PM sent. Thanks Kevin

 

Got them.

 

Did you get the chance to put a smoke machine on it?

 

couldn’t find an actual smoke machine but tried a cigar and didn’t see any leaks

 

A smoke machine introduces smoke and pressurizes the intake to push it out any cracks or openings. Not sure how you'd create the pressure needed just using a cigar.

 

>>> Homemade Smoke Machine <<<

>>> Here it is in use. <<<

 

Ok. I got the fuel pressure gauge from Kevin. Thanks Kevin. The pressure reads 57 psi while idling and jumps to 60-64 psi when hitting the throttle. Does this tell you anything. What should the pressure read?

I didn’t jumper the fuel pump or have a vacuum at the regulator. Should I have done this?


Thanks in advance

Ashley

 

Ash,

The fuel pressure goes up as the throttle plate is opened.

The system tries to keep a constant differential pressure between the fuel rail and the inlet tract where the reference point is for accurate flow metering purposes.

At idle the engine pulls a deep vacuum of about 20 inches Hg- that is equivalent to a vacuum of 10 psig or an absolute pressure of 5 psia. When the throttle is fully opened under full load the pressure in the inlet tracts is closer to atmospheric pressure so the pressure will rise by close to 10 psi. You get 7 psi difference revving the engine under no load so that sounds credible to me. However if you did not have the vacuum line connected then not sure what is happening.

 

The vacuum line was connected. I didn’t have a vacuum pump hooked up. I thought I read that somewhere.

 

Fred,

you listed 10 psi as normal, I’m getting 5 times that. You think my readings are normal?

 

Ash,

I suspect you may be getting a tad muddled between readings for the absolute pressure in the inlet tract and the pressure in the fuel rail. This is quite a common issue. For purpsoes of discussion atmospheric pressure is zero at sea level if we are talking gauge pressure. In reality the absolute atmospheric pressure is 15 psia at sea level and when the pressure drops below such number the system is now into the "vacuum regime".

The fuel injection system is controlled by opening the injectors for a pre-defined amount of time so as to be able to pass a pre-determined amount of fuel but that quantum is based on the premise that there is a fixed differential pressure between the fuel rail and the inlet tract. To keep the logic sound the fuel pressure regulator has to be modulated such that as the pressure in the inlet tract varies so does the pressure in the fuel rail thus keeping a constant differential pressure no matter the air flow. At a full vacuum the pressure would be zero psia or a column of 30 inches of Hg [mercury] as engineers prefer to use to define vacuum. When the 928 engine is at idle with the throttle plate closed the engine will pull a vacuum of about 20 inches Hg [that is approx 10 psi of vacuum or an absolute pressure of 5 psia]. When the throttle is fully opened air is literally flying into the inlet tract such that there is a large pressure drop through the runners and the pressure at the point of fuel injection is now close to atmospheric pressure or 15 psia in absolute terms. The fuel injection system is thus pressurised to keep a constant differential pressure between the rail and the plenum and that differential pressure is approx 3.5 bar or 50 psi and when the motor is running flat out a pressure gauge mounted on the fuel rail will show 50 psig. Allow the engine to idle, the pressure in the inlet tract will drop to about 5 psia and the pressure on the gauge will then drop to about 40 psig to compensate. Undo the vacuum line on the FPR and the pressure will remain constant at 50 psig no matter the engine speed.

Quite a simple concept once one gets one head around it.

 

Hey Guys,

I'm stumped! Before I take the intake off to see if there is an air leak, is there anything additional I should check? I've done all of the recommendations throughout the thread but she is still running very rough.

 

You did not put a smoke machine on it.

If you have an intake leak - it will be exposed.

 

I did purchase a smoke machine and used it today. I didn't see any smoke escaping the system. I introduced the smoke at the hose that connects just in front of where the trottle cables attach. Is that the correct spot?

 

That is a good spot. I disconnect the hose at the 'Y' fitting that goes to the brake booster, and hook in the smoke machine into the 'Y', and run the air/smoke pressure at 1 psi.

How did you block off the MAF?

Dang it, I thought for sure you'd have found your problem.