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280Z Rough Idle/Sputter

Engine L28

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#1 lizardexit

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Posted 19 July 2016 - 04:01 PM

So I have a 76 280Z 2+2 with a rough idle when cold.  It also doesn't really like starting from cold.  Have to give it gas to get it going.  Once it heats up it is fine.  But here is the kicker, on cold idle it seems to suck air up the tail pipe, which stops when it warms up.  I held a lighter up to the tailpipe and watched the flame go up the tail pipe.  It also smells like it is running rich, with a glorious off throttle backfire and some black smoke.  Timing is good, fuel system has been pretty much completely replaced, compression was also checked, all 6 cylinders right around 135-150, not great but decent.  I am wondering if it could be a valve lash issue?  Not sure what else to check.



#2 NewZed

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Posted 19 July 2016 - 04:28 PM

Timing is good,

fuel system has been pretty much completely replaced,

I am wondering if it could be a valve lash issue?  Not sure what else to check.

You should give numbers.  Timing, fuel pressure, pretty much anything that can be quantified.  And what does timing do when you rev the engine.?  Does it advance?  Valve lash is easy to check, even if you don't adjust it.  Turn engine, stick in feeler gauges, write down numbers.



#3 lizardexit

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Posted 19 July 2016 - 05:10 PM

Unfortunately I had a mechanic check the timing, as I do not have a timing light.  So no idea if the advance is working, but I was told timing was right at spec.  Would issues with the advance cause a rich condition off throttle?  The timing advance housing is rusty, no idea how it looks internally.  As for fuel system, it is right around 36-37 PSI (brand new regulator, injectors, pump, and rubber lines).  I will pick up some feeler gauges this week to check valve lash.



#4 NewZed

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Posted 19 July 2016 - 06:38 PM

Is this car new to you, or is it an old car with a new problem?

 

You said that the engine runs fine when it's warm.  Valve lash increases as the engine heats up.  Checking lash would be a good idea.  You might have a valve not closing when cold, causing a dead cylinder.  That's very bad.



#5 lizardexit

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Posted 19 July 2016 - 06:45 PM

Car is new to me.  Picked it up in May non-running from Virginia.  It had been sitting for roughly 9 months.  Replaced basically the whole fuel system and half the brake system.  New fluids, battery, and some wiring got it on the road.  I'm still sorting out the kinks before it becomes reliable transportation.  This one issue has been the hardest to solve so far.  Well, besides the reverse lights the refused to work properly.  I will be checking/adjusting valve lash this weekend.



#6 NewZed

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Posted 19 July 2016 - 06:52 PM

Check your Air Regulator.  It's purpose is to let extra air past the throttle blade to increase the idle speed at cold start up.  They can get gunked up inside, or the electrical connection can corrode.  You should have about 1000-1200 RPM idle speed for a few minutes right after a cold start.  You might just have too low idle speed.



#7 lizardexit

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Posted 19 July 2016 - 07:00 PM

I'll check on that. That would make a lot of sense if it isn't opening. It does not like to start cold. Usually have to give it a decent amount of gas to get it going. Is that tied into the thermotime switch at all? Or does it operate independently on a cold start? I'm not at all convinced my thermotime switch/cold start valve are working at all, but I have not gotten around to checking them.

#8 lizardexit

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Posted 19 July 2016 - 07:05 PM

Nevermind. Found it in the fuel injection bible. I'll take a look at that tommorow. That would make a ton of sense though. Idle stays pretty constant hot or cold. If anything, it creeps up when the car warms up.

#9 lizardexit

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Posted 21 July 2016 - 05:35 AM

Quick update.  AAR appears to be working, still cleaned it out some.  Also found out I have no vacuum leaks which is shocking to me.  I will be adjusting valve lash Saturday.



#10 Leskinen

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Posted 28 July 2016 - 10:27 PM

Hi

Have you checked the thermotime switch? In my case it caused terrible idle (also the engine died every now and then) and sputter, the mixture being way too rich.
1975 280z fairlady ## dnsf.org ~ JuiceS





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