What are the trouble codes and how does one get them on a 1986 Omni w/carb

What are the bugs? To get codes for cars with fuel injection, cycle the ignition switch three times from "off" to "run" without ever cranking the engine. After a few seconds, the Check Engine light will turn off, then it will flash the two-digit codes. Count the first series of flashes, wait for a short pause, then count the second series of flashes. That's the first two-digit code. After a longer pause the next code(s) will flash the same way. The last one will be "55" meaning the end of the message. Some carbureted cars had Engine Computers for the ignition timing and to run the mixture solenoid in the carburetor. They can also store and flash fault codes.

These were dandy little cars. For the running problems, first be sure the carburetor isn't loose. If it is, the fiber gasket under it shrinks and causes a vacuum leak. Tightening the nuts isn't the solution. You gotta replace that gasket.

The dieseling was a common problem. There's two things to check. First of all, look at the idle speed solenoid on the back of the carburetor. Be sure it is moving freely and working. It may not be strong enough to push the throttle open when you turn on the ignition switch so you have to tap the gas pedal after the ignition switch is turned on, but it IS strong enough to hold the throttle open once it's already open. That solenoid retracts to lower idle speed when you turn off the ignition switch. That is done to reduce the dieseling. Very often that solenoid got stuck due to corrosion. The center shaft could be adjusted to set the idle speed and people did that to maintain proper speed while the engine was running, but that defeated its purpose since it couldn't retract when you stopped the engine. I never tried to fix one of those solenoids; I just replaced them.

The second thing to look at is the AC compressor clutch if your car came from the factory with AC. If you simply disconnect the negative battery cable, the clutch should engage for one or two seconds when you reconnect the cable. The same thing should happen each time you turn off the ignition switch. That is done to run the compressor and put an additional load on the engine. That also reduces the dieseling. If the AC doesn't work or someone disabled it by removing the compressor clutch relay, it can't do its thing.

Anything that causes the mixture to be too rich will promote dieseling too because of the extra fuel going into the engine. You should have a "Lean Burn" computer, or it might be labeled something like "Engine Management" Computer, or something like that. I thought they used a Check Engine light but I can't remember for sure. Regardless, the temperature gauge on the dash and the computer use different coolant temperature sensors. The one for the gauge will have a single purple wire. The computer's sensor will always have two wires. Both of them will be on the front of the head right behind the thermostat housing where the upper radiator hose is attached.

Be aware too that if you do anything that causes coolant to drain from the head, you will have to burp it to get the air out when you refill the system. If you don't do that, it will overheat because thermostats do not open in response to hot air. They must be hit with hot liquid. To burp it there are threaded hex plugs or sensors that can be removed. Depending on the year, those plugs can be on top or on the driver's side, or both, or you can unscrew any sensor. There could be up to three of them on top and on the side. Fill at the radiator until coolant runs out of what you removed, then leave it loose so you can recheck for any remaining air after the engine is started.