Adjustable Fuel Pressure Regulator

[siteunseen]

I've bought a cheap universal off ebay but it, like others, leaks down after awhile. I've read Zed Head say that the Aeromotive Tech guys said theirs does it too and Wade Nelson put a check valve in the line before the fuel rail on his. I've found a retired Nissan mechanic that's getting mine fine tuned now that he's put that adjustable FPR on. So if my new Bosch pump has a check valve in the pump wouldn't I want another in the return line? I have brand new injectors so I hope they're not the cause. Or has someone found one that doesn't leak down? I need more pressure because of the larger fuel rail and some other things I did. And probably the drift? in the ECU Fast Woman talks about. Thanks for any helpful replies.

Cliff

Edited November 28, 2012 by siteunseen

[FastWoman]

Hi Cliff,

I'll be interested to see how your project pans out. Obviously you're using a different approach to solving the ECU drift. I played around with that idea on my own Z, but I found that completely blocking the return line only gave me about 45 psi, which didn't get me quite to the right mixture. I was guesstimating I'd need maybe 50-55 psi. Higher flow injectors were another option, but I had already replaced mine.

I don't think you would need higher fuel pressure because of a larger fuel rail, but I don't know what other things you did. Just keep in mind that higher pressure will richen your mixture. Having an adjustable pressure will magically give you the ability to tune your mixture. Just make certain your new FPR will regulate down to the stock 36.3 (?) psi.

I suspect I bought (and promptly returned) the same Chinese Aeromotive look-alike FPR off of ebay. I found it did a very bad job of regulating pressure, I suspect because it had a small fuel orifice and diaphragm. The Nissan orifice is very large, I suspect, so quite a lot of fuel bleeds by without much spring compression. I've never seen an Aeromotive, but I suspect they're not that much different. I think the fundamental design is different from Nissan's.

You shouldn't need a second check valve in the return line, and in fact that might just be an impediment to free fuel flow. Sadly, I don't think there's any reasonable way to bottle up pressure in your fuel rail if the FPR is going to let it go. If the pressure sags, then fuel will boil out of the injectors immediately after engine shut-off, and then the fuel vapor will have to be purged under pressure for a hot start. You can do this by installing a primer switch in the cockpit. It should be a momentary switch that will deliver +12 to the fuel pump relay. With that switch, you can re-pressurize your fuel rail and purge most of the air. Unfortunately that last bit of fuel vapor inside the injectors is a bear to work past, so you will probably have hard hot-restarts. I wish there were better answers for you.

PS Another approach might be to build an adjustable FPR with parts from a Nissan FPR. If you know a good machine shop, maybe they could cut the working parts out of the can and build them into a new shell with an adjustable spring tension. Hmmmmm....

Edited November 30, 2012 by FastWoman

[siteunseen]

Thank you so very much for your time in explaining things. Would you please give this a look and see what you think http://www.summitracing.com/parts/msd-2222 I don't know boost from a hole in the ground but if my intake vacuum rises with the RPMs I think this would work.

Edited November 30, 2012 by siteunseen

[siteunseen]

Well I've learned the difference between boost and a hole in the ground.:disappoin And manifold vacuum decreases with RPM. Is there anywhere that pulls more vacuum with RPM? I've got to get my car back! It's driving me nuts.

[Zed Head]

The purpose of the FPR is to maintain a constant pressure differential between the fuel in the injector and the environment at the tip. So that the ECU can control fuel supply accurately with injector open time. So if pressure goes down in the manifold (vacuum reading goes up) then fuel pressure in the rail will go down also. If pressure goes up n the manifold (boost) then pressure goes up in the fuel rail.

The FPR in your link in Post #3 should work fine as long as you want to go richer than stock in your tuning, which it looks like you do. Actually, it looks about like what FastWoman was describing, a stock regulator that's been modified to allow spring pressure adjustment. 45 psi is actually a pretty big increase over stock and should supply about 12% more fuel than 36 psi. The unanswered question about the Summit FPR is whether or not it will leak down like the Aeromotive and your EBay FPR.

[EuroDat]

Would it not be easier the change the spring tension in the AFM or the resistance in the temp sensor circuit to adjust air/fuel mixture? If leak down is a problem, replace the FPR with an OEM.

Just a thought

[FastWoman]

Zed has it right. Boost is simply positive pressure, and vacuum is negative pressure. I think the basic design is the same. Boost works to enhance spring pressure, and vacuum counteracts it. Spring pressure + boost (or - vacuum) = fuel pressure. I'm uncertain whether the FPR you reference is adjustable, but it looks like it probably has a large diaphragm. I also have no idea about leak-down, but probably (?) being the same basic design, I would guess (?) it holds pressure. You could always ask the Summit folks.

All of that said, I have to echo what EuroDat has said. My conclusion was that changing the coolant temp sensor circuit's resistance was the least kludgy way to richen the mixture without altering/changing out expensive components. For only a few bucks, I could tune my mixture with a tiny screwdriver. I woudn't have to go with a leaky non-OEM FPR or change out for a noisy higher-pressure fuel pump.

I did decide the AFM spring tension fix was probably not a good idea, because it would change the relative response properties throughout the air-flow range, causing the AFM to peg out at a lower draw, with no further enrichment beyond that. From what is described right in the FSM, the CTS circuit alters the pulse width the same way throughout all operating conditions (i.e. even enriching the mix with WOT and wide-open-AFM). What I may lose with this alteration is some accuracy in mixture during warm-up. However, my engine seems to run just fine when it's cold, even in the dead of winter (OK, a Virginia winter, but just the same...).

Anyway, the alteration is very simple -- just a 20 turn variable resistor in series with the CTS wiring. I believe I used a 10 kOhm variable resistor.

[siteunseen]

Does an FPR have to have vacuum? Would a constant pressure of 36psi be ok at idle as well as WOT? I found a guy without the vacuum on a adjustable FPR and he says it "works like a charm". He has my same set up it looks like. I apologize for being hard headed sounding but I have no knowledge of resistors and am a little hesitant to go outside of my abilities.

If I do use the resistor is that something I could get at Radio Shack or auto parts stores?

Radio Shack has 1 that looks like a volume knob, with 3 post, I would need 2 post right? In and out?

They're a couple of bucks vs. $70 for the FPR. Probably should try that 1st.

Edited December 9, 2012 by siteunseen

[Zed Head]

I don't think that you ever really said what you're trying to do, but it looks like you're trying to make things richer. I agree with FastWoman, that adjusting the spring in the AFM will affect low RPM and high RPM (low and high air flow) differently so is not an ideal way to do things. Adding a resistor will make things richer at all air flow levels.

I used a 5k-ohm audio taper potentiometer (volume knob) from Radio Shack, part #271-1720 to adjust the resistance. I installed it originally just to be able to add lots of fuel to try and fix the hot start problem, but then found it was very useful when I ended up with a lean AFM, that had a lean flat spot at low RPM. Basically right after taking off from a dead stop, the engine ran through a flat spot then jumped ahead when it hit the good spot. The pot allowed me to tune out the flat spot, but it also turned out the whole curve was lean so it ended up as a really nice tuning tool, for my specific lean-running AFM. For the record, previously I had had a rich-running AFM on the engine but had no way to adjust besides an adjustable FPR. Which leaked down as described.

Anyway, get the audio-taper model, the linear model is much harder to control. And 5k is really too much range, I only use the first 1/4 turn of the knob, so a smaller range would probably work even better.

[siteunseen]

That's what mine is doing too. Flat spot then it screams ahead. A fellow that's helping me thought higher fuel pressure would clear it up but it hasn't. Thanks for the part number Zed Head.

[Zed Head]

No problem. Get some bullet connectors and about 8 ft of wire also and you can just unplug the wires to the coolant temp sensor (bullet type) then plug in the potentiometer wires, with no wire cutting, and mount the pot inside the cabin. Tune while driving. If it doesn't work, just unplug it and go back to where your were, no damage. Good luck.

[FastWoman]

Try a 20-turn potentiometer instead! It's designed for very fine adjustment with a screwdriver. You can order the things off of ebay. Here's a 5 kOhm one:

http://www.ebay.com/itm/20-Turn-Potentiometer-450-20-5K-ohm-Precision-Pots-From-VRN-/270918474052?pt=LH_DefaultDomain_0&hash=item3f13ffd944

Oh, and the FPR does need vacuum. Otherwise the EFI will deliver a richer mixture on idle or deceleration and will deliver a leaner mixture on wide-open-throttle.

Edited December 10, 2012 by FastWoman