P0131, P0151, and misfire code

1996 Chevy Silverado, V8, 5.7L, 130k miles

The engine runs and idles ok. But when I accelerate hard, it lacks power at higher RPM (5000). At this point, the dummy light starts blinking (according to the manual it means misfire) and then goes steady after normal driving. The codes are P0131, P0151, and the random misfire code. Since two O2 sensors are in alarm, I figure they aren't the problem.

I've replaced and gapped the plugs, but nothing else. I've been thinking about replacing the plug wires and distributor cap, as they have never been changed.

Any suggestions?

yeah complete the total tune-up and check for a wiring problem to the 02 sensors! Fuel pressure and egr operation are worth checking as well!

Thanks!

When the "dummy" light blinks like that, it means your engine is missing badly enough to cause damage. That's why it blinks rapidly....to stop you from revving to 5000 RPM, since...ya know....that always fixes it. Usually a rapidly blinking light will stop most people from driving their cars if for no other reason than "hey, there's a blinking light on my dash, neat! Now how high can this engine rev?"

Now, down to business... Your O2 codes are indicators that there is either a very lean condition in your engine, or bad o2 sensors(less likely).
A tune up is a good idea, but won't cause a lean condition. Your random misfires can definitely be caused by a lean condition! So check:
vacuum leaks, fuel trim readings(if you have a scanner) and report back. Also check for any dirt or oil buildup in the MAF, because there's a chance that could be a cause as well.

I haven't had much time for this lately. I ran a quick check of the MAF. Using a frequency meter, I get 2400Hz at idle and 6000Hz at 5000RPM. The book says 2000Hz idle and 8000Hz "at top speed". Is that enough deviation to be trouble?

Also still haven't checked fuel pressure and egr. Kid's baseball games are keeping me busy.

I have a question about the MAP. When checking voltage on the unit, it runs about 1.2V at idle. When the throttle is pushed slowly, I see no change. When the throttle is pushed fast, the MAP momentarily jumps to 4.5V for a second and then drops back to 1.2V. Is this normal? Shouldn't air pressure be constantly higher at full throttle?

are you referring still to your MAF or map sensor? Do you have both?

both. 96 chevy vortec v8. It has MAF in the air intake, and a MAP on the manifold. I was checking the MAP this time, and that is where my question lies. I've assumed the MAF to be healthy.

With no load and throttle slowly pushed, don't expect to see a lot of fluctuation in Air pressure in the manifold. WHen snapping throttle you'll see a sharp decrease in vacuum on accell and sharp increase in vacuum on decel throttle snap. Looks normal to me.
You're looking in the wrong area. Read back through the posts and perform the recommended procedures and then post back.

Thanks. I needed to know how it operates, because I had read that a bad MAP can cause a lean mixture at high RPM.

I've checked for vacuum leaks using propane around the hoses and fittings. Nothing exciting there. I've checked the MAF, MAP, and TPS sensors. I wanted to make the easy checks first. Now I need to go buy/borrow some tools (fuel pressure guage, vac guage, obd2 scanner) to get much further.

Youth baseball is still keeping me busy. Only a few games left.

Using propane won't give you any results unless you have a scanner to watch your fuel trims. Also Using it outside or near a fan will cause the propane to blow away. So, propane's a good idea, but you need to watch and see if your STFT drops suddenly into the negative range double digits WHILE applying propane or throttle body cleaner to the intake man(COLD).
Sometimes the leaks will go away on a warm engine(manifolds and heads expand and seal the leak)
Don't forget it's possible to have a leak internally into the crankcase that won't be discernible from spraying th. body cleaner/propane at the intake.

Baseball is more important, by the way. Go Royals!

I though you were contemplating 02's?

I thought as much, while doing it. I need to borrow or Ebay a scanner.



I honestly don't have a clue what I'm doing. I'm checking the cheap stuff first, and then I'll go after the rest. I'm going slow, because there isn't a drivability problem unless I stomp on the gas. It has plenty of power in the "normal" driving range. In the last 2-3 days it has become hard to start, needing 3-5 seconds. Gas pressure/flow problems?

fuel problems including injectors and regulators bleeding off are always good possibilty's with those trucks! So are the pumps themselves and the harness to it!

It sounds like this could be a good learning experience. You'll get it soon!