Just now, Yarb said:

Went back and looked at the Rock Auto listing and they listed 70- 80lbs max. Sure seems like a lot of pressure. What was the flow like returning to the can? Probably pretty strong I would think. Anyone know the specs on the Bosch pump?

Flow into the can was strong, guessing at least a gallon a minute.

I ran my 78 out of a 5 gallon can while tuning/ working on the engine waiting on my new tank. Had both the supply & return hoses in the same can. I remember looking at the amount returning to the can and don’t recall it having that sort of volume.

Correction, the product spec says 35.5 GPH, probably about right.

https://www.oreillyauto.com/detail/c/import-direct-fuel/import-direct-fuel-pump/ifd0/e16078?srsltid=AYCX9vJXt7n_LTyqblYtBGOzPqksw-8Xzk6cE2fukNB_OpfoDhfJFu_u

You could pull a vacuum on the FPR port and see what happens. If pressure drops it's not the pump overpowering the FPR. The FPR is just a valve with a spring behind it. The diaphragm works against the spring when there's vacuum.

The typical pressure gauge can be damaged by exceeding its max pressure. There's a tube inside that gets deformed. Does the gauge sit at zero when there's no pressure on it? It's probably a Bourdon type.

https://www.streatcontrol.com/a-closer-look-at-the-four-main-types-of-pressure-gauges/

https://www.bourdon-instruments.com/us/en/product-overview/pressure-gauges/c/36822

https://blog.wika.com/us/knowhow/8-common-reasons-for-pressure-gauge-failure/

Wouldn't it be pretty easy hooking everything up like stock to "close the loop" and see what it reads? Let it run, the injectors will flow and should lower the PSI reading you are getting. Just guessing though.

Thanks everyone for the feedback, I'll be putting it to use.

Here's the game plan.

1. Get a cheap fuel pressure gauge to verify readings of my Aeromotive gauge.

2. I also found this FPR on amazon. Made in China, so I won't hold my breath, but at this point I just want to try something that is actually advertised as 2.5 BAR (36.3 PSI). I believe it's for a Volkswagen, but I can definitely make it fit the Z if the pressure is right. (Link to Amazon 2.5 BAR FPR)

3. As siteunseen said, I'm going to complete the loop again and use the return line to the tank, now that I know there's no restriction there.

I've bought cheap gauges at Lowe's in the swimming pool section. They're used on the pool filter.

Have you had the fuel rail off? It looks pretty clean.

https://www.lowes.com/pd/STAR-Water-Systems-Gauge-100-LB/1000667311

Edited November 5Nov 5 by siteunseen

8 minutes ago, siteunseen said:

I've bought cheap gauges at Lowe's in the swimming pool section. They're used on the pool filter.

Have you had the fuel rail off? It looks pretty clean.

https://www.lowes.com/pd/STAR-Water-Systems-Gauge-100-LB/1000667311

Nice, good to know. I picked up a $13 guage from amazon.

Yes, fuel rail was removed a while back, cleaned the inside as best I could with acetone and compressed air. Didn't feel overly restrictive at all when I blew threw it, so it should be good. Cleaned up the outside too while I had it off the car👍

Alright, a couple updates...

I went ahead and rigged up a test to measure the "cracking" pressure of my 4 FPRs I have on hand, along with any variation in pressure readings between my Aermotive guage and my newly purchased cheap amazon gauge.

From left to right: OEM, OEM, ZCarDepot, HiSport PR4052 2.5 Bar (china made amazon unit, Link here)

And here's my setup. Air compressor regulator setup to deliver 50PSI at the gun. Fuel pressure gauge teed into the FPR inlet. FPR outlet open to atmosphere.

Results were:

FPR	Cracking Pressure (Aeromotive)	Cracking Pressure (cheap Amazon)
ZCarDepot	38	36
OEM-1	44	42
OEM-2	40	38
HiSport	38	36

A couple takeaways:

• The cheap fuel pressure guage is consistently 2PSI less than the aeromotive unit. And I tend to believe it because the HiSport FPR was advertised as being 2.5 BAR, and that's exactly where it opened.

• ZCarDepot FPR appears to be 2.5Bar as well.

• OEM units may drift higher with age

Anyways, so I'm now fairly confident I'm getting an accurate pressure reading. After installing this new cheap gauge, I'm getting 39 PSI (pump on, engine off), and 33 PSI at idle. Still don't understand how I can see the proper 36 PSI cracking pressure using air on the bench, but when installed on the car, I'm seeing 39 PSI. Bugs me. I do still wonder if my non-oem fuel pump is part of the problem.

A short side quest, I decided to hack up one of my OEM FPRs and scavenge the mounting base so that I can use it with the HiSport FPR. Worked well.

Hack saw to remove the mounting base...

Clean it up with the wheel of death...

Add a elbow and boom, new FPR that uses the original mounting base...

For the heck of it, here's the inside of the OEM FPR...

Based on what I've read from other forum posts, I was targeting an idle fuel pressure from 28-32 PSI. I'm at 33, so I'm going to have to call it good enough for now. Especially because I believe solving the next problem will actually solve the fuel pressure problem as well.

Ugh, Intake Manifold Pressure!!!😵‍💫😵‍💫

I cannot figure out why its not in range...

I can't seem to get anything above 15.5 at idle. Needle is steady.

Here's what I've checked:

• Timing is 10deg. BTDC

• Idle is about 750RPM

• New spark plugs

• Confirmed PCV valve rattles

• Went through the FSM procedure where you measure continuity across various pins at the big connector that goes to the ECU (drivers kick panel), everything OK.

• Removed water temp sensor for ECU, tested per the FSM, all OK.

• Checked and checked and checked for vacuum leaks. I don't hear any. Sprayed brake cleaner all around various ports and connections, nothing. Everything appears tight.

• Tested EGR system per FSM, appears to be functioning properly.

• Removed spark plug wires individually while running and confirmed drop in idle, indicating all cylinders are firing.

• Did a compression test while mildly warm (forgot to hold down gas pedal as well). Good results nonetheless:

  • 1- 162

  • 2- 162

  • 3- 162

  • 4- 165

  • 5- 165

  • 6- 162

• It starts, idles steady, and seems to rev fine (maybe a slight miss upon blipping the throttle)

I'm stumped, any suggestions?

PS: I did come across this video from @Dave WM in which he demonstrated a significant increase in manifold pressure by using the 500ohm potentiometer mod. I'd obviously prefer to find the root of the issue before resorting to this mod, but I'd be interested to hear other's thoughts.

Edited 10 hours ago10 hr by kcpope