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Old 05-16-2009, 07:56 PM
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MPFI 3.1L gm engine hard starting, fuel related

dont have a pressure tester so im stuck there.

got a 96 or so very common multi port 3.1L engine that is taking extra cranking to get started. put new plugs in it and no change at all. i know its fuel related

any ideas how i can figure out if its the pump or the regulator or ?

i doubt it would be the MAF, or MAP sensors, right ?

actually i think we better put a new filter on it first now that i think of it and at least eliminate that much.

thanks for anything

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Old 05-17-2009, 09:41 AM
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You can make a temporary pressure tester from an oil pressure gage.

An oil pressure gage that goes to 100 psi works about the best, IMHO- it has a small fitting size that will take a 5/16" fuel hose.

You put a piece of fuel hose over the fitting, hose clamp it securely, put the other end onto the fuel rail fitting (remove the valve from it first) and hose clamp that securely.

Then the pressure can be checked.
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Old 05-17-2009, 09:56 AM
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Is it hard to start both when hot and cold?

If it is both you could have a bad pump or pressure regulator. You can eliminate the regulator by blocking off the return line temporarily and see if the issue goes away.

A defective temp sensor can also cause hard starting under certain temperature conditions.

The other common issue is a leaky injector that will actually flood the engine when its not running and will cause extended cranking.
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Old 05-18-2009, 04:53 AM
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Another common problem is the relay it supplies power for a few seconds to prime the system.After starting ther is an aoil pressure switch on a gm that takes over this is done so that if the engine dies in an accident etc. fuel flow is stopped.Sounds like while cranking once you get oil pressure it starts?
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Old 05-18-2009, 04:16 PM
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relay no

nothing to do with relay and electrical

the pump always primes and runs etc.

i found the problem though a bit ago

i remembered that we had a 90 something celebrity with the same mpfi style injection engine but 2.8L instead(really same as 3.1L anyways) and it had this same problem where it was difficult to start and took much cranking times to start almost always.

the culprit:

ruptured fuel pressure regulator

raw fuel getting sucked up the vacuum hose to it

so yeah.....................

unplugged it and capped it, and sure enough it fires right up now.

the chevy celebrity all over again de ja voo hahaha

Last edited by fast68; 05-18-2009 at 07:39 PM.
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