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Posted by: warrenoliver Oct 10 2011, 07:25 PM

My '73, 2.0, Djet has been running great all summer! I noticed lately that it smells like gas when running and my mileage is poorer than usual. Yesterday when cruising at freeway speed it began running differently and would occasionally buck or shudder. It would run OK if I gave it more gas or let off on it a little. Today, it really smells like gas when running and sometimes runs really rough or wants to stall. When it runs rough, it smokes black smoke (running rich).
I put a vacuum tester on the MPS and can draw no vacuum. Is that how the MPS is tested? I had engine off with the ignition off when I tried to pull a vacuum.
If the MPS is bad, will that cause the rich/rough running?
I have placed a WTB in the classifieds. Any ideas on where to get a good used one?
Help, I wanna drive a little more before we get the white stuff.

Warrenoliver

Posted by: dlee6204 Oct 10 2011, 07:42 PM

If the MPS isn't holding vacuum then that is likely your problem. When they go bad, it usually runs extremely rich. I have one for sale in the classifieds...

http://www.914world.com/bbs2/index.php?showtopic=146883

Posted by: somd914 Oct 10 2011, 08:12 PM

Yes, testing is simply drawing a vacuum down with with engine off to I think 16" (don' recall for sure), and it should hold vacuum for several minutes (I have read anywhere from 5 mins to 20 mins). Concur that you need a new (used) MPS.

Posted by: somd914 Oct 11 2011, 03:32 AM

Another thought as I have been down a very similar road. If by chance you discover your MPS is good, e.g. a leaking vacuum connection on the MPS when you tested it, I'd check the cylinder head temperature, but check it from the ECU connector. I had a frayed wire at the harness connector to the CHT. Car ran very rough and very rich, would hardly idle, and sometimes didn't want to start leaving a strong gas aroma.

A good troubleshooting link if you haven't seen this one:

http://members.rennlist.com/pbanders/djetparts.htm

Oh, also check the MPS hose to make sure it isn't shot.

Posted by: Spoke Oct 11 2011, 07:38 AM

When you say "draws no vacuum" for the MPS, do you mean you can't pump it up at all or it leaks down rather quickly?

The MPS senses the vacuum in the intake and adjusts the injector pulse width to match the vacuum. The MPS is the only sensor to the ECU to let the ECU know how much fuel to allow through the injectors.

High vacuum = throttle closed = narrow pulse and little fuel.

Low or no vacuum = throttle open widely = wide pulse and lots of fuel.

I was able to fix my MPS with no vacuum by taking it apart and sealing things up. Now it holds vacuum very well.

Posted by: warrenoliver Oct 11 2011, 06:05 PM

QUOTE(Spoke @ Oct 11 2011, 08:38 AM)

When you say "draws no vacuum" for the MPS, do you mean you can't pump it up at all or it leaks down rather quickly?

The MPS senses the vacuum in the intake and adjusts the injector pulse width to match the vacuum. The MPS is the only sensor to the ECU to let the ECU know how much fuel to allow through the injectors.

High vacuum = throttle closed = narrow pulse and little fuel.

Low or no vacuum = throttle open widely = wide pulse and lots of fuel.

I was able to fix my MPS with no vacuum by taking it apart and sealing things up. Now it holds vacuum very well.

When I try to draw a vacuum on the MPS, I can not get any vacuum. I hooked a plastic hose to the MPS and sucked on it - no resistance at all!

I have located a good used MPS so I hope that fixes it. Once I get it, I might take the old one apart. Does anyone rebuild these things?

Warren

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