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914World.com _ 914World Garage _ "1.7 lost power, won't idle"-Now Runs! But...

Posted by: porschefool Aug 1 2004, 01:18 AM

Hey gang,
'72 1.7 runs fine now, all I had to do was pull the vac line from the MPS. Why the hell would THAT do it?
Can I live without it?
Doesn't make sense..

Thanks to all who gave input!

Posted by: McMark Aug 1 2004, 01:33 AM

Does it idle or drive fine? Very different situations. I believe you might have gotten it to idle without the vac line to the MPS, but you sure can't drive it. If it's driving fine, then what you're calling the MPS isn't really the MPS. It just ain't possible.

Posted by: porschefool Aug 1 2004, 11:02 AM

Just got back from proving you right...
It was too dark/too many beers to test drive last night, but this morning...
yeah. Under load, it ain't happening.
Does that mean it IS the MPS that's gone rotten on me?

Posted by: McMark Aug 1 2004, 12:21 PM

If it's running fine when the MPS is not plugged in at idle then something is way off. It shouldn't idle very well. Thinking about it logically it seems that your car is running very lean and by pulling the MPS hose you make the computer think it's at full throttle and pump in more gas. I'd pull your injectors and see if they're spraying nice.

Unbolt both injectors on one side and pull them up and away. Unplug the coil (black wire on the + side) and unplug the connector from one of the injectors. Place a towel or rag under the injectors and have a friend crank the engine. You should see a fine even cone spray from the injector that is still plugged in. Replace the plug on the unplugged injector and pull the plug on the other one. Repeat. Then do the other side. Be careful because gas is explosive. Use your brain.

If the gas comes out in a stream you have dirty injectors. It should be a nice strong fan spray. If all the injectors have the same bad spray pattern it could be that your fuel pressure is too low to get a good pattern. Check the fuel pressure and then test the injectors again.

Make sure not to lose the rubber seals on the ends of the injectors when you pull them. They must be there or you will add more problems.

As a test, you might try pulling the temp sensor connection as well. It's a two wire connector that is plugged into a sensor on the air plenum body. Pulling that connection should enrich the mixture and let you see if you really are running lean.

Posted by: anthony Aug 1 2004, 01:12 PM

You can do a basic vacuum test with a MityVac on the MPS. See Brad's D-jet troubleshooting web page:

http://members.rennlist.com/pbanders/DJetParts.htm#troubleshoot

Posted by: porschefool Aug 1 2004, 04:44 PM

Ok.
Slapped the meter on the MPS contacts, they look okay.
Pulled 20 in. of vacuum..didn't hold. Dropped to 10 within a minute or so.
Am I screwed?
Who's got that case of NOS units laying around?

Posted by: McMark Aug 1 2004, 05:03 PM

You should be still be able to get that motor running and drivable. Maybe not perfect, but better then it is. You have more problems than the MPS.

Posted by: anthony Aug 1 2004, 07:04 PM

That's not a huge amount of leakage. One can usually find used 1.7L MPS for pretty cheap.

Posted by: porschefool Aug 1 2004, 08:26 PM

It eventually leaks down to zero, it only takes about two minutes.
Hmmm, I'm gonna have to deal w/that, even if there's something else going on.
Fuel press is go, spark/timing is go, damn thing idles smoothly and revs freely with the MPS vac line off..what else could be wrong? Am I way too rich or lean?
Now that it will at least idle smoothly, what can I do w/MPS disconnected to figure out my mixture?

Posted by: McMark Aug 2 2004, 01:06 AM

Check your injectors. It'll only take you about an hour.

Posted by: Bleyseng Aug 2 2004, 07:57 AM

At idle the TPS tells the engine to be on the idle circuit. As you rev the motor the MPS is rich or at WOT 0hg.(the vacuum pulls the MPS to lean). Find a good MPS and then adjust everything as the motor should be idling 1500-2000 with the MPS hose off.

Geoff

Posted by: porschefool Aug 2 2004, 01:01 PM

My idle (with the MPS vac line off) is around 900.
She revs freely and smoothly, but of course, can't be driven (a minor detail, to be sure)
Just sounds so damn smooth...
I guess I'll check the injectors as suggested. At least that won't cost me another 50 bucks.