Running rough? It's not fuel injectors
Q When I first start my '96 5.0 GMC engine in the morning, it runs very rough. If I hold the accelerator down for about 30 seconds the engine will smooth out, then start and run great the rest of the day. The truck has 44,000 miles on it, and I have run a pint of Techron through the engine about every 4,000 to 5,000 miles since the truck was new. I have replaced the spark plugs, rotor and distributor cap and cleaned the throttle body. The service adviser at the dealership says the only thing that will cure it is a new set of injectors for $860. What do you think?
A I can appreciate the service adviser's hunch regarding the fuel injectors, as GMC/Chevy pickups and SUVs of this vintage employed a somewhat problematic system known as central sequential fuel injection. But, because you've proactively babied the fuel system with Techron fuel additive and the engine runs great when warm, I think there are other culprits to be checked that have a higher probability of resulting in a fix.
In a follow-up message, I asked you to differentiate between “rough” and “slow” running when cold, and you nailed the solution path by telling me the engine runs far too slowly when cold-started and if helped with any throttle, smooths right out. This points straight at the idle air control, or IAC, system as being the cause of this symptom. Fuel-injected engines with traditional cable type throttles, now replaced with electronic throttle controls, employ an IAC device to allow air to bypass the throttle during fast idle periods and in compensation for accessory loads.
IACs vary in method from simple electrically pulsed solenoids to your more sophisticated stepper motor type. On General Motors vehicles, rather complex electrical signals are sent from the electronic control module, or ECM, through four wires to the IAC, which rotates its internal pintle to any of 256 possible positions. My hunch is your IAC is dirty or sticky and doesn't move well when cold, or a faulty electrical connection may be preventing movement of any kind.
Verifying proper GM IAC electrical commands can be made simple by using an inexpensive IAC testing tool containing a pair of bi-color LEDs, or light-emitting diodes. The tester is temporarily connected to the truck's IAC wiring connector. With the engine started, one looks for both LEDs to rapidly change in color or flash from red to green. If this occurs, the ECM and wiring are deemed OK and the IAC is the assumed culprit, based on the process of elimination.
You might try removing your IAC, which has an easy two-screw attachment, and cleaning the pintle tip and throttle housing passage. If this fails to restore proper operation, it's probably worthwhile to simply renew the part ($12-60 depending on source/quality). I don't often recommend throwing parts at a car, but sometimes it's faster/cheaper than performing the diagnostic procedure. 44,000 miles? Wow! This sounds like a pampered and probably immaculate truck.
Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at email@example.com; he cannot make personal replies.