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Hard to start when warm Idles Rough- 75 280Z


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Hi guys,

I have always had this problem to an extent, but it got alot worse today. First off my car is a 75 280Z, Usually when I start my car and the engine is already warm it tends to idle rough kind of like it is choking on fuel a bit, but then after you drive it for about 20 seconds it is fine, but today it got alot worse where it took me up to five times to get it started, it sounded like it was gunna start right up but as soon as i let go of the key it died, finally I was able to hold it at about 3000 rpm for about 30 seconds and i slowly let off the gas and the engne stayed running. This only happens when the engine is warm. Im not sure what is causing this, but if anyone has any suggestions they are greatly appreciated.


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Hello Jason,

It sure sounds like a bad Thermotime switch to me. It's located at the T stat housing. to give you a quick explaination- What happens is when the coolant temp. is below a certain degrees, The Thermotime Switch then completes a ground for the Cold Start Valve. The Cold Start Valve is only suppose to operate during starting and when the coolant temp. is below the specified degree. Then when the engine coolant reaches ABOVE the specified temp shown in the FSM, the ground is suppose to break which cuts off the Cold Start Valve.

What is going on with your Z is the Thermotime Switch is not functioning correctly and is providing a ground to the Cold Start Valve even when the engine coolant temperature is warm and the Cold Start Valve is injecting the extra fuel when not needed which is flooding it. Then once you get that extra fuel burned off by racing the engine for 30 seconds it's fine again.

More than likely thats the cause of your problem.

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If that was the case, I would think that it would run noticeably rich all the time. Try checking the fuel pressure while it's running and then turn it off and see if the pressure holds or goes away. If it goes away, you most likely have a bad fuel pump check valve.

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