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Rescue: 1988 Bronco Upper Engine and Fuel System Fixes

HOT ROD logo HOT ROD 5/30/2018 Hot Rod Network Staff

The Bronco's existing Air Flow Research (AFR) Renegade cylinder heads (PN 1456) are its largest small-block Ford castings. AFR's 220cc intake runners were probably way overkill for a mild 349ci street engine in a 6,000-pound Bronco, but the heads were nearly brand new and the existing valvesprings had 140 pounds on the seat, plenty for a hydraulic-roller cam. However, bolting on the heads revealed yet another major issue: The original installer used the wrong ARP head-bolt set, one designed for use on 289/302 production-style heads machined for "normal" -inch fasteners that screw into the block's threaded -inch holes.

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But the big Renegades have -inch holes for compatibility with the -inch-threaded cylinder-block head bolt holes found on many OE 351W blocks and aftermarket race blocks. ARP's special stepped small-block Ford head bolt was the easy problem-solver here. New bolts in hand, the heads were mated to the block using Mahle Original head gaskets.

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The Bronco's computer-optimized hydraulic roller grind survived the previous engine's debacle, but assembling the upper valvetrain revealed overlooked lateral misalignment between the roller rocker tips and the valve-stem centerlines, fixed by adjusting the AFR heads' interlocking pushrod guideplates.

The valvetrain geometry was off as well. Every engine is different. Many factors affect geometry, including head brand and design, head gasket compressed thickness, head or block milling, valve-stem lengths, valve-seat location and height, lifter cup heights, and cam-base circle diameter. Going to slightly shorter pushrods was the answer here.

The Bronco's aluminum timing cover was corroded, so Sanchez replaced it with a good used one attached with ARP bolts. Now it was down to the oil pan; in theory, all Sanchez had to do was clean and inspect the old oil pump and oil pan, then bolt everything together using a Mahle one-piece oil-pan gasket that, Sanchez says, "fits any Ford small-block motor, not just selected late-models." But nothing was simple on this rig. The previous engine had no dipstick or in-block dipstick tube, so Sanchez modded a Fox-body Mustang tube and dipstick to work.

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Sanchez threw in 6 quarts of Lucas 10W-30 motor oil, then pre-primed the motor using an electric drill and a gutted distributor. "Pre-prime before installing the intake manifold," Sanchez firmly recommends. "That way, you can be sure that oil gets to where it needs to be and not where it shouldn't be."

With the drill still running, the hydraulic-roller lifters' valve lash on the adjustable valvetrain was set to zero lash plus turn. But after the drill was turned off, several lifters quickly bled down, indicating possible internal lifter damage caused by the original missing oil-galley plug. Playing it safe, Sanchez replaced all 16 lifters with new Ford Performance lifters.

With the engine making good oil pressure and the new lifters installed and lashed, Sanchez installed the lower half of Edelbrock's two-piece EFI intake. A special intake gasket set is needed to match the big 220cc AFR head's intake ports (best fit: AFR PN 6812 or equivalent). Next in was the distributor (now equipped with a hydraulic-roller-cam-compatible Melonized gear), new ACCEL fuel injectors, and the upper intake half.

The Fuel System Fix

"It's lucky the original engine only lasted 15 minutes. If it had run any longer, the fuel hoses would have burned down the whole truck!" —Mark Sanchez / AEW

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After dropping the motor back in, Sanchez found serious fuel-supply system issues. The Bronco was using fabric-covered, rubber-cored, AN-style fuel hoses and fittings. With today's street fuels, the core can rot from within, eventually clogging the fuel injectors. Of more immediate concern, the too-short, poorly routed hoses hit the exhaust pipes. Checking this out, Sanchez saw the -inch feed hose and the -inch return hose had been reversed at the tank in a crude attempt to make a higher-capacity, longer, 5.0L-H.O. Mustang fuel pump fit in the Bronco's tank.

But on a Bronco, the in-tank unit serves as a low-pressure lift pump that feeds an inline high-pressure pump the builders totally forgot about! Sanchez installed a new, correct Bronco lift pump inside the tank plus a new high-pressure inline pump, redoing the fuel lines using stock-type plastic tubing and crimp-on fittings. At the engine end, the AN-style fuel logs were replaced with factory fuel rails.

The Results

Engine top-end issues were successfully resolved and the fuel system was fixed. But there were still electrical, instrument panel, and cooling problems. We'll tackle them next month and finally (we promise!) get the Bronco up and running on the chassis dyno.

Lessons So Far

Sanchez doesn't mince words: "Nothing on the Bronco was done right. Whoever worked on the car previously and built the motor has a reverse Midas touch: Everything they touch turns to crap." Use reputable techs with a proven track record!


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