I’ve got an issue or potentially a few issues with a 1993 Volvo 850. As soon as it hits operating temp, the fuel pump relay clicks off and the car dies. If you let it cool off a couple minutes it’ll start right back up and drive fine until you hit operating temp again.

The fuel pump, relay, and filter are new in the last 200 miles (problem did not go away after changing them)

The only code is for the MAF sensor
It will barely run or sometimes not run at all with the MAF unplugged

We’ve proven the temperature correlation by driving with no thermostat for a while. It’ll drive as long as you want below operating temp

What am I looking at here? I’ll order a (rebuilt) MAF sensor ($200 ugh) and I’m thinking maybe camshaft position sensor? I’ve heard about the cam end seal getting moisture in the cap but again it’s like a light switch. You hit the certain temp and you’re off, you cool a couple degrees off and you’re back.

 

Closed loop usually means when the O2 sensor takes over (minimally) controlling the mixture. It's in "open loop" when first started. Usually after a minute or so, once the control unit thinks the cat is hot enough to do its job.

What do you consider to be closed loop? Have you tried driving with the o2 sensor disconnected? (although I think your definition of closed loop is not what i think it is)

Cam sensors for those cars do fail - but there is generally always a code - and with those (sensors) if there is a code and even if the car is still running - I would strongly suggest to the customer to replace the cam sensor. Have seen too many fail.

 

I seem to be mistaken then, I thought these OBD1 systems were in open loop until you hit thermostat temp, say 185. That’s how my 1988 Dodge is with TBI. My mistake for assuming that.

In that case, let’s just say the issue happens around 180-190 degrees.

As I said the only code (I checked fuel and ignition codes) is for the MAF sensor, but considering how it will not run with the MAF unplugged there’s something more here.

 

That's certainly possible - my experience fixing Volvos for a living - I relied on what I saw. A customer complains that the car starts (cold) and drives down to the stop sign then stalls - we find the O2 sensor (when the car changes from open to closed loop) made the car stall. A new o2 sensor solves the problem (this was years ago, mid 1980s)