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This Old Horse: Fuel Pressure Gauge

EFI Mustang FPG upgrade

Basic law of hot-rodding: the more you choose to modify, the more you have to modify. Build a 400-horse EFI 302 and you're going to need to update the five-speed, the radiator, the clutch, the diff, (perhaps the axles), motor mounts, fuel, ignition, computer mapping, hoses, etc. What else? Surely we've forgotten something. Okay, you'll need a fuel pressure gauge.

Fulmination Fix

This much-flogged Mustang sports a wicked new EFI 302 that had begun coughing back into the manifold. Fuel demands change with a new engine and, in most cases, you're starting from an educated guess. Break the engine in a little fat, start fiddling with pressure when there's 1000 or so miles on it, read plugs and listen for bad sounds. Tricks aside, though, it's good to have an idea where the pressure's at—EFI Fords have a particular range in which they're happy (38-46 psi @ WOT)—and within that there are a lot of variables to be concerned with. You need to know. With a backfire in the manifold, we had to be wrong on fuel.

Ford's sold a billion small-blocks, and the EFI variant is common. Fortunately, most of them were manufactured with a smart nipple on the OEM fuel rail that makes installing a standoff fuel pressure gauge simple. The factory rails are good for 400 hp on a small-block (the pump is the weak link, then the regulator, the lines under the car, and finally the factory rails) so you don't have to yank them if you're keeping things reasonable. Unfortunately, Ford chose to manufacture the fuel-T nipple with a strange 100-year old Saginaw machine thread that would have died an ignoble death had it not been on all those Fords.

A fuel pressure gauge is in direct contact with fuel. There's gas coming off the rail through the braided steel line attached to the back of the gauge, and you can't be routing raw 93-octane into the passenger compartment. Wouldn't be safe, or legal. Therefore, the gauge lives in the engine compartment, which isn't much of a problem as that's where you are while you tune. You've seen the FPG mounted on the cowling of some cars, above the hood and in front of the windshield, and you can buy isolator kits, which use a hydraulic link from the engine compartment to the cockpit that mirrors fuel pressure with a diaphragm, but both of these add complexity. Monitoring fuel pressure from the engine bay is fine. Why complicate?

Parts Improvisation

It's likely we're going to have some or all of what's necessary for little bolt-on jobs already in the garage. True to form, there was a lighted AutoMeter mechanical fuel pressure gauge in a box of 5.0 parts, and a length of braided stainless steel hose just perfect for fuel-pressure duties crawled out of some WRX parts (complete with standard 1/8th female fittings). We would fabricate a bracket out of some scrap steel, fasten things with pop rivets or bolts, and a trip to the parts store down the hill produced what we thought would be the correct unions. Correct, except for the weird little Saginaw-threaded OEM fuel rail "T" that none of the parts guys had EVER seen.

You have to dig deep to find parts like this, but we hadn't expected it to be so tough to locate stuff for such a ubiquitous engine. Every place we called came up dry, though we learned that Ford Motorsport used to make these adapters. So our collective head rang like the bell tolls One, and we called GRC Performance.

An old buddy from our top-speed and road-race days, Umberto Gizzi at GRC Performance (grcperformance.com/949.457.1875), has done everything you can to a Mustang, building early- and late-model in his Mission Viejo shop. We got him on the phone and he promptly floored us with four words: "I'm making 'em myself."

The Blind Man (don't call him Bert) had apparently been in need of the adapter for customers, so he reached out to some engineer buddies and had the thing built from scratch. Tidy little brass 90-degree fittings, Saginaw-to-1/8th, male-to-female, designed to fit into the cramped space in front of the starboard head where the pressure nipple lives. Umberto had a dozen in stock, dropped one in the mail and one day later we were in business.

Scheme-matics

This is not complex. You'll want the engine cool and for fuel pressure to bleed off. Use a rag you don't like to catch any leftover fuel dripping from the rail. Employ flare wrenches whenever possible. Plan the route for the braided stainless line, which will be full of fuel, to avoid hot parts like headers or the block. We bent a small piece of mild steel strap into an "L" and fastened it to the inner fenderwall where the OEM induction box used to live. We used two small bolts (one hole already in the fenderwall, the other purpose-drilled), and bombed the bracket Krylon Black before installation. You can hook up the gauge light to the hood lamp at any time. Original plans to use a gas-proof thread compound didn't hold up to the peculiarities of the fittings on the gauge, so we switched to gas-safe Teflon tape. We sheathed the stainless-steel braided fuel line in plastic wire loom to keep it from damaging things it rubs against, and zip-tied it into place.

Once everything looked attached and sensible, you want to cycle the ignition, turning to Run and back off so the fuel pump can pressurize the system. Check for leaks. A blast around the block and we reexamined things in the driveway, noticing an excellent new leak in the radiator, somewhere near the petcock but not exactly. Fortunately, it's a hi-po Fluidyne aluminum unit that shouldn't be tough to patch once the fissure is sourced. Stay tuned for that. While under the car for a look at the radiator, we caught a glorious new puddle of expensive adjustable oil bleeding a sympathy stain onto the pavement from the port-aft shock. Even better.

With a little fiddling it became apparent we'd been turning the regulator backwards—the motor was running at about 35 psi, which was down from 38 or so set at break-in. We stretched things to 40 psi at no vacuum (turning the Holley regulator counter-clockwise) and once we patch the Fluidyne, we'll let you know what leaks next. Oh, and This Old Horse has ceased coughing.