idle hunting when restarting warm then, gets better

[francesconyc]

I’ve read many articles about this issue, and before I take the car to the mechanic (for other work), I want to understand the problem better so we can zero in on one thing. I trust my mechanic, but they’re human—and despite their experience, they’ve sometimes missed the culprit on the first try.

Here’s the long-standing issue, which got worse after I replaced the entire exhaust system (new Sport Dansk muffler, catalytic pipe, heat exchangers, injectors, spark plugs, and the 02 wire that was nearly crumbling).

Symptom: Idle hunting, from 900rpm instantly goes down to 500-600rpm fluctuating (see video) after restarting the car once it’s warm. after few seconds seconds goes back to normal, and then after another few seconds it start the same flutuations, in cycles steady>fluctuation> steady> etc

Video: https://youtu.be/lmDKJHmeteM

When it occurs: Mostly in slow-traffic areas (aka poor cooling conditions); rarely after sustained highway driving (~60 mph).

Pattern: The longer I drive on slow roads, the worse it gets.

Duration: the entire cycles of steady idles/fluctuations lasts 1–5 minutes before returning to normal

Cold starts: Never a problem—only when the engine is warm.

Driving: besides the Idle the car runs great, super responsive, great acceleration. only the idle is annoying.

It feels like something “digital” or sensor-related is telling the engine to go richer/leaner—some posts mentioned the WUR, for example. I’m inclined to rule out an air leak or failing gasket, because those would cause rough running anytime the engine was warm; in my case, it always returns to a smooth idle.

That it only happens “after a while” makes me wonder if a temperature sensor is at fault—if it were purely a temp issue, I wouldn’t expect the reading to normalize so quickly. Bottom line: my prime suspects are the CDI box itself, the WUR, or the O₂ sensor.

ps. maybe my idle in the video s lower then the desired 950, i can def check with a timing light, and i can adjust the idle screw to make it higher and see if it "solve/patch" the problem for now

[almostordinary]

Video isn't showing up.

I'm also on the hunt for a similar issue, that my car has experienced in the past. I tend to go overboard as I love to service this car. I am just down to replacing the CHT sensor, as this and the speed sensor caused my very similar issue in the past.



My CHT threads weren;t the best, so just waiting for a M10x1.0 tap to clean the threads and reinstall

[mysocal911]

Originally Posted by almostordinary

Video isn't showing up.

I'm also on the hunt for a similar issue, that my car has experienced in the past. I tend to go overboard as I love to service this car. I am just down to replacing the CHT sensor, as this and the speed sensor caused my very similar issue in the past.



My CHT threads weren;t the best, so just waiting for a M10x1.0 tap to clean the threads and reinstall

Two different engine systems; his is CIS & yours has a DME ECM.

[almostordinary]

oh thanks for clarifying i'm in early morning skim mode.

[PeteKz]

Usually too much idle timing. Check the timing at idle and at high RPM, with and without vacuum hoses connected, to make sure the dizzy is working correctly.

[AndrewCologne]

@Francesco

You have a lambda based engine management, correct?

Did you followed the proper initial setup instructions?
https://nineelevenheaven.wordpress.com/the-911-sc-3-0-engine-with-catalyst-and-lambda-control/

If with correct a set up the issue continues, then ... attach a DMM .. to pin 2 to the ECU connector below the passengers seat and check if the voltage of the o2 sensor while your problem occurs and check the voltage, ... it should swing within 0.1 and 0.9 volts.
If not then check the duty cycle at the test port in the left engine compartment (also explained in the link above) ... I guess your ECU is driven mad cause of a non proper initial CO setup.

[francesconyc]

Originally Posted by AndrewCologne

@Francesco

You have a lambda based engine management, correct?

Did you followed the proper initial setup instructions?
https://nineelevenheaven.wordpress.com/the-911-sc-3-0-engine-with-catalyst-and-lambda-control/

If with correct a set up the issue continues, then ... attach a DMM .. to pin 2 to the ECU connector below the passengers seat and check if the voltage of the o2 sensor while your problem occurs and check the voltage, ... it should swing within 0.1 and 0.9 volts.
If not then check the duty cycle at the test port in the left engine compartment (also explained in the link above) ... I guess your ECU is driven mad cause of a non proper initial CO setup.

oh interesting, i will try the ECU voltage test!

I believe the dizzy/timing etc is correct (I remember checking it a while ago abd also mu mechanic i think they did)


to me it seems definitely a richness mixture problem: also because i forgot to mention that my exhaust system (i told you i replaced all exhaust and new Dansk muffler Sport) now pops a lot after I get the symptoms mentioned above (which i love it haha, but i understand it maybe be due to gas to be too rich? right?)

what else regulates the richness/lean of the mixture? is it the ECU only or also the WUR?

[francesconyc]

Originally Posted by AndrewCologne

...your ECU is driven mad cause of a non proper initial CO setup.

what do you mean by this? you mean that i need to setup like in this video?
https://www.youtube.com/watch?v=Z5OGEADEB1Y


the thing is the car starts great, great idle 950, also it runs/accelerate great, regardless of those symptoms.

if my problem was richness/lean, wouldn't I havethe same issue all the time (at start, wamr, hot, accelerating...) and also including when driving above the idle rpm?

my issue is literally just the idle....

[aoncurly]

Your idle hunting symptoms are identical to what my 81SC had. For me, it was a bad O2 sensor. You said you replaced the O2 wire, but what about the sensor itself? After replacing the sensor and readjusting the mixture, the idle was fine.

Was the O2 wire replaced at the loom end where the plug (on left side of engine compartment, not at exhaust end) is? My understanding is that wire is shielded and can be difficult to splice correctly. Although not related to the hunting issue, I also had my plug end connector replaced as it was crumbling, and my mechanic said splicing the wire was very difficult. He had to cut into the loom to properly connect the new plug. The crumbling plug and wiring caused other mixture problems, like surging and just general bad running. After replacing that, and readjusting the mixture, my engine ran much better.

Good luck.

[AndrewCologne]

Originally Posted by francesconyc

what do you mean by this? you mean that i need to setup like in this video?
https://www.youtube.com/watch?v=Z5OGEADEB1Y


the thing is the car starts great, great idle 950, also it runs/accelerate great, regardless of those symptoms.

if my problem was richness/lean, wouldn't I havethe same issue all the time (at start, wamr, hot, accelerating...) and also including when driving above the idle rpm?

my issue is literally just the idle....

I mean, do a proper CO initial set up.
Look for service garage where they have the professional equipment to do that.

[proporsche]

your idle is too low on the video-i`d disconnect the oxy sensor and see what is up.Most likely running rich

Ivan

[francesconyc]

Ok, so I did a trip to the suburbs of Philly last week — 75 miles going and 75 miles back.

Trip notes:
• It was a very hot day, like 88°F or so.
• I keep the car in an underground garage with a good cool temp.
• When I start the car in the morning, it starts right away. At warmup start, the idle goes up to 1200–800-ish for 30 seconds or so, then adjusts slowly and sticks to 950.
• From garage to highway, all good — the idle was always sticking to 950 like a clock.
• On the highway, always good for 75 miles, idle perfect at every single traffic light and gas station.
• Parked the car, went to lunch/strolled with friends etc. for 3–4 hours.
• Started the car to go home, all good — no roughness, idle at 900-ish.
• Back to highway/cruising, all good, idle perfect at every stop at 950 again.
• During the full trip, the oil temp was always at the first white mark (I think 210°F?)

⸻

Yesterday I drove the car again to the city.
Lots of traffic and never able to cruise (1st–2nd gear only). Roughness came back again — but mostly only at the first restart (this is odd).
E.g., I drive 5 minutes — all ok. I stop and restart right away — it gets rough. It’s almost like the “reboot” goes wrong…

⸻

A few days ago I also made a couple of quick tests:
• I removed the oil cap when the car was warmed up, and the car almost dies (which I read somewhere it shouldn’t).
• During the rough idle, I removed the connector of the fuse in the front seat of the Jetronic box — the car died right away.

⸻

My takeaways:
• The temp doesn’t matter much.
• Regardless of whether the engine is warm or not the roughness seems to always happen at restart, never on the first start of the day.
• It feels like cruising (a steady throttle at 2–3K rpm) helps “cleaning.”
• During the roughness period, the muffler POPS a lot — maybe because the mix is too rich?
• It seems something happens when not cruising, which I don’t think is temp-related anymore. Could it be the fuel? Fuel vapors?
• when the rouchness happens is worse at restart and it clears after 15 mins. after that there seem to be a cycle every 10-15 second cycles (15 seconds at 700-ish rpm, 10 second back to 950 ish) (maybe is the ecu reading and adjusting?)

⸻

My next thing to do:
• At this point, I’m gonna buy a new Lambda sensor — which is cheap and im not sure when the prev owner changed it.
• Im gonna buy a rebuilt WUR. mine is 43 yrs old. time to change it no matter what.
• Since im in the upgrade frenzy, i may buy an AFR gauge since at this point i also have to do other works at my mechanic shop next month.


Questions: can someone explains what is actually happeing from the electronic standpoint? i think at this point i knwo how CIS work but this part is missing.
- is it possible the the box in front needs to be replace os it is unllikely? whats indie that box?
- is it possible to totally remove disconenct the o2 senror? i dont need to pass the emission test anymore, so why bother?

[francesconyc]

Originally Posted by proporsche

your idle is too low on the video-i`d disconnect the oxy sensor and see what is up.Most likely running rich

Ivan

yeah i know i adjusted since, now is perfect 950

[francesconyc]

Originally Posted by aoncurly

Your idle hunting symptoms are identical to what my 81SC had. For me, it was a bad O2 sensor. You said you replaced the O2 wire, but what about the sensor itself? After replacing the sensor and readjusting the mixture, the idle was fine.

Good luck.

my conenctor was crumbling and i replaced wiht one sold by Sierra Madre which is much easier

i also just ordered a new sensor. good call

[AndrewCologne]

Originally Posted by proporsche

your idle is too low on the video-i`d disconnect the oxy sensor and see what is up.Most likely running rich

Ivan

In his case the frequency/interval of his oscillating idle fast, means its typical for a leaned out mixture and is the exact same rough idle when the OX relay fails.
Too rich mixtures at idle result in an idle which oscillates slower.

[AndrewCologne]

@francesconyc

Did you check your OX-Relay? Maybe it sometimes fails due to an inner loose contact/connection

[francesconyc]

i'm def starting to think it's running lean. i think my mechanic replaced all the muffler / exhaust pipes they didnt do a full fine-tune , im gonan try to push 1/8 the adjustment scre to make it tad richer and see

also watching a bunch of videos of lean vs rreach sounds it sounds not about surging but the oppsite.. i think we are closer to the culprit

[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.

[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!

[francesconyc]

ok one more thing i noticed:

now that the popping is completely gone (maybe a very light gurgle once in a while) and the car restart perfectly when warm


one think i fele is different is when i start the car from totally cold (sitting 24h or more) is that the idle is a bit low 750-850 (cant tell for sure) until the car gets warm.

if i understood correctly, when the car is warmed up the o2 sensor start working right, but not before it reaches a critical temp?

so i wonder what it that regulates the car when warmgn up is it the fmaous WUR?