Hot start issue

[maj75]

New to me ‘75 280z.  It has less than 60k miles.  It was driven infrequently for the last 15-20 years.  Drained the full tank and the gas at the bottom was very old and dark.  Car starts fine when it’s cold.  Runs well.  Idle is the tiniest bit rough but holds.  The issue is hot start.  It will stutter on cranking and take longer to start.  Then it barely runs and sounds like it’s running on less than 6 cylinders.  I have to hold down the gas pedal and it will miss and buck for a few seconds.  Then it clears up and runs ok.  I can drive it normally and it will idle.  It has new air and fuel filters, plugs, wires, points and distributor cap.  I’m going to throw in a can of Techron today, but thought I’d post here.  Searched and found plenty of cold start threads but no obvious hot start ones.

[Dave WM]

Next time you stop the car pop the hood open for 5 min, see if that resolves the hot start.

Do you have an owners manual? The manual has a blurb about hot start may need you to apply full throttle during crank.

are you running non ethanol fuel?

I suspect since it got hot here if FL recently you are just experiencing the normal for 75's EFI woes. later models added vents and fans for a reason.

Edited March 19, 2021 by Dave WM

[rcb280z]

Also try popping the hood a little to vent the heat out while you are away from the car and see if that helps. It'll give you an idea of where to go next while trying to diagnose it. The 77 and 78 have louvered hoods to vent the heat. So there must have been an issue in the earlier models.

Edited March 19, 2021 by rcb280z

[Zed Head]

4 hours ago, maj75 said:

  Searched and found plenty of cold start threads but no obvious hot start ones.

Use the term "heat soak".  It's been discussed in depth in the past.  Aluminum fuel rails seem to help, they dissipate the heat, apparently.  It's a very common problem.

[Reptoid Overlords]

Any noticeable smoke on hot startup?

[cgsheen1]

We have a '75 "museum car" in the shop. It's all  mostly original with only 8,400 (ya, I DID say 8,400) miles. Usually runs like a dream but it "heat soaks", "vapor locks" when the weather is warm and you drive it to the store and let it sit for 10-15 minutes and then want to start it again... I thought this was mostly an Arizona thing but MANY Z owners from all parts of the country (and Canada) have had the issue. Nissan made many modifications over the years in attempts to solve it. The problem was mostly solved when they started using ECU's that "primed" the fuel system and pushed air bubbles away from the fuel rail. Moving the fuel rail away from the exhaust manifolds was an even better idea... With the intake manifold sitting on top of the exhaust you'll always jump through hoops to keep heat soak to a minimum.

Edited March 19, 2021 by cgsheen1

[mailnome]

That does sound like heat soak.


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[maj75]

Sounds like the solution is popping the hood.  I do have the manual, so I’ll check that out as well.

Thanks for the quick responses!