Won't idle with oil cap on...ideas??
 

Built a fresh 408 L92/L76 and seemed to run fine, just really rich. Initial dyno tune laid down 500/451 rwhp/rwtq at a rich 10.5 AFR. Now the shop tells me that it wont idle unless it has the oil fill cap off and everything seems ok, but he says that it's not even showing the iac counts in the datastream? It's supposed to go back to Jim at Speed on thursday, so I'm not real sure where to go from here... If you guys have any ideas, please help! Thanks, Jess

Check your PCV system for restrictions. I've seen this happen on other types of cars, so just a thought anyway

According to my shop, its set up right. I sent him a pic of how its supposed to look. So if thats the case, not sure where else to look...

I don't really know what else would have anything to do with the oil cap on one of these cars.. I am going to look at a few things and see what i can come up with. at least a few ideas anyway

Alright, I appreciate it. The pcv thing was what came to my mind as well, but being as I'm in CA for a couple more days, its kind of hard to run and look at it with him...

Ah something just hit me. you said your IAC was not counting right? Find out if it was just stuck closed or open, If its closed it could very well be your problem... and it could be trying to suck the air out of the crank case because its stuck or failed closed

basically since the computer cant compensate for the un metered air it needs and it being closed up the vacuum in the intake is pulling it though the pcv system and when you take the cap off there is more air there for it to breath so it idles up

I'd check the IAC motor as well but I experienced a very similar issue when I installed the blower on my car.

I didn't have any restriction in my pcv system, in fact I didnt have a pcv valve at all. I just had a hose running straight from the tb to the line on the valve cover.

With the car running, it created so much vacuum that you couldn't remove the oil cap and the car would make a screaming noise similar to a really squeaky belt.

So, I would recommend double checking the PCV system, make sure you dont have too little restriction as well as too much.

Thanks guys! If any more ideas pop into your heads, let me know. I let my guy know about these two...

Sounds like the PCV is fine, it's the other half of the crankcase ventilation that's an issue. If you're not getting any air into the crankcase, then there's no actual airflow through it. Once you seal the crankcase up by putting the oil filler cap on, the airflow ceases. I'd be checking the crankcase vent on the other side.

So which port is the crankcase vent? The rear ones on the valve covers are the ones that pull air out of engine, right?

The front port on the passenger side valve cover is the vent. In stock form a hose runs from that to the tb. I have a short hose and a small filter on mine.

Just had a similar.issue the pvc hose and valve cover vent hose got switched. So it is has a vacuum leak with the cap off or a breather installed. With it on it pulls the crank case into a vacuum. If the car was tuned with the leak the iac won't be able compensate. I could get 12in of vacuum in my crank case with it sealed up. Pvc should go to intake vacuum port. The breather vent should go to the tb. I believe.

Alright, thanks guys. That's kind of what I was thinking and my shop knows about it, so hopefully it's that simple. Thanks again, Jess

https://ls1tech.com/forums/generation-iii-external-engine/540617-where-pcv-valve-98-camaro.html here is a post of the pcv system on a 2000 ls1

Just talked to my shop and the hoses were flipped, so now the iac can hardly keep up to let it idle. So Jim at Speed will have his way with it on thursday and should be able to lean it out and make some more power from it. Woohoo!

Good deal.

The PCV is supplying air that the IAC can't or won't. Questions being
why it needs that much more air, or if the air needs haven't changed
but IAC function did.

Turns out they had the PCV and breather hoses flipped at the front of the engine. Had it's final tune today and Jim got another 16 RWHP bringing the total up to 516. So not too shabby...

How's the idle?

It idles pretty well, just a little high for my taste-about 1200 hot, 1700 cold start...