Failed 26 year old original db2, what’s inside? What goes wrong?

Thewespaul

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Tearing down some cores today and found I had a factory sealed db2831-5028 in my pile. This is a non turbo calibration for 92.5-94.5 7.3 idis with a torque screw housing.

When these pumps fail, it typically starts out as driveability issues, non functioning advance, poor idle, smoking exhaust, that develops into hot start issues and eventually no starts, I’m going to show in this tear down where these problems originate in the pump, what parts are failure prone, address some db2 myths and go through some of my solutions when I rebuild them.
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First comes off the top and side cover to drain the fuel, then nose cone is removed to pull the transfer pump assembly. The transfer pump blades and liner are the first moving parts to contact fuel, so these components are usually quite telling for what the rest of the pump will look like. This liner and blade set shows very normal wear with no large particles in the inlet filter or rust/discoloration from water or other fluids getting into the fuel. Good signs.
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This liner and blade set would be performing fine with this wear, so the ability for transfer assembly to move the proper volume for actuating the advance system and filling the plungers is not the cause of this pump failure. Remember this pump creates volume displacement, not pressure.



Next the governor assembly is removed, your biggest wear item here is the metering valve. Normal wear on this metering valve, but noticeable uneven wear on the back side of the valve due to the design of the governor assembly putting pressure on the back of the valve at all times.

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Thewespaul

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Next I remove the advance system, the light load advance arm showed pretty good wear with the roller worn down to the point of damaging the face cam. The roller will need to be replaced along with the face cam in this situation.
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Next comes off the advance piston plugs, the driver side plug has an internal seal for the smaller advance piston that actuates the internal advance servo inside the advance piston. This seal is a common leak point. Notice the wear on the smaller advance piston and scarring on the advance servo.

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Thewespaul

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Next the main advance piston is removed. This is a hardened steel slug with a chromium coating sliding in an aluminum housing, it depends on the tolerance between the bore and piston to create a differential of pressures between the driver side with housing pressure and the passenger side with transfer pressure to create an operable advance curve. Without a proper tolerance to seal the bore from transfer side pressure that is the 10x the housing pressure side there is a significant volume loss due to leak by into the the housing, and out the return instead of being used to fill plungers and actuate on the piston.

Notice the significant scarring and wear on the advance piston. It is completely through the chromium layer and has transferred some of the aluminum housing onto the piston, this gives transfer fuel a path straight out the return.

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With the cam pin and head locating bolts removed, out comes the head and rotor assembly.
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Thewespaul

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With the rotor locks removed, we can start inspecting the heart of the pump, the head and rotor assembly. Looking into the head bore, we see some faint wear on the discharge port side that is mirrored on the rotor right pass the discharge ports. This is acceptable wear, and outstanding for the pumps age. This head and rotor has much life left in it and will run again in a 90cc pump.
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Next I look at the consumables, the cam ring shows normal wear without any pitting so it too will not need replacement, the plungers themselves show minimal wear but are an earlier plunger design without the lubricating rings so I will be replacing and fitting these to the rotor with low lubricity plungers. This core is turning out to be a good donor.
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Thewespaul

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Now for the last bit of disassembly, put comes the delivery valve assembly, very minimal wear here on the delivery valve and stop, these were working fine but I will be replacing the valve with a higher flowing upgrade, and replacing the stop and spring to ensure a clean idle and injection cycle.
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The shoes and rollers show some pretty good wear, both these will be replaced.

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With the snap ring removed, the driveshaft can be pulled out of the front of the housing and the bearing, thrust plate and thrust washer inspected. These parts look really good for their age, with the thrust washer of wave washer taking the brunt of the wear. If you ever get a pump with significant in and out play on the driveshaft this is your cause, it’s more common for this thrust washer to be in pieces than intact by the time I get them. The front section of the shaft that the bearing inside the front of the housing rides on is lubricated by engine oil, there are three separate cup seals that keep diesel and oil separated, judging by the lack of elasticity in the seals and the runnieness of the oil in the bearing these seals were leaking diesel into the crankcase.
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Thewespaul

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Now we have a bare housing and head and rotor assembly, let’s talk about what went wrong here.
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It’s evident that the advance piston showed the most dramatic wear, and it’s definitely responsible for this pump’s failure to start hot. The plunger bore is immeasurably worn, and the rotor to head fit is unchanged from factory, the claims that after 100k miles the plungers are worn out and cause hot starts or the head to rotor fit deteriorates to the point of not starting is terrible misinformation. These pumps need a certain amount of volume from the transfer pump to actuate the advance and get the plungers moving in a hot start situation where tolerances are opened up the most. The volume doesn’t change at cranking unless the transfer pump fails, it’s the tolerance of many moving parts that creates a lack of bypass for that transfer pump to create pressure. If there’s a significant leak at the advance piston, metering valve or any moving part exposed to both transfer and housing pressure the volume created by the transfer pump just goes out the return with minimal pressure since there is very little resistance to flow.

That sort of situation is at its worse when the pump is hot and these tolerances at their greatest. When cranking the engine spins about 150 rpms. The housing pressure inside the pump is typically 10-15 psi, if you don’t have a good delta between that and transfer pressure you will have zero plunger movement and no fuel injected since pressure is at an equilibrium on both sides of the plungers. Stanadyne requires at least 20 psi at cranking to create a 5-10 psi delta between both sides of the plungers to get ample movement and displacement to start the engine.

So what can be done to prolong your pumps life and prevent these issues? The biggest killers for these pumps is lack of lubrication, filtration and contaminants. Make sure your filtration system is well maintained and you get good quality fuel, upgrade your filtration system if you want to really extend your pumps life and don’t ignore air intrusion issues as they will cavitate these parts and wreck havoc over time. Use a good lubricity additive, I won’t name brands do your own research and decide what you think works best for your application and budget, anything is better than nothing, and if you want your fuel system to last, don’t run a tank on dry diesel alone.

Now onto part 2, where we get to do some upgrades for performance, and endurance!
 

aggiediesel01

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That’s really cool info Wes, thanks for sharing. My question is how on earth does the advance piston create so much wear? It moves back and forth as much as what, maybe 1/2” all while bathed in fuel? I get that the hardened chrome on the advance piston is harder than the aluminum and the wear will be on the softer material but as compared to other steel to aluminum wear points in the pump this seems excessive. Is there some other inherent design issue here like maybe a resonant vibration or insufficient load bearing area for the mass of the piston?
 

Thewespaul

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They all get that way very quickly, in a matter of 20-30k miles, it’s the lowest point of the housing so contaminates end up in that bore when the pump is shut off, and with the cam pin going through the advance piston into the cam ring you have upwards pressure on the advance piston pushing it against the top of the bore. There’s also no other steel on aluminum moving components in the pump, everything else is steel, nylon or brass bushed. This part is pretty much in constant motion as well when under regular operation.


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ISPKI

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Just a hunch but, while chromium and hardened steel are extremely hard, aluminum oxidation is harder. We send tooling fixtures made of aluminum out for anodizing for exactly this purpose, can't even scratch it with a surgical blade. The aluminum likely degrades slowly over time, losing oxidized particles and depositing them on the chromium surface, might end up acting like super fine grit sand paper. If any water was introduced the system in any volume, that would probably accelerate the oxidation reaction exponentially and result in rapid wear of the components. That suggests that the lubricants we use aren't solely beneficial for their lubricating properties but also for their ability to displace rogue oxygen and provide a barrier on the aluminum bore.
 

hacked89

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Great writeup Wes. I see the cam ring in its solo picture and in the head and rotor one, but can you describe how it functions in the perspective of controlling fuel flow?

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