1978 280Z Runs rough? - Page 5 - Nissan : Datsun ZCar forum :Nissan Z Forum: 240Z to 370Z
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post #41 of 56 (permalink) Old 07-12-2019, 04:23 PM
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Dave - are you suggesting I try an ethanol free gas?
I seriously doubt that would account for the problem, I just prefer to run the cars on what they were designed for. They have a very primitive EFI system, so less variables the better. Does the problem present only at Idle?
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post #42 of 56 (permalink) Old 07-13-2019, 08:52 PM Thread Starter
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Ok thought I would check the valve gaps (even though a local guy told me he checked them!!). Counting valves from the front had to correct gaps in 2,6,10,11 out of the first 11. 11th is intake so trying to get it to .008 and it won’t get that tight? The top of the rocker moves freely up and the others do not. I assume something is missing? Tried to show on the photo that I can pull the end of the rocker up? Thanks for suggestions and help
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post #43 of 56 (permalink) Old 07-13-2019, 10:47 PM
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is the lash pad in place? Should not have a problem adj to .008.


I should have asked if you have done a compression test, just assumed that was covered. A leak down test even better.
poor compression will look like a miss, guess its possible to show up as the engine warms up. a Cold and Warm test should be done as well.
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post #44 of 56 (permalink) Old 07-14-2019, 01:57 PM Thread Starter
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Ok I got the gap set. Now on to the rest of your suggestions. I let u know. Thanks
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post #45 of 56 (permalink) Old 07-14-2019, 02:47 PM
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Ok I got the gap set. Now on to the rest of your suggestions. I let u know. Thanks

what was the deal with the problem of not getting it set?
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post #46 of 56 (permalink) Old 07-14-2019, 08:46 PM Thread Starter
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To hard to explain. Maybe just mental fatigue after a long day. I thought that I spun the top nut one way and then the other and if would not tighten either way. I had it so loose that the rocker was lifting off freely so I figured something was wrong. Thanks
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post #47 of 56 (permalink) Old 07-14-2019, 08:56 PM Thread Starter
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Ok. Worked on the timing and got the Vacuum up to 16-18 range. Got a compression tester and I think I found the problem. Should be 164-178lbs and mine are as follows. 1-108, 2-105, 3-110, 4-113, 5-111, 6-110. There is no doubt that when car warms and runs rough that 1&2 are not combusting. When u pull off those plug wires it runs the same. Sorry I know this should have been the first thing I checked. Realizing my local mechanic does not know what he is doing. He checked them and said they were all about the same. I now know that means nothing if they are all below spec. Can I just assume the only fix is a major engine rebuild. Thanks
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post #48 of 56 (permalink) Old 07-15-2019, 09:41 AM
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those are low but
a few things to consider. You have to be sure your equipment is reading correctly, Cant tell you how many times I have been misguided by faulty equipment or hook up. IF possible check equipment against a known good working engine and see if you get normal readings.


Next, should not mater much but did you have the throttle plate wide open? did you have all the plugs out (helps with cranking speed)?
Did you try putting a little oil in each plug hole (wet vs dry test).


You may want to do a leak down test, this will tell you if you have a ring issue vs a valve issue. they are all the same so I would assume IF they truly are that low, most likely worn compression rings, the leak down test will help know for sure (you will see low % and hear the air escaping thru the block (valve cover/dip stick/from the opening that lead down the front cover where the timing chain goes).


as far as the power test (1&2 not changing RPM). this is where you really should focus. Start with listening to the injectors, make sure you hear them clicking. For spark, pull the plugs out, leave them attached to the plug lead and rest the metal body of the plug on the valve cover, check for spark.


If both check (click and spark) pull the suspect injector and one that seem to be working, check to see if they both spray the same while cranking. Its possible you have a slightly clogged injector. You could even just swap the #3 (good injector) with &1 or &2 (the bad cylinders), and see if the power test fails on #3 after the swap (and passes on the cylinder that got #3 's injector).


Cant overstate the importance of making sure you compression readings are really that low. Also the factory specs are a bit optimistic IMHO, my 75 gets about 150-155 across all, below factory spec yet is runs just fine. I think more typical reading on a engine that has 100k or more miles would be 125-150 and it would run fine, just down on max power. Confirm those low numbers before going to far. a wet test will prove out the ring issue, if you jump up to 170psi from 110psi with an teaspoon of oil added, then yes your engine is worn and would be something you need to consider.
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post #49 of 56 (permalink) Old 07-15-2019, 04:16 PM Thread Starter
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Don’t want to ask a stupid question but I’m not sure how to open the throttle plate? Is this done by depressing the accelerator??
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post #50 of 56 (permalink) Old 07-15-2019, 06:25 PM
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yes just hold it open any way you can. I generally use a remote starter (push button) that plugs into the solenoid on the starter, and the battery +. this way I can be outside the car while cranking and don't have to worry about disabling the ign or fuel injection. you can just reach over and hold the throttle open while cranking (find the throttle shaft that runs along the top of the intake manifold). Use a good compression tester, the kind that screws into the plug hole is best (vs the kind that you have to press into the plug hole). Generally it will take about 4-5 compression strokes to max out the reading.
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