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My service manual (90) says a code 53 is system over voltage. It means the voltage detected at the IGN input of the ECM is greater than 17.1 V for 2 seconds. That would be your charging system over doing it.

 

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did you ever figure this out ive got the same code my 92 305tbi?

 

blurocz i have the same exact problem, the stumbling at 2kish, and getting code 53 on long cruises.

I do not have an EGR valve provision on my after-market intake manifold. The sensor for it is still hooked up and sending a signal to the ECM. From what i have read the stumbling is caused by the computer adjusting the timing to compensate for the EGR and since it isn't there its excessive timing.

I would check the vacuum lines going from the EGR solenoid to the valve because it sounds like your solenoid isn't getting vacuum from the valve. And if those check out check the EGR for proper function by pressing up on the diaphragm (it's on the underside of the valve) at idle and seeing if it effects the idle.

 

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Did you try to press the EGR diaphragm while at idle? it shouldn't be hard to press in at all. If you have to really put pressure on it i bet the valve is shot and not sending and pressure at all to the solenoid.

The vacuum lines should be easy to check since there is only 1 going from the solenoid to the valve.

Just from what you described (the stumble) and you are getting the same code and symptom as me, and same year car/ecm, I'd put money on that valve is too stiff and not functional.

 

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i just hollowed my cat and got a code 53 cause of exhaust readings. maybe your issue is in the o2 sensor or something