Ongoing saga Volvo 850 overheating stalling under load
 

Volvo 850 1996 B5252 auto wagon imported from Japan to a Pacific Islands countryMy reliable 850 blew a head gasket 3 years ago, and I left it as it's too difficult here, no dealers etc. But 2 months ago i found a good mechanic and he changed it. Gasket now fine, water system now fine, fan comes on correctly, no overheating. No OBD2 codes (using a generic Bluetooth sensor and Torque Lite app).However, the trouble is it stalls and loses power when warmed up. Starts fine and revs high when not under load, cold or warm. When cold pulls as normal, starts up a steep hill, no problems. But as soon as the temp gauge is at operating (a smidge over 3 o'clock) it begins to lose power under load. If running on the flat it keeps going but will stall when trying to pull away quickly on the flat or on on a hill. In queued traffic I just release the brake and it will pull away slowly under idle but if I put my foot down it stumbles and stalls, same on a hill start. The engine will start straight away and rev right up to the red line, when in Park, but any load and it stalls. Once cooled down it is fine again. A trip to Australia and I have now replaced the HT leads and it does reduce the problem, the engine does run better, but still the problem remains to a lesser extent. The coil is also new, most of the sensors are new. It doesn't overheat. The head gasket is fixed, no signs whatsoever of any remaining issue there.Other symptoms - it tends to idle a bit high, 1000, and when warm sometimes settles into a higher idle maybe 1200. Also the ignition switch is on the way out, when you start you have to move the key back a touch to complete all the circuits (indicators and shift lock etc). But can't see how that could cause the issue only when hot.I'll change that when I can next Uk trip (it's hard to get things here). One other thing, it has no thermostat but its a tropical country. The cooling system seems fineCould it be fuel system? Hopefully not low compression?
Would really appreciate any help. Thanks!

dunno why you'd think a car will run better without its thermostat - there's a big difference to an engine at 80F than 195F... Have you checked the engine coolant temp sensor (it plugs into the thermostat housing - just follow the top radiator hose back to the block). I'd replace that, install a correct thermostat and replace the overflow tank cap for starters.

I am in the USA, and we did not get the 2.5 liters here, but of course the principles are the same. This is not really easy to diagnose. I think the HT leads are interesting; Since the engine does rev unloaded, it's not terribly obvious that you have an air measurement problem. Those are the two areas I would be interested in.

Lots of people have had a plugged up catalytic converter. That is something I have heard about, but I never had one plug on my own cars. This does sound suspiciously like it could be that.

Volvos, it seems to me, have an unusually low voltage ignition system. I can always tell, as a driver, if I have spark breakdown. I don't know if you can tell or not, but if you think that is the problem, I would suggest pull the plugs and see what you have in there. You need a very low voltage plug with a very low gap. That is the way the 850's were. It can be whatever the manufacturer recommended. i don't have any recommendation except to say if it's iridium or platinum, I'd throw it away.

if it's not that, then it seems to me that you must have a failure to accurately measure air flow when it is increased, or a failure to deliver fuel at higher intake manifold pressure.

With Torque app you should be able to see how hot the engine thinks it is. You can also watch how much air flow is measured. Measuring fuel pressure under load is going to be a little bit more work.

Hi firebirdparts, thanks very much for those insights. I will pass them on to the technician. I'm using the recommended multiple ground spark plus that have three electrodes, and according to what info i have should be used with the gap as delivered. But looking at them, the gap is quite large, well over a mm from the copper central electrode to each of the three ground electrodes.

What could one do if it's the catalytic converter (beyond replacing it)

I remain puzzled why it pulls perfectly until warmed up. I note the other contributors comment about the thermostat (thanks!) we'll put the new one in, but then it will warm up quicker and reach a higher operating temp.

Will also replace the distributor rotor head and ignition switch.

I don't see any codes at all with my bluetooth OBD2 reader with Torque, it does say connected to ECU but I suspect it's not getting any data from it.