So few weeks ago this strange problem started happening.
Car pulls great in first gear throughout the RPM range.
Then comes 2nd gear and around 4500 RPM to 5500 it's as if the rev limiter kicks in from time to time, car feels as if it looses HP and struggle to reach the orange line.
SES light does not come on. problem happens when the car is cold and worm.
I have new platinum plugs and AC DELCO cables.
I've attached a scan tool and it shows IAT at 155 even if temp outside is 40.
So I pulled out the IAT sensor and checked it's OHM, in the house where the temp is 70, it was showing 2690 which is perfect, I heat it with a hair dryer and it drops to 700, again great. So I am off to the car to check the voltage going to the sensor, it's showing 4.90V which should be fine.

Not shure what else to check. 02 sensors were showing 680 when the engine was worm, did not yet check it under stress when the problem happens.

Any ideas?

 

Could be the optispark or the coil. My car used to do that, it ended up being the wires in my case though. It kind of sounds ignition related to me, but what is strange is the fact that it does fine in 1st gear... So that leads me to believe its not ignition, at least not totally. Sorry i cant be of more help

good luck

 

Thanx ManicHaze
Plugs and wires are all new, no more than 1000 miles on them

Any other ideas?

 

OK,
Baffled by the strange results of the IAT sensor test compared to the computer info, I decided to just unplug it....guess what, the car pulls great now, no hesitation and no stutter, I am going to Autozone today to get a new sensor and just throw away the old one.....
This is strange, and I can't explain it but it helped

 

The saga continues
Went to autozone and got a new IAT sensor. I plugged it and the santool is now showing 168F.....Checked the sensor and it shows the correct OHM according to the outside temp.
Voltage on the connector is showing 4.9V which should be fine....
Any ideas? could it be the PCM?

With the IAT sensor disconnected, car works great but eventualy SES come on and it throws code #23 Manifold Airt Temp too low.

 

My guess is a burnt or decrepit wire for the IAT sensor follow the wiring, check the voltage INTO the PCM (dunno which pin or whatever)

 

electrical, try replacing coil, or try spark plugs

 

Hey golanr i was having a miss from 5000-6000 and i saw your thread so i unplugged my IAT and i didnt have any miss, well after a while my SES light came on and my car started missing again. Is this what your's does? The car runs good for a while with the IAT unplugged but as soon as the pcm notices it, it'll begin to run crappy again?

 

Hey blaze309
Well, eventualy it turned on teh SES light throwing IAT too low (#23) but I did not notice a miss....I did not drive it that much though...just allowed it to go into "closed" mode and verified miss yes/no. as with the Sensor it was happening immediately.

I think that StickMaster is on to something, I will check that as well, the Voltage on the connector should be 5.0V, mine is showing 4.9V....not a lot of difference but who knows?

Have you tried swapping your plugs and wires?

 

My car got a full tune up about 15 thousand miles ago.

 

Id check that all your grounds are good as well, that can usually clear up some electrical gremlins

 

Maybe your fuel pump is getting weak so the pressure is dropping the the upper rpms. Just a thought.

 

Thanx hsyr,
I already ordered a Fuel pressure gauge it should come in the next few day and I will check it, but the computer is showing equal pressure on both rails and it is around 110-128 (This is not PSI value)

StickMaster,
How do I check grounds? Where shoudl I start? Interesting that you mention that, I have a new alternator and battery, the Volt Meter in the car is hovering between 13.9-12.9...The scantool is showing 13.7-13.0...

 

Clean off ALL the contacts:
the patches on the passengerside fender well (whatever its called) there should be a bunch of wires there all comming to a point one positives and one for negatives i belive
your battery terminals
brakets that go from the alternator to the engine

steel wool it, whatever, just make sure they are clean and making good contact also you can spray it with some electrical cleaner.

 

Thanx StickMaster, I will do that.

One more piece of info, I read in most threads here that with IAT dissconected, the PCM should show a temp of -40. Well mine is showing 353!!!

Seems like connection related (ground) or bad PCM now..