Surging or hunting at idle

[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

[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

[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.

[asavage]

. . . 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.

[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