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diesel tractor: positive (back) pressure in air intake line

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  • diesel tractor: positive (back) pressure in air intake line

    hello friends, got a bit of a head scratcher for y'all:

    little back story: I've got a 1990 Ford 1320 tractor, with a I-3 diesel under the hood. 4-5 years ago it seized on me, tore the whole block down, bored it & rebuilt it. Since then, it runs really nicely, however it is VERY slow to cold start. Once I've started it, it pops off nearly instantly.

    It has glow plugs, though I find them pretty ineffective & i suspect their use prior to the rebuild damaged the precups, which ultimately shattered & seized between the rings & cylinder walls.

    Temp has a lot to do with it, but even @70F ambient, it takes close to a minute to start. during the winter it just wont start...through some trial & error I found that if I pulled the intake hose & blew my hot air gun in there, it pops off as if I had already started it (seconds). That works, but is quiet cumbersome, so I build a 12V heat gun with a tee to incorporate it into the intake line:

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    As pictured, the end of the heat gun barrel(fan) remains open, which I dont really want since that bypasses the filter. My plan was to build a hinged cap over the fan @ the end so that I could just flip it up or down. That should be pretty each given the negative pressure with the intake during operation will suck the flap in....or so I thought. To my surprise, with the engine running it does not suck air through the fan. Instead, I have positive pressure coming out the fan, enough the back-drive the fan. Engine runs fine, I had full power and I can still make a cap to seal the end, though it will need to be designed differently to contain the positive pressure. I could just press forward, but that positive / back pressure in the intake concerns me. Here is an overview of the system:

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    I cant, for the life of me, figure out where in the engine cycle pathways could align such that air could come back through there?

    How do I have positive pressure @ the fan and negative pressure at the intake hose(input end) ?

    also any insight into why the heat gun is soooo much more effective than the glow plugs? --btw I also tried a set of glow plugs in the intake line, that was the next best to the heat gun

    thanks all!
    "it is no measure of mental health to be well adjusted to a profoundly sick society." -- krishnamurti
    "look deep into nature, and then you will understand everything better." -- albert einstien
    "any intelligent fool can make things bigger and more complex...It takes a touch of genius - and a lot of courage to move in the opposite direction."

  • #2
    Just an under-educated guess here:

    Where the intake system on a diesel has no throttle to create a negative pressure the amount of air in the system is equal to the amount required per intake stroke minus any resistance in the system. The most common source of resistance is a partially plugged air filter. With a clean filter and it's relatively free flowing system there could be a thermally induced increase in volume as the air hits the warm intake manifold. That extra volume would escape through the owner added fan duct.

    As an example of warm air expanding we can go back to the iconic P51 aircraft of WWII. It had an air scoop under the fuselage to feed the engine cooling system. The heated discharge air from that system had enough increase in volume in it's contained duct work to provide a marginal amount of added thrust when exiting the back of the system which was enough to overcome the added drag from the intake.
    Last edited by tom_d; 10-01-2022, 07:28 PM.

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    • #3
      You also have moving column of air that is flowing and then being stopped by the intake valves opening and closing. That could cause a bit of positive pressure. It could also be from the flow being interrupted by the additional piping you have welded in there.

      lg
      no neat sig line
      near Salem OR

      Comment


      • #4
        A diesel engine requires compression heat to ignite the fuel, a glow plug is merely an aid to warm the small per-chambers in order to get the ball rolling. It should function as designed if all is as it should be.
        A friend of mine has a similar tractor and it starts well in below freezing weather without using his block heater, although it does flash up faster with it's use.Sweet little tractor.

        Have you ascertained that all of the glow plugs are functioning when intended? Does the compression meet factory specs? Are the injectors clean and popping off at the required pressure and atomizing fuel into a fine mist?
        Deficiencies in any of these systems will compromise the engine's ability to start when cold and conversely a warm engine will mask those deficiencies to the point that one would not suspect anything wrong

        As noted above a moving column of air has momentum and at low air demand situations and with a clean filter the air does pile up a bit in the intake tract. The system is designed to be fuel throttled and not air throttled so there will, or should always be, an abundance of air, so yes not unusual to see a slight positive pressure in the intake tract.

        The operation and longevity of the engine is dependent on a clean supply of air so I trust you have not operated the engine as picture when working the tractor.
        The intake air pre-heater is a good idea shared by many engine builders however all of the ones I can remember seeing incorporated the heating grid inline with the intake tract, usually in the intake manifold. The flow of intake air is heated as the engine is spun over and the grid is taken out of the electrical circuit automatically through the use of a temp sensor that switches a relay off as the engine warms up. However a manual switch is much easier to install and just as effective.
        Home, down in the valley behind the Red Angus
        Bad Decisions Make Good Stories​

        Location: British Columbia

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        • #5
          Originally posted by Willy View Post

          Have you ascertained that all of the glow plugs are functioning when intended? Does the compression meet factory specs? Are the injectors clean and popping off at the required pressure and atomizing fuel into a fine mist?
          glow plugs have been bench & field tested.
          compression- I haven't done since the rebuild, as I recall I was about 10psi over spec on all 3 cylinders.
          injectors- a sticky injector was a contributing factor to the seize, so I made a little bottle jack pressure tester. they are all good, and if they weren't the engine would be bogging down, its not.

          In below freezing, I wouldn't even attempt to crank the engine without using my block heater. though thats more out of consideration of oil flow during cranking.

          As noted above a moving column of air has momentum and at low air demand situations and with a clean filter the air does pile up a bit in the intake tract. The system is designed to be fuel throttled and not air throttled so there will, or should always be, an abundance of air, so yes not unusual to see a slight positive pressure in the intake tract.​
          thats fascinating. highly unlikely my air filter is particularly clean though. the only part of that explanation that might not mesh is that the pressure is extremely consistent, no pulsing detected. I suppose that could be being smoothed by the back drive fan.

          so I trust you have not operated the engine as picture when working the tractor.
          yah, I ran it for an hour or two today. it was blowing air OUT, not sucking anything in. Even if it did, both ends of the heating tube are fitting with very fine SS mesh. yah its not a filter, but nothing significant is getting in there. Long term, I agree it should be sealed off during operation.

          re: heating grid --i've seen those, though nothing small enough to fit my machine. My sudo version of that was 3 additional glow plugs in the intake line. It was helpful, but not enough. It really seems like you need the bare nichrome wire, hence my little hot air gun. Also, having worked with nichrome wire before, I know it gets brittle & breaks. even a small chunk of that would thoroughly destroy a cylinder wall. I didn't want to put another screen on the main line as it would reduce air flow.

          larry_g
          It could also be from the flow being interrupted by the additional piping you have welded in there.
          why would that be? after my mod the intake is still a 2" clear bore with no obstructions, identical to OEM.



          thanks for the ideas everyone, its really helpful to have some people to bounce ideas off of.
          "it is no measure of mental health to be well adjusted to a profoundly sick society." -- krishnamurti
          "look deep into nature, and then you will understand everything better." -- albert einstien
          "any intelligent fool can make things bigger and more complex...It takes a touch of genius - and a lot of courage to move in the opposite direction."

          Comment


          • #6
            Does it start well after having the block heater plugged in for an hour or two?
            Home, down in the valley behind the Red Angus
            Bad Decisions Make Good Stories​

            Location: British Columbia

            Comment


            • #7
              A few things come to mind after reading all of the above. The positive pressure in the intake - I can't see for sure from the pictures but is the air intake located behind the radiator cooling fan? If so it could cause a bit of a positive pressure in the intake. You could check that by opening up the air cleaner with the engine running. If it is coming from upstream that should cause it to temporarily stop. The glow plugs in the precups are only designed to heat the small amount of air in the precup. If they are manually operated some manufacturers specify how long they should be on before cranking the engine. You really need to properly time the heat cycle, don't just guess. If my old memory is correct the Cat engines required a minimum of 30 seconds and even longer if the ambient temperature was colder. But after saying that - years ago I had a Mazda pickup with a diesel engine in it from the factory. It had an automatic glow plug cycle with temperature sensors. These were high heat glow plugs that took only a few seconds to heat up. You turned the ignition switch on then waited for the glow plug light on the dash to go out before cranking the engine. If you didn't wait she would not go. Another thing that can make a diesel hard starting in cold weather is having summer fuel in the system. The fuel companies switch over to winter fuel which is a lighter grade some time in the autumn. Then back to summer fuel in the spring. Just a few thoughts which may or may not help.

              Larry - west coast of Canada

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              • #8
                my guess is you have 1 cylinder that isn't working.. short video of it running? There should be a butt ton of air sucking in though that opening.. (not having to go through the filter..)

                Or.. cam timing off? blowing some air back through the intake?

                Hmm - I think second.

                sam

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                • #9
                  If you have good compression and good fuel delivery,but it's still a hard start on a warm day. You might want to check your injection timing. On a 70* day it should light off in one or two revolutions without glow plugs.
                  I just need one more tool,just one!

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                  • #10
                    ADDED DETAIL:
                    since this has veered more into why the engine doesn't start better in the first place, I need to add this: "time at rest" factors in somehow. harder to start after 2 days of rest than 1. My theory on that is I suspect a very very tiny leak(its on my list) somewhere in the injection lines/fittings that causes the system to become slightly un-primed. I also don't get any fuel out the exhaust when it first starts cranking, when fuel hits while cranking (but not burning) it turns white -->grey and eventually a big puff of black when it starts. Pretty clear exhaust after there, unless under heavy load.

                    Originally posted by Cuttings View Post
                    A few things come to mind after reading all of the above. The positive pressure in the intake - I can't see for sure from the pictures but is the air intake located behind the radiator cooling fan? If so it could cause a bit of a positive pressure in the intake. You could check that by opening up the air cleaner with the engine running. If it is coming from upstream that should cause it to temporarily stop. The glow plugs in the precups are only designed to heat the small amount of air in the precup. Another thing that can make a diesel hard starting in cold weather is having summer fuel in the system. The fuel companies switch over to winter fuel which is a lighter grade some time in the autumn. Then back to summer fuel in the spring. Just a few thoughts which may or may not help.
                    thats fascinating, I've been trying to figure out how that positive pressure could be there, ie whats the source? Momentum in the air column makes a little sense, but the radiator fan acting like a sudo ram-air could account for that. Yes, that intake is right by the fan, in fact its inline with the OD of the fan, where the most air moves, I'll bet thats not an accident. would also explain why I have no airflow @ the fan when I'm down at idle. Theory is easy enough to test, I'll do that tomorrow.

                    I haven't lite the precup glow plugs for years out of concern for damage. The heat cycle in the manual is only like 15 seconds, thats ineffective, I'm unwilling to push it farther. You got a little more room for guess work when you're just heating the air coming in.

                    not sure about summer/winter fuel. But I do add an anti-gel additive to my diesel during the winter.

                    Originally posted by skunkworks View Post
                    my guess is you have 1 cylinder that isn't working.. short video of it running? There should be a butt ton of air sucking in though that opening.. (not having to go through the filter..)
                    Or.. cam timing off? blowing some air back through the intake?
                    Hmm - I think second.
                    sam
                    i'd be happy to take a short video, anything particular you'd like to see demonstrated? or you just wanting to hear it?

                    I fully expected that too. I brought duct tape out to temporarily seal the port, thinking it would just suck right on🤣🤣.
                    I am all too familiar with how that engine runs when I'm down a cylinder, i've been there, she's firing on all cylinders with as much power as its ever delivered.

                    I dont know that I have any mechanism to adjust cam timing, I'll have to check the manual. Its been a minute, but I remember checking the lifters(?) on the intake & exhaust. I feeler gauge was involved & I had them in spec. Itd probably be worth checking them again. Is that the timing adjustment you're referring to?

                    blowing air back through a leaky intake valve is worth gaming out...wouldn't you expect a pulsing type flow? I also would expect that to dirty up my intake manifold, which it hasn't. thoughts?

                    Originally posted by wierdscience View Post
                    If you have good compression and good fuel delivery,but it's still a hard start on a warm day. You might want to check your injection timing. On a 70* day it should light off in one or two revolutions without glow plugs.
                    I'll look into the timing, I think there's some shims in there for adjustment. That's one of the few parts I've never taken apart

                    from sitting cold, if I stick my heat gun on the intake, no block heater, no glow plugs, it WILL start inside of 2 cycles at any temp above freezing. Below freezing I always use the block heater, so I dont know if it would or wouldn't start without it. my guess is, it would.

                    Originally posted by Willy View Post
                    Does it start well after having the block heater plugged in for an hour or two?
                    no, not particularly, not without hot air on the intake. It does crank much better with everything warm & flowing, which is really why I do it. especially if I gotta crank on it for a minute+.


                    Last edited by mtraven; 10-02-2022, 04:11 AM.
                    "it is no measure of mental health to be well adjusted to a profoundly sick society." -- krishnamurti
                    "look deep into nature, and then you will understand everything better." -- albert einstien
                    "any intelligent fool can make things bigger and more complex...It takes a touch of genius - and a lot of courage to move in the opposite direction."

                    Comment


                    • #11
                      I don't claim to be able to explain it, but I have read that exhausts can have areas with low pressure and areas with high pressure, and the art of exhaust tuning is to get the high and low areas in the right places for maximum efficiency. Maybe the same applies to your inlet tract.

                      I don't understand why air seems to be blowing out of the inlet tract but the engine, which is running, must be sucking air in. Where is the engine getting its inwards air flow from?
                      'It may not always be the best policy to do what is best technically, but those responsible for policy can never form a right judgement without knowledge of what is right technically' - 'Dutch' Kindelberger

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                      • #12
                        EVERY diesel that I've been around has a vacuum on the intake.
                        If it was mine I would re examine the head, valves, and timing.
                        This makes no sense from 50 years of dealing with engines.
                        Does this have a turbo on it??

                        olf20 / Bob
                        Last edited by olf20; 10-02-2022, 10:34 AM.

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                        • #13
                          I’ve been around several older diesel tractors that started remarkably better if you gave the hand primer pump a few strokes before you try to start it.

                          on edit:the fact that it starts good when it’s been running recently and poorly when it’s been sitting fits that scenario perfectly.

                          on second edit:cranking speed makes a difference too. We have a 110hp. Allis Chalmers 8010 that wanted ether at any temperature below 80F. A new gear reduction starter made a new tractor out of it.
                          Last edited by SVS; 10-02-2022, 01:31 PM.

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                          • #14
                            I would say that SVS is on the right track as far a poor starting after sitting for a few days is concerned. A few strokes on the lift pump priming lever to build pressure back up should make quit a difference. If that does it the problem is likely the outlet valve in the lift pump leaking slightly. If the picture I found is correct, you should be able to remove the pump and take it apart to check the valve. It may just be a little dirty. Some of those type of lift pumps also have a screen in them that should be cleaned once in a while.
                            Larry - west coast of Canada

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                            • #15
                              A friend had a very hard starting JD diesel. Finally came down to the solenoid in the injection pump was not getting voltage. I don't know how it ran at all but it did. After repairing circuit it was a good starter.
                              ​​

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