Although I have a tech manual line drawing that shows the IAT sensor location, I am asking for some help before I go any further. I see three possible sensors or wire connectors. Can anyone guide me to the right clip?

 

The IAT sensor location is underneath the Plenum on your "91".

 

The IAT sensor toward the rear of the plenum (near the distributor) on the bottom side. If you stand on the driver's side and look on the bottom of the plenum between the 2 runner pairs, that is it. If you are just checking the sensor, it has a detachable pigtail... when you find the sensor, just trace the wires back and there should be a harness right near the distributor that you can unplug to do a check with a meter. Just be careful with the "car" end, as it can get lost back there.

You *might* be able to change the sensor from between the runners, but most likely you would need to pull the plenum.

SB91

 

My SES light came on today, and code 23 came up.
Sorry to hijack your thread, but i also have a '91.

With a scanner the IAT is reading -40*C
On a '91 can this be a cause of the poor performance I have been having once the engine is warmed up?

How hard are these things to replace?

Cheers

 

i would think that -40c would cause you to have some issues.replace the iat and let us know what you get.

 

Could be.... if you are getting a code, then the computer has identified an issue. I am guessing that since air temp is a part of the closed loop formula, it is probably staying in open loop. I would check the sensor itself like I described above just to make sure it is not a wiring issue. There is a table of resistance/temp values in the FSM.

SB91

 

As far as I know, IAT is only used by ECM for EGR operation and has nothing to do with closed or open loop.
Code 23 and a -40 reading, suggests that there is an open in circuit or sensor. It may be just unplugged. There is a connector at IAT and about 6 inches away, is another connector(one you can access to diagnose circuit)
This is from my 86 FSM, but may prove useful.

http://members.shaw.ca/corvette86/Code%2023.pdf

 

Thanks Agent 86.

I'll check my FSM to see if its the same.

after doing some more reading found that 90-91 is a Speed Density, which uses the IAT, but not sure how. Anyway I'll get on that and see if its the sensor, cables etc.

Cheers!

 

Yes, speed density may use the IAT, for more than EGR operation. Wasn't thinking at full throttle, when posting.

 

Yes your 91 has the speed density sys. witch does not have a mass airflow sensor. Speed density systems use throttle angle, air intake temp, and water temp to set the fuel ratio from a fixed chart in the computor. the oxygen sensor can lean or enrich fuel by changing the pulse width to the fuel injectors. Unlike mass airflow sys. that more accurately mesure air flow to the engin, speed density sys. are more limited. if you do engin modifacations to speed density sys. you have to replace the chip. To check if your in closed loop mode on obd1 sys. (96 and older cars) just use a paper clip to connect ''a'' and ''b'' termial on ALDL conector under the dash. (top roll, the two on the right side). then start the car, the check engin light will flash off and on ''fast'' when in open loop mode,and ''slower'' when in closed loop mode (when the engine warms up). I beleave this is called the field service mode in the factory service manual from GM. You can drive the car in this mode and the check engin light will tell you if your running rich (light stays on) lein (light goes out). You gota love thease OBD1 cars that you run basis dianostic's with jut a paper clip. I have owen my 91 convt. sence it was new and still love it. hope this thread helps out.

 

The 90-91 certainly does use the IAT, in this app. refered to MAT sensor (Manifold Air Temp.) for fuel calculations. The function of a MAT sensor is to provide input from the intake temp. (ecm logic use is a product of combustion). Relocating it changes fueling values based on temperature values/look up tables in the bin file/eprom (located in functions as: "Mat compensation counts v.s.mat" and "MAT Inverse Term Lookup Delta Mult. Vs. Airflow" ).

 

So basically the serious power loss when warmed up I am experiencing can be the fault of the IAT?

Ordered a new one, will post back once it arrives in HK and is installed.

Cheers

 

It's possible particularly based on the -40 input (obviously incorrect data). In addition I would want to know the input of all the other sensors (cool. temp. 02 and so on). My advice is to make sure that everything is right.

 

Coolant sensor is spot on (two years old), matches ambient air temp at startup, stays at 89*C warmed up crusing.
O2 is half year old, seems to be doing its job.
IAT ranges from -40 to -35*C so theres definitely something wrong with the sensor, and the SES light comes on and off as it ranges down past 40*C.

Let me do another full scan and I'll post back all the sensor data.

I hope I dont need to pull the plenum to replace this....

Also, my engine seems to carb up quite badly, could this be a fault of running rich with this sensor malfunctioning? Seafoam does a great job cleaning the valves and lasts for about a week or two, then the slightly rough idle comes back... (cleaned the IAC and TB too)

someone was trying to tell me that the air in HK was particularly bad for engines as a lot of cars experience excessive carbon buildup on the throttle blades. I have to clean mine every week or two.

 

It can be done with the plenum on,i did mine last fall. I used a ratchet wrench and had a helper on the other side of the car to assist me.

 

Alright, got the new IAT on, and the power is back. There was infinite resistance on the old sensor, which was the coolant type. I guess a readout of -40C was dumping waay too much fuel, which is probably why the engine keeps carbing up?

The new one looks like it was designed for air, and theres a lot more power when fully warmed up!

Forgot to say, it is a PITA to remove and replace, especially connecting the plug back up! but well worth it!

 

Hey 91' White Roadster when you say. "You can drive the car in this mode and the check engin light will tell you if your running rich (light stays on) lein (light goes out)". Does the car have to be in open loop of closed loop when you drive/run the test, or it doesn't matter? I have a C2 Wideband in my 90' vert, i'd like to run the FSM test and see what both of them say at idle and WOT. Can you explain what the top roll (you mean top row?) is on the obd1 port? Can you post a pic of the pin layout?

I had a rich condition come out of know where, I changed out the coolant temp sensor, & TPS, running way to rich on the Wideband at idle, Barely blip the throttle it would dip into the 10's & the motor would hesitate kinda.. black sut on the ground bye muffelrs. New sensors fixed the AFR at idle & no more sut on ground or black tips, but still kinda rich at WOT in the 12's & 11's hardly ever in the 13's. Car runs strong though. Yesterday after I adjusted my shifter linkage to use first gear again I fired it up and was letting it warm up and it through a SES light after a few minutes, but went off while it was still running, I noticed on the Wideband that it was super rich at idle again, like the same issue from before. So i thought maybe the 02 is smoked cause ive been driving it with the super rich issue for such a long time. I put a new O2 in it, fixed the AFR & idle and ran hard. Fired it up today still good. If it does it again, im getting a new IAT & MAP, not that much money $100, might as well get a knock sensor too.

But really should bye a scanner for this car that gives me lots and lots of info. Going to get the OTC NEMISYS. Expensive but does a lot. I'll post again if the rich condition comes back. Any extra info always appreciated.

A = Ground
B = ECM Diagnostic
C = Ride Control Diagnostic
F = T.C.C. (VIN 8) (Transmission Converter Clutch)
G = CCM Diagnostic (ab 1990)
H = Antilock Brake
J = E&C BUS DATA
K = SIR Diagnostic (ab 1991)
L = SIR Serial Data (ab 1991)
M = Serial DATA (ECM - CCM - EBCM - A/C Programmer)

 

Put a new IAT sensor in, seems to be good at idle on the Wideband and Not Going Rich when throttled is blipped.