Wouldn't that point to the AAV being suspect or something else that is temp sensitive? It's been a LONG time since I had to R&D L-Jet, cracking open some cob web filled brain files here.

On a bone stock car, what could cause high idle during cold warm up?

Sucking air through the supercharger and the intercooler may be flipping this switch in the other direction since how the car has to suck through those restrictions.

Crap....the perfect test is disconnect the intake piping after the intercooler. Thinking out loud here, the IC may not be that much of a restriction, especially at idle.

 

Here's a photo through the AAV from another recent thread where the owner is doing a Top End Refresh on a '78. This is at rest without power to it. Thought it might be of interest since it shows the cold aperture opening.

 

Carl told me once if I ever wanted to see what my car in stock form would drive like that I could just remove the belt from the supercharger and drive it that way, so I'm assuming pulling air in through the supercharger assembly without it spinning must not be that restrictive, but certainly it's more restrictive than not pulling air through it. When I sent my supercharger off to have the case vented, ceramic bearings installed, and the vortex billet impeller, I just slapped the K&N filter onto the old intake pipe and drove it that way for a couple weeks - not the same car without the forced induction.

 

Correct, the car will run fine, but you can feel the loss in power sucking through all the equipment. So in a case of needing to do some quick R&D, something's might be wrong with the blower.....pulling the belt is an option.

I'm just pondering in a case like there where we are threading a needle where minute changes could affect the outcome, might be safer to go 100% in the other direction.

I'm not saying my methodology is the only way, this is just how I would approach it.

 

Careful that AAV opening shown in post 32 above is for a K-Jet car - I think the opening for the L-Jet is different.

 

Great picture, Pete. I think I can guess at the size of that opening and start at say 20% for my restrictor.
Thanks,
Dave

 

Hi guys,
I have two data points:
Flow restrictors in inlet tube to AAV: (Original warm-up RPM 2000 @ 70 degrees F ambient garage temp)
3/16" inside diameter restrictor (3/8" socket 1/4" drive): reduces to 1700 RPM
1/8" inside diameter restrictor (Rubber stopper with drilled hole) reduces to 1250 RPM.
I'm pretty happy with that, and only have a couple of minutes to evaluate because engine warms up and AAV closes electrically.
I left the 1/8" restrictor in place and will try a cold start tomorrow.
BTW in my excitement to try this I forgot to try 0 boost condition with SC piping disconnected. Maybe tomorrow.
Thanks again,
Dave

 

Dave, this is great. So it looks like you're on the right track with your diagnosis and what needs to change to achieve the desired result. What's the ID of the rubber hose from the AAV as I'll want to find something that looks good and functions well once you determine the proper aperture size.

 

Hi Pete,
I will measure tomorrow. I plan to drill a new stopper to 3/32" and see what RPM I get. At that point I will give data on hose and stopper. This will allow me to reconnect my idle TPS switch also, whether that is of any value or not remains to be seen. My target RPM is 1100 to 1200. We will see.
Thanks,
Dave

 

One more data point in my experiment.
Today's cold start:
I put a metal tube through the hole in my rubber stopper flow restrictor. The inside diameter is 3/32". My new warm-up RPM is 1100. I am going to try this a couple of more cold starts before I settle on it.
BTW the metal tube is a cut off injection needle from a Tony's Cajun Turkey Marinade Kit. It looks like a tremendous restriction, but I can still blow through that opening quite strongly.
The ID for the connecting tubing for the AAV seems to be about 5/8" although I'm sure it is some metric size. The rubber stopper I had on hand from Ace Hardware, probably a #1 or #2, anyway the large end is just under 5/8" so the tubing fits over it. I will update.
Thanks,
Dave

 

Hi again guys,
Well my current setup seems very consistent. It starts up cold (70 degree ambient temp) at around 1000 RPM. It gradually creeps up to 1100-1150 RPM then settles down to 800 RPM fully warm idle. I think this satisfies my target for a warm-up program, so I think I will consider this problem addressed pretty well, if not solved on a root cause level. I know there is air flow through the cold start system, so I'm pretty confident that it will function pretty well in colder weather too, but time will tell.

BTW, for those not too familiar with L-jet, here is my brief summary of the cold start system and the function of the AAV:

The cold start system consists of a separate air intake and fuel injector. The air intake flow is controlled by the Auxiliary Air Valve AAV based on its own internal timer mechanism and the engine temp. Similarly the cold start fuel injector is controlled by the thermotime sensor or switch also based on its internal timer and engine temperature, and only functions during cranking of the starter. Once each of these components have timed out (up to 3-5 seconds for fuel, and 3-5 minutes for air), the whole system remains essentially dormant until the next startup cycle. Of course the main fuel injection system will also be controlled by temperature and O2 sensor feedback, but these only apply after the cold start system gets the engine running. Earlier and later cars accomplish this process in a slightly different way, through different components, but this is the L-jet method.

Thanks,
Dave

 

Great work Dave. Did you end up at the 3/32" aperture/restrictor or something different?

 

I left it at 3/32" using the marinade injector needle I mentioned.


Thanks,
Dave

 

Well I need to give a huge shout out to Petza914.
Pete also has a 928 Motorsports supercharged 16V 928. His is K-jet, mine is L-jet, but we both use an auxiliary air valve to control the warm-up RPM of our engines. He apparently found the same high warm-up idle situation that I have described here. Based on my rubber stopper with 1 3/32" marinade needle orifice AAV flow restrictor, Pete designed and fabricated a beautifully machined aluminum 3/4" male-male hose coupling with a 1/4" central threaded hole that receives Holley main jets. He sent me several jets to choose from: sizes 70, 80, 90, and 100. These are orifice size in thousandths of an inch. I screwed in the 100 size which is close to my marinade needle size. I installed his restrictor in my AAV tubing, and it works great! I get my nice, stable 1100RPM cold idle that gradually levels off to 800 RPM when warm.
I wish I had taken a picture before I installed, but I always forget "before" pictures of any of my projects. Maybe Pete has a picture of his device, because it is a thing of beauty. Many thanks to him.
Thanks for all the interest, and once again I see what a great group of people are here on Rennlist,
Dave

 

Dave, thanks man. I'm glad the concept you proved out worked and thanks for doing the leg work to test your theory and help us out.

I actually found a guy on ebay that makes reducing fittings for welders and told him what I was trying to do. He said he'd make something for me and it was his idea to thread it to accept Holley Main Jets (3/32" I think), which I thought was a great idea, as the variety of orifice sizes in those jets should allow for variations from car to car based on air moved by the supercharger, mixture settings, etc.

I haven't installed mine yet, but may tomorrow.

Here's some pics. I had a few made as I got a price break at 5. Sent one to Dave and one to Carl, am keeping a couple for myself, but have a spare if someone else is interested and I could have more made as well.