RedHot85Vette

My Car: '85, L98, stock, OBD I (no scanner). What does the ECM want to see at startup, sequence of events, especially Air Temperature (from the MAT), before it goes into Closed Loop? Preferably verifiable documented responses via GM service documentation, Chiltons, Haynes, others of the like, Authentic C4 Experts, etc...

John

jv9999

When the engine is up to temperature and the O2 sensor becomes active. That's what my service manual says.

xrav22

https://www.corvettebuyers.com/c4vettes/l98.htm

bjankuski

Interesting read but has errors.

bjankuski

I have copied a picture of my manual that lists the conditions required to go from open loop to closed loop. There is no mention of the MAT sensor but that does not mean it was omited from the write-up. I have also copied a tune for the 1985 car and this shows the temperature the car enters closed loop and the seconds of delay before the car enters closed loop depending on the starting temperature of the engine. This may have something to do with the MAT or maybe just the ECT sensor. I am not a computer code reader so I have not looked through the GM tune to see everything that needs to be satisfied to enter closed loop.

RedHot85Vette

I have this book, and the information within the above reference is what I want to confirm.

Also within the book, there is a section on recommended mods for better L98 performance. The first mod advises to relocate the "IAT" to the Air Intake housing... otherwise the ECM will get a false read of the outside air temperature. This advice assumes the GM engineers must have been out of their minds mounting the device where they did. BUT... the "IAT" he's referring to is my "MAT", which is located at the rear of the Air Plenum, drivers side... it hangs upside down, under the plenum, and has a black "umbrella" type connector, attached to a harness, about a foot long, and connects to the main harness, near the EGR Solenoid... I believe it has an Orange wire and a Black. I believe the sensor is designed to do as it's name requires, measure the temperature of the air IN THE MANIFOLD ( NOT the incoming outside air). I believe that if the MAT is confused with an "IAT" (designed to measure the incoming air), and is relocated to the intake housing, it's THEN that the ECM will get a false reading... being that it won't be getting the Data that it's looking for (140 degrees inside the intake manifold).

So unless I see data indicating otherwise, I'm not relocating my MAT. I'm going to assume that the Engineers did a fine job placing the sensor where it is, allowing it to do the job it is designed for, and one that makes the ECM very happy.

QUESTION: Regarding the O2 Sensor. If I don't get a "flashy" code (OBD I) indicating a bad O2 sensor, can it still be bad. Isn't it recommended that we replace it every 50K miles or so? But why replace it on such a basis? Is it a Preventive Maintenance thing with the idea that they tend to be at their life's end at about 50K? What about the maxim, "if it ain't broke don't fix it"? As far as I can tell, my O2 sensor looks every bit of 118K miles old as is my car but I'm not getting any flashy codes that says it's bad. I am getting somewhat of a pungent, or strong exhaust odor, and people have noticed. I am thinking maybe that might be due to a bad O2 sensor, although other bad sensors, like maybe a bad IAC, might be causing that problem as well. I feel like I have a lot of work to do. Could a bad muffler or small holes, or leaks from an exhaust system pipe cause that condition... or how 'bout a bad Catalytic Converter. I'm thinking about replacing the Cat with a glasspack or straight piece of pipe. Any tips will be greatly appreciated. The exhaust of my car sometimes reminds me of my old '64 Comet, what had NO emission controls whatsoever, not even a PCV valve. It had a "draft pipe" which is a pipe that came out of the cylinder head, or block, went straight down under the engine/car... and as you drove, the poisonous gas emissions blew away in the "draft".

"Once the Oxygen sensor (mounted on the exhaust pipe) reaches operating temperature of several hundred degrees, the Manifold Air Temperature (MAT) sensor shows an intake air temperature of more than 140 degrees..."

RedHot85Vette

BRAVO!!! Great Stuff! Thanks!

I'm replacing my ignition control module with an ACCEL version. I have the timing to do. I have a new PROM coming that will eliminate the smog pump, which I ripped out during my renovation project. What a bunch of junk! I put a pulley in it's place. Next is the AC. Don't need it. Don't use it. If I ever change my mind I'll put it back. It's a lot of weight and engine bay clutter for as much as it was used. Never converted. Still R12. It works, gets very cold, but the Compressor rattles so much, like it wants to explode. It's going bye bye, maybe today, if RockAuto gets me the bypass pulley today.

bjankuski

The statement that the MAT must ne at 140 degrees before the car enters closed loop is definately wrong.
-Closed loop operation can be obtained at 104.5 degrees the intake air would be less then that.
-The intake air temperature in the winter would never even hit 100 degrees much less 140 degrees
-My own car has the intake air temperature sensor mounted in the air cleaner housing and the car enters closed loop at 122 degrees where the factory closed loop temperature is programmed. (It has been this way for 20 years)

RedHot85Vette

On the '85 the MAT is mounted in the plenum, at the rear, so it's measuring the temperature inside the plenum. I believe it's possible that the air in the plenum can reach 140 degrees ... that plenum gets pretty hot after running a bit. And maybe if I didn't do a throttle body coolant bypass, the air in the plenum would reach 140 quicker and help to maintain that temp as the car runs no matter what season it is. Maybe that's what the engine techs at GM intended?

What year is your car? In '85 there is used a MAT mounted in the rear of the plenum. I believe it tests heated air inside the plenum. Later models may have a sensor mounted in the air cleaner housing or throttle body (IAT), but that sensor is designed to operate in the air cleaner housing or throttle body, testing the temperature of air before it reaches the plenum. I don't understand how your car can enter closed loop at 122 degrees... is that 122 degrees of air in the intake housing?! How can the fresh air flowing through your housing get a temp so high so fast.

Hey! Can you let me know what year your car is? I'd like to look up the part number for your IAT.

I guess the real simple question is: is my MAT or IAT, whatever you'd like to call it, located where it should be, and providing the ECM with what it wants for closed loop? What's your opinion? I'll get whatever, move whatever, it's no big deal to do.

bjankuski

My car is a 1986, the IAT was in the plenum and i moved it to the air cleaner 20 years ago and the car enters closed loop when the water temperature hits 122 degrees, while the air temperature is at ambient. I do not believe the IAT is used for closed loop operation. I run the IAT in the air cleaner beacuse a drag race and the plenum gets heat soaked and takes too long to cool to give me accurate readings for a drag race (I used to data log my runs). On a street driven car the plenum readings should be pretty accurate after being driven for a few miles.

vader86

Basically the CTS must reach a minimum temp, the timers (which vary by engine temp) must expire, and the O2 voltage must be within a certain range, which indicates that it has reached a certain temperature range.

When a car drops out at idle when its warmed up, its because of the O2 sensor cooling off.