SOLVED 1994 Volvo 850 B5252S 2.4 10v rough idle when warm, but fine when cold Topic is solved

[mazzaw12]

Hello everybody.

I'd like to share my experience solving a weird issue with my 1994 Volvo 850 2.4 10v Engine B5252S.

TL:DR the voltage regulator was ruining the fuel injection and messing with all the sensors. I changed and it miraculously
healed.

So, if that out of the way...

Last January we had some harder winter for Berlin, dropping below -10C (20F I guess) and it developed a strange issue.
On cold start it would run great, but after the engine temperature needle started moving it would start bogging and back firing and had almost no power until 2000rpm, like if I had my choke closed on a carb.
Over 2500 rpm it would run mostly normal for a few minutes and start misbehaving again. I'd get out of throttle for a few seconds and it was back to normal. When stopped It would go down to like 500 rpm and shake like hell. Before launch I'd have to rev it up a little or else I had no power. Revving it up in 1st gear until 4-5000 rpm would also fix it. So the solution was to drive like a maniac the whole time.

My first though was it could be a MAF sensor. It doesn't have one. So I changed the intake temperature sensor and it worked fine for a day ??? After that it became worse than before.

Secondly I cleaned on the PCV return on the intake that was full with oil condensation. No result. Searched Vacuum leaks, no result. OBD1 told me nothing other than battery voltage too high or too low. I ignored it at first because it always charged at around 15V.

I drove it like that for another 2 weeks until I was at work (I'm a motorhome mechanic) and was checking a battery on a motorhome and I had the idea of checking my own battery. It was showing only 28% life. But I had changed it last October, less than 6 months ago, how could it be? The charging test showed that it was charging with 16V and peaks of 16.5V, uh.
I decided to change the Voltage regulator because it was obviously not working before anything fried and with it came the hope that maybe that was the cause.
After that it started reading 14.5v charging voltage, cool. I took a shower (it's a dirt work to change the voltagem regulator without removing the alternator, but possible) and went for a ride and it started showing 11V ??? What?? How?? It was 14 right now. I suspected of the battery and drove to buy a new one, now it was at 12.5V, so no charging...
Also on the dashboard half of the warning lights were missing including the alternator one. I grounded the small lead to the alternator and every light came back on. I tried providing 12v directly to the alternator to excite it, but no change either

Then I took the voltage regulator out again to see what was my mistake. Below it there is a little metal tab that grounds it, I pulled it a little higher and it just worked. Charging with 14.3V, running smoothly since the last 2 weeks and even getting better gas milage.

I hope that my little story can save someone from this gremlins