Fuel primer mod., 1995 DTA408 (would also work on other older inline 6 cyl's))

[sojourner]

I know it's been a L-O-N-G time since I posted anything on my project. One reason is, I spent 8 months working away from home and couldn't make any kind of progress being over 500 miles away. I have been running the bus regularly after returning home and had an issue with the primer pump diaphragm cracked and causing an airlock in the fuel system. I tried to no avail to find an aftermarket fit so I decided to replace it with one modified to fit. The fuel primer on my engine is nothing more than a displacer. The check valves to make it a pump are in the fuel filter housing. Fortunately, the threads in the fuel filter body for the displacer are generic 1/4" FNPT so making an adapter to fit a primer off another engine is pretty easy.
First, I ordered a basic and generic primer pump used on too many engines to list here.





The threads on this unit are M16 x 1.50. I then ordered a brass 3/8" FNPT x 1/4" MNPT adapter:



The plan was to cut new threads into this adapter to allow the primer to thread into it. As it turns out, the primer threaded into the adapter, albeit too loose for my liking. There are a few ways to address this problem:

1. Helicoil the adapter with a M16 x 1.5 Helicoil insert.

2. Sweat in a 3/8" x 1/4" NPT bushing, drill and tap to M16 x 1.5 threads.

3. Mix up some JB Weld, put it on the threads of the primer body, snug it together and let the epoxy set up and seal things.

4. Since pipe threads are tapered, trim off the top of the adapter until I get a tight fit on the threads farther down the bore to get the strength to get a good crush on the copper washer to seal things off.

Since I have a lathe I chose option #4 with #3 as a backup plan.

I trimmed off 1/8" of the adapter and test fit the primer with the copper crush ring off. It fit much tighter but didn't allow the primer to fully seat. The fix for this was to run a 3/8" NPT tap in a bit until I got the primer to barely seat hand tight. Then to take it apart, install the copper crush washer, put it together and tighten it down. Worked like a champ with the ability to tighten down very securely in a vise. I then leak checked the whole assembly. Held beautifully with no leak down.

Here's the finished assembly next to the old diaphragm unit:




Here's the unit installed:



I will say, this modification is a HUGE IMPROVEMENT over the stock unit!! It will prime the system in a much shorter time than the OEM one that came on my engine (which took forever).
Hopefully this will help others out there!!

[Tango]

Brilliant mod! Many thanks for sharing. This could easily be adapted to just about any engine solving an all too common problem. Thanks again.

[djdalfaro]

Hi,

Sorry to resurrect this thread, but I'm going over possible common issues I may run into with my bus, in order that if/when they do happen I have a head start on how to solve them.

So I understand what a primer does (primes the pump), but the unit you've installed looks a whole lot different, and I see no bulb to pump. Can you give me a quick rundown on how it works?

Thanks,

[sojourner]

Originally Posted by djdalfaro

Hi,

Sorry to resurrect this thread, but I'm going over possible common issues I may run into with my bus, in order that if/when they do happen I have a head start on how to solve them.

So I understand what a primer does (primes the pump), but the unit you've installed looks a whole lot different, and I see no bulb to pump. Can you give me a quick rundown on how it works?

Thanks,

All a bulb, diaphragm or plunger does is displace fuel to draw through the check valves (inflow - plunger - outflow). I replaced my diaphragm type with a much more reliable, available and less expensive system.
It's just another way to do the exact same thing.