3.2 surging cold idle

[inaminit]

I have been reading old threads about 3.2 warmup issues, but I have not found any discussions that relate to my problem: The cold idle at startup will surge between 1000 and 1300 rpm for about 35-40 seconds, then settle out at about 1300, then gradually drop down to about 900 as things warm up. Runs fine after warmup.
1986 3.2 stock engine transplanted into a 76s. Control box gone through recently by Ingo. Rebuilt AFM swapped in-no change. Swapped in a spare idle control valve-no change. Cleaned grounds-no change.
Now I am going to get serious - I have bought an AFM meter, an oscilliscope, and a breakout box. I need some ideas on a test program. It seems to me that the 35-40 second surging period is a clue, but I havent found anything that points to a particular malfunction that only lasts that long.
Please give me some test program ideas-thanks in advance!

[Michael7810]

How old is your O2 sensor? I had a similar problem in my 944T that went away when I changed the 132k mile original O2.

[GH85Carrera]

I bet it is a vacuum leak. Just a guess. Mine used to do the same thing. When I dropped the engine to do the rebuild we found the intake was surly leaking as some of the intake bolts were not tight. We speculated that cold, the air leaked, but as soon as the heads warmed up, it tightened up that tiny bit to seal the leak.

After the rebuild, same electronics, with cleaned injectors and and a fresh rebuild it idled just fine. Stone cold at 17 degrees or blazing hot after a long day of running at the track. Like a modern car does.

[917_Langheck]

How many turns out from closed is your idle air bypass screw - the one under the yellow cap when the car was new?

The idle air control valve is having to over compensate for too much bypass air for that idle situation, thus the initial hunting.

[76FJ55]

I vote air leak as well. those are very similar symptoms as I had on my 84 once I got it running from its 12 year slumber. turned out to be leaking intake manifold gaskets. This is a very common problem on the 3.2 as there is a plastic insulator between the head and intake runner at each port with a paper gasket on each side.

your 40 second initial time may be associated with the warm up time of the O2 sensor before the system goes closed loop and starts compensating for the unmetered air, but other than that I doubt it is driven by an electrical issue.

[NotStock]

I just fixed a very similar problem. I went down the same road as you with my 3.2 swapped SC. Oscilloscope, wideband, the whole deal. The oscilloscope is handy for injector and spark issues but I doubt you have either. For me, the jury is still out on the helpfulness of the wideband without datalogging.

I say all that so that suggesting its probably a vacuum doesn't come off as the default answer. Vacuum leaks caused the same symptoms in my car of fluctation in idle at start up that went away when warm. I fixed the leaks and now it fires up and idles steady. Like you I tried two AFMs and two ICVs without any change.

I'd suggest getting a smoke machine. You don't seem to be scared to buy tools and jump in, so I'd just go straight to the smoke machine first. I did it and I am a smoke machine convert. My brake booster line was a huge vac leak and the thermoswitch on the back of the intake as well. All found with the smoke.

I hooked the machine up to the fuel pressure regulator vac line. I removed the airbox and made a block off plate out of 1/8 aluminum that attached where the airbox bolts to the AFM. Suggest starting there, getting it sealed up, then going further if you still have issues.

Something I did a lot while troubleshooting was to check the temp on all the exhaust flanges with an IR gun. Now that the car is all dialed in the exhaust flanges all are within a few degrees of each other when running. In hindsight the driver's side flanges were hotter back when I had the brake booster leak. And when I was chasing spark issues the dead cylinders were cold (but thats a whole other issue). Anyway, might be a helpful quick check for you. I know I checked it a lot while I was in the thick of it.

Good luck.

[NotStock]

Can't imagine it causing surging, but is your idle switch working? It messes with timing at idle. Likely not an issue, but worth verifying that your car knows the throttle is closed.

[Discseven]

[Cairo94507]

Discseven - Nice video, thank you for posting.