SD25 Blow-By Query

SD diesels were widely available in the US in the 1981-86 Datsun/Nissan 720 pickups, and in Canada through '87 in the D21 pickup.

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gregsimms
Posts: 2
Joined: 1 year ago

SD25 Blow-By Query

#1

Post by gregsimms » 10 months ago

Hi folks. I have a 1985 720 running a SD25 with 160,000 km (99,400 miles) on the clock. My question/query is about blow-by and how much you are seeing, if any, in your SD25.

I was reading that if you are getting a significant amount of gas escaping from the oil cap, then you are generating poor compression and likely experiencing blow-by. Doing this test on my SD25 I get a steady put-put of gas escaping from my oil cap, enough to flutter the cap when placed on top. Bad news. I was pretty surprised given the low mileage on the engine. I also have great starts at low temperature and still getting the factory 7-8 L/100km (29-33mpg) fuel mileage. Oil does not feel diluted by unburnt diesel, but I don't trust myself. I have noticed low power at highway speed, but what should I expect with 70hp diesel.

Are you experiencing the same amount of gas coming from your oil cap port? Is there a blow-by tube on these models? Could a malfunctioning crank case ventilation be causing this? Yes, I do plan on doing a compression test.

I appreciate all your help! Any and all suggestions are welcome.

waynosworld
Posts: 517
Joined: 7 years ago
Location: Vancouver Washington USA

Re: SD25 Blow-By Query

#2

Post by waynosworld » 10 months ago

Welcome :)

I have never really checked to see how much blow-by either of my SD25 engines have, but I will say I found an SD25 engine on the side of a guys house that likely had less than 50,000 miles on it and it was great around town but gutless at freeway speeds, not as gutless as an SD22, but gutless never the less, whenever I came to any hill/incline on the freeway I usually ended up floored and half the time I ended up dropping a gear just trying to keep it moving above 50mph, but I have a lead foot as I drive 75mph on the freeway, I drove it this way for 7 years and I am lucky that I didn't blow that engine up after learning about exhaust gas temperatures(EGTs), as I put an EGT gauge in my daily driver and was very surprised how fast these engines get when you floor them, it only takes 2 minutes floored for the EGTs to rise above 1400 degrees going up an incline/hill and this is without a turbocharger.

A couple years ago I turbocharged my SD25 engine in my 521 kingcab, this is a very long story but I will keep in short in this thread, I should have turbocharged this engine when I bought the turbocharger for it 6 years before I actually did turbocharge it, you see I bought another vehicle that had a turbocharged SD25 in it, but it really was not set up for a regular guy to drive, the EGTs would get way to high just trying to drive down the freeway at 70mph, it appeared to be alright at 50/55mph the way the guy I bought it from had it set up, I was afraid I would blow it up so it sat in my driveway for a year, then one day I decided to figure it out and I finally did, I then turbocharged my 521 kingcab and since that day I decided I would never drive a diesel vehicle again without a turbocharged engine, it is a pleasure to drive now and I get better mileage than I ever have, and the EGTs are lower than they have ever been considering the way I drive.

Here are my 2 threads about my turbocharged vehicles, the way I did it is not the only way, but with the turbocharger I used on both engines it is the only way to do it with them turbochargers.

http://nissandiesel.dyndns.org/viewtopic.php?f=5&t=3380

http://nissandiesel.dyndns.org/viewtopic.php?f=5&t=3474

I would suggest you put the oil cap on, start the engine and make sure that the crank case breather tube near the starter is not plugged, if it has no blow-by there then you need to figure out why.
71hp is not a lot, especially for a 3000lb vehicle, diesel engines are known for having torque, not HP, with my turbocharged SD25 engine I get about 31mpg going 75/80mph down the freeway, I get 25mpg in the city, if I could drive 50/55mph I would likely get over 40mpg but I cannot drive 55mph, I did make a 120 mile trip once in another 720 with an SD22 in it that I drove under 55mph and I got 35mpg.

I will start my truck up in a little while and see how much blow-by it has and let you know.
I know the voices are not real,
but they have some really good ideas.

gregsimms
Posts: 2
Joined: 1 year ago

Re: SD25 Blow-By Query

#3

Post by gregsimms » 10 months ago

Hey, thanks for the reply. Not sure what the information on the turbo stuff has to do with it, but it's interesting. Let me know if you did the oil cap test.
cheers

waynosworld
Posts: 517
Joined: 7 years ago
Location: Vancouver Washington USA

Re: SD25 Blow-By Query

#4

Post by waynosworld » 10 months ago

I suppose I wrote all that stuff about the turbocharger because to me it seems like I was beating the engine to death trying to keep up the speed limit on the freeways without a turbocharger, it is 70mph here in WA state, I was using 2/3rds of the pedal or more just driving down the freeway, now with the turbocharger if I go 65/70mph I use only the top 1/4 of the pedal even on the hills, it will go right up that hill without moving my foot, a cruise control would actually work in this truck now.


OK, I went out and started my engine and pulled the oil filler cap off and at an idle it had quite a bit of blow-by, but when I revved the engine a little the blow-by went away or it was a lot less, I could not tell if it went completely away as there is a lot of air moving around in there as I have a custom setup, there is no radiator shroud.
I know the voices are not real,
but they have some really good ideas.

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