Subarumor Posted April 10, 2014 Share Posted April 10, 2014 My 1993 California Loyale passed smog for every test except it was 3pts high on the low-speed hc test. I have an SPFI, 5speed, with 180k miles on the car. I noticed the fuel pressure regulator vacuum line has zero vacuum at idle and when I rev the engine! I blew some air into the passage and it only exits from a port ABOVE the throttle plate! According to theory, it's supposed to get manifold vacuum below the throttle plate so it can actuate at idle to reduce fuel. So why is it ported to above the throttle plate at atmospheric? What's funny is I don't have any idling or other problems associated with a faulty fuel regulator, yet it appears it's not doing anything. The shop manual shows the SPFI engine has the port above the throttle plate too. However, it's below the throttle plate for the MPFI engine! Anybody have an explanation for such a strange setup? Is the regulator in the SPFI just a 'fuelish' invention in this case? Quote Link to comment Share on other sites More sharing options...
skishop69 Posted April 10, 2014 Share Posted April 10, 2014 (edited) That is a normal set up. The spring inside the regulator works to keep it closed with no vacuum applied. This restricts fuel flow to minimum pressure. Since it is ported above the throttle plate, there is no vacuum until you open the throttle. This then creates vacuum above the throttle plate which acts against the spring increasing the fuel pressure. The more throttle, the more vacuum, the more pressure. As for the high HC, You have a few possibilities. Leaky fuel injector, bad regulator, weak coil, or cap/rotor, plugs or plug wire issues. Check the condition of all your ignition components and your timing as well as your air filter. Whatever it is, it's a small issue and may not even be noticeable. Edited April 10, 2014 by skishop69 Quote Link to comment Share on other sites More sharing options...
Gloyale Posted April 10, 2014 Share Posted April 10, 2014 Pressure is raised as Vacuum is applied. At idle there is no Vac to the regulator...so pressure is at it's lowest......And free Revving in Nuetral doesn't load up the engine enough to really produce alot of above throttle vacuum when observing it. What you would want to do is Put a vac pump onto the Regulator and hit it with a few pump see if the idle goes up. MPFI setup is opposite.....since many of them are turbos....the less Vac the Higher pressure fuel. Quote Link to comment Share on other sites More sharing options...
naru Posted April 10, 2014 Share Posted April 10, 2014 (edited) My 1993 California Loyale passed smog for every test except it was 3pts high on the low-speed hc test. I have an SPFI, 5speed, with 180k miles on the car. I noticed the fuel pressure regulator vacuum line has zero vacuum at idle and when I rev the engine! I blew some air into the passage and it only exits from a port ABOVE the throttle plate! According to theory, it's supposed to get manifold vacuum below the throttle plate so it can actuate at idle to reduce fuel. So why is it ported to above the throttle plate at atmospheric? What's funny is I don't have any idling or other problems associated with a faulty fuel regulator, yet it appears it's not doing anything. The shop manual shows the SPFI engine has the port above the throttle plate too. However, it's below the throttle plate for the MPFI engine! Anybody have an explanation for such a strange setup? Is the regulator in the SPFI just a 'fuelish' invention in this case? It is not foolish. On the contrary,it is the only way it will work. Regulator needs to maintain a constant pressure difference between the fuel supply and the injector tip for the ECM to meter fuel accurately. Since the SPI injector is above the throttle plate,regulator needs to measure pressure above the throttle plate. Throttle bore pressure is not the same as atmospheric pressure,hence the need for a regulator. MPI injectors are below the plate as is the regulator vacuum line. Edited April 10, 2014 by naru Quote Link to comment Share on other sites More sharing options...
naru Posted April 10, 2014 Share Posted April 10, 2014 Pressure is raised as Vacuum is applied. At idle there is no Vac to the regulator...so pressure is at it's lowest......And free Revving in Nuetral doesn't load up the engine enough to really produce alot of above throttle vacuum when observing it. What you would want to do is Put a vac pump onto the Regulator and hit it with a few pump see if the idle goes up. MPFI setup is opposite.....since many of them are turbos....the less Vac the Higher pressure fuel. This is just plain WRONG. More vacuum =lower fuel pressure Pressure differential between the injector tip and the fuel rail needs to be a constant 21 psi. Quote Link to comment Share on other sites More sharing options...
naru Posted April 10, 2014 Share Posted April 10, 2014 That is a normal set up. The spring inside the regulator works to keep it closed with no vacuum applied. This restricts fuel flow to minimum pressure. Since it is ported above the throttle plate, there is no vacuum until you open the throttle. This then creates vacuum above the throttle plate which acts against the spring increasing the fuel pressure. The more throttle, the more vacuum, the more pressure. As for the high HC, You have a few possibilities. Leaky fuel injector, bad regulator, weak coil, or cap/rotor, plugs or plug wire issues. Check the condition of all your ignition components and your timing as well as your air filter. Whatever it is, it's a small issue and may not even be noticeable. You have it backwards too. More vacuum = less fuel pressure. Quote Link to comment Share on other sites More sharing options...
Gloyale Posted April 10, 2014 Share Posted April 10, 2014 This is just plain WRONG. More vacuum =lower fuel pressure Pressure differential between the injector tip and the fuel rail needs to be a constant 21 psi. More vacuum at the regulator.........Less overall vacuum in the engine with throttle open. The pump needs to put out more pressure when applying more throttle i.e......more ABOVE THROTTLE vacuum. Maximum intake vacuum is closed throttle at idle......that is when pressure at the injector is lowest. Quote Link to comment Share on other sites More sharing options...
naru Posted April 10, 2014 Share Posted April 10, 2014 More vacuum at the regulator.........Less overall vacuum in the engine with throttle open. The pump needs to put out more pressure when applying more throttle i.e......more ABOVE THROTTLE vacuum. Maximum intake vacuum is closed throttle at idle......that is when pressure at the injector is lowest. Frankly.I am surprised you do not understand this. Manifold vacuum is irrelavant.The injector is not in the manifold. Quoting the FSM "The pressure regulator adjusts the fuel pressure to 147 Kpa (21 psi) compared to the throttle bore pressure of the throttle chamber." More vacuum = less pressure so to maintain a 21 psi difference fuel pressure must drop too. Don`t feel too bad,Being more familar w/MPI,I was confused the first time I measured fuel pressure on on a SPI/TBI car and saw how unresponive fuel pressure was to throttle opening compared to a MPI car myself. Quote Link to comment Share on other sites More sharing options...
Subarumor Posted April 11, 2014 Author Share Posted April 11, 2014 Well, it all sounded so clear when I read SkiShop's explanation . . . but then it got muddy with Naru's admonitions. I believe it's a semantical misunderstanding over Naru's imprecise use of the word 'vacuum' (ie: not explicity stating which vacuum he's referring to). Anyway, I'll assume it's working in reverse of the MPFI system so all is ok. All ignition components were recently replaced, timing is at 18degs, air filter is clean so those aren't an issue. Diagnostic test showed it was actually running lean, but getting some soot and fumes at the tailpipe! Confusing. All I can conclude is that it's pumping oil out the tail (it's using about a 1qt/800mi nowadays with no external leaks evident) which would explain the lean mix concomittent with the 'high' hc. I replaced both cats 2-4yrs ago but maybe they're fouled. Thanks for the feedback. I don't have any more ideas. Quote Link to comment Share on other sites More sharing options...
naru Posted April 11, 2014 Share Posted April 11, 2014 That would be the ported vacuum that the reg is hooked too(AKA throttle bore pressure) Manifold vacuum does not enter the picture. Precise enough? Quote Link to comment Share on other sites More sharing options...
skishop69 Posted April 11, 2014 Share Posted April 11, 2014 (edited) To clarify manifold vacuum better... Manifold vacuum is at it's highest with the throttle closed. The full volume of the cylinders is still trying to pull that amount through the teeny tiny space between the throttle plate and the throttle bore. It can't. Physics won't allow it. As you open the throttle, the vacuum decreases because the cylinders are now able to more easily ingest more air past the throttle plate. That is why turbo cars use vacuum supplied below the throttle plate. The turbo turns vacuum into pressure in the intake and the tubing prior to the throttle plate. The spring has a different rating so when vacuum turns to pressure, it helps the spring open the diaphragm. I was wrong about most MPFI systems. I put my mouth (fingers) in overdrive while my brain was in neutral. Naru was correct. Most MPFI systems use full vacuum behind the throttle plate acting against the spring to pull the diaphragm back and close off the fuel feed line opening the return line (low pressure). As the throttle is opened, vacuum decreases and the spring forces the diaphragm to close off the return and open the feed line to allow more fuel and pressure to the injectors. We were both correct about how each system operates though the MPFI set up is more widely used as Naru stated. You need to know which type you have and all my MPFI Subies have been turbo so I shouldn't be speaking on a non turbo set up. I apologize for confusing the situation. Edited April 11, 2014 by skishop69 Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.