Surging or hunting at idle

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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mbehensky
Posts: 4
Joined: 12 years ago
Location: Soquel, CA

Surging or hunting at idle

#1

Post by mbehensky »

I just purchased a 1981 Datsun diesel pickup with 217k miles. I got it fairly cheap because it won't idle smoothly (unless you turn the mechanical governor up to the point where it has no power at all). After reading various posts here it seems like it probably has something to do with the pneumatic governor (as a Mercedes diesel man I'd never heard of such a thing before).

Is this likely to be the diaphragm? It seems like stiffness in the diaphragm or some sort of stickiness in the linkage could cause it to have a hard time settling on an rpm.

I found another post about surging at medium speed which mentioned an air leak at the intake/exhaust gasket, but it was unclear where that was.

Thanks for any help...

Max Behensky
1981 Datsun 2wd pickup
1982 Mercedes 240D
1987 Mercedes 300D wagon (cracked head)
1981 Mercedes 300D 2.5 Turbo (bad transmission)
Nissan_Ranger
Posts: 267
Joined: 14 years ago
Location: Canada

#2

Post by Nissan_Ranger »

There are any number of reasons that can cause rough idle and surging. A perforated diaphragm will usually manifest itself as decreasing throttle responsiveness and rapid loping idle with black exhaust smoke. Those problems increase as the breach in the diaphragm increases.
Rough idle can be caused by:
Poor fuel quality (try changing supplier and install fuel conditioner)
Intake manifold gasket air leaks (Check fasteners)
Cracked/leaking butterfly control hoses (Check and replace as needed)
Sticky injectors/poor spray pattern (Try Fuel conditioner and lube additive and proceed to an injector tune-up if one hasn't been done and the additive has no positive effect.)
In any case, you should ensure that your fuel system has been recently serviced.

HTH,

N_R
The old 'six gun' was as popular as the cell phone in its time and just as annoying when it went off in the Theater.
Tracteur Tom
Posts: 11
Joined: 12 years ago
Location: France

#3

Post by Tracteur Tom »

Also check out all the fuel lines for leaks and change the fuel filters. The diaphragm is easy to check, just release the 4 cover bolts. Do the simple things first before worrying about injectors etc.
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asavage
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Re: Surging or hunting at idle

#4

Post by asavage »

mbehensky wrote:. . . something to do with the pneumatic governor (as a Mercedes diesel man I'd never heard of such a thing before).
:) If you were a real Mercedes diesel man, you'd be extremely familiar with the pneumatic governor, as it was the exclusive province of MB through at least the 200D (and I think the 220D). With the 240D, MB finally dropped the pneumatic governor.
Regards,
Al S.

1982 Maxima diesel wagon, 2nd & 4th owner, 165k miles, rusty & burgundy/grey. Purchased 1996, SOLD 16Feb10
1983 Maxima diesel wagon, 199k miles, rusty, light yellow/light brown. SOLD 14Jul07
1981 720 SD22 (scrapped 04Sep07)
1983 Sentra CD17, 255k, bought 06Jul08, gave it away 22Jun10.
mbehensky
Posts: 4
Joined: 12 years ago
Location: Soquel, CA

Problem Solved

#5

Post by mbehensky »

Well, I bought a new diaphragm from southwest diesel (http://www.swdiesel.com) and took out the old one. It had a huge hole along one of the folds, and after replacing it the truck runs smooth as silk.

There was a huge amount of solidified cod liver oil in the area behind the diaphragm; I cleaned it out with solvent and q-tips before putting the new diaphragm back. I tried 4cc of synthetic engine oil as lube; some people say its OK and it can't work worse than the fish oil.

If you do this job, here are some tips:

1. It is a pain in the butt to get the cotter pin out of the pin to remove the diaphragm. Right angle long nose pliers would be a huge help (I couldn't find mine and spent an hour poking at the cotter pin with screwdrivers trying to straighten it out).

2. As others have stated, the new diaphragm comes with a spring loaded inner section that goes through the pin. My diaphragm had it tied loosely in place through the pin hole. If you're not careful when you put the new diaphragm in place, this will come out and you may loose the spring or insert. I was working on my driveway at night, and managed to find this after it fell out through pure luck. Be careful.

Max Behensky
1981 Datsun 2wd pickup
1982 Mercedes 240D
1987 Mercedes 300D wagon (cracked head)
1981 Mercedes 300D 2.5 Turbo (bad transmission)
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