Changed 1982 exhaust to a true dual exhaust system

[tj_82_vette]

I have changed my 1982 exhaust system to a true duel exhaust system with no catalytic converter. I purchased this system from Mid America Design.

I have a lot more horsepower now since I have less backpressure with out the catalytic converter but it seems to run too rich at idle.

When idling, there is like an air puffing sound coming out of the left exhaust.
I was told that means it is running too rich because the backpressure has change and the computer can not compensate for the change in back pressure and is making run too rich.

The car accelerates great and runs fine accept at idle after it has warm up.

Is there a way to adjust the intake system? I know that it is computer controlled. Is there another chip that will correct this problem?

Thanks, Tom

 

[CyberVette81]

That's a good question. I have an '81 that I am looking to replace the entire exhaust system (put on headers and true duals) and have been wondering how this will affect the computer control. With all the knowledgeable folks on this site, I'm sure someone will have the answer.

 

[shakey]

After the exhaust change have you unplugged the battery for about 30 minutes to reset the computer?

It may help

 

[tj_82_vette]

Have not disconnected the battery yet for 30 minutes. If I drive it for a couple days. Would turning it off and on reset the computer over a period of days or is it by just disconnecting the battery resets the computer.

Thanks, Tom

 

[photovette]

The puffing sound could be a bad cylinder--either caused by lack of ignition, bad spark plug, or valves. Could also be an obstruction--did the '82 have an EFE/heat riser valve; it may not be opening or the butterly valve came off and is stuck--this happened to my '77.

 

[shakey]

That I know of ,you have to disconnect it for a period of time. The off and on won't do it.
That was always a GM thing IMHO.

I could awys be wrong but at least that is cheap advice L.

 

[wishuwerehere82]

It could be a missing cylinder, or could have a burnt valve or a bad cam lobe or the TBI's might need to be rebalanced, or your left bank IAC motor might be stuck.. How is your O2 sensor installed? is your TPI sensor set at .525 volts at idle? There's no butterfly valve in the system.

 

[tj_82_vette]

Thanks for all the input guys. I will start out with changing the plugs on the left side.
I just changed them last fall but will change them to weed out that part of the problem.

Where is the IAC motor and how do I get it un stuck. I'll will check this after I change the spark plugs.

The checking voltage of the TBI may be out of my leage. I will look at my chiltons
corvette book. Will also chaeck the IAC motor part as well.

 

[wishuwerehere82]

The IAC(Idle Air Control) motors are servos that regulate the input air at idle for each of the two TBI's. They are located on the front of each TBI unit with a connector attached to the wiring harness. They look like 1" diameter cylinders and are about 2" long. If you remove the connector and unscrew the IAC, there is a conical shaped valve at the end of the motor shaft which goes into the throttle body and regulates the amout of air that goes into the throttle body at idle and controls the fuel/air mixture.
Undo them one at a time and reinstall the wiring harness connector with the IAC out of the TBI. Turn on the ignition, but don't start the engine. The shaft should quickly extend out of the motor housing. If they twitch or don't move at all they are probably bad. Clean off the tip of the valve with carb cleaner and a cloth and reinstall, then try the other one. Replacement is the only way to fix them.

There is a very good article in the March 2000 edition of Corvette Fever Magazine (Pg. 14) on tuning and calibrating the cross fire injection system. You may be able to get a back issue at your local library. It covers balancing the TBI units, cleaning the IAC's, setting the TPI sensor.

The TPI sensor is a on the throttle linkage of the rear TBI on the drivers side, facing the fender. The blue(Lower) and black (center)wires that are probed with the ignition on to set the .525V at 650 RPM. The mounting screws are loosened and the TPI pivots to change the setting.

Start with the spark plugs.

 

[JohnGrawcock]

I have changed my 1982 exhaust system to a true duel exhaust system with no catalytic converter. I purchased this system from Mid America Design.

I have a lot more horsepower now since I have less backpressure with out the catalytic converter but it seems to run too rich at idle.

When idling, there is like an air puffing sound coming out of the left exhaust.
I was told that means it is running too rich because the backpressure has change and the computer can not compensate for the change in back pressure and is making run too rich.

The car accelerates great and runs fine accept at idle after it has warm up.

Is there a way to adjust the intake system? I know that it is computer controlled. Is there another chip that will correct this problem?

Thanks, Tom

Tom,
Unless I'm mistaken, you’re running a single O2 sensor off of the right (passenger side) bank. The computer gets no feedback from the left bank so modifications to the control system wouldn’t have any effect. However, when everything is operating correctly this setup works fine, because both throttle bodies are driven by the same signal from the ECM. The only problem it that if something goes wrong on the left side the control system cannot adjust for it or even report a code. Your problem is likely something more basic.

Here are a few quick thoughts:

Checking for a dead plug isn't a bad idea. The fastest way to find one that is not firing at all is to connect your timing light to each one in turn and see if it will trigger. If the light will not trigger the plug is definitely very dead.

While you have the timing light handy compare the spray pattern coming out of both injectors. It should be the same. If not, one may be dirty or worn. As long as you’re looking at the throttle bodies check to see if they are at least roughly synchronized (fancy way of saying that the butterflies are in the same position).

I'm not thinking TPS. This should set a code.

The IAC might be a possibility. Neither one is stuck open. This would cause a very high idle. However, if the IAC on the right throttle body (the one that controls the left cylinders) stuck closed you should run rich on the left side. If this is the case you might be able to feel a difference in the suction at the air ports located directly above the IAC motors.

If you get as far as pulling plugs you could use a compression tester to eliminate that scary valve suggestion someone else made.

Hope some of this helps.

Good Luck,

 

[brusso]

The car accelerates great and runs fine accept at idle after it has warm up.

Thanks, Tom

Did the O2 sensor get put back in. I assume that you got the "Off Road" system. I didn't think that there was a provision for an O2 sensor, in those pipes. If this is the case, that would explain what is happenning. When cold the computer runs the engine in open loop mode. In other words the O2 sensor input is ignored. The computer uses a fixed reading, for the O2 sensor input, until the engine warms up. Then the computer accepts the input from the O2 sensor or closed loop. If nothing is there or sensor is bad, car runs like s!@# at idle when warm. Sound familliar? I'm not saying that this is it for sure. There have been many good responses thus far. Just something else to think about.

 

[tj_82_vette]

Thanks guys for the great replies.

There is a provision for a O2 sensor on my new exhaust system. I bought a new sensor from Mid America.

I am going to start with changing the the plugs on the left side this weekend and then go check the IAC motors.

Thats a good thought about the O2 sensor. I am going to see if I get that puffing sound when I first start up while the computer is controling the idle. From what I understand now is after the car is warmed up the O2 sensor takes over. MAybe the O2 sensor is bad ?

Thanks for all the great responses.

Tom

 

[brusso]

Thats a good thought about the O2 sensor. I am going to see if I get that puffing sound when I first start up while the computer is controling the idle. From what I understand now is after the car is warmed up the O2 sensor takes over. MAybe the O2 sensor is bad ?

Thanks for all the great responses.

Tom

It's a good possibility. Wealth of info on this forum. We'll get it!

 

[tj_82_vette]

Well I checked the exhaust when first warming it up and I can hear the puffing a little.

I then changed the plugs and the puffing is still there at idle after warm up. It's hard to hear when you first start it and the car is idling high but at normal idle it very evident.

I was going to check the IAC motors but I understand they are just for idle speed and the car seems to be idling properly.

I know of a mechanic from another fellow 1982 vette fan from this website. I can't remember his name but I do the have mechanics phone number he gave me. The mechanic's business is called Duff's garage in Tyngsboro Mass and is supposed to know 1982 Vette cross fire engines real well. I have an appointment for next Tuesday.

Will let you guy's know what the outcome was for your future diagnosis and interest needs.

Thanks Again, Tom

 

[tj_82_vette]

Wll guys, I took my car to Duff's garage like I said I was and he found the problem real quick.

The problem was a vacuum hose that fell of it's connection. The vacuum hose is located under the air filters. I must of knocked it off somehow or it just fell off.

The car runs great now with no puffing sound. It also sounds great witth the true dual exhaust from Mid America and the Dynomax mufflers.

Thanks to all for your suggestions !!



Tom