White smoke from exhaust during prolonged downhill engine braking on HDJ81 (3 Viewers)

[Magic Spool Bus]

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.

 

[roscoFJ73]

Citroen diesel doing the same.

I've only recently started living with a diesel. Today, I started the car up (on holiday) and drove out of the car park, turning towards the nearest village - a few miles away, all downhill. Consequently I hardly pressed the accelerator pedal and after we got to 30mph we coasted the rest of the way into town (hitting about 55mph without right-pedal). I noticed there was a fairly big trail of white smoke trailing us as we coasted down the hill, and when we stopped in the town (engine still running, just getting some cash) white smoke and an acrid smell were coming from the exhaust. Power delivery was fine when using the right-hand pedal. Got to final destination (20 miles of right-pedal usage later) and smoke had cleared totally. No smoke from car when driving back later on, but we didn't coast downhill for a mile or two and the car had been sitting in winter sun all day.

So... should I be concerned? Is this just unburnt fuel (injected in coasting to keep cylinders warm, normally) caused by the edge case of running cold down a hill, spinning the turbo up from the air-pumping through the cylinders (no throttle ofc) cooling the engine down etc (Boyle's law iirc?)... or is it more sinister?

Posting this, as never had a diesel before, and it might just be a quirk of the fuel (drove a diesel van a few years ago, but it wasn't mine and so I didn't pay as much attention to potential for blown engines...) and how the engine operates...[quote/]

 

[roscoFJ73]

F600 with same problem and he is getting backfiring also

white smoke down hill - Diesel Bombers

12 Valve 2nd Gen Dodge Cummins 94-98 - white smoke down hill - So we have a 97 f600 with a P pump 12v w/allison auto at work with 380,000miles and we use it to run appliances up and down I70. The driver is telling me while going down hill at pretty good altitude probly 9000ft it just billows out...

www.dieselbombers.com

 

[Magic Spool Bus]

F600 with same problem and he is getting backfiring also

white smoke down hill - Diesel Bombers

12 Valve 2nd Gen Dodge Cummins 94-98 - white smoke down hill - So we have a 97 f600 with a P pump 12v w/allison auto at work with 380,000miles and we use it to run appliances up and down I70. The driver is telling me while going down hill at pretty good altitude probly 9000ft it just billows out...

www.dieselbombers.com

Solid info on that one rosco! I wouldn’t imagine mine is bad injectors but overfueling sounds likely as I’m tuned at sea level and quite rich. Obviously at altitude we get even more rich because a loss of air density. Maybe backing out the fuel screw would be the answer when knowing we are going to hit these high altitudes.

 

[AussieHJCruza]

Solid info on that one rosco! I wouldn’t imagine mine is bad injectors but overfueling sounds likely as I’m tuned at sea level and quite rich. Obviously at altitude we get even more rich because a loss of air density. Maybe backing out the fuel screw would be the answer when knowing we are going to hit these high altitudes.

If your engine is turbocharged the elevation won't make any meaningful difference when on boost, I wouldn't bother turning down the fuel for the sake of off-boost situations

 

[roscoFJ73]

I can't see it harming the engine. I would try a lower gear and let the rpm rise and see what happens

 

[BDA130]

FWIW, I’ve been done many a Colorado pass in my HDJ81 and not experienced this. Just an idea, I use AMSOILs all in one diesel addictive that has a cetane boost...think that might help?

 

[Magic Spool Bus]

FWIW, I’ve been done many a Colorado pass in my HDJ81 and not experienced this. Just an idea, I use AMSOILs all in one diesel addictive that has a cetane boost...think that might help?

I run Stanadyne and sometimes power service diesel which is a cetane booster as well so I wouldn’t imagine it is.

 

[Loober]

@Socal81 I was having pretty much the exact thing happening when engine braking for a while, had the head rebuilt, which mostly involved replacing valve seals, did not touch the pump or injectors, put it back together and it went away. Still is noticeably smelly but thats explainable due to the unburnt fuel, but no more smoke or dead spot in power when getting back on the throttle, thats what made me convinced that's what was causing it, otherwise a very weird coincidence.

 

[Magic Spool Bus]

@Socal81 I was having pretty much the exact thing happening when engine braking for a while, had the head rebuilt, which mostly involved replacing valve seals, did not touch the pump or injectors, put it back together and it went away. Still is noticeably smelly but thats explainable due to the unburnt fuel, but no more smoke or dead spot in power when getting back on the throttle, thats what made me convinced that's what was causing it, otherwise a very weird coincidence.

Man I really hope this isn’t the cause because I just had the head checked and they said the valve seals and everything looked great... it’s strange that it would only do this at altitude though and then runs completely fine at sea level if it was the head. Maybe I’ll get some head studs soon and do the head again

 

[Loober]

Mine was doing it at all times in prolonged descents not just at elevation, short stints I was not noticing it, which made sense, since the oil had to have enough time to accumulate to cause the effects. The dead spot in throttle could be linked to a slug of oil that had to be burned before the fuel could start to be combusted again, so that also make sense in the valve seal theory. The effects of elevation definitely adds to it.

 

[roscoFJ73]

The dead spot in throttle could be linked to a slug of oil that had to be burned before the fuel could start to be combusted again, so that also make sense in the valve seal theory.

Its quite possible you have a gooey mess of oil and half burnt diesel stuck to the insides of the combustion chamber which makes combustion difficult. As it happening to all diesels great and small, I wouldnt touch anything.

 

[mxtaco48]

Older thread here but I have a 1HDT swapped into my HZJ73. I live In Texas but on a trip to NM, once I got to 10,000’, I would Smoke on downhills too. Not only that, it would “backfire”. Once I got Back down out of elevation, it quit doing this. It was very strange.

 

[Lumpskie]

Older thread here but I have a 1HDT swapped into my HZJ73. I live In Texas but on a trip to NM, once I got to 10,000’, I would Smoke on downhills too. Not only that, it would “backfire”. Once I got Back down out of elevation, it quit doing this. It was very strange.

Sounds like it runs rich at higher altitudes with thinner air. Out of curiosity... Do you have an EGT gauge?

 

[mxtaco48]

Yes, I do. EGT, Boost and coolant temps. BTW, this trip was last year, last week of July-first week of Aug. On downhills, it would feel like the engine was almost creating pressure and when I'd slightly tap the gas pedal it would release this pressure causing the backfire sound.

I agree about it running rich. Next time, I'll adjust my IP at those elevations. I was less knowledgeable at the time about the issues and how to tune the engine.

 

[Magic Spool Bus]

Yea mine did the whole back fire white smoke at 10,000 but it still runs great now. My EGT during this was 300F pre turbo nearly. I wonder if it was getting so cold inside the cylinder that the fuel would not ignite correctly especially with insanely high back pressure through a tiny turbo hotside going down hill. Because after the back fire the engine would run normal, EGT would come up and smoke would go away. If you let it get too cold again coasting, the process would start over.

 

[Lumpskie]

Backfiring, in my experience is caused by unburnt fuel in the exhaust. I wonder if your injectors need a little cleaning or you are running rich at idle (fuel screw turned up). It also makes me wonder how rich you are running under full load. (EGTs would be high uphill in that case) 300f on a long downhill seems about right... shouldn't cause smoke in and of itself, I wouldn't think.

 

[Magic Spool Bus]

Backfiring, in my experience is caused by unburnt fuel in the exhaust. I wonder if your injectors need a little cleaning or you are running rich at idle (fuel screw turned up). It also makes me wonder how rich you are running under full load. (EGTs would be high uphill in that case) 300f on a long downhill seems about right... shouldn't cause smoke in and of itself, I wouldn't think.

Yea that fuel screw has been turned up but my AFR gauges during these events are maxed out at 99:1. Under full load foot flat 20psi on that old grunter my AFR would 23:1 up top with yes... excessively high EGT. Injectors and pump have both been rebuilt about 20k km ago but doesn’t mean they couldn’t be the issue

 

[mxtaco48]

Backfiring, in my experience is caused by unburnt fuel in the exhaust. I wonder if your injectors need a little cleaning or you are running rich at idle (fuel screw turned up). It also makes me wonder how rich you are running under full load. (EGTs would be high uphill in that case) 300f on a long downhill seems about right... shouldn't cause smoke in and of itself, I wouldn't think.

I have a freshly rebuilt IP and the fuel screw was set at whatever setting was factory.

 

[Magic Spool Bus]

My logic may be off but I don’t see how being rich would cause this unless it’s absurdly rich. For me full load would have been 24:1 AFR at sea level of course this would richen up at this altitude by how much idk the AFR gauge never reflected anything wildly different other than a few points richer at times but nothing alarming like 16:1 AFR at any time under any condition.

The reason I believe it’s something to do with high back pressure is because compression breaking down hill compounds back pressure quite a lot. For example on a steep downhill I would see 15psi in the inlet and 30psi in the exhaust manifold with no load and no fuel applied. EGT would be very cold even colder then on a start up.

My event would go something like this. I’m coasting downhill so steep I need to apply breaks to not exceed 70mph, I notice the engine almost sounds like it has shut off (the engine got noticeable more quite while coasting compared to normal) this prompted me to apply fuel to see if I could hear the engine come alive. Because it very much sounded like I was coasting along with an engine shut off while coasting in gear... somehow. So 1st I applied maybe 10% fuel into the pedal. This would produce lots of white smoke. Okay let’s add even more foot pedal... then I would get a big back fire, lots of black smoke and normal engine operation.

What was also observed is if I coasted all the way to the bottom of a hill, stopped to idle for just a moment, returned to driving and all would be normal without any smoke.

My head has already been tested and refurbished, injection pump and injectors rebuilt so I doubt it’s either of these.

again I’m not speaking facts just speculating