Magic Spool Bus
SILVER Star
I wanted to add to this mystery as well. Like @KiwiDingo mentioned I had this same exact thing happen in Colorado while descending a steep mountain pass on the highway. The truck would start just fine at high altitudes and would white smoke until warm and then it was clean exhaust again. Leaving Denver and coming down that pass I was in 5th and required even some brake pedal to not exceed 110km/h to give an idea on steepness. I started to notice something odd because the engine was so quiet it seemed to not even be running. I started adding some fuel pedal and had no power. Glanced back and lots of white smoke. I gave it even more fuel pedal and it backfired and blew lots of black smoke and then had power again. If I let it start compression-braking again I would go through this process every time until I descended below 8-9000ft of elevation. It pulled up hills and flats fine at this elevation and was only present when descending. This was 6 weeks ago.
Fast forward to now I had advanced my injection pump timing 1mm while back at sea level. This weekend I just went up again to 10000ft of elevation. When descending it tried to do this same thing again but no backfire and much less white smoke when trying to come back into throttle while descending but still the same issue. Once below 8000ft, it stopped again.
One thing I noticed is EGT pre-turbo was maybe 250-300F during these events so yes the engine is getting really cool. I also notice that I have this flat spot in power when going from no throttle to maybe 10% throttle. Hard to explain but imagine you are going downhill and you decide to give some throttle, the engine start picking up and then goes a little dead until you give it some more, and then it feels fine. I'm on a brand new short block from Toyota with maybe 15000 miles on the engine. I had my head reconditioned and bench-tested they were in great shape as the original engine only had 93000km on it. My pump and injectors are rebuilt by a Denso shop so I don't think it's my head or pump is the issue but I'm wondering if this flat spot is an injector related issue. This flat spot is exactly where it's white smoking a lot when doing this big compression braking on hills.
Definitely a mystery but I believe its main culprit is such cold cylinder temps. Advancing my pump timing seemed to help a bit though.
Fast forward to now I had advanced my injection pump timing 1mm while back at sea level. This weekend I just went up again to 10000ft of elevation. When descending it tried to do this same thing again but no backfire and much less white smoke when trying to come back into throttle while descending but still the same issue. Once below 8000ft, it stopped again.
One thing I noticed is EGT pre-turbo was maybe 250-300F during these events so yes the engine is getting really cool. I also notice that I have this flat spot in power when going from no throttle to maybe 10% throttle. Hard to explain but imagine you are going downhill and you decide to give some throttle, the engine start picking up and then goes a little dead until you give it some more, and then it feels fine. I'm on a brand new short block from Toyota with maybe 15000 miles on the engine. I had my head reconditioned and bench-tested they were in great shape as the original engine only had 93000km on it. My pump and injectors are rebuilt by a Denso shop so I don't think it's my head or pump is the issue but I'm wondering if this flat spot is an injector related issue. This flat spot is exactly where it's white smoking a lot when doing this big compression braking on hills.
Definitely a mystery but I believe its main culprit is such cold cylinder temps. Advancing my pump timing seemed to help a bit though.