PPR O-Rings

[sdg3205]

Gentlemen,

I'm working through a hot start issue on 4916 under the tutelage of Toby.

Here's the testing I've done:

1. The accumulator holds an indefinite vacuum of 19 in Hg, so no tear in the diaphragm is apparent.
2. PPR removal shows the usual problem o-ring on the nose cone as fine, as is the rear one. The tiny little black one (see below) is however all chewed up.
3. With the PPR back in place in its current state, my K-jet gauges (in line at the distributor input) show the system primes to 5.5 bar, but upon release of the jumper, drops completely in less than 30 seconds.
4. Cold start valve is not leaking - tested.
5. All injectors are new (cracked and mixture adjusted earlier)
6. Now, same test with the jumper - prime the system to 5.5 bar, clamp off the fuel send line from the pump (kill the jumper) and the system holds 2.75 bar for 45 minutes.

This tells me the pump check valve is toast.

BUT

Is 2.75 bar of rest pressure for 45 min OK? Or is that low and possibly due to that tiny little black o-ring. If so, does anyone sell a kit with ALL 3 PPR o-rings (PLUS the copper washer)?

Special T seems to sell just the 2 main o-rings and the copper washer

IMG_3806.jpgIMG_3805.jpgIMG_3804.jpgIMG_3803.jpgIMG_3802.jpgIMG_3801.jpgIMG_3800.jpgIMG_3808.jpgIMG_3807.jpg

[Bitsyncmaster]

I think only the small O-ring at the end off the PPR is what holds rest pressure. I don't think you even need the other O-rings to hold rest pressure.

[Ron]

Seems it could do this?
Leak_PushUp.jpg

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PPR_O-RingLocations.jpg

[sdg3205]

Interesting Ron. Any idea about a replacement for that little guy?

[Ron]

I dug one out of a pile of old trans kit boxes (all nitrile).
I've only had one (far right in pic) cause a problem. I caught it when I went through the FD, but missed a sliver that wound up stuck through the relief valve hole...
I can match one to one in a spare FD and send ya one? PM me where.

[Rich]

Now, same test with the jumper - prime the system to 5.5 bar, clamp off the fuel send line from the pump (kill the jumper) and the system holds 2.75 bar for 45 minutes.
This tells me the pump check valve is toast.
BUT
Is 2.75 bar of rest pressure for 45 min OK?

Yes, it's fine.

According to p.D:02:01 of the DMC Workshop Manual:
Primary Pressure

Checking value: 4.9-5.5 Bar
Setting value: 5.1-5.3 Bar

Rest Pressure leakage test

Rest Pressure: 3.3 Bar
Min. Pressure after 10min: 1.7 Bar

So it looks like the Primary Pressure is OK and your system - when you clamp the fuel hose - passes the Rest Pressure Leakage Test by a significant margin. And that was done with the old o-rings in place, meaning they were OK at least relative to the hot start and the rest pressure.

I don't know about using vacuum to test the accumulator. What matters is how it acts when there's fuel in it at 5 Bar or so. Did you test the Rest Pressure with hose clamped between tank and accumulator? That would result prove everything past the clamp - including the accumulator - is OK.

When your system can pass the Rest Pressure Leakage Test without clamping any hoses then your hot start problem will be gone.

[Bitsyncmaster]

The O-ring I use is a Viton size 1.9 X 3.8 mm.

I think this is the part number:

http://www.mcmaster.com/#9263k637/=to9s3g

I think stock size is 2.0 X 4.0 but I like the smaller one so the piston does not stick inside the FD.

[Ron]

I dug one out of a pile of old trans kit boxes (all nitrile).

OOPS! I meant to say (all viton)"

[David T]

Yes, it's fine.

According to p.D:02:01 of the DMC Workshop Manual:
Primary Pressure

Checking value: 4.9-5.5 Bar
Setting value: 5.1-5.3 Bar

Rest Pressure leakage test

Rest Pressure: 3.3 Bar
Min. Pressure after 10min: 1.7 Bar

So it looks like the Primary Pressure is OK and your system - when you clamp the fuel hose - passes the Rest Pressure Leakage Test by a significant margin. And that was done with the old o-rings in place, meaning they were OK at least relative to the hot start and the rest pressure.

I don't know about using vacuum to test the accumulator. What matters is how it acts when there's fuel in it at 5 Bar or so. Did you test the Rest Pressure with hose clamped between tank and accumulator? That would result prove everything past the clamp - including the accumulator - is OK.

When your system can pass the Rest Pressure Leakage Test without clamping any hoses then your hot start problem will be gone.

Are you SURE you clamped the right hose? If you clamped the return hose by mistake the PPR could still be leaking and the fuel pump check valve is OK. I also do not think you can properly test the accumulator by using vacuum.

[Ron]

I also do not think you can properly test the accumulator by using vacuum.

+1

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Just in case- I didn't mean to imply that the tiny o-ring was causing the problem, just that it could have.. It (because it is chewed) and a check valve (failed test) sounds like the the best route to me.

FYI- "In order to fully seal off the control-pressure circuit with absolute certainty when the engine has been switched off, and at the same time to maintain the pressure in the fuel circuit, the return line of the warm-up regulator is fitted with a check valve. This (push-up) valve is attached to the primary-pressure regulator and is held open during operation by the pressure-regulator plunger. When the engine is switched off and the plunger of the primary-pressure regulator returns to its zero position, the check valve is closed by a spring." (Gasoline Fuel-Injection System K-Jetronic, pg20) [See 2nd pic above]