A complete tune up has been done, but at 4rpm it just chugs, there is just dead acceleration in the thing leading me back to the PCM I think. The tps is fine and the IAC was just replaced.

 

OK, you say it chugs at 4000 RPM, what gear/speed is it at? If you are doing 60-70MPH or more, that's not a surprise. Just because the speedo goes to 100+ does not mean the car was designed to do 100+. Sometimes it's a mechanical limitation and other times it's a limitation of the PCM (by the factory).

Did you test the TPS with an analog volt meter to see if it has a dead spot?

Here's another possibility.... Have you had anyone connect to the car with a scan tool and read the data direct from the car's computer? The reason I say this is because you may have a problem in the cluster (bad motor on the tach or someone serviced the cluster and did not line the tach needle properly). A friend of mine has a car that the temp guage goes through the roof (but the car is not over heating, and when I connect a snap-on scan tool, it reports normal temp readings).

The last time the car was tuned up, was the upstream O2 sensor (the sensor on top of the rear exhaust manifold) changed? If so, was a AC Delco OR Delphi sensor used? The data that sensor collects is used to dictate other variables such how much fuel is used. If that sensor is reading bad, there is an outside chance it's making the car idle high. I believe it's recommended to change it every 70,000 miles. The reason Delco and Delphi are important is that the car from the factory is tuned to respond best with a AC Delco one (I've also heard form my GM guru Delphis are reliable). Other brands may work, but not as well OR prematurely set check engine codes.

There is an outside possiblity nothing is truly wrong with the car. I'm not saying it's not a faulty PCM, but the odds are very slim, it's very rare a PCM is bad (and when one does go bad, you typically have larger problems, such as a car that doesn't start or a large list of weird and unexplained problems).

 

Hopefully Matt can chime in, our guru of this engine. These things honestly do rev high and well. I believe they stock go to 7k where ours stop at 6k. The power is in the top-end supposedly, so there is definitely a problem.

I'm just unsure what it could be. Maybe a fueling issue?

 

It chugs when it hits 4000rpm just trying to rev it in park sitting in the driveway. It has no acceleration when trying to punch it to merge on the freeway, it just doesnt speed up. I have had a code reader hooked up to it but no codes were displayed

 

There's a rev limiter in park and neutral of 4k so you don't damage the car by revving it in those 2 positions.

 

Yea.. but it refuses to go over 4000rpm when accelerating trying to merge on the high way, hits 40-43 mph and just the acceleration is gone even with the pedal floored. after that its a slow crawl to get any speed

 

What you just described in your acceleration problem makes me wonder if the entire drive train is suffering. I know from a couple experiences a plugged up CAT will cause HORRIBLE issues, such as lack or loss of power at most any speeds.

Unfortunately, I do not know how to properly inspect a CAT for failure, but perhaps that's the next spot to inspect.

Again, also just as good mainenance, I'd replace the upstream O2 sensor (make sure it's not giving the computer garbage data that could be resulting in this issue).

 

Oddly enough it turned out to be the Mass Airflow Sensor which was coated in oil, cleaned it and threw it back and it works excellent now. Some times it is the last thing you think to check.

 

That's awesome news! Thanks for the update!

BTW - Of course it's the last thing you'd check, because once you found it, why would you check anything else?