Soot build up

Discussion in 'Diesel powered automobiles' started by vtec-e, May 11, 2009.

  1. vtec-e

    vtec-e Celtic MPG Warrior

    During my "reasonably scheduled" hot runs to clear the exhaust of soot build up i often get quite taken aback at how much comes out. It's spectacular actually! One day i pulled out of a junction and some cars built up behind me. I was going up a slight hill and i started my hot run. Well, the cars were lost in smoke and soot and they all backed off, presumably waiting for some spare parts to fall out! It did this for maybe 10 seconds and then cleared.
    What bothers me is that all that soot could be captured but how? And where exactly is it building up in the exhaust system? I could, if i was sure it would work, make a centrifugal/cyclone type collector to collect the soot. But if it's building up on the turbo itself or the manifold then there's not much point. Any ideas anyone?

  2. npauli

    npauli Well-Known Member

    That doesn't sound good. Does your oil turn black quickly?

    I think soot forms from fuel in the combustion chamber that it either too cool or doesn't have enough oxygen to burn well. For example, there's a little gap between the cylinder wall, piston, and first ring that meets both criteria. There's probably some complex series of reactions that determines how much soot forms and how much gets burned off.

    The industry has already figured out how to remove soot from the exhaust stream - a diesel particulate filter (DPF). Any on-road diesel in the US since 2007 has one.
  3. vtec-e

    vtec-e Celtic MPG Warrior

    Not really abnormally. I ran a flush before last oil change and the new oil is still almost like new in terms of colour. I've been giving it a bit of welly once per trip for maybe 30 seconds and it smokes only a little. But if i dont do it for a few weeks then it chucks out soot (under pedal to the metal) like there's no tomorrow, for about 5 seconds then it's clean. My mpg is good and it pulls well.
    I looked into this with my other car and got the impression that it was normal for soot to build up after a lot of gentle driving and that the best way to keep things clear was to drive hard once in a while.
    Npauli, i'm thinking you're suspecting something's wrong. Whats the prognosis?!:)

  4. npauli

    npauli Well-Known Member

    After your first post, I was guessing one of the following:

    1) Something was wrong. (fuel system, air system, piston rings, wrong oil, etc. could be anything)
    2) Everything was working as designed, but the system just wasn't designed for whatever you were doing with it.

    After your second post, #2 above seems like the most likely. Easy way to tell = drive like a non-hypermiler for a little while, and see if it improves. Sounds like you already did that. You can be sure that the system was designed for non-hypermilers. I know Europe allows more soot from diesels than the US, but I'd think visible smoke would be a no-no for any modern diesel under the kind of conditions it was designed for.

    If it were my car, I'd try to keep the soot buildup to a minimum. I wouldn't know if that soot build up would hurt anything or not, but without knowing I'd try to play it safe. My daily commute has one nice hill climb that gives my engine some regular exercise, so I haven't had to do anything special.
  5. vtec-e

    vtec-e Celtic MPG Warrior

    Thanks for that. I've been trying my hand at P&G lately and there has been no soot worth talking about during acceleration. It is common knowlege around here that you have to work diesels so a short burst now and then should help clear things out.
    Regarding the oil type/quality; God knows what was in it before i changed it so that may have been a factor. When i drained it out it was jet black. My new stuff still has a slight golden hue to it after a few weeks driving.
    What i'll do is both drive like before (gentle accel + steady state at 80kph +DFCO) and then non-hypermile it. I'll give each a week and see what the soot build up is like.

  6. vtec-e

    vtec-e Celtic MPG Warrior

    I've been thinking about exhaust gas velocity and how it relates to exhaust diameter. Driving the car like we are told, ie hard and fast, leads to higher exhaust gas velocity which results in a better cleaning of the system. Hypermiling leads to much slower exhaust velocities and hence, poorer cleaning of the system.
    If the exhaust diameter was less, it would increase gas velocity and keep things clean at a more sensible rpm. Would this affect performance at higher rpm? Probably, but how much?
    Now, i'm not going to go and make a custom manifold with smaller diameter pipes but if the soot was building up after the catalytic converter then a replacement exhaust with a smaller bore might help. But then it might mess up my mpg. What do the experts here think?

  7. Nevyn

    Nevyn Well-Known Member

    Smaller doesn't only create faster flow, but it also increases backpressure. I'm not sure what effect that would have on a vehicle like yours.
  8. npauli

    npauli Well-Known Member

    I don't like the idea of trying to reduce the diameter of the exhaust for several reasons:

    1) Like Nevyn mentioned, smaller exhaust will mean more backpressure, higher exhaust temps, etc., and generally won't be good for your engine health or mileage.
    2) DIY exhaust modifications can be tricky and/or risky. People upsize their exhaust all the time, so aftermarket parts abound. But try to go smaller and you're on your own.
    3) The original problem you're trying to solve (soot build up) might not even be affected by the exhaust pipe at all - for all I know the build up might be in cylinder or in the exhaust ports just past the valve.

    I'd vote for trying to prevent the soot build up just by letting the engine get a little more exercise a little more often. If that doesn't do it, you might try to investigate if that soot could have come from burning oil or something else not functioning as intended.
  9. seftonm

    seftonm Veteran Staff Member

    Interesting how your 1.4 D4D can make so much more soot than my 1.9. I can't make enough soot to make cars back off, even if I want to. I try to blow accumulations out a few times per tank, with a run at high rpm's and load, to hopefully make plenty of exhaust and heat to keep things clean. With my car, I know that some of the soot build up is in the exhaust. Where exactly, I'm not sure, but it can build up on the turbocharger if it's not exercised occasionally. There is also some buildup in the intake from the EGR, and finally, some of the smoke I see in my mirror is simply from the combustion when I am trying to blow things out.
  10. vtec-e

    vtec-e Celtic MPG Warrior

    Thanks for the info guys. I'm currently a few days into my week of my usual driving style. On tuesday next i will revert back to an "almost normal" driving style for a week. Then see what i get out of my exhaust.
    Mike, you see thats the thing. I hadn't been doing any hot runs to clear out the system since i got the car so there was bound to be a big build up. I guess it's something i'll have to keep an eye on and do every few days for a few minutes. Incidentally, high rpm's in a lower gear on a downhill stretch dont cause excessive fuel consumption. If i maintain my speed but drop a gear....or two, the engine uses no more fuel than it would have in 5th. That might have to be a new tool of mine.

  11. seftonm

    seftonm Veteran Staff Member

    I hope that works, ollie. High rpm's with low load aren't enough for me to keep things clean. There simply isn't enough exhaust flowing. I need to have a decent load on the engine plus the high rpm's to blow out the soot buildup.
  12. jcp123

    jcp123 Caliente!

    Don't feel bad...I don't have soot issues, but I got some carbon buildup in my gasser which was holding a valve open and causing a miss. Buildups happen to all of us!
  13. Harold

    Harold Well-Known Member

    Have you checked the water filters? Can you clean them or are they the throw away type? Sounds like water to me! Hal:)
  14. vtec-e

    vtec-e Celtic MPG Warrior

    I did funnily enough. I opened the drain valve and diesel came out. But thanks for the reminder, i'll check it again. What would water do to cause soot building up?

  15. Harold

    Harold Well-Known Member

    I had a friend who had a Citroen diesel [French auto] and it smoked like crazy! I asked him if he had cleaned the water filers and he said it had none. Well anyway I cleaned them for him, and no more smoke! It was blowing black like you wouldn't believe. Sounds like you have it covered! Hal
  16. groar

    groar X-Frenchy: very

    Funny, I was going to ask this question.

    The EGR entry is known to keep soot while closed and release it when opened.

    The more I HM, the more I can see black fumes in my mirror. I think the longer NICE-On coasting and the fact I rarely push a lot on the accelerator by now are making things worse.

    Now when I'm keeping the minimum load to keep the minimum speed on up-hills, I can see black fumes during all the climbing-up. Before I was going with the flow, so much faster than PSL... and was using a bigger load.

    Also I don't push the accelerator on a few places where i used too...

    In a couple tanks I'll have to go to the garage for maintenance. I'll try to ask the question.

  17. vtec-e

    vtec-e Celtic MPG Warrior

    Hi Denis, it'll be good to hear what answers you get. I've just finished removing my intercooler for an inspection and it is clean. Well, there was a light coating of oil on the inside but thats from the crankcase ventilation afaik. There was no sludge or carbon built up. I had a peek down to the turbo blades and everything looked clean there. At least on the compressor side anyway....
    I'll try a high load/high rev hill climb this evening on my way to work. Then i'll do it every day for a week to see if it persists or cleans out. If my mpg doesn't suffer then i'll have to keep it up or suffer later with expensive repair bills.

  18. Harold

    Harold Well-Known Member

    Ollie, in Ireland do they use a winter and summer grade of diesel? Try a different station next fill-up? Hal
  19. vtec-e

    vtec-e Celtic MPG Warrior

    No change from summer to winter as far as i know. I use the Topaz diesel (formerly Shell) and they have an additive that cleans the fuel system and engine. It's a popular station so there's little chance of me getting stale diesel.
    Anyway, i gave it some welly up a hill earlier and it threw out a little smoke/soot but nothing too dramatic. I kept it up for about 20 seconds on the first hill and it cleared before i got to the top. Then i tried it on another hill but it hardly smoked at all. My trip fuel consumption was 2.9L/100k after all this so i guess i'll be doing this once a week from now on.
    I talked to a workmate about this and he wondered if wrapping the exhaust in that insulating wrap might help burn off any soot building up. It could work but i don't want to go to the expense and hassle if there's no chance of it working. Thoughts anyone?

  20. vtec-e

    vtec-e Celtic MPG Warrior

    During my commute home this morning i went for the "heavy right foot" approach during acceleration a few times. Actually, i planted it once or twice...... Small amount of smoke but again, nothing alarming. After a few of these there was no more smoke. My steady state consumption was better. Where i would have been doing 3.0 i was instead doing 2.2 to 2.7L/100k. Based on this i expected my trip average to be better than normal. Well, considering the few times i wellied the car it was brilliant! 2.9L/100k! I did get the feeling that my steady state consumption was getting worse after a few minutes but with the terrain going up and down and wind changing direction, it was hard to tell. Which is a pity because it makes it harder to analyse this soot/carbon buildup problem.
    I get the feeling that P&G is the way to go with diesels as it keeps things clean at all times. I'd better get practicing......


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