ULTM8Z

So the more finely tuned I'm getting this car, the more I'm noticing other things... (that I want to fix before I release my next Miniram start bin).

There's a slight degradation of idle quality that seems damn near proportional to oil temperature.

The ignition coil upgrade I posted about several weeks ago improved the idle quality, particularly noticeable at colder oil temperatures.

Which is why I think this high-oil temp thing is more pronounced than it was... just a more noticeable contrast between hot and cold oil temps.

Basically, when max coolant temperature is reached, but oil is still "cold", is when I have the best idle quality. From there, as oil temperature increases, idle quality degrades just enough to be annoying.

It dawned on me that maybe the proliferation of oil vapors increasing with temperature may be causing the issue and that the PCV valve may be letting them get into the air stream at idle.

So question is, anyone with a performance camshaft, what are you doing for a PCV valve? I'm currently running a factory TPI valve, but I'm thinking that may not be the best choice.

BTW, I don't think anything in the calibration drives fuel delivery vs oil temperature (unless I'm mistaken). But it wouldn't matter anyway since I don't have a oil temp sensor hooked up to the ECM. I have a sensor, but it's only for my dash gauge.

ULTM8Z

Ok, so I just proved to myself the PCV has A LOT do with the tune.

Had a couple of PCV valves laying around the garage and tried them out. One had no effect (I think it's the same p/n).

The other one made the car run way worse...idle quality deteriorated badly... in fact my 128 BLM shot up to 150 at idle and was all over the place in cruise. Retuning for 128 at idle did not smooth the idle back out.

I also had an 1970 LT-1 valve... but the valve flutters like crazy and causes an erratic idle.

Also, when I disconnect the valve altogether and plug the vacuum line, the idle smooths out quite a bit at hot oil temps, though not much effect on BLM.


So the question then is what is the ideal valve to run with moderate cam that's more radical than stock? I'm pulling about 12 inHg in gear and about 14-15 in park, for FYI.

JP86SS

iTrader: (1)

PCV is a controled air leak drawing fresh air into the block and controlling by orifice the amount drawn into the intake.
Different size engines have different needs for the amount of air "flushing" needed.
I don't have any specs on them but one valve flows different than others as I have toyed with some I had laying around.
I run the stock TPI valve only because I don't know what else to use.
It needs to balance how much TB blades are open against PCV flow and IAC steps.
Its another game to play, minimizing the PCV to only what is needed would be best practice.
Not sure how you prove what amount that is.

ULTM8Z

Actually, it dawned on me earlier this morning what the problem could be (or at least a major contributor).

Remember that separate little IAC manifold I built for my Miniram to “slay the split BLM beast”? Not too long ago I resurrected that thing and hooked the intake hose up to the PCV valve figuring it’s just another source of manifold vacuum and it would quiet the loud sucking noise through the small tube (which it did).

Problem is, since the idle air is now drawn almost exclusively from that source and delivered to the cylinders completely independent from the main plenum, my overall idle air temperature is NOT at all reflected by the IAT sensor in the intake tract. Put another way, If my oil temperature is hitting 180-200°F (which is probably pretty close to the air temperature inside the block), and the IAT sensor is telling the computer the air temperature is only 100-110°F, that’s a huge discrepancy in fueling.

Granted that other PCV valve made the car run worse, but disconnecting the PCV completely from the intake manifold made it run markedly better (probably because having the IAC hose pull air directly from the environment much more closely matches the reported temperature from the IAT). It could be that the current PCV valve is in fact correct (I think it’s a factory L98 that I bought a long time ago), and the other one was incorrect (have no idea what it is). Which would mean it’s simply a matter of re-routing the IAC hose to a fresh air source.

Ugh, this is what happens when I deviate from factory configuration… I end up chasing my tail on essentially R&D….

Hopefully this weekend I can make the modifications and see what happens.

Six_Shooter

iTrader: (2)

This is definitely an interesting discussion and look at other aspects of tuning that are not always explored.

Keeping an eye on this one and think I will try some of my own experiments.

ULTM8Z

You're not kidding... I seem to get all the off-the-wall problems...

Dominic Sorresso

Well they are "good" problems which contribute to the hobby.

Back to the topic. On the LT-5 crankcase, venting is an issue given the higher rpms of the motor. We actually have 2 PCV valves drawing oil vapor from each injector housing which then gets re-routed into the plenum. Recently, I have installed a catch can significantly increasing the hose lengths before arriving back at the plenum. I assume now that the vapor has a good bit of time to lose heat. This has made a fairly dramatic reduction in how sooty my exhaust tips get. Also found out (inadvertently) that I had a slight air leak at the catch can and re-installed with a much better seal. Helped the idle. In fact, the LT-5 has a notorious idle hunt where the PCV fittings are usually the source. All connections get zip-tied.

Fast355

iTrader: (2)

I run a 5.3 fixed orifice PCV valve in all my engines. Just ask for a 2003 C1500 5.3 application, cost about $2.00 at O'Reillys or Autozone.

eseibel67

The problem with those fixed hole valves is that they whistle.

Fast355

iTrader: (2)

Not the ones I have run. The chattering of the stock valve was way more noticeable. My Express is dead quiet and the valve is sitting right under my right foot.

ULTM8Z

Thanks for the tip. I may try that.

Fast355

iTrader: (2)

Fixed orifice is on the left.

RBob

iTrader: (1)

I saw that checking the pictures on Rock Auto, just an empty shell with a small opening. It also appears that there isn't any other control device, such as a solenoid to control the ventilation during decel or such.

I wonder what the thought is behind GM using that type of valve.

RBob.

ULTM8Z

Probably just figured, "hell with it"...

Six_Shooter

iTrader: (2)

Probably customer complaints about a "rattle in the engine"...

ULTM8Z

I tried the fixed orifice valve. Didn't make too much difference.

But, per the other thread with solving the voltage issue, that actually did make quite a bit of difference.