Engine Intermittently runs rough?

1990 NISSAN 300ZX
110,000 MILES • 3.0L • 6 CYL • 2WD • MANUAL
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ROD300ZX
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My car's engine started running rough while at a stop light. After I parked for an hour and restarted the car, it ran fine going home. The next day the car ran fine, but two days later it had the same issue. I changed the plugs and coils, and the car ran great for 2 days. On the 3rd day, I started the car, and it ran rough even after stops. Today, I started up and it ran fine and gradually deteriorated.
Here are the tests I attempted: The ECU had code 55, which is 'no codes'. I unplugged the MAFS and the engine stopped, which I think means it's okay. I looked at the throttle body and it is dirty, especially since the PVC occasionally throws some oil down the line. I have no idea if the EGR is working. The gas cap usually has back pressure and I burp it. One time I drove without a gas cap. Fuel filter was new about 2,000 miles ago. My previous mechanic is 'unavailable' to work on this problem... where do I go from here?
Apr 19, 2023 at 9:02 AM
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STRAILER
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Is sounds like the EGR valve can be getting stuck open, I would remove the valve and test it, here is a video to show you how and the EGR location in the images below:

https://youtu.be/DqUCbbKdWf4

this guide may help as well:

https://www.2carpros.com/articles/stall-at-idle

Check out the images (below). Please let us know what happens.
Apr 20, 2023 at 8:53 AM
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ROD300ZX
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Your analysis sounded correct based on the information i knew at the time...
I took the car to a mechanic I've used several times. He was able to identify that the cylinder#4 fuel injector was not firing due to lack of computer signal. I bought and installed a replacement control board to fix the problem. It's been a slow fix due to hard to find parts, but my car is finally running better...
Thank you for your analysis, I'll let you know when the next bizarre issue arises!
Jul 3, 2023 at 11:32 AM
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STRAILER
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Glad you could get it fixed, thanks for letting us know. Please use 2CarPros anytime we are here to help.
Jul 4, 2023 at 9:45 AM