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1977 zcar L28 6 cylinder low power, smoking

4zcar crazy

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compression test 135 on all except 1st cylinder by fan was 90,  spark plugs wet but evaporate within a minute, fouls out spark plugs and won't start again. It emits a lot of grey smoke all the time and some blue smoke after warming up some.

This is a teenager's project car for autocross that we are helping him with. Suggestions would be greatly appreciated!


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If the compressions are all within 5% and it's not burning oil it should run... ok. Lets see if we can get the numbers up.


Check and set the valve lash.

Check and set the timing.


Gray smoke sounds rich.


There is a water temp sender to tell the ECU when the motor is cold so it will run a richer mixture. If not working the ECU will think it's in warm up mode all the time.




Under the air filter is the air meter. It has a door on a light spring and the more air entering the more the door moves. This movement tell the computer how much air and it figures out the mixture to inject. If the door is sticky the mixture will be wrong.

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Wow, so much information!! Thank you! The car sat for about 15 years without running. 

Checked what we thought was the mass air flow unit, door on that does not appear to be sticky. How do we clean it, with throttle body cleaner?? Is this the same as the air meter? 

Will look up the leak down test and do that this next weekend.

Since the car has not run for a long time is the water temp sender something we should replace?

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Thanks.   Very helpful.  I was planning to try adjusting the valves next.  All injectors are brand new from Autozone.  So, what causes too much fuel or a leaky injector.  We are using non ethanol fuel.  


Also, all the plugs are damp when we take them out, and evaporate in a minute or less.  The cylinder with the low compression may be wetter.  I will have to check.  It smokes like crazy as it warms up and if we get on the accelerator.  So, have to clean the spark plugs after each time we drive it.  It has only been driven around the block.  Would it help to license the car and try driving it on the highway? 


Would that help the compression rings and oil ring loosen up?  We r going to look at the crank case ventilation system for possible plugging.  And we plan to try another computer.

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It could be the cold start valve, or the cold start timer staying on to long or stuck open. the timer is located in the T-stat housing, and the cold start valve (injector) is on top of the manifold pretty much behind the throttle valve it will have a steel fuel line going to it. The cold start system is basicaly the "choke" for EFI.


Checking the PCV system good but more then likely not the problem, check the rubber hose coming from the metal tube out of the block to the pcv valve, this should be rotten and collasped on itself. also clean the throttle plate and body (itake manifold) with carb clean and a small wire brush. 


You really need to find a mechanic who knows L-jetronic fuel injection, other wise your more then likey going to spin your wheel and waste a lot of money.


But I will say the best thing you can do is clean all your eletrical connection and then put a little die-eletric grease on them (within the fuel injection wiring harness, and do not forget the grounds.......very important........... and often over looked. This has been the main reason for poor running Z.

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You won't get the compression up. The rings are shot.


If the valves needed adjustment you'd have like 30. 90 means the valves are seating but the rings are leaking.

Seriously? leaking valves can cause any range of leakage just like rings, the only way to know is with a leakdown test. the blue smoke would more likely be rings of coarse but being at 90 does not rule out valves at all epecially if you are talking about just being out of adjustment. 


plus what TFM1066 said, I have had the cold start valve stick on me it will make grey to black smoke (and bad ass fire out of the tail pipe if you light it)

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Thank you for all the awesome input. Looks like my husband and kids have about 18 hours of research and testing to do this weekend. I'm leaving town for this one!! Husband says cylinder leak down tool has been ordered so wish them luck. Will update you after this weekend. Again thank you so much!!!!

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Finally getting back with some feedback.  As for the compression in the first cylinder closest to the fan, there was no clearance in for the intake valve.  Adjusted to specs.  Compression test went from 85 to 125.  So that was interesting.  I guess the valve was staying slightly open. So, thanks for the help on that.


As for the Fuel pressure, it is about 60.  I think you all said it should be about 28 or so.  Replaced the FPR, but no change.  Maybe the fuel pump is not correct and has too much pressure.  I will remove return line and see if the pressure changes.  I do not understand how the pressure is regulated or generated in the fuel system.  


Cleaned most all the electrical connections for the sensors.


Still getting too much fuel in the cylinders.  The plugs are wet from fuel.


If I unplug the cold start, does that disable the extra fuel added to the system?


Have been checking grounds.  The ECU is apparently the correct model for the 77 280z   A11-600000

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you would need to disconnect the steel fuel line going to the cold start valve, or remove it from the manifold and plug the hole in the intake, which ever is easyest for you,


60 PSI is to high, that is to much pressure, it should be @ 32-35 with the vaccum line hooked up and then go up @ 8-10 pound with the vaccum line disconnected. Make sure there is no fuel coming from the little vaccum nipple on the regulator.


you could have a bad regulator or the wrong fuel pump supplying to much pressure or a pinched return line causing pressure to be to high, this need to be fixed first, dont worry about checking the cold start system or anything else until you get the pressure down.

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According to the FSM you should be at 30PSI at idle and 37 at wide open either your regulator is bad or the return line back to the tank is clogged. The regulator bleeds off excess pressure from the rail and sends it back to the tank there is a vacuum port that references manifold pressure in a 1 to 1 ratio so there is always 37 PSI grater than the manifold. At 60 psi you will be injecting almost double the required amount of fuel causing your black smoke. Verify your return line is clear (it can be pinched closed if the car was jacked up by the inner frame rail incorrectly) or it could just be plain clogged if it is clear then replace your regulator.

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Back with more on the smoking z car.   Finally found the problem with the fuel pressure.  U all were very helpful again.  It was the return line.  It was clogged about a foot (metal line) from the rubber line that goes into the gas tank.  Totally plugged with black goo and some rust.  Got it unplugged after many hours.  yuk.   


And the pressure now reads 36 to 38.  No more gray smoke!  and it runs without fouling the plugs from being too rich.  Yeah.  Made a minor adjustment to timing and it has some power now.  


The bummer part.... now the smoke is blue.  Probably was blue and gray before.   It's not bad when it's idling, but under acceleration is much more.  And upon letting off gas, u get a momentary cloud out the exhaust pipe.  Any ideas....maybe valve seals if I m lucky.  Its too much blue smoke to drive it and not get some not so nice gestures from the folks behind the exhaust pipe!!


I am going to try and adjust that mixture? (screw) on the lower corner of the MAF back to where it should be.  And I need to adjust the (what we thought was the idle mixture screw) -- the big 2 inch spring thing with an adjustable screw in it on top of the air inlet tube (just above the throttle sensor).  I notice that the prior owners had broken the exhaust manifold bolt on back of motor (last one).  So, we probably need to remove the top end of motor eventually anyway.  


Any suggestions.  Thanks for all the help.

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A huge blue cloud on start up after sitting warm for 20 min. and less when driving is usually the valve seals. They aren't that expensive and you can change them in a couple of hours without removing the head and no special tools but a valve spring compressor. This may not fix the problem, but won't hurt to do. If you end up rebuilding, they are already done.

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Alright,  I will warm it up and let it set for 10 minutes and start it up.  If I get a big cloud of blue smoke, then it is probably just the oil seeping past the valve seals into the cylinders.  And the seals are removable without removing the head.  Cool.  I assume I have to just put rope in each cylinder so that the valves don't drop into the cylinder?  Is it fairly straight forward to remove the cam shaft and the timing chain and reline them back up?

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