sd22 720 can push start but not crank start narrative

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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deezler
Posts: 1
Joined: 12 years ago
Location: SF eastbay california

sd22 720 can push start but not crank start narrative

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Post by deezler »

my sd22 720 pickup runs fine, but it's very hard to start by cranking, especially when it's hot.
Strangely it's easy to start by pushing it.

Battery and starter are strong.

I just drove it 80 miles to a wedding and back. Had to push start it to
get back. It runs well and has ok power at high rpms. lower than
expected power/torque at low rpms.

The fuel pump, glow plugs, filters and fuel flow/pressure are good.
The DPC is disconnected, injector pump control lever (off, run, start) is controlled with a string, I verified that's doing what it's supposed to. So the DPC circuit/motor isn't the culprit.

I have clear (parts unlimited) fuel hoses so it's easy to see/feel fuel flow, pressure, and bubbles. there are no air leaks. the fuel pump works. I put the new style bosch priming pump on it.
Fuel goes all the way through the system and back out the return hose,
both with the primer and with regular fuel pump while running. The overflow return check valve seems to open with the proper amount of pressure

Cranking does not blow white smoke out the tailpipe like it should.
There is no black smoke when starting or running.
While running, pinching the vacuum line to the driver's side of the throttle body makes the motor race. So the IP diaphragm is not broken.

The idle speed control knob stops the motor when pushed in all the way. When the truck was running its best, this knob did not do this.

Here's a clue. I once pulled the idle speed higher while driving in gear. The car lurched/surged a few times, even thought my foot wasn't on the accelerator. Maybe the jerking from push starting wobbles something important and that's why it starts then? I should try having someone bump my truck while cranking it...

More facts of the case.
The behavior is the same with bio and petro diesel.
I removed the TVV, thermal vacuum valve, a.k.a. California monster which blocks access to the rearmost glowplug. I did that a long time before the problem and it didn't seem to affect anything.
I removed the altitude compensator just now, it didn't solve the problem.

Just did a compression test, the two rear cylinders get 440psi and 420psi, I didn't check the front two cylinders because the test fitting is blocked by the presence of the injection pump. This engine is supposed to only have 50k miles on it, that's consistent with this good compression.

A friend just suggested maybe I have a sticky rack, the bump from push starting jars it so it moves.

any other theories? stuff to try?
if I get another injection pump, where and what kind should I get?
a viton rebuild kit for the injection pump? where to get?

thanks, Tim

p.s.
update - the fix: A friend's dad suggested it's a sticky rack. I saw the fuel was clear and brown sludge on the bottom of a fuel hose. So I worried that residual rubber from the old fuel hoses was coming out of solution and gluing random stuff together. I drained the petrodiesel and replaced it with biodiesel. I added a few ounces of diesel fuel additive snake oil containing "glycol ether" intended to "treat up to 240 gallons". Then push started the truck in reverse gear and ran it for a long time while doing vibrato on the gas pedal. That seems to have wiggled/dissolved things loose. So now it starts on the second glow by cranking.
I replaced the fuel hoses more than a year ago, but there's still apparently rubber in the fuel, so I better only use bio to keep it dissolved until I've done enough driving to consume the last of that resigual rubber.

is there a viton seal kit for this engine? viton fuel filter gasket?
1982 720KC SD22
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