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intermittant advance on 93' 6.5 but the cold advance always works

Could be a worn piston bore,and too much psi leaks past after the fuel is hot. Usually if the piston is stuck it will stay that way. Do you have lots of white smoke ?
 
I have white smoke..a little blue in it when going down hill and giving it a little fuel. It won't take just a little fuel you have to get those rev's up before it can burn it. But if I click the cold advanced solenoid on. The timing is advanced and no problem.
 
nealpellecchia said:
I have white smoke..a little blue in it when going down hill and giving it a little fuel. It won't take just a little fuel you have to get those rev's up before it can burn it. But if I click the cold advanced solenoid on. The timing is advanced and no problem.
Click to expand...

"a little blue in it when going down hill and giving it a little fuel"
DON"T DO THIS!!! These old IDI engines are famous for smoking on downhill runs with a little fuel. All you are doing is washing the rings in diesel. Either get on it or stay completely off the throttle in situations like this. Don't give it just a little fuel as the white smoke says it isn't being burned. Getting on it puts enough heat in the engine to burn the fuel after the downhill cooled everything off. The Precups, glows, and anything hot cools off on a downhill run with the fuel off from just airflow through the engine. You have to get things hot to burn fuel again and will have some rattle as the precups heat back up.

As you have seen there isn't a "automatic" way to compensate for a long downhill run that cools the precups off. Turning the cold advance on as you have seen helps, but, as soon as the precups quickly come back up to temperature you are suddenly running too much timing stressing everything and eroding glow plugs.

If you don't have any other issues this is "normal" for this engine and just requires driving habits to match the engine design limits. (Yes, cruise control can smoke you white on downhill grades so you may have to turn it off on some hilly terrain.) As posted above the DB2 has a limited life before it needs to be refreshed. A bump in advance of the pump timing to the engine can buy you more time depending on other issues.
 
What are your other symptoms aside of smoke going downhill? How many miles on the IP? Again you may not have a problem.

The advance piston repair requires a pump rebuild and isn't done on the engine. It's not a bad job to change the IP. A bad job is changing the starter or PS pump... :eek:

The timing is controlled by pressure on the advance piston. If the piston leaks it bleeds pressure off and the wrong 'retarded' timing happens. When the cold advance turns on it slams the door on full pressure (should say flow) beyond what the normal advance uses and overcomes even serious piston leakage. Cold advance is just brute force vs. the normal timing flow/pressure.

Think of a worn shock absorber still pushing the rod out even though it is too worn out to control the vehicle.
 
377,000 mi. sluggish when not working very peppy when working. it needs pressure to advance yet cold advance causes no pressure and advances it to the max. should say flow? where? So cold advance slams the door on the leak at piston?? I almost got it. Is there a diagram? I assume advance turns the rotor or head. How does the pressure do that and 0 pressure do that too?
 
I was ticketed once for it all the way down the hill. Of course you don't see it until your told or you happen to notice it. Yes I coast and then give it a quick gun and it pops a little smoke and then takes the fuel. yeah unburned fuel is what I thought. But cold advance doesn't heat anything up it just allows the accelerator to keep up with the engine speed. I think.
 
You need more light load advance. Adjust the retard cam and the allen screw. Do the cam first ,probably hitting the ramp to early .
 
yeah, when we tried to push in the piston with a screw driver a couple of years ago they said it didn't do much but they were trying to sell me a rebuild job. Well over 100,000 miles ago. maybe 200. ha ha. But I picked a pump up off an engine and studied the linkage and saw the arm on the side that pushes the piston with throttle pressure directly. Is your cam attached to this? Pictures anywhere? Thanks.
 
The cam that pushes the arm,its fastened to the throttle shaft,but can be adjusted. If you have tps or vacuum regulator its behind that.
 
Best explanation of how it works link

With that many miles likely you need a rebuild. The cold advance overwhelms the leak so it advances anyway, but, normal advance is ruined by the internal leakage because the flow is less. Less flow into a leak = less pressure. Less pressure = less movement of the piston thus less advance.
 
Timing is achieved by balancing housing pressure with transfer pump pressure and the "leaky" piston is in between. Housing pressure goes to zero so the advance piston gets 100% of the transfer pump pressure and flow to slam it full advance. Without cold advance the housing pressure is higher and the piston leaks - well there isn't as much force available and the piston due to the leak isn't as advanced as it should be.

Their explanation of the light load advance is confusing and in error. From the site:

A light load advance mechanism provides advance when the engine is operating at low speed or under light load, when the transfer pressure is too low to move the advance piston.

The light load advance is actuated by an external face cam and rocker lever assembly when the throttle shaft rotates (on the 6.2L and 6.5L engines, this mechanism is on the passenger side of the pump). The lower end of the rocker lever pushes on the end of the servo advance plunger.

As the throttle shaft rotates, the face cam pushes on the rocker lever using a “see-saw” action, which depresses the servo plunger and advances (No opening the throttle and ramping the lever on the cam retards the timing. The correct word here is retard... and in more ways than one.) the timing through the power plunger’s linkage to the cam ring. At a predetermined angle, the face cam flattens out, so that additional throttle movement does not affect the servo.

http://www.oliverdiesel.com/tech/howdb2pumpworks.htm

The HPCA solenoid is located under the fuel return outlet, under the pump housing cover. It is activated by the coolant temperature switch, which is mounted on the rear of the passenger side cylinder head. When coolant temperature is low the temperature switch is closed, energizing the HPCA solenoid (rear pump terminal connected with a green wire), which lifts the check ball off its seat in the return outlet. This reduces housing pressure to near zero, so that the transfer pump pressure behind the power advance piston can easily advance the cam ring.
 
At the risk of a double post lets re-visit the light load retard lever. With the engine at idle use a screwdriver or crowbar and depress the bottom of this lever all the way in. The engine should go retarded and stumble badly possibly even smoking white. This proves that this retard mechanism is working. If the engine stays running the same with the lever pressed in you need a pump rebuild. Lets see: you can push the pin in and retard the timing at idle - this means the lever doesn't provide advance as you press the throttle proving the site error and confusion.The advance is already in the pump via static timing to the engine. So the lever reduces timing as you increase throttle.

This mainly has to do with ignition delay of diesel after it's is injected. Same RPM same throttle can have different total timing due to the temperature of the precup. Hot precups evaporate and light (burn) diesel faster thus giving you the peak torque at an earlier crank angle after injection. High throttle generally means hotter precups to a point temperature doesn't matter.

As you have light load indicated by light throttle naturally there is less heat in the engine specifically the prechambers. So it takes longer for the diesel injected to evaporate and burn. So the pump is advanced by static timing to provide advanced timing for this condition. As you demand more throttle the load goes up and you have more heat in the precups. At this point you don't need the extra timing as the fuel evaporated and lights faster - same RPM just different load and fuel. So to reduce timing the lever runs up the cam as the throttle increases reducing timing.

I don't recommend this due to range limits. But take the downhill smoke example at very light throttle that is fixed by the 6 degree cold advance. You could advance your pump on the engine enough to stop the smoke at very light throttle. Then you compensate for the advanced timing by bringing on the light load timing retard quickly by tightening it up via the screw adjustment and adjusting the cam position on the throttle shaft. Problem is with this theoretical adjustment is where the cam ramp flattens out: when you reach the flat spot you need more retard to continue adding throttle without going too advanced.
 
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It appears the site I linked to has been "dead" since 2005. So I doubt I can get them to correct it. It has clouded my understanding for years.
 
Truck drove beautifully last night. Then down hill with a little gas to keep up speed and it took all the gas, no smoking as advance kept up with the rpm's. Lots of fuel left in tank, usually out after this trip. No smoking, peppy, just feed it fuel and effortlessly it performed, it was a pleasure. How come? Intermittent fuel tank problem? It was a concern once. Drilled a hole in the gas cap. Good tank of green bio diesel? I mean really green.
 
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