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Thread: 1978 280Z Runs rough? Reply to Thread
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  Topic Review (Newest First)
07-22-2019 08:30 PM
chaseincats
Quote:
Originally Posted by Dave M View Post
That's correct.


common areas include cracks in the boot from the AFM to the TB, disconnected hoses, missing or damaged O rings on the fuel injectors. Harder to see is the EGR stuck open if used. it would leak into the exhaust manifold. its something to test but not with the smoke test. Other things to look out for is damaged PCV hose, and PCV valve. best to just look at the hose (disconnect it and examine) and just replace the PCV valve if no history on last done. In a pinch you can shake and see if it rattles (good) if no rattle some brake clean can be used to clean out the gunk. I prefer to just replace, but get a good one, Nissan OE if you can.
Yes, this.

Also (and this was my prob) the bladder in the vacuum advance pod on the distributor was cracked so smoke was coming out of my distributor cap haha. After capping that line, the car wouldn't even idle because the PO cranked the idle screw up so high to compensate for the leak.
07-19-2019 02:24 PM
Dave M
Quote:
Originally Posted by ctfisher View Post
Yes Iím still not 100% on how u check for leaks using smoke. So I remove the rubber boot between the air cleaner and the air flow meter. I seal up the open end of the air flow meter with something like a plastic cup. I then pump or blow smoke into the intake manifold top right port that connects to the brake booster?
That's correct.


common areas include cracks in the boot from the AFM to the TB, disconnected hoses, missing or damaged O rings on the fuel injectors. Harder to see is the EGR stuck open if used. it would leak into the exhaust manifold. its something to test but not with the smoke test. Other things to look out for is damaged PCV hose, and PCV valve. best to just look at the hose (disconnect it and examine) and just replace the PCV valve if no history on last done. In a pinch you can shake and see if it rattles (good) if no rattle some brake clean can be used to clean out the gunk. I prefer to just replace, but get a good one, Nissan OE if you can.
07-19-2019 12:50 PM
pilgrim You got me. Seems like you'd want to apply smoke on the outside of hoses and look for it to be sucked in, but how to do that with the engine running is beyond me.. (I have learned that I'm not good at finding vacuum leaks.)
07-17-2019 09:31 PM
ctfisher Yes I’m still not 100% on how u check for leaks using smoke. So I remove the rubber boot between the air cleaner and the air flow meter. I seal up the open end of the air flow meter with something like a plastic cup. I then pump or blow smoke into the intake manifold top right port that connects to the brake booster?
07-17-2019 08:26 PM
pilgrim Vacuum leaks can easily cause rough running. Good idea to check thoroughly.
07-17-2019 07:19 PM
ctfisher Sorry have not had time to do additional tests and won’t have time until next week. I’m going to redo the compression , do leak down and check on some of your other suggestions. My vacuum seems ok but I may try the smoke test again. I tried it before but don’t think I did it correctly.
07-15-2019 06:25 PM
Dave M 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.
07-15-2019 04:16 PM
ctfisher 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??
07-15-2019 09:41 AM
Dave M 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.
07-14-2019 08:56 PM
ctfisher 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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