mps problem?

[LittleBlue914]

i was messing around in my engine compartment last night, fiddling with idle, and checking on stuff, when i accidentally pinched the hose running to my M.P.S. my car revved up to 900-1000 rpm and stayed there perfectly probably the best i have ever herd it idle..

the only other way i can get it to idle is to adjust my furl mixture knob two clicks clockwise and then play with my idle screw, this makes the exhaust smell like it's running very rich and the idles it isn't as good as when i pinch the vacuum hose to my M.P.S.


so my question is is my mps bad??? - it seams to hold vacuum fine though
is something else bad any ideas???
or would it hurt anything to just pinch this hose and drive it like that???

any ideas would be greatly appreciated thanks again George



this is what i am referring to my M.P.S.

[LittleBlue914]

i have been doing some home work on this and i think, if i have read correctly, when no vacuum is placed on the M.P.S. it runs in rich mode, and when it detects vacuum it goes lean, so as long is i get it to idle fine with the M.P.S. plugged off, not running to rich or anything i should be good to go right???
i think i am going to take it for a test run Monday or Tuesday when i get time and see if it runs lean enough if not back to square one... any kernels of wisdom would be greatly appresciated


thanks much George

[Dave at Pelican Parts]

The MPS is how your fuel injection figures out how much air is going into the engine. When you pinch off the hose to the MPS, it sees no vacuum, which makes the FI think that you are at full throttle. It won't inject any more fuel no matter what the throttle setting at that point, which is not exactly what the engine needs.

I don't know what your original problem is, but unplugging the MPS is a band-aid at best. It would be a good idea to find a Wide Band O2 meter to see what the mixture is doing at idle and off idle. A good dyno will have one you can use on the dyno, you can buy an LM-1 here on Pelican, and you may be able to pick one up elsewhere.

--DD

[3D914]

George,

Another consideration is this the correct MPS for your engine. IIRC years back the only rebuilt MPS' being done were calibrated for the 1.7L engines. I think I had one on my 2.0L, as I was never able to get a good balance between idle and off-idle - and eventually went to MegaSquirt.

THere was a web site that had a complete breakdown of data on the stock FI, parts and appropriate years. Hope its still around. Maybe someone who recalls can chime in.

Enjoy!

[Dave at Pelican Parts]

Brad's '70 914 2.0L

[jjbunn]

Originally Posted by Dave at Pelican Parts

The MPS is how your fuel injection figures out how much air is going into the engine. When you pinch off the hose to the MPS, it sees no vacuum, which makes the FI think that you are at full throttle. It won't inject any more fuel no matter what the throttle setting at that point, which is not exactly what the engine needs.

I don't know what your original problem is, but unplugging the MPS is a band-aid at best. It would be a good idea to find a Wide Band O2 meter to see what the mixture is doing at idle and off idle. A good dyno will have one you can use on the dyno, you can buy an LM-1 here on Pelican, and you may be able to pick one up elsewhere.

--DD

Hey, Dave - If he pinches off the hose to the MPS, wont the MPS see the same vacuum as just before it was pinched? I mean, the vacuum between the pinch and the MPS will be preserved, right?

I'm trying to understand all this MPS stuff, as my 2.0 914 has a persistent cold start problem - it needs about three goes before it will keep running then runs well after a few minutes. The MPS is the first thing I checked.

[Dave at Pelican Parts]

If there were a perfect seal through the whole thing, the MPS would see the same vacuum. But there just about never is, so it will generally start seeing ambient air pressure (0 vacuum) pretty quickly.

--DD

[jjbunn]

Originally Posted by Dave at Pelican Parts

If there were a perfect seal through the whole thing, the MPS would see the same vacuum. But there just about never is, so it will generally start seeing ambient air pressure (0 vacuum) pretty quickly.

--DD

My expectation would be that, if you pinch off the hose to the MPS, and the engine speed changes, then the MPS or the hose connection at the MPS, must be leaking. In other words, it's a good test for a faulty MPS. Otherwise the MPS would see no difference in vacuum level, and the engine brain would not change the engine conditions. Is this too simplistic?