I just got a 88 firebird 305tbi m5 and it has a rough hesitation and wont idle when the engine is still cold. I replaced the plugs and wires and set the timing to 8a and it seem to be running pretty good, But again after it cooled down i had the hesitation again. I also replaced the MAP sensor, Still nothing. I ran my code scanner on it and came up with code 23; MAT sensor, TPS sensor, Mixture Control solenoid, or IAT sensor. Im new to this tbi setup so is there any one of these that is a consistant problem with these setups? Thank you.

 

I had same problem and just replace my MAT sensor, runs like new

 

Whats a MAT sensor? Do you mean MAP sensor?

 

No its MAT, manifold air temp.

 

Anyone else got any suggestions... Since ive had the car, which is like 5 days, ive changed the plugs and wires, MAP sensor, IAC valve, MAT sensor, set the timing(it ran best at 8 deg advanced, when engine was warm) and put on an open element k&n. The last thing i just did was checked the TPS volts, i got it to set at about the exact opposite of what they were saying to set it to on here, so im guessing maybe the TPS is fryed now too?? The final reading was about 4.75v at somewhat idle(car wouldnt idle itself so it was right about 800-1000 rpms) and about .75v at wide open throttle. I tryed switching the probes on the voltmeter every which way and thats the closest i could come up with.

 

Oh and one other thing ive been kinda monitoring is the voltmeter has been running a notch or two below 13v, could i have a draw somewhere thats causing my problem??

 

Do you have access to a scantool, or a laptop? Might be faster then shotgunning a bunch of parts. Id look at the CTS output first, then the fuel pressure next. The TPS looks OK, although the readings are in reverse(wrong leads being measured).

 

The fuel pressure seems to be ok, seems to be running rich and im goin through tons of gas. As far as the CTS, the guage on the dash goes up when i crank it but then drops back to 100 and doesent move. Its been real cold the past couple days and when i leave for work, i put it in gear and have to slip the clutch to keep it from stalling, thing just has no low rpm power under a load until it warms up. Even then it still doesent feel like its giving its all.

 

FastTA


CTS (coolant Temperature Sensor) and Dash Temperature Gauge are two separate circuits. The Dash Gauge temperature sensor uses separate sensor. The CTS is more important for ECM to properly deliver fuel mixture. CTS sensor is located next to thermostat housing, a bit to the left, on intake manifold.
Dimented is absolutely right – get a basic GM OBD-1 scan tool or ALDL cable with a laptop. There is no point of replacing sensors, actuators blindly. It gets expensive after a while plus you may not solve the actual problem.

For example a busted (open, high resistance) CTS sensor will make ECM to think that engine is very cold (-40 C) and will rich-up mixture. With a scan tool or a lap top you will be able to see if ECM sees proper temperature.

Read stickies

RF

 

The one you are speaking of by the t-stat housing is actually the MAT sensor (manifold air temp) and it is one of the sensors that came up on code 23 which was the lone code that ive had flashing.So, I replaced it, and still nothing, but the code isnt flashing anymore. Thanks to all for the help tho.

 

the two holes with red circles around them are where the the coolant sensor for the ecm could go. the gauge sensor is on the drivers side of the block or head i believe. hope this helps

 

The MAT sensor is in the air intake system. most likely in or around the air cleaner. IE.. "Air INTAKE tmep." Most often the air intake temp sensor is a open element set up. However. In reality, there is no real difference between the AIT and the clt sensor other than the open element will give faster temp change readings than the closed element. You can use a clt to test the mat if the plug fits. You cannot use a open element mat to test the clt because it will short out. You can test a mat, clt by putting your probes to the connectors, and holding the element in your hand or fingers for open element, and watch the voltage change. They actually use clt's as mats in some applications such as pu's or severe duty.

good luck
mrtater

 

Ok just to get this straight, my car has an MAT which is on the pass. side of the intake, an IAT which is screwed in to the air cleaner and the CTS which should be on the drivers side head. I replaced the MAT but i havent touched the IAT... Is there a way to test the IAT without a scanner?? If not i think its only 12 bucks at autozone and it looks pretty shot anyway. Im still thinking maybe theres a problem with the TPS or maybe the throttle body needs a rebuild. Thanks a lot guys.

 

Not Exactly

IAT - Inlet air temperature is typically screwed into air cleaner assembly
MAT – Manifold Air temperature is found screwed into manifold runner, typically aft of TBI.
CTS – Coolant temperature sensor is typically mounted into a manifold water jacket near thermostat.
Instrument Cluster Gauge sending unit is typically screwed into drivers side head.

The first three sensors IAT, MAT, and CTS are temperature variable resistors that provide ECM with temperature data. Mrtter99 is correct that all three are very similar in terms that resistance variation over operating temperature which is nearly identical. In case of IAT and MAT the two are same.

Typical CTS is 12146312 and it has the following basic performance:
Temp C …. Resistance (Ohms)
-10 ……… 16,321
0 ………… 9,516
20 ……….. 3,555
50 ……….. 992
80 ……….. 338
100 ……… 180.3
120 ……… 102.2
Complete data sheet can be found here:
http://www.pe-ltd.com/Downloads/coolant_temp.pdf

For 1985 Camaro with 305 G or H engine used 25036979 CTS



Typical IAT/MAT is 25036751 and it has the following basic performance:
Temp C …. Resistance (Ohms)
0 ……..… 9,256
20 ……… 3,457
50 ……… 965
80 ……… 329
100 …….. 175.3
120 …....… 99.4
Complete data sheet can be found here:
http://www.pe-ltd.com/Downloads/air_temp.pdf

For 1985 Camaro with 305 there were two different IAT sensors
1992-1985 5.0L (F) 305" 2nd Design 25036979
1985 5.0L (F) 305" 1st Design 25036751


There are many different instrument Cluster Gauge sending units out there - you will have to look it up by application. You can check a sensor by measuring sensor resistance with a DVM, at a known temperature.
I hope this helps

//RF

 

my car was running like crap and wouldnt stay on, ended up being the coolant temp sensor, it screws in to the intake manifold right behind the t-stat. try that, only a few bucks and it was causing my car to die on me every time i went to move it. made a huge difference.

 

Ok, you can get a general test, (working/not working) on the IAT simply by holding the element between your fingers and watch the voltage change with a voltmeter, as it warms up to body temp. Or use a hair dryer. If your CLT has the same plug, Use the old one you replaced if it is still good to see if it makes a difference in your cold start problem. The problem with open element temps. is they break easy. I learned alot about how all the sensors work by reading about them on the megasqurit web site when I built my first MS. You would be amazed how simple they all are.

mrtater

 

Just curious but did you disconnect the proper cable before doing the timing?

 

Ok thanks a lot guys. Iwent out and looked, still only see the two sensors but i dont have a light right now so ill check it out tomorrow, anyone know where i can find a diagram for a 88 firebird, my chilton manual is useless. Can i check out the resistances and all with a voltmeter, if so what should it be set on. As for the timing, i did disconnect the tan wire.

 

Fast89

A small lesson in electronics.
You do not measure resistance with a voltmeter – resistance is measured with ohmmeter. Most modern (combination) meters in the last 30 years combined ability to measure voltage, current and resistance all in one convenient, low cost package – DVM (digital volt meter). You can get them at just about any hardware, automotive, electronics store. Personally I prefer Fluke DVMs – they are well designed, rugged and should last a life time provided that you do not abuse them. They are not cheap.

The CTS should have black/red and yellow wires attached to connector.

In the photo below a CTS sensor is being measured on my work bench – sorry about a mess. DVM is set to resistance scale and it reads 2,781 Ohms which is just below 2,830 Ohms which corresponds to 25 C. Having a connector pig tail (from J-Yard) makes measuring resistance very easy. BTW, do not attempt measuring resistance of the CTS sensor while it is connected to the harness – big no-no!

Sorry, but I do not have wiring diagrams – check E-Bay for Helms Manual or original factory shop manual.

//RF

 

Finally got her going!!! Turns out the fuel pump wasnt putting out enough volume under a load, had good fuel pressure at idle tho so it kinda fooled me. She runs great tho now with all the new sensors and everything else i replaced, too bad i finished up just in time for the rain. Thanks for all your help guys.