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Sd25 Fuel Adjustment


SLO720

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Finally got to drive my SD25 truck down the road and I believe it is running somewhat lean. The trip I made was totally on level ground and never over 70mph. Truck would read up to 1000f on the egt gauge at WOT. Engine only has 58k miles one and ran amazing just on the warm side. I would like to richen the system not to where it's billowing smoke but just enough to hopefully bring temps back down. Engine the truck originally came out of spent its entire life on a mountain and I'm assuming that's why it's so lean. Idk if the altitude adj works it is there though. Also feel like I need to step up to a taller tire or 3.33 gears. Truck feels like it's really topping out at 65mph. Was a great experience to finally drive it regardless!

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With diesel a temperature increase is opposite of a gas motor. Increased fuel increases temperature for a diesel motor, and an IDI motor (SD25) will hold more heat in the head than a DI motor. Also, increased fuel will increase chances of black smoke, as you alluded to. If the motor spent it's entire life at a higher elevation it is probably set up with a leaner fuel mix.

 

1000f is not excessively high (do you have an EGT gauge?), but is not low for a diesel either, especially not turboed and on flat ground. If the motor is at a lower elevation now than it was set up for it is probably running a bit rich.  A turbo probably would help you, but the SD22/25 is not easily turboed because of the vacuum controlled intake system. Contact wayno (on this site) for how he turboed his diesel motors. He is one of the few who have successfully turboed his motors. It really depends on how you hook up the turbo. He has at least two Datsun diesel SD25's that he dropped into 520/521 pickups and turboed them.

 

Don

Edited by 620slodat
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The more fuel it gets the hotter EGTs it will run, before I put my turbocharger on my 1969 Datsun kingcab diesel powered truck I put a EGT gauge on it to see how hot it ran without a turbocharger, I could not believe it, over a half mile slight uphill climb crossing the I205 bridge into Washington going 65mph it climbed to 1400 degrees and I was not even at the top yet, that surprised me, I had no idea but it was almost floored keeping it going 65mph up that slight grade, they run hot when floored.

I had bought a SD25 turbocharged engine I bought that ran very hot EGTs, I could not drive it that way and that is why I put that EGT gauge in my 521 kingcab diesel, to see how hot it ran.

I figured out how to get my SD25 engine to run properly on my 1980 Datsun 720 diesel truck piping as a draw thru setup, it ran fine until about 64mph on flat ground, it sort of hit a wall and I was hardly pushing on the pedal, I could get another 6mph out of it floored, it was running lean and would go no faster on level ground and the EGTs were very low(600 degrees floored), I asked everyone on the diesel forums what they thought the issue might be, I even removed the smoke screw to help it run more rich with no change, them Nissan diesel forum guys went crazy when I told them I removed the smoke screw, to this day I have not put it back, anyway there was that knuclehead guy in the diesel forums selling his turbocharged SD22 diesel engine on craigslist, I called him and talked to him about how he had turbocharged his engine, he told me this complicated way with valves and hoses, but something he said made me think of something.

I went home and put a valve in the vacuum line that controlled the fuel mixture using a "T", as when the turbo was spooled up as a draw thru it created a lot of vacuum in the venturi making the engine run lean, so I would open the valve releasing the vacuum created as a draw thru and took it for a test drive, before I was lucky to get 70mph, when I opened that valve it easily went a 100mph so that made me think of something else.

So I decided to turbocharge my 521 kingcab diesel engine and pipe it the way I thought it might work after talking to that knucklehead guy , I had everything I needed as my SD25 engine had a turbocharger on it when I bought it, but it looked so jury rigged( it had the waste gate wired shut), I removed it and ran it without the turbo for 7 years(this was back in 2009), I put it all together just like it was stock, now the idea I got was that whenever anyone put a turbo on their SD22 diesel the vent line was always vented, as if you leave it where it was it would have turbo pressures in that line and that is why so few have succeeded in turbocharging their SD series engine, it runs away because the injection pump has pressure which floors the vacuum operated injection pump, it is scary to drive it that way and that was how the SD25 diesel engine was piped that ran very hot but slightly differently, so anyway I piped my 521 diesel exactly like it was stock without a turbocharger, I piped the turbo onto the end where the air filter housing pipe connected, and I piped the air filter hosing to the turbocharger in-feed, now everything is stock on that diesel engine, if I didn't pipe the turbo output to the throttle body it would run normally, all the hoses were where they were supposed to be, I test drove it and it drove great, it had lots of power and when I let off the pedal it acted normally, that vent line needs to be under turbo pressure just like the other vacuum line is under turbo pressure, the two vacuum lines cancel each other out and the truck runs great, that is the secret, you just put a turbocharger in front of the  air inlet and leave everything stock, I cannot believe no one else ever figured this out, even the diesel forum guys didn't believe me at first, I had to make a drawing and post it.

Now I found more power thru some issues I had, if you put a "T" in that vent line and bleed off a little of the turbo pressure in that line it lets the other vacuum line apply a little more pressure to that side of the diaphragm and it runs a little richer, but if you vent to much the engine starts to run away like that line is vented, it is a fine line when you start playing with the pressures in that vacuum line, the more you bleed off pressure the more fuel it gets, the hotter it runs until it runs away, but all you have to do is let off the pedal and that cuts off the air and it will quit running away, my 521 turbodiesel engine runs around 800 to a 1000 degrees all the time unless I am going up hill for an extended period of time when it is hot outside, it can get up to 1200 degrees pretty easy if I put my foot into it trying to keep it going 75+mph all the time, on level ground not an issue, but up a hill it can run fairly hot unless you back off the pedal and drive normally.

 

I used 3.50 gears in my Datsun 521 kingcab SD25 diesel truck(no turbo), when I turbocharged it I found a set of 3.3 gears and installed them as it had the torque to turn them gears with the turbocharger, your SD25 powered 720 will not likely turn them gears on the freeway and help you go any faster except down hill, you will be dropping gears on any significant hill just trying to keep it going 50mph, you need a turbo to turn 3.3 gears, you need a turbo to turn 3.5 gears, hell you need a turbo to turn 390 gears when you come to a hill on the freeway, you are only going to go so fast with 71hp, I am likely pushing somewhere between a 100hp to 120hp now, but my 521 kingcab turbodiesel does not like to tow anything, it runs hot when I try to tow my work trailer, but my 1980 datsun 720KC even with 3.3 gears in the rear will tow most anything, the former owner of that engine had the injection pump turned up, I do not know how it was done, but that truck don't really like the freeway, it will go really fast on the freeway but my foot is deep into the pedal going 70mph while my 521 kingcab turbodiesel I am hardly pressing on the pedal going 70mph.

OK, my 521 kingcab turbodiesel has a 1985/86 Subaru XT turbocharger, it is a great truck to drive, it is AWESOME on the freeway, the 1980 Datsun 720KC turbodiesel has a 1998  VW Passat turbo in it, it does great around town but doesn't really like the freeway, 1985/86 Subaru XT turbochargers are rare, it took me years to find a second one as a backup.

Edited by wayno
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Happy Thanks Giving everyone 🙂

 

So my plan is to turbo charge, but mount it in the bed with it's own oil/coolant reservoirs and respective pumps. This will keep temps down, not interfere with brake booster and means I don't have to worry with a custom manifold. I have seen your diagram and had it at one point from another friend. The being able to go up hill with light gears right now doesnt bother me as I plan to only drive it locally and it's very flat land around here. I kind of figured there was a lean condition simply bc it doesn't smoke at all. It smoked once upshifting for a hill and I havent been able to duplicate it since. 

 

I'm just excited to have this truck on the road right now. Truck is like 5-6 different colors as of current and truck has been updated to an 85-86 ST bed and interior. Will post pictures when I get a chance. 

 

And yes I set it up with an egt gauge. Truck has done excellent what little bit I have driven it. I plan to do the valve stem seals on this truck as I feel like they may be the cause of the blue tinge of smoke at start up. I also have everything to do that and will use one of the older diesel forums for reference on that. The motor only has 58k miles on it and the body has been sitting since 1999 so I'm still working out bugs in various places.

 

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Edited by SLO720
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I am kinda wondering why that first photo of the pyro shows 400 degrees when the engine is not running and the light in the gauge is not on, it should be at zero.

The turbo needs an oil feed from the engine to have the pressure needed to lubricate it, you also need to drain that oil back into the engine oil pan, same with the coolant,  it needs to recirculate but that is a little easier, just a couple long hoses coming off the heater core system but that system then needs to be open/flowing all the time.

When the engine smokes on start up for the first time each day when it is cold they all puff this white smoke, every time the engine misses you will see that puff of smoke, that smoke is atomized un-burnt fuel and it will have a strange/funny smell, when it is really cold out it smokes a lot as the engine misses a lot on start up when it is really cold, that is if it will start at all, I have block warmers in my diesel engine blocks and I have a induction air warmer also, when above 40 degrees I use the induction air warmer as it starts easier in a hurry, when below 40 degrees it is a toss up, it will start hard and miss a lot unless I use the block warmer, below freezing I use the block warmer at least an hour before starting it otherwise it might not start, I have never tried to use the induction heater below freezing as when I installed it it never got into the 20s/below freezing after that.

I do not think the rope method can be used to change out valve seals on these diesel engine as both the injector and glow plug are in a pre-chamber with this tiny hole leading to the combustion chamber, you might be able to use air with the right adapter but maybe it is not your valve seals that are the issue, if it continues to smoke on start up after the engine quits missing then it might be the valve seals.

My SD25 engine pushed my 521 kingcab and 720 kingcab around fine on level ground before the turbo era using 3.50 gears in the rear, but I was dropping gears on hills, sometime 2 gears just trying to keep it going 50mph, that was very frustrating for me, now I could use a cruise control, I don't even have to move my foot on the pedal, it goes right up the hill, but I have to keep my foot in the pedal going down the hill also, my foot is in the same spot all the time at a given speed, at 75 to 80mph I am likely three quarters into the pedal all the time and I get just over 30mpg at them speeds, I do not know what I would get driving 55 to 60mph as I cannot drive that slow on the freeway, I expect close to 40mpg, the guy that owned the SD25 engine before I bought it from him said he got 48mpg, but he was also using propane injection, I still have that setup but don't use it.

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This pyrometer for whatever reason sets on whatever it was last when you turn off the ignition. When the ignition is turned on it resets to zero. And I planned sealed systems with respective reservoirs and pumps. I also note the low temp running down the road. I'm pretty sure I pulled the thermostat out of the housing and never replaced it. I only had one thermostat gasket and the thermostat I ordered came in wrong. 

 

It's also a tinge of blue on a cold start and that's what has me concerned. I've started this truck at 8*F and it did okay. I definitely know what you mean with the missing have a 6.2 Detroit that misses badly. I just don't feel like fighting the glow plugs to replace them (the military 6.2s glow plugs blow out badly then snap off on removal). The sd25 has fresh slow glow plugs in it and I have a tool to air up the cylinder for these diesels for seal replacement. The adapter is made by "Lang" and is a TU-15-3A if I read the box right.

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