The Combo

See all 37 photos Tom Eyeington (right) built an LS-powered 1981 Trans Am as a 16th birthday gift for his son, Alex (left), but it stalled and vibrated terribly.

In 2010, Thomas Eyeington bought a derelict 1981 Pontiac Trans Am (TA) with a tired Poncho 301 and TH350. His plan: Build a modern high-tech hot rod for his son Alexander's 16th birthday. Tom swapped in a 0.030-over, 6.0L LS truck engine with a custom hydraulic-roller cam (222/230 degrees at 0.050, 0.580/0.590-inch valve-lift, 112-degree lobe-separation angle) that actuates the valves in GM L92 rectangular-port aluminum heads. Fuel and spark management is via MSD's Atomic EFI port fuel injection, MSD ignition coils, a 90mm throttle-body, and 42-pound injectors. Billed as "plug-and-play," Atomic EFI can theoretically construct and refine its own fuel and spark curves using self-learning algorithms. A handheld tuning device allows monitoring and some user-initiated changes.

MSD's separate handheld controller manages a rebuilt 4L80E electronic overdrive automatic trans with a 3,000-rpm-stall lockup converter. Both engine and trans controllers communicate with each other to coordinate gear shifts, TCC (torque converter clutch) lockup, and the fuel and spark curves.

A custom driveshaft hooks the trans to the original GM corporate 8.5-inch 10-bolt Posi rearend fitted with 4.10:1 gears. RideTech's bolt-in four-link rear suspension with coilovers replaced the stock buggy springs.

See all 37 photos It may look like just another 1981 TA, but under the stock shaker hood lurks a built 6.0L LS truck motor managed by MSD's Atomic EFI and shifted by a 4L80E automatic overdrive.

The Problems

After a host of additional chassis, interior, and exterior refinements, Tom says he "duly presented the car to Alex on May 8, 2014—his 16th birthday—running, but with numerous nagging issues." Despite the subsequent attention of several tuner shops, two driveshaft shops, and a rearend rebuilder, two major issues were never resolved:

Stalling after warm-up when placed in gear or when coming to a quick stop after hard acceleration.

Severe driveline vibration at any speed.

The Dead Hole Fix

We sent the Keller, Texasbased TA over to Machinehead Powersports west of Austin. Owner Kevin Crocie does hard-core tuning, hard-core engine builds, and custom suspension setup.

See all 37 photos Machinehead Powersport's Kevin Crocie builds trick engines, develops custom chassis packages, and tunes carbureted and EFI systems.

"The car was trailered in," Crocie says. "I started it up and could immediately hear a constant misfire." He used a temp gun to check for a "cool header pipe," and determined cylinder No. 1 wasn't firing. Using an inline "blinky-light" tester between No. 1's ignition coil and spark plug, Crocie found a bad coil, replacing it with a new MSD unit.

See all 37 photos The TA's -ton truck LQ4 6.0L Gen III small-block had a constant miss. Motor running, Crocie used a temp gun to check each header pipe (A); a low reading on cylinder No. 1 indicated a "dead hole." Crocie then attached a Cornwell inline spark tester between that dead cylinder's coil and spark plug. "It didn't flash (B), meaning the plug wasn't firing," Crocie says. "I swapped the coil to a different cylinder, and the dead hole followed the bad coil. If it didn't, I would have checked the spark-plug wire and coil-trigger wires. If they had passed, I would have checked the injector." Crocie installed a new MSD coil (C); later, another was needed when the No. 4 coil died.

The Stall Fix

See all 37 photos The TA stalled when placed in gear with maxed-out IAC counts—eventually traced to a possibly glitchy 4L80E pressure switch manifold.

Decks cleared, Crocie started working on the TA's big stall. He noticed weird idle air control (IAC) motor readouts on the MSD controller. An IAC is to EFI what a choke is to an old carburetor: At cold-start, both help keep the car idling. IAC "counts" (a numeric value) are high when cold, but like a choke pulling off, they should decrease as the vehicle warms up. In this case, Crocie says, "After the TA was fully warmed up in Park/Neutral, I was seeing a 100 to 160 IAC count readout, when at hot-idle it should be at 5 to 20 in Park on the MSD. If I put the car into gear, IAC counts jumped to 200—the max permitted on the MSD system—and the car would stall, even though the computer was way over-advancing the spark trying to keep it running."

Other than a defective IAC valve, high counts can be caused by:

Carbon buildup on the IAC pintle or idle-air bypass passage

Vacuum leak or engine mechanical issue that causes low idle vacuum

Incorrect throttle-stop screw adjustment

See all 37 photos An IAC (A) is a stepper motor-actuated valve that extends (for lower counts, less air) or retracts (for higher counts, more air) a pintle in the throttle-body air bypass passage. IAC counts should be high at cold-start, decrease during warm-ups, and be low fully warmed-up. A sudden load increase causing a momentary, sudden, low-rpm, vacuum drop-off—like going from Park into Gear—can increase the IAC count as it tries to maintain a preset idle rpm. Crocie tried lowering the TA's too-high counts by adjusting the throttle-stop screw (B) to open the throttle-blade slightly more at idle. The TPS (C) checked OK.

The TA had no carbon buildup or vacuum leaks, but at 12 in-Hg, idle vacuum was a little on the low side for self-tuning EFI—yet not out in left field for a performance cam. Crocie thought that providing more bypass air independent of the IAC might reduce the IAC count and raise the idle speed, preventing the stall, so he increased the throttle-blade opening amount via the throttle-body's stop screw, then installed a higher-flow-rate PCV valve of unknown origin from his spares pile. "Everything now seemed happy in the computer at hot idle, but the count still spiked to 200 when placed in gear, and the timing was still advancing from 9 to 15 degrees BTDC at idle up to 30 degrees—way too high.

"I noticed whenever the coolant temperature was only 150 to 170 degrees, the idle would almost maintain coming out of Park. I installed a Mr. Gasket high-flow thermostat that opens at 158 degrees instead of the stock unit's 192, then set the computer-controlled electric fans to turn on at 165 degrees and go off at 140; they now ran constantly. Coolant and inlet air temperatures dropped and the computer-set, low-speed advance went down to just 8 degrees. In-gear IAC count was cut in half, down to 100, so I knew I was moving in the right direction, but there was still an occasional random stall going into gear.

See all 37 photos Replacing the stock 195-degree thermostat (left) with Mr. Gasket's high-flow, 160-degree unit (right) plus turning on the electric fans at a lower temperature dropped coolant and inlet air temps, resulting in more stable low-end spark advance and lower IAC counts, but some random stalls still occurred shifting from Park into gear.

"Now I began to suspect the trans. Its big electrical plug passed all MSD-spec'd voltmeter electrical tests with flying colors, but during the procedure I suddenly noticed the handheld controllers for the engine and trans always reported the trans being in Second gear, regardless of the actual detent position. No wonder the computer was confused. It couldn't compensate when the other sensors like the TPS [throttle position sensor] and VSS [vehicle speed sensor] contradicted it."

So Crocie brought the trans over to Eagle Transmission in Austin, where Brian Day pulled the trans apart and checked the pressure switch manifold, which is supposed to tell the computer which gear the trans is in. "Brian thoroughly inspected the trans and cleaned everything up, but never actually found anything in particular," Crocie says. "But after he put the trans back together—voila!—the mysterious problem went away." No longer confused, the computer self-learned and the stall was cured.

See all 37 photos The 4L80E's master connector is located at the rear of the main case on the driver side (A). Crocie performed electrical continuity tests on its terminals with a voltmeter. It passed, but while testing, Crocie glanced at the handheld trans and engine controllers. "I suddenly realized that no matter what detent I selected—Park, Reverse, Neutral, or any forward gear— the controller said the trans was in Second!" Nearby Eagle Transmissions in Austin thought the pressure manifold switch (B) might be glitching. Located between the filter and valvebody, it's like a computer touchpad, but actuated by hydraulic pressure instead of your fingers. Thoroughly cleaning up the trans and pad solved the problem!

See all 37 photos

See all 37 photos "Maybe particulate matter in the trans had goofed something up," says Crocie. "But now the computer was happy, the IAC counts were normal, the stalling stopped, and overall driveability continued to improve as the computer's self-learning ability could now apply the proper strategy over time." Compare the "BEFORE" data (trans supposedly in Second when it's physically in Park) to the "AFTER" data (trans properly reported in Park, IAC and timing stabilized).

The Vibration Fix

With the car driveable, Crocie went to work on the bad vibes. His initial report: "On a test drive, the review mirror begins shaking above 15 mph, and as the speed increases, it gets progressively—exponentially—worse, spreading to the entire car. It occurs in any gear. By 70 mph, it feels like the car is tearing itself apart. I can see dust floating around in the air!

"I put the car up on the lift for a visual inspection. One of the first things I check on engine/trans or suspension swaps is the rear pinion nose angle and overall driveshaft phasing. The TA has a four-link, coilover conversion that uses thin-wall polyurethane bushings at the pivot points. There's not a lot of movement or deflection, which makes proper phasing critical. I noticed the pinion angle was perfect with the rearend hanging on the lift and the coilovers connected, but that's not real-world. Once I disconnected the coilovers and positioned poles supporting the rear differential simulating real-world, loaded ride-height conditions, the pinion nose was 2.7 degrees down compared to an imaginary line drawn through the trans yoke and driveshaft centerline—way too much. For a general-purpose street car, on a four-link I like to see 0.2 degrees pinion down with a stiff suspension at normal ride height. I lengthened the four-link's adjustable upper arms to achieve this, then cycled the suspension through its full travel range. There was only a 1.1-degree gain through full sweep—very good.

See all 37 photos To minimize vibration and friction, Mark Williams says the trans output shaft centerline should be parallel to the pinion centerline within degree in both vertical and horizontal planes (but not "pointed" directly at each other). The combined front and rear U-joint operating angles should be within 2 degrees.Mark Williams

See all 37 photos The TA's pinion angle was 2.7 degrees down with the suspension loaded at static ride height—way too much. Crocie corrected that by lengthening the RideTech four-link's upper arms (arrows), reducing pinion angle to just 0.2 degrees nose down, with only a 1.1-degree change through the full range of suspension motion.RideTech

"Yet there was little or no reduction in vibration, so next I put a buddy in the car and had him spin the motor up to 3,000 rpm in Drive with the car up on the lift. Standing under the car, the driveshaft was oscillating so hard, I couldn't focus on it, even with my prescription glasses on. It looked like velvet, there was so much vibration." A dial-indicator check revealed excessive runout at each end of the TA's driveshaft. Not only was a new driveshaft needed but the oscillation had also damaged the trans yoke bushing in the extension housing, requiring it to be replaced, too.

See all 37 photos With vibration still present after correcting the angles, Crocie checked for driveshaft runout. "Rotating the driveshaft by hand with a dial indicator on each end, I found excessive runout—0.021 inch in front, 0.029 at the rear. The end caps weren't welded on concentrically."

See all 37 photos The vibration fix: a properly balanced and phased Mark Williams's aluminum driveshaft with beefy 1350 U-joints and a new trans yoke.

For the new custom driveshaft, Crocie went with a Mark Williams MasterLine 3.5-inch-od, 6061 aluminum unit. Every Williams shaft is electronically high-speed balanced with the trans yoke. Special assembly fixtures guarantee proper weld-yoke phasing during assembly. "I've never had a problem with anything Williams makes," Crocie says.

See all 37 photos These are the critical dimensions needed for a custom driveshaft. Mark Williams prefers to supply the trans yoke as it makes the balancing job more accurate and determining the right length easier: Just measure from the pinion yoke U-joint centerline to the back of the transmission extension housing (E). The car must be on the ground at operating weight (gas and driver).Mark Williams

See all 37 photos Williams' beefy MasterLine 6061 aluminum driveshaft includes a billet trans yoke and sealed Spicer U-joints. The precision, washer-like balance weights actually bolt on (inset). "Our balancing spec is 25- to 30-percent tighter than most competitors," says Williams' Andrew Dickson. "We spin balance at 4,000 to 8,000 rpm and can go as high as 10,000, depending on the 'shaft's critical speed."

See all 37 photos

See all 37 photos Weighing in at 14.05 pounds, the new Mark Williams driveshaft is 4.35 pounds lighter than the old 18.4-pound steel driveshaft it replaces.

Williams won't install anything smaller than a 1350 U-joint on its hard-core shafts, but GM corporate 8.5 10-bolt rearends use smaller 1330 U-joints at best. Tom's Differentials supplied a flange that fits the GM pinion but accepts the larger U-joint.

See all 37 photos A Tom's Differentials pinion-yoke adopted the new driveshaft to GM's 8.5-inch 10-bolt corporate rearend. The pinion-yoke retaining nut also preloads drive-pinion bearing "crush." To avoid complete rearend teardown, measure the force needed to rotate the old yoke. Install the new one and tighten the nut to 200 lb-ft. Continue tightening in 10lb-ft increments until rotational force matches the old value—here, about 1517 lb-in.

See all 37 photos Another often overlooked critical preload spec is the pinion flange's U-joint straps. The center of the strap should contact the U-joint before the end-tabs bottom against the yoke flange's surface. With the strap retaining bolts finger-tight, you should be able to fit a 0.020—0.030-inch feeler-gauge between the strap and flange; if not, shave 0.020—0.030 inch off the bottom of the cap's flat ends.

See all 37 photos Vibration also damaged the trans extension housing rear bushing—the old yoke had 0.050 inch clearance! TranStar offers a replacement 4L80E bushing. With it and Williams' yoke, the clearance is now a nice, tight 0.0020.003 inch. You'll need to remove the housing to install it and then a new rear seal. The housing-to-case O-ring-style gasket is reusable.

The Results

Except for a slight converter lockup "hunt" in overdrive at 53 to 55 mph under cruise, the TA now drives smoothly; as the computer learns, it keeps getting better and better. Alexander drove the TA back to his dorm in College Station, Texas: a flawless two-hour drive.

See all 37 photos Pedal to the metal with IAC counts normalized, vibration cured, and two bad coils replaced. Don't mess with Texas.

Lessons Learned

Crocie advises, "When you notice something's wrong with the car, something's wrong. Don't drive away from the repair shop for good until the problem is fixed for good. Don't get lost on what the car is doing; find out why it's doing it."

See all 37 photos

See all 37 photos Tom Eyeington (right) built an LS-powered 1981 Trans Am as a 16th birthday gift for his son, Alex (left), but it stalled and vibrated terribly.