MAF/MAP folks (TBI or TPI)

[dyeager535]

What kind of valve cover oil fill "plug" was GM using on the early EFI stuff?

Never thought about it before, but an "open" in a valve cover (non-stock breather or bad fill cap seal) will actually affect how your engine runs. Got to thinking about it, realized the later GM stuff is running screw in oil fill caps. Picked a Vortec valve cover set up this weekend due to this.

Tested it with my setup, sure enough, engine running, loosen up the oil fill cap, idle change.

Trying to wrap my head around this, would the MAP setups have the same problem?

 

[southernspeed]

I've noticed that, I guess it's the extra air it can suck through the PCV valve when the motor is open.
My 89 (ish) motor has the old style 'half twist' metal cap, my '91 has the plastic screw in 'ratcheting' type cap. Apart from vacuum sources which are in effect closed, the TB can't suck fresh (below throttle plates) air apart from the PCV. The idle changes when you stand on the brake too and it gets a mouthful from the servo. Another reason I don't like open element rocker cover breathers. I guess they introduce too much fresh air into the engine, reducing internal vacuum and allow more to be sucked through the PCV???

Damn Mr. Yeager, we're getting very 'deep' and thoughtful here!

 

[Can Can]

When you say the idle "changed", what exactly do you mean? Was it less smooth, slower, etc...??

Now you've got me thinking......

 

[dyeager535]

I'd have to go check again. I just loosened the cap and instantly noticed a difference. Tightened/loosened a couple of times, but since I knew it was "wrong" didn't leave it loose to observe the effects closely.

Of course, that means my truck would have to be up and running. It didn't seem to get worse per se, but it certainly changed. On the EFI setups though, even a dead cylinder can be "overlooked" because of the computer control keeping idle smooth. So while it may appear minor to the ear or tach, it may be a bit more severe. Would have to try it with a scanner hooked up and watch all the data.