'It's the TPS, dude! This is our third F-body project car in the past 5 or 6 years, and we hear that tip from the local help nearly every time there is an undiagnosed problem. In reality, the TPS hasn't failed on any of our cars. It's the computer, dude! The ECM has barfed at least once on every one of our TPI cars, and when you go down the diagnosis flowchart, what's usually at the end is a little dialog bubble smugly stating "Faulty ECM." Bummer. That's a $100 headache.
But the car has potential, and in the following months were going to build this sucker to try and redeem ourselves for that red Formula we ruined a few years back, and to make it simpler than the black '91 Camaro we did with a blown 383. We'll also keep this one smog legal. Sound easy? Not in California. We've found from past experience that these cars have to be nitrous-oriented if you want to go fast and not spend a fortune in the name of compliance.
So treat this as an introduction to another third gen that we are going to make fast. Look for day-to-day stuff and little details at CarCraft.com and look in the magazine for the big fun. If we can do it, there's no excuse for you.
Day One:The Trouble BeginsThe car is a 150,000-mile '91 Firebird Formula that Freiburger picked up for $1,500. It has the original Tuned Port 5.7L that uses a speed-density EFI system rather than the mass airflow sensor used on pre-'90 cars. A quick check of the Service Parts Identification label located on the inside of the console door below the hatch on the driver side revealed the four RPO (Regular Production Order) codes we were interested in: B2L, MD8, GU5, and G80. The B2L indicates this car has the 5.7L (350) V-8 instead of the 5.0 (305). The MD8 is the 4L60 overdrive automatic. The GU5 means the car has the 3.23:1 ratio in the 10-bolt, and the G80 is the code for limited-slip. Other cool RPOs to look for: G92=performance axle, GU6=3.42:1 differential, N10=dual exhaust, and F41=heavy-duty suspension package.
It's actually a lot of car for the price. The only problem was that it barely ran.
Day Two: Budget Code ScanningWe've found that with each TPI car we bring home, we're destined to spend $1,000 on stuff just to get it running right. We saw bad credit looming when the Firebird would hunt in Neutral and stall completely in gear. So we started the diagnosis. The first thing to do on these cars is to check for engine codes. The best way is to use a scanner like the Actron CP9145 Super AutoScanner. It relays the ECM signals in real time and can read and store codes. Jeff Smith had one lying around, but this is Car Craft so we decided to use the field-service technique by jamming a piece of wire between the two upper right terminals on the ALDL (Assembly Line Data Link) under the dash. The engine light flashed the number 12 three times then gave us a code 32 which indicated a faulty EGR solenoid (according to the factory service manual we had left over from the '91 Camaro days). It made sense to us, so we pulled the plenum off the engine to get to the EGR after using a test light to probe the connector harness for power. The EGR was clogged with carbon and stuck open so we replaced it.
Day Three: Tune-up TimeWith the EGR problem solved, we moved to the basic tune-up with plugs, fuel filter, distributor cap, and rotor. The No. 5 and 7 plugs were actually easier to reach from underneath the car. We changed them on the lift, where we also changed the fuel filter (it's on the floorpan under the driver-side rear seat) and a broken transmission mount. We also installed new U-joints and a rear transmission seal. Done
Day Four: StrandedWe got a two-day pass to get the car smogged and reregistered (which really meant we were going drag racing) then made a few blasts near the shop and headed for the freeway. We made it about 15 miles before the car began to lose power. Uh oh. We shifted down and increased the revs but we knew we were in trouble and looked for a place to stall. We found a closed off-ramp and pulled to the side just as the car died. It would crank but not run. Fortunately, California has a state-funded fleet of tow trucks that roam the freeways looking for chicks in white Firebirds with high-heels and flat tires. They stopped for us and dragged the 'Bird off the road
Day Five: Rich. but in a bad wayBack at the shop we checked the timing to make sure that the chain hadn't fragged or the distributor gear hadn't given up. To check the timing on these cars you must first disconnect the EST (electronic spark timing). It does what the vacuum advance and centrifugal weights do in a regular distributor, but it uses math in the ECM to make the timing adjustments. The base timing checked out at 6 degrees and seemed to be OK. Fresh tune-up, good timing, and the motor wasn't blown up. The only thing left was fuel.
The cool thing about Tuned Port cars is the fuel rail is pretty easy to diagnose. The fuel is pumped from an in-tank electric through a fuel-pressure regulator that bleeds to a return line to control pressure. There is a port on the passenger side where a fuel-pressure tester can be added directly to the system. Our pressure-gauge needle was buried past 50 psi. Oops.
Day Six: Swap The RegulatorThe fuel regulator diaphragm had been eaten through and wouldn't hold vacuum, so the pressure was going sky high and causing a rich condition. It was supposed to be from 34-47 psi max. After the fix, we drove it for some gas and idled around the deserted warehouse district near the shop. The car powerslides great. Donuts are easy too.
Day Seven: Why Bother?Finally, we were ready to have fun and install our pile of mods, including SLP's headers, after-cat, and some other TPI goodies. So we got to the shop and fired up the car, but then the idle became rough and the engine stalled. Bummer. No engine light, and no codes in the computer.
At this point we began to hallucinate and checked the TPS. Smith had delivered the scanner, so we plugged it in to the ALDL and took some readings. With the key on and the throttle closed the TPS read less than 2.5 volts, so it wasn't jammed open. With the engine off, we slowly floored the throttle and watched the TPS reading progress to 5 volts at WOT. Rats. TPS was fine.
Day Eight: No Trigger FingerWith a fresh brain we went through the diagnosis. Fuel pressure? Check. Codes showing on the scan tool? Nope. TPS working? Check. RPM indicated while cranking? Ruh-roh! Neither the tach nor the scanner was giving us an RPM reading while the engine cranked. That meant one of two things: The distributor module was toast or the ECM wasn't functioning properly. Actually, they are both related. The ignition module sends a reference pulse to the ECM while the engine is cranking and to the coils to throw some sparks. We pulled the No. 1 plug and grounded it then cranked the engine. Big blue sparks. We also pulled the four-wire connector off the distributor and touched the purple and white (rpm reference) wire with a test light connected to 12-volts. The scanner didn't read rpm. We were close to the problem. The last thing to do was check continuity to the ECM from the purple and white wire to the distributor. We peeled open the harness by the A/C accumulator (that's where the EST is, remember?) and spiked a continuity tester there. The wire was good from the ECM to the distributor. Our flow chart ended with the statement, "Faulty ECM."
Day Nine: Tire SmokeThe new computer cost us 100 bones plus another $75 for the core. We plugged it in and the car fired right up and ran smoothly so we went out for some test laps. It's good, but the gas gauge is still broken, the radio doesn't work, and a lot of that worthless '91 GM water-based paint flaked off the hood and roof when Freiburger washed it. That just means there's more work to come. Stay tuned!
' The 7730 ECM box is actually the same for the 3.1L, 5.0L, and 5.7L cars, so look to the wrecking yard for spares in six-cylinder and eight-cylinder speed-density F-Bodies and Berettas, then swap in your PROM chip.
|THE TRUE PAIN OF OWNERSHIP |
|Description ||PN ||Brand ||Price |
|'91 Firebird Formula ||N/A ||N/A ||$1,500 |
|Battery ||N/A ||Delco ||75.00 |
|Distributor cap ||DR134B ||Standard ||18.81 |
|Rotor ||DR133 ||Standard || 3.81 |
|Spark plugs ||R45TS ||ACDelco ||14.72 |
|ECM ||780-7730 ||Standard ||107.97 |
|EGR valve ||214-5540 ||ACDelco ||62.50 |
|Fuel filter ||G3727 ||Fram ||10.88 |
|Fuel pressure regulator ||PR10 ||Standard ||62.68 |
|Heater hose ||N/A ||N/A ||2.71 |
|Intake plenum gaskets ||23468 ||Corteco ||10.82 |
|Transmission seal ||96135 ||National ||1.88 |
|Transmission mount ||2394 ||Anchor ||16.54 |
|U-joints ||5346 ||Precision ||20.64 |
|Hood assist rods ||901322 ||Monroe ||41.40 |
|Hatch assist rods ||901020 ||Monroe ||74.40 |
|TOTAL ||$2,024.76 |