1990 Camry fuel/AC/electrical unknown issue
 

I have a 1990 Camry in excellent condition. Awhile back it began cutting out intermittently so I replaced the fuel filter because it hadn't been changed in awhile. No change. I took it to an auto repair shop and they replace the idle air control valve. No change. I returned it and they replace the A/C Idle up relay switch. No change. After each replair/replacement it ran fine but after a short period it began acting the same. Last week I drove it extensively for two days with no issues. Today it had problems starting. The continuing symptoms included cutting out (like a fuel pump malfunction) whenever I decelerate and accelerate after shifting (standard transmission). Although it didn't quit today it still balks as if it may quit. The mystery for me is that it is an inconsistently occurring issue; it happens randomly. After returning home I dumped some Iso-Heet into the tank and took it for a longer drive and it drove fine. Did that fix it? I don't know because, on occasion, the issue improved when the engine was warm, but not always. Short story: I cannot pinpoint a cause. I'll continue with daily drives to see if the problem continues.

Might be ign coil,

There may be also a cold fuel start injector that injects into the intake when coolant is cold. This seperate from the fuel injectors. If the injector leaks there may be too much fuel.

Question: If the cold fuel start injector is the issue would it be hard-starting each time? The reason I ask is that it is sometimes hard-starting and sometimes starts like nothing is wrong. It is puzzling. Thanks.

Forgot one thing. About a year ago I had the coil replaced. Bad coil? Mechanic said that that was not the issue. Not so sure, myself.

Did the HEAT fix the issue?

Any correlation between engine issues and things like outside air temp, rain, engine hot/cold, gas tank full/half/low, going up hill to place a load on engine, etc?

Is the problem getting worse?

As the injector could be intermediate issues. Sometimes they leak. But issues typically limited to starting. If hard starting when engine or outside temps are warm then perhaps injector is OK. Its for cold temps and cold engine.
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Is the ign coil built into the distributor or remote?

The car has a built in diagnostic system which will output any codes using the check engine light.