MAF or COMPUTER?

Hi , Have a 97 Sonoma with the 4.3 vin x engine. Lately truck runs rough at idle and the idle speed jumps all around. No codes are set. I have the Palmer Performance software so I can read in real time the computer as well as turn off codes etc. Suspecting the MAF sensor , I unplug the MAF sensor and the roughness stops, although power is down a bit. Also sets a code. Before purchasing a new unit I decided to swap the MAF senser from my '99 Buick Century which runs great and uses the exact same MAF sensor. This is where things get interesting...

After the swap the truck continues to run bad and the Century continues to run great. So now I know both sensors are good.

Hooking up the computer I find the Century idles with about 5 gallons/second air flow. Right on target. Also hooked up the computer to a 2002 Buick Lasabre and see about 6 gallons/second air flow. Again, due to the larger engine, right on target.

Pull the service manual for thr Sonoma and specs say airflow at idle should be about 5-6 gallons/second airflow. Hooking up the computer to the Sonoma initialy gives me about 5 and engine runs good. Keeping it hooked up I notice the airflow reduces to about 2.5 gallons/second and idle is terrible. Keep in mind this is a known good MAF sensor.

I then check wiring from the sensor plug back to the computer, the ignition feed and ground with and no shorts opens etc. Plugging in the sensor with the ignition on but NOT running shows the computer thinks it's getting 1.92gallons/second. Unplug the sensor and the computer shows 0. Each of the 2 known good sensors always gives me exactly 1.92 gallons/second.

Just to check, I again hooked up my computer to both the Century and Lesabre and the computers show 0 gallons/second with ignition on and engine NOT running.

My logical conclusion is the computer is bad. Am I missing something?

MAF sensors measure grams of air per second. Don't worry about your 1.92 volt reading, (not grams per second) with the ignition on and engine off, it's not causing your problem. Before you go throwing parts at it, check fuel pressure and leakdown. Low fuel pressure will cause the MAF sensor to show low air flow when the engine is running because the engine is consuming less fuel. Low fuel pressure usually does not set a DTC but, if if does, it's usually a MAF fault and many people replace the MAF sensor needlessly. Ignition in run position, engine off, fuel pump running, fuel pressure must be 60psi to 66psi and must remain above 55psi for 3 to 5 minutes after the fuel pump shuts off.

I actually had a pump problem several weeks ago and replaced the pump, regulator , filter,and cleaned out the lines. But, last night, just to be sure, I rechecked pressure and fuel pressure is within specs. Any other ideas?

Fuel pressure is within spec? That's good, is leakdown also within specs? If leakdown is excessive, the check valve in the pump is probably leaking which can also cause insufficient fuel volume. Keep in mind that just because the parts are new, is no guarantee that they're good. Make sure the pump is getting battery voltage AT the pump on the grey wire, and that the black, (ground wire) at the pump shows less than 5 ohms resistance to ground.

The unsteady idle could also be due to a vacuum leak or a lazy oxygen sensor, either of which should turn on the SES light though. Are you using AC Delco plugs, distributor cap and rotor? How many miles on them, and on the plug wires? A worn distributor gear can cause a rough idle too. Have you checked misfire history files? Your Palmer software might be able to access that. Just a few things to check ;)

Let me throw this out there while I'm thinking.

Does the rate go to 2.5 gpm as soon as it goes to closed loop? Is this the way it works. Or another way to ask might be when cold is everything ok and as soon as closed loop occurs is that when it falls apart?

I Haven't thought about thatyet. I'l give it a try