F-Bods,

About a year ago I rebuilt my '93 LT1 and installed a 268 HR XFI cam from Comp. Bottom end is stock, .030 over. Exhaust ports raised to improve flow, headers, and finally a new tune from FastChip. It took me a while before I ordered the new tune, but in the mean time I had the car dyno'ed to check AF at WOT. Everything is fine around town and the car made 350hp/355 torque at the wheels but was lean at WOT. I installed the chip, opened up the throttle blades according to the documentation. Drove it for a while to make sure things are okay.

Here's what's going on.

- When I start the car cold, idle quality is poor (lumpy & stinky). If I try to drive it, it surges, very low power, and will idle up & down.
- Checked everything with my code scanner and the poor drivability and idle occurs in open loop. Once the ECM goes closed loop, it runs fine (or at least drives fine.)
- While scanning the engine data, I noticed the IAT (Intake Air Temperature) sensor was reading 150+F. This is in the fall in Texas, so the air temp is probably in the 70s max.
- I replaced the sensor. Measured ohms on the old and new and saw a difference, so I figured the old was bad. I installed the new one and it reads 190+F. Ambient temps were in the 40s.

What should this sensor actually read? It's a resistive element, so it shouldn't matter which way it gets connected since it has a clip.

Bottom line, I think it's running lean. I have an AF meter that arrived yesterday. I want to get that installed so I can check, but still need some feedback on the IAT.

TIA,
E

Hello,

Check your wires that go from the pcm to the IAT sensor. It sounds like you have a wire that has rubbed through and grounding out. The computer is seeing almost no resistance and in its program charts that equals extreme hot(160-190F). God bless.

-Chvrlt283

I have been trying to fix this thing off and on for several months. I have confirmed that the sensor(s) are good. My ground wire is good too. The resistance of the brown wire between the ECU and the sensor is about 7 ohms, so I think that is okay too.

My manual says I should read 2V DC on the brown wire with the key on and the engine not running. The manual also says that I should have 3V DC with the engine running. I get 2V DC in both cases. A local repair shop says the that the ECU for GM uses 5V DC as a reference voltage and 3V is too low.

Can someone who's IAT is working, please take these two measurements from a '93 and report back the results? I don't know if later years are different so cannot confirm if a 94-02 will also work. Additional data can't hurt.

I believe the ECU is bad for this circuit, but want some additional confirmation before I go buy a new ECU.

Thanks in advance,
E

Hello again,

Sorry for the late response, I hope it is still of some use. 7 Ohms is way too high. The resistance on just a wire should be no more than 0.2-0.3 ohms. I think the wire between the IAT sensor and the PCM is compromised and is grounding out. Follow the wiring harness and check for any damage (rubbing, chaffing, critter chew marks, or any where that it may have been flexing). I hope this helps and God bless.

-Chvrlt283

Yeah, that resistance is way too high. Usually, it's the connection at the IAT or at the PCU that is the problem. By the way, the voltage at the IAT is .6 to 5.0. The PCU will reset whatever voltage is read a the time of startup to zero as a base for setting the block learns. However, the IAT should not vary from the .6 to 5.0 volts. If the IAT is out of whack, then so are the calculations by the PCU for setting basic mixture and that will cause the car to run badly as you state. Add any potential IAT voltage problems to a bad wire and I think you see the problems. If I understand your posting, 3.0 volts max from the IAT is way too low.