idle hunting when restarting warm then, gets better

[pmax]

Originally Posted by aoncurly

If you're getting pops (backfire), it sounds like a mixture related issue. A couple of things come to mind when I had some similar problems. For the backfire and rough running, I took my car to a mechanic and he did something with the air sensor plate. It literally took him like 5 minutes. He adjusted something, and said, "It sounds much better!" Sent me on my way and didn't even charge me.
More recently, I was getting some general rough running issues and my mechanic first adjusted the mixture, but after it still did not feel quite right. Took it back and he said he checked my plug cables by putting a towel over the engine compartment while the car was running. He said it looked like a Christmas tree with all the crossfire sparks in there. He replaced my plug cables, checked the mixture and the car ran much better.
Mixture problems are tricky and are related to so many things.
You said that your exhaust pipes were replaced. I hope the O2 sensor bung was properly welded or factory welded, as introduction to air in the exhaust is going to mess up the O2 sensor readings.

Your mechanic's a keeper.

[pmax]

Originally Posted by francesconyc

I think i did fix it.
Finally found a 3mm allen key long enough.
I finally tried adjusting the rich/lean screw.
I didn't feel any notch as someone described, but i think i turned clockwise 1/12th or so...

now the car doesn't pop anymore on deceleration and it restart keeping the same 950 rpm all the time... I need to drive in more different conditions than today but today I drove 20 miles around the city and so far so good.

I'm genuinely shocked that such small tweak solved multiple symptoms at once.. wow

I still need to keep an eye on the hot-restart problems i had in the past, but I'm happier now!

Or you enrichened the mixture enough to mask the actual problem but in your case, the new exhaust likely required it.

Going back to the on/off nature, I'm guessing it's the lambda trying unsuccessfully to correct the above.

[Mtsurfposse]

The popping noise you heard was excess gas burning up in your exhaust. When you hear “the kids” crackling their way down the street they tweaked the ECU to extra rich.

As you experience different driving temps and engine heats up things change. I’ve had similar things happen and I won’t go down my rabbit hole as you seem to have an easy fix but something in the 10-20 minutes is how long it takes for the oil and the block and everything to reach temperature. So when your dialing things in again, get it all good and hot first

[francesconyc]

so driving even more these days i basically noticed this:

Only during cold start warm-up:
• When driving slowly in 1st/2nd, then clutching in or coming to a stop:
• RPMs drop sharply (to ~400) then recover immediately to ~800–900
• Once engine reaches operating temp (~210°F), this behavior disappears
• Warm idle is perfect sticking at 950 ish (need to borrow a strobe gun to say for sure)


I wonder if its also due to the fact they my Deceleration Valve is disconnected / hose capped– like many POs used to do.

it's literally a problem of the first 5-10 minutes until the temp reaches 210F then it;s gone, so it;s not a big deal. but i also wonder i maybe rising the idle RPM screw can help a bit

[kltarga72]

You are on the right track. Drive the 911 a good 15 minutes before you adjust the fuel mixture. Write down the symtoms, and make small tweaks like you mentioned. Write down which direction (rich cw or lean ccw) and how much you turned the mixture screw.

I had the same problem with the hunting idle, mine was eventually fixed by going lean. Again make small adjustments, drive for several miles. It took me several weeks to fine tune the fuel mixture, check and adjust ignition timing, also fine tuning the idle adjustment screw. Test with a cold and hot start noting if any thing changed.
I also made sure that all fuel pressures were correct and no vacuum leaks prior to making adjustments.

[AndrewCologne]

@ Francesco
Read the full page:
https://nineelevenheaven.wordpress.com/the-911-sc-3-0-engine-with-catalyst-and-lambda-control/
If you follow all steps your car will run like when it left the factory.

Adjusting the CO on a lambda-based SC without a CO gas tester:

Drive the car until the engine reaches full operating temperature.
Connect a digital multimeter (with duty cycle measurement capability) to the test port on the left side of the engine compartment — red lead to vehicle ground, black lead to the green/white wire of the test port — and observe the duty cycle at idle.
It should fluctuate around 50–55%.
If it doesn't, adjust the CO mixture screw as follows:

-> If the duty cycle reads lower, the initial mixture is too rich → turn the CO screw counterclockwise (to the left).
-> If the duty cycle reads higher, the initial mixture is too lean → turn the CO screw clockwise (to the right).
After adjustment, check cold-start behavior again.


About the idle drop ...
If the idle speed drops extremely low during a cold start, the missing deceleration valve is not the cause.
I have mine disconnected as well — my CDI is controlled by a digital, map-based ignition unit that relies on full manifold vacuum to assess engine load across the entire range.

So, a missing deceleration valve only causes RPM to drop more quickly to idle — not below it.

Have you checked your auxiliary air regulator?
Make sure it opens properly during cold start and fully closes once the engine is warm.