baddad

My 1st z project car & a total noob to the nissan engine. I now have an 81 280zx n/a with 77m miles. Car needs TLC but is in decent shape overall & running wise. When I did an oil change, I went to start the car to circulate the oil to make sure that levels were fine & the car wouldn't start! Finially after repeated cranking & putting the pedal to the floor did it start & run VERY poorly as long as I kept playing with the throttle. Long story short, my oil cap was still off & when I reinstalled it the car ran fine. I've never had anything like this before. If you take the cap off while running she will drop R's big time spit, sputter & die.
Next week I was checking the PCV valve. I pulled the hose off while running to check the vacuum & the very same thing happened. I have never had any other car do this & even the factory manual states to check the PCV this way.
I'm totally baffled on this one, it almost acts like a TPS but what would crankcase vacuum have to do with it? I do have other issues (the cold start valve isn't receiving signal & I'm consuming alot of oil) but I don't see see any correlation to this particular problem.
I turn it over to the pros, Bob

Niku-Sama

i can see PCV doing it as i have seen it happen on alot of cars. but i have never figured out the oil cap thing, mine does the same thing, it doesent die but at idle it drops really low. if your car is dieing with it off you might check out your idle speed, sounds like its a little low.

duowing

This would mean your car is running correctly. In most cases pulling the oil cap is simply a way to check for vacuum leaks. If you can pull the oil cap off and the car either dies or starts running very poorly it means your intake system is in good shape, and you have no vacuum leaks or very small vacuum leaks.

baddad

This is my first dance with an older fuel injected car. I've not run into this with carbed engines. Is this typical for this engine? Bob

duowing

Usually it is, on my friend's 97' BMW 328is, in order to get to the O2 sensor wiring harness you have to remove the plastic covers that go over the fuel rail, etc. You need to remove the oil filler cap for this. So for some reason we forgot to put the cap back on. We started up the car and it was idling horribly, we couldn't figure out why. Then I noticed the oil filler cap was off. Put it back on and straightened up.

wake2wake21

ok check this out. on intake side there is a box that tells the car how much air its getting(mass air flow sensor) now if u look on your valve cover there is a hose coming off and running into the intake side(bypassing your mas) taking the oil cap off bypasses MAS causing the same as a big vacuum leak.
noone has told me this it is my educated guess while observing my zx do the same thing. so if anyone knows im wrong please let me know
i believe this has to do with emmisions right..... yeah emisions suck

Ratfink

Other emmisions crap or not, removing the cap will do this on a car that is running a pcv system.

With eh throttle plate/s closed, the car will idle. It will also give it the correct lean mixture of fuel needed to make it idle well. If the PCV is working correctly, you are actually supplying a vacuum to the crank case of the engine, and that is combined with the volume of unused space under your cam cover.

If you take the cap off, you are letting that obtained vacuum in the system balance itself with atmospheric pressure. This means that the hose from the pcv valve is now supplying the engine with more air than is needed to make a proper air/fuel ratio. Your car will likely die, or sputter and run like crap.

I need to hook the pcv up on my car. I just have it venting to the air. Which means my piston rings are not sealing as well as they could be.

Thanks for the reminder.