Fuel pressure issues

[siteunseen]

I put my pressure gauge in today and got some questional readings. Gauge attached to hose from filter reads 48 p.s.i, hooked to the fuel rail drops to 25. Unplug vacuum from FPR goes to 34. Plugged vacuum line from intake and ran it without FPR getting any vacuum. Runs waaay better. If I let it sit at idle for a few minutes it loads up. A couple of rev ups and it clears up. I'm hard headed and stuck on the idea that the FPR is bad. $60 dollar part, but if it ain't broke..... Any advice?

Edited September 4, 2011 by siteunseen

[siteunseen]

Now that I can attach photos I just wanted to say my fuel gauge is exactly as FastWoman suggested, I had to get "T" from Napa as there is no TrueValue around here and used 2 5/16" hose barbs. Next I'm going to put in a vacuum gauge off the intake somehow. Love those gauges!

[FastWoman]

A gauge on the vacuum! Hmmmm... Sounds like a good idea! :-)

Your fuel pressure does seem a bit low. It's not EXTREMELY low, but your mix would be richened a bit if you were running at the spec pressure.

The fact that your engine runs way better with the vacuum line disconnected from the FPR means that your engine is ordinarily running too lean. It might be running too rich with the vacuum unplugged, hence the fouling of your plugs (?).

I suspect the calibration of our analog ECUs might have deteriorated on many/most of our cars, so your engine might not be exactly right even with spec fuel pressure, but it will be CLOSE. You might want to replace your stock FPR with an adjustable FPR -- a good one, such as made by Aeromotive, not a Chinese knockoff (been there, done that, don't recommend it). Then you can adjust your fuel pressure to tune your engine to the correct mixture.

This is a solution I considered on my own Z, which was also running lean. Unfortunately I estimated that I'd have to run the fuel at around 55 psi to compensate for MY ECU's drift. My fuel pump was incapable of such pressures. Instead of running non-stock fuel pressure, I opted to alter my coolant temp sensor circuit resistance, which corrected the mixture problem. But if I could have solved it with fuel pressure, that's what I would have done!

[siteunseen]

I've done some looking around and found a MSD#2222 adjustable FPR from Summit Racing for $95 or $79 off E-bay. Or I could get the original equipment Bosch #W0133-1607752 for $99. I could never figure out how to "alter my coolant temp sensor circuit resistance" as you did. The MSD is a universial FPR with a range of 36psi to 45psi. I kinda' like it. Opinions?

Edited September 5, 2011 by siteunseen

[Zed Head]

34 and 25 aren't that bad, as FastWoman suggests. The 25 psi with the vacuum hoses connected is an indication that you actually have fairly good intake vacuum, since intake vacuum is what moves the FPR diaphragm to drop the fuel pressure.

The 48 psi before the fuel filter (I think that's what you meant, not from the filter) seems kind of high. You might have a partially clogged filter. But I don't know what a pre-filter pressure should be, so can't really say.

Your car should run fairly well though with the vacuum hose connected to the FPR, at those 34 and 25 psi numbers. It might be somewhat lean, but not dramatically. What do you mean by "waaay better" and did you let the engine get to full operating temperature? It will run way better with a rich mixture when it's cold, but will run worse when it's hot.

You might do a little more investigating before you spend the cash on an adjustable FPR.

[FastWoman]

Zed, I'm thinking siteunseen meant the 48 psi is static pressure -- a gauge at the end of the fuel line, with no outflow. (Is that what you mean, siteunseen?) Static pressure is supposed to be 45 psi or better, according to the FSM.

If you're talking about 48 psi before the filter, with everything connected up and flowing, then yes, that's rather high.

[Zed Head]

I pondered that but wasn't sure. All I see in the pictures is a Tee fitting. So I conjectured.

[siteunseen]

I clamped off fuel line right after the gauge before the metal fuel line at the front of the valve cover. It went to 48psi. Then when clamp was released and the fuel entered the fuel injection system it dropped to 25psi. Then I plugged the vacuum coming off the intake to the FPR and also plugged the bottom of the FPR it stays at a constant 32psi. I adjusted the valves cold & hot, set the timing at about 11 or 12 degrees BTDC and the car seems to run better than ever. Now that there is no vacuum associated with the FPR would that be considered a "non-rising regulator"? And will that be a negative move on my part? Maybe cause some kind of damage? I've been checking the plugs and they are a nice off-white color. Side note: this will all be re-adjusted when I do internal engine rebuild this winter. Thanks for your help.

[FastWoman]

OK, your fuel pressure is pretty close to correct on all measures.

You need the vacuum input to your FPR. This maintains a constant pressure differential between the fuel rail and the intake manifold (the path through which the fuel is sprayed). If this differential doesn't remain constant, your EFI won't meter the fuel accurately. You'll be relatively lean on heavy throttle and relatively rich on cruising.

[siteunseen]

Ok I just drove around the neighborhood for awhile. It has rained all weekend so I didn't put it "under load on the open road" just quick throttle advances in my garage. It ran really well at 1st, cold, but then went to crap when heated up. So I hooked the vacuum back to the FPR and it went back to low 20's psi. Barely made it up a hill near my house but after the hill it ran really strong. Maybe I should take off the AFM and clean it and the connector maybe even figure out how to adjust it? Or maybe the FPR really could be bad. I'm stumped but so happy this is my biggest worry. Thanks again.

[Z train]

Get a fuel pressure guage and run it back out of the hood and tape it to a windshield wiper and go for a ride with a friend.You need to know what FP is under different conditions as it will help to NOT chase your tail on this issue.

[siteunseen]

That's a very simple solution for reading Fp. And your exactly right, I am chasing my tail because I don't know my tail from my elbow with all these variances on a 34 year old EFI system.