Inconsistently low RPMs after idling for several minutes
 

In the few weeks I've owned this car, I'm noticing a pattern of poor idle behavior if the car sits for a few days and the engine is cold and the behavior seems inconsistent. The behavior also never happens immediately on start; starts fine. After about 10 minutes, the car will sometimes drop down below 900 RPM like it was going to die off. Sometimes it does, sometimes not. Yesterday was difficult. The car sat for about a week. Started fine, pulled out, idled fine. After a period of time, again maybe ~10 mins, RPMs dropped down again and this time shut off. When I tried to restart, when it would finally try to turn over it barely wanted to run. Pumped the accelerator it would finally run and then I had to nurse the throttle to get it to not die - car stuttered and backfired. I continued to give it a little throttle for a few minutes and then, all of a sudden, it just kicked above 1,300 and stayed there

. I drove it around town and it was fine. Restarted fine the rest of the afternoon.

I'd like to get to the bottom of it to avoid being stranded somewhere and looking for some advice. Not sure if it's a tune issue? Plugs, coils, MAF, TPS? Get a compression test? The car has a dataloggit system and FC Commander and assuming that may be helpful in getting some more data. I can post a video...

https://cimg6.ibsrv.net/gimg/www.rx7...47bca132c0.jpg
Single turbo on RP Street-Spec port
Rebuilt / competition Apex seals 2003
550cc/1600c fuel rail system with RP competition fuel pump
Jacob's Rotary Coil and ProPak
MSD DIGITAL DIS-2
Last tuned, according to records, in 2003

Appreciate the advice, as always.

What is vacuum? Afr? Timing? When this all happens?

you may have more than one problem.

your start from cold sounds like it is related to the cold enrichment settings. generally you have extra fuel provided on cold startup and as the coolant warms the enrichment goes away. at that point your tune sounds like it is lean. what is your AFR when the idle drops? just add fuel.at that point. log your enrichment and AFR... probably a simple fuel fix.

Thanks for the initial replies. Really appreciate the knowledge here.

Just pulling a few # ...
Today, pulled the car out.
Initial idle - 1350 to 1460 RPM
Boost -380 to -390
Timing 7 to 8

Around ten minutes in, there was a 2 min period (never stalled)....
Idle fluctuation - 900 to 1700 RPM
Boost -110 to -390
Timing 4 to 24

After that, settled back down.

Edit, see I didn't get AFR results. Will test further and report back.

Heatsink, high inlet temperatures and oil contamination of intake/idle valve can mess with things. I'm limping my factory turbos through till I buy a place with room to pull the car apart easily, I am getting intermittent stalling due to oil contamination of the intake. I thought the throttle damper had failed to start with.

There's a lot to speculate about, and it's all speculation without data.

Based on your description of how it runs a while before the symptom starts, then goes a while before it suddenly stops. I'd say it's less likely to be a mechanical fault, and more likely to be tune related.

First, targeting AFRs of 14.7:1 at idle is a recipe for a surging/unstable idle.

The time frame you mention makes it sound like it's temperature dependent. Having it enter closed loop AFR control too soon can cause it to fight against the warm-up enrichment.

So look at datalogs and see if in fact the symptom occurs relative to certain temperatures. Look at your tune and see what events/settings are relative to those temps.

Also the fuel and ignition tables should be set to have very little change in the region of idle. If the values jump too much from cell to cell, any little change in load or rpm can change the timing or fueling, increasing instability.

It's also best to turn off closed loop fuel and idle control when you're trying to iron out problems like this.