i open my oil cap and car bog down like its missing 1 cyl or 2 a cycle

[M-type]

title says it all
oil pump?

[NismoS-tune]

oil pump? lol have you done a compression test lately? just curious. that's an odd one. Leave the cap on = win

[Shane_H]

The vapors from the crankcase, including small amounts of blowby gasses, are sucked into the intake manifold by the pcv system. This keeps the crankcase depressurized, and vents the harmful vapors. They are sucked into the intake for reburning. When you open the oil cap, the pcv system sucks in a bunch of outside air which is full of oxygen. The engine sucks the air into the intake, and causes it to run lean, and idle bad. This is especially evident on an engine that uses an airflow meter, or mass air flow system. The mass airflow sensor cannot meter that extra air as it does not pass through it. Its the same as a vacuum leak. Engines that use a MAP sensor instead of a Mass Air Flow, CAN compensate immediately, since they measure the pressure in the manifold, not the actual airflow into the intake. If a vacuum leak is induced to a MAP equiped engine, the idle will probably just go high, but still run fine.

[cschepp]

Wow great explanation!!

[M-type]

Compressions are #1 120PSI; #2 125PSI; #3 135PSI; #4 125PSI; #5 140PSI; #6 130PSI

[needavdub]

Compressions are #1 120PSI; #2 125PSI; #3 135PSI; #4 125PSI; #5 140PSI; #6 130PSI

I think that may be on the low side for a RB, Mine were 151-153 across all 6 cylinders. Also the 20psi deviation between the high and low #s doesn't seem good...

[Supraclean]

If those are correct somebody needs a rebuild......

[M-type]

yea direct from nissan

what do you think it is?

[nelsonmxmarc]

Yea, low numbers..

Did you do this dry? (didnt put any drops of oil in the bore beforehand)
Motor was cold/warm/hot? (cold should yield results ~10 psi lower than hot. Rings seal better warm-hot through expansion and will give higher numbers)
How many cranks on starter? (people generally say 5, not necessarily true.. have someone watch the gauge and crank til it stops going up)

[nelsonmxmarc]

Oh right.. Nissan did it.... God knows how they tested it.. I'm assuming a leak down test seeing as they should be better equipped..

[M-type]

yea nissan did it cold and dry this morning

i requested a leak down test tomorrow to confirm findings

but everyones elses is alot higher 150/155 issh

[nelsonmxmarc]

I see.. So the numbers are still quite low regardless, but you'll see a variance of 10 or so psi - again still quite low.. 160-170 is about stock, while 150 is generally quite high, its a good number to aim for..

Worst case scenario is a top end rebuild.. If you plan to daily drive this car next summer, i would certainly look into the most cost effective option of going about that, because what i'm seeing here is the beginning to a pretty typical top end job..

[redrocket]

im sorry this may sound dumb and i might be wrong but isnt the rb a closed pcv system. wouldnt opening the oil cap allow loss of pressure. out of the 8 rbs i have whether it be friends cars or my own they all want to die as soon as you losen the oil cap. is this not normal?

[BRN32]

Tested my motor twice, first test, cold and closed throttle got a whopping 60 across the board?!? Second test, was warm with a weak battery. About the - five turns(real slow,lol) produced 120 on first two cylinders before the battery puked.

I figure it'll be higher with a good spin, but will need a rebuild in a year or two.

[M-type]

waiting for a my shops dyno to be dailed in then im rechecking compression and dynoing it but the car runs great