomorrow, might even hook up the spare ecu. I might even try some other plug wires

I wonder if this is related to the advance weights binding in the dizzy? Have you lubed it in the last year? 1 drop of 30 weight in the center of the dizzy shaft, annually per the manual, to lube the advance weights. It may cause the hiccups you describe.

No I've not done that.