77 280Z Dropping Fuel Pressure once Warm

[siteunseen]

Put your gauge before the filter and see what you have.

[FastWoman]

^^^ good idea!

Heap, you asked about the possibility of a stuck-open injector. I think if your pump is healthy, it should be able to supply all the injectors in their full open position at 36 psi. However, if in doubt, look at your plugs. A bad injector will be associated with a plug with very different looking deposits.

[Z-Heap77]

All:

We finally got around to receiving and replacing the FPR today, but we still have the same results. The car starts out fine, but once it warms up the pressure drops off and applying any throttle causes the idle to break up. So we've eliminated the FPR as the culprit, what should I be looking at now? Is there still potential for the pump to be the issue? It just seems strange for the pump to be working just fine before we removed a single part from the car and then immediately upon completing the intake swap, the pump is no good. I feel like there has to be something else I should be looking at, but at this point I'm lost.

Any suggestions?

[rossiz]

i just had fuel pressure issues due to clogged fuel filters. i had a clear fram G3 between the tank and pump, which was filled w/brown crud in ~2 months of use, and the main filter (which was installed by the PO prior to the fram G3) was filled w/junk too.

your pump may be able to supply the pressure, but if the flow is restricted you'll have problems.

the filters are cheap and easy to replace.

[FastWoman]

Have you had a chance to try what I suggested in post #5? That would tell you whether you have an issue with fuel supply, whether it's the pump, bad electric to the pump, a clogged pickup screen, a clogged filter, a clogged line, or whatever. If there's a problem anywhere in the system, you'll see it in the pressure and return flow rate. If you can re-create the problem in your driveway this way, then you can narrow down the cause, component by component. If you can't re-create the problem this way, then you're probably looking at the wrong system. (For instance, maybe the EFI is at fault.)

Also, have you read your plugs yet?

Edited June 6, 2014 by FastWoman

[Zed Head]

Sorry for the bad advice. If you had a pile of spare parts like I do you could have double-checked the FPR.

Thinking back, people have had cases where something loose in the tank covers up the screen on the outlet port of the tank. Either rust particles or gas tank sealer or just junk that ended up in the tank. Then when the pump turns off it floats away, and the cycle starts over.

I would let the pump run on battery power in the garage, without the engine running, and just listen. You'll hear a change in pitch from the pump if there's something happening back there. Then crawl underneath and run a line to an external tank. Do the same thing. If the pump runs fine for extended time on the external tank, but not the car's tank, you'll know where the problem is.

Forgot to say - make sure your gauge is good. They go bad. #5 and #6 here - http://www.classiczcars.com/forums/fuel-injection-s30/52413-78-no-start-w-out-fuel-priming.html#post457924

Edited June 6, 2014 by Zed Head

[siteunseen]

Or you could put a gauge on the hose going into the bottom of the fuel filter, straight from the pump. My Bosch hits 50 psi and doesn't fade when I put a gauge on the fuel supply hose straight from the pump, before the fuel filter. Shop Aqua EZ Pressure Gauge at Lowes.com

[Z-Heap77]

I happen to have another new fuel filter so my dad is going to through that on today and see if there's any change in how it's running.

I haven't had a chance to do the test in Post #5 as it was a holiday weekend back then and wasn't able to purchase the additional items needed at that time. I'm going to gather everything up this week and plan to do that next weekend. The car resides at my dads house in Md and I'm in Va so I don't get any time during the week to mess with it.

I'll report back my findings after I run the test on the pump and I'll pick up an extra gauge that I can try to run inline before the filter as well.

Thanks All.

[siteunseen]

That gauge at Lowes is one Fastwoman found a while back for another fellow having the same problem. I didn't know they had that one, the one I found was for an air compressor, so when I was at my Lowes I saw it in the pool supply section. It has a 1/4" nipple that will thread right down into your existing 5/16" fuel line, tighten the clamp, they call it "dead headed" when it stops the circulation of fuel. Remove the coil wire then turn it over a few times and the gauge should hold the reading for a good while if everything's OK.

[FastWoman]

Don't dead-head the fuel flow for too long, though, because the pump uses the fuel for cooling.

[Z-Heap77]

So.... The nightmare continues. Before I could pick up the hose and additional gauge my dad had ordered a new fuel pump along with a few other things that we needed. So I put off purchasing the hose and gauge and we just went ahead and installed the new pump. Unfortunately the results were identical. Mediately upon starting the car the gauge on the rail was only showing 22 psi. I left the car running for quite a while to see how the car would run and what the gauge might show. It eventually got down to about 18 psi while at idle. Removing the vacuum from the FPR produced at most 30 psi.

One thing that was a little odd was that the car didn't seem to break up like it was previously. I didn't drive it but I was able to make it break up just applying throttle while I'm neutral.

So I'm going to pick up that gauge from Loews that you listed in previous post and I'll apply it before the filter. Should I do that in the dead head layout or just put it online with the T fitting that I can screw the gauge into?

Also what should I be looking for? If I put the gauge before the filter it should read close to 50 psi? What if I'm not getting that? What should I check then?

[Zed Head]

I would replace the rail gauge with the new gauge first. I think that your gauge is bad. A quick swap will at least confirm that you have a problem.

The gauge before the filter would show if you had a clogged filter (big pressure drop), but the FPR should still take pressure to 36-37 psi with no vacuum hose connected. A clogged filter would just make things take longer but pressure would still get there. The fact that the vacuum hose causes an 8 psi change is normal, the starting point is just off. 36-37 (no vacuum hose) and 28-29 (vacuum hose) psi will be what a good gauge will probably show.

Your fuel supply is probably fine and your problem cause is somewhere else. Again, sorry for the bad advice, I just assumed that the pressure numbers reported were good. Should have asked, since I have three gauges of my own that I swap around to the various devices that use them, just to be sure they're all working correctly, because a bad gauge screwed me up for a few months on a bad FPR.