[93celicaconv]

Some random guess. Maybe you didn't clean iacv well? Maybe coolant temperature sensor is bad? Look at fuel trim, too much gas or air?

The OP affected the idle symptom issue by only removing a vacuum hose connected to the EGR valve (eliminated the idle problem when warmed up). I would think the OP would be best served by digging into that element first, not the IAC or coolant temperature sensor.

OP, if the engine runs normally when cold, but doesn't when warmed up, and with a warm engine with the engine idling (no foot on the gas pedal), I really don't think your EGR VSV is the problem at all.

I'm attaching a picture of a sticker that has the vacuum routing diagram for a 1995 Camry 5S-FE (2.2L) with California emissions, just for reference. I am also attaching a service manual section from my 1993 Celica service manual - it looks to me like the EGR system on a 1995 Camry 5S-FE is essentially identical to that on my 1993 Celica 5S-FE. If you look first on page EC-38, that page will help you a lot. There is a table at the bottom that ties the diagram boxes labeled (1), (2), and (3) above it. Look at that very closely and spend some time understanding it. I think it will help you understand next steps needed to verify what is wrong.

Note in the table, when the engine coolant temperature is below 131 deg F, the ECU will not supply power to the EGR VSV. That means the EGR valve should be closed when the engine is running, no matter if you are on the highway or have it idling. Next note that when the engine coolant temperature is 140 deg F or above, the ECU may or may not energize the EGR VSV. The engine coolant temperature is that which the engine coolant sensor for ECU is telling the ECU. The ECU will also not energize the EGR VSV if the engine RPM is above 4000. The ECU will also not energize the EGR VSV is the throttle plate is not beyond port E on the inside of the throttle body. The ECU gets that information from the throttle position sensor on the throttle body. The ECU will only energize the EGR VSV when the throttle plate is moved beyond port E on the throttle body.

With all that said, you stated the engine runs & idles smoothly when cold/cool. This tells me your engine coolant temperature sensor for ECU is functioning properly, and the ECU is responding properly by not allowing the EGR VSV to be energized. However, once the coolant temperature is above 140 deg F, the ECU properly removes that restriction on not energizing the EGR VSV, which is also correct. But, the ECU should be not activating the EGR VSV if the throttle plate is not beyond port E (which if it was, the engine should be at a very high idle speed). This tells me you may have a problem with your throttle position sensor, or at least the signal getting back to the ECU. Has your throttle position sensor ever been removed? If it was, it needs to be calibrated when reinstalled to ensure it has the output of a closed throttle plate when the throttle plate is closed. If not calibrated properly, or not working properly, your ECU may be getting a signal that the throttle plate is opened further than it really is. There may be other root causes that produce this symptom, but the fact that your EGR VSV is energized when the engine is in a slow idle (throttle plate closed) tells me the problem is in the throttle position sensor and/or the wiring going to the ECU. The ECU could also be defective in that it can no longer read the position location coming from the throttle position sensor.

If it was my car, that is where I would be focused on right now.

 

[93celicaconv]

As you say, EGR is likely not the source of the problem. I still plan to remove and clean it so that I can rule it out as a possible issue.

I’ll look into the throttle position sensor (calibration as well) and the coolant temperature sensor. Is there any simple way to diagnose an ECU as being potentially faulty? My imagination would be that these are expensive and difficult to change; I’ll do some research.

Aside from not being able to pass emissions, is there any negative effect to continuing to drive with the deactivated EGR? Am I going to ruin my Catalytic converter by sending unburnt fuel into it?

I said the EGR doesn't appear to be the root cause of your issue. The EGR appears to be active at a state where it should not be, so the EGR is causing (in my opinion) the idle problem. But from what I can deduce without actually personally diagnosing your car is that the throttle position sensor maybe the cause of why your EGR is active when it should not be.

Your fuel tank should be under some pressure - the vapor pressure of the fuel. In winter, your gasoline will have a higher vapor pressure than in summer, as winter gasoline needs to additional volatility to ignite in cold winter temperatures. But temperature plans a major role in gasoline vapor pressure. The vapor pressure of gasoline drops as the temperature drops. I don't know where you are located, so unless you live in a relatively warm winter area, you really shouldn't be noticing significant fuel tank vapor pressure. Your EVAP shouldn't be considered faulty if your fuel tank has a high pressure - EVAP is not meant to control fuel tank pressure.

If your coolant temperature sensor is faulty, your car should remain at cold idle speeds well after the engine has warmed up. Does it? Is your coolant temperature gauge in the normal location after the engine has fully warmed up? Also, if the coolant temperature sensor is faulty and reading too cold (keeping you engine idle speed higher - but still steady), your EGR should not be active as the engine doesn't think the coolant temperature is above 140 deg F. By how you previously described your engine's operation, there wasn't anything there indicating a coolant temperature problem.

Unfortuantely, an ECU doesn't know when it has a faulty capacitor. Toyota's built in the era of yours are notorious for having ECU capacitor failures. Sometimes when taking an ECU apart, a faulty capacitor can be identified (it has leaked and also the corrosive leak may have permanently damaged your circuit board too) or the capacity may have swelled a little. Often though, a capacitor has no visual signs of failure. It is difficult to test capacitors for failure when they are installed on a circuit board. They are easy to diagnose if removed from a circuit board. But most folks are not up to going that deep into an ECU.