If caught in stop/go traffic in hot weather the engine goes into a weird limp-home mode where it says full or normal power is not available, but no check engine light present.

It takes 10-15 min to cool and the engine starts like nothing ever happened and all is well.

The temp gauge does not rise above normal, and the cooling fan operated normally.

Ring any bells?

Thanks-Bob

“... Ring any bells?...”

Yes it does, however in our case it was accompanied with a “Reduced Engine Performance” message on the DIM. Shutting the engine off and restarting would clear the problem and them it would repeat a few minutes later. Worse in hot weather. Our problem turned out to be the aging connection at the ECM.

Check for fault codes as Klaus suggests. Especially look for any related to throttle position.
Some things to try that don’t cost much money:

Remove the throttle body and clean it thoroughly.
Inspect the wire harness where it connects to the throttle body.
Remove the ECM, spray the contacts with electronics cleaner and reinstall - reseat it and unseat it a couple times to scrape the pins clean. ($15 tool required)
When the car is in limp mode check the fuel pressure at the injector rail schrader valve. A cheap Harbor Freight gauge is adequate.
Resist the temptation to take the car to a mechanic that wants to start out by throwing expensive parts at it.
Let us know what you learn.

Hi, Thanks!

Yes, I scanned the car today and got 6 ECM codes, mostly throttle position, and other throttle body and sensor codes, plus an implausible Mass Airflow reading.

I unplugged the throttle body plug and sprayed it with contact cleaner (the plug feels like it slips in w/o any force, making me think the female contacts are not gripping the male pins well) and re-inserted it several times.

This is at least the second ETB the car has had installed. I replaced it 2 years ago, and noted at the time the retaining "clip" that locks the plug into the ETB was broken off making me think I was not the first person to visit that area so I used a tie wrap to hold the plug FIRMLY and fully into the ETB.

For good measure I sprayed the MAF sensor with MAF Cleaner, but of course today it was already running like a Swiss Watch (or these days a Seiko).

I reset the faults and will see if this resolves the problem for no I guess.

Bob
BTW, I had a 2001 S60 T-5 a few years ago that ate electronic throttle bodies like popcorn. Volvo paid for the last one under their extended warranty but only after I did a video of my "Live Data" to show them the problem. Up to then 4 service visits with the problem were only "could not duplicate" with an offer to replace my throttle body again for ANOTHER $1800 of my money since they could not determine it was bad (but the Live Data video made them say uncle). I loved the car, but after 2 transmissions in 110k miles and the current one could not be made to shift properly despite several re-flashes and erasing adaptations, I reluctantly let go of it. Of course back then the factory was still using carbon track TPS and not Hall Effect Sensor units. Hopefully my new ETB is a Hall Effect unit in the S80.

My understanding is that Volvo sells the throttle wire harness with connector. I don’t know if it’s pig tailed and you solder it in or if it’s the complete run that you re- pin into the ECM connector.
Those Bosch throttles are normally quite reliable. Hopefully you’ve found the issue with the loose connector. I had to tie wrap mine in also due to a broken catch.

Do you own a scanner? Plug it in on one of these drives and see if there are any 'pending' codes. My old 850 does not like to creep along in traffic at idle speed & 1st gear - it says my cat is not functioning correctly.

It could also be that the ATF is too hot...
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Keeping it running is better than buying new

Yes, definitely check the condition of your ATF. My 93 850 went in to limp home mode when the fluid would overheat. No check engine light. As you said, after the car cooled everything was fine.
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Will I buy another Volvo??? We'll see....

Yes, that's job 1 for today. I'm hoping something is stored in memory-possibly a non generic "P" code that is proprietary my Autel Maxi-Das will read.

I know lots of things that don't trip a CEL will be held in memory.

Strange "limp home" where the engine stays almost at idle. Never had a limp-home before, but I thought it would allow maybe 30mph. I was lucky to keep it running to get it to the shoulder to let something (not the engine which was mid-scale) cool down.