Hot start problem resolved due to failed check valve in the DMCH fuel pump

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Originally Posted by Bitsyncmaster

Buna-N will swell with todays gas. Look at that O-ring after it sits out on the bench awhile. It should get smaller. I did a swell test measurement back on the old .com forum but don't have a copy of my results.

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Interesting. That might be why my old o-ring was noticeably larger. I did a lot of research on the material and it said it was compatible with gasoline. What other applications would Buna-N be used for?

I still have a Viton check valve on the way so I'll probably just swap it out and be done with it. Any idea how long it takes for Buna-N to deform?

Who has a good source for Viton o-rings for the PPR?

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Originally Posted by opethmike

Yes, you definitely have to use lubricant before sliding in.

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Yep, that's what she said...

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Originally Posted by kings1527

Interesting. That might be why my old o-ring was noticeably larger. I did a lot of research on the material and it said it was compatible with gasoline. What other applications would Buna-N be used for?

I still have a Viton check valve on the way so I'll probably just swap it out and be done with it. Any idea how long it takes for Buna-N to deform?

Who has a good source for Viton o-rings for the PPR?

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This is where I get mine:
http://www.mcmaster.com/#metric-o-rings/=p3nifs

I use 1.9 mm X 3.8 mm but I think the stock size is 2.0 mm X 4.0 mm. Either works but I guess the smaller one gets more seal since it has less surface area. Viton is a little harder then Buna-N.

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Originally Posted by Bitsyncmaster

This is where I get mine:
http://www.mcmaster.com/#metric-o-rings/=p3nifs

I use 1.9 mm X 3.8 mm but I think the stock size is 2.0 mm X 4.0 mm. Either works but I guess the smaller one gets more seal since it has less surface area. Viton is a little harder then Buna-N.

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I fail to see how a smaller one gets MORE seal with LESS surface area. :hmm:

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Originally Posted by DMC5180

I fail to see how a smaller one gets MORE seal with LESS surface area. :hmm:

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Constant pressure to less surface area will compress the o-ring more than a larger surface area.

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Originally Posted by Bitsyncmaster

This is where I get mine:
http://www.mcmaster.com/#metric-o-rings/=p3nifs

I use 1.9 mm X 3.8 mm but I think the stock size is 2.0 mm X 4.0 mm. Either works but I guess the smaller one gets more seal since it has less surface area. Viton is a little harder then Buna-N.

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Thanks perfect, thanks for the info Dave. So that'll be the size for the smaller o-ring. Do we know the size for the bigger o-ring? Or does that one prove to not be much of a factor for holding rest pressure when compared to the smaller o-ring?

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Originally Posted by kings1527

Thanks perfect, thanks for the info Dave. So that'll be the size for the smaller o-ring. Do we know the size for the bigger o-ring? Or does that one prove to not be much of a factor for holding rest pressure when compared to the smaller o-ring?

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Don't worry about the larger O-ring. It really stays dry and does not need to seal.

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Originally Posted by kings1527

Ok, so I got mine taken care of. I TRIED to order the Viton check valve from US Plastics and it was taking FOREVER to them to get them back in stock. So I went with the same check valve but with Buna-N instead since they had those. And of course, about 3 days after I order the Buna-N, they get the Viton back in stock. :facepalm:

So I installed it and yes, super easy to do. But I still had my hot start problem. After researching it, I know that lack of rest pressure is really only due to three things under most circumstances:

1) Fuel pump check valve
2) Accumulator
3) PPR O-Rings

Since the PPR O-rings were the path of least resistance at this point, I went there first. I had some that I bought from Hervey :nervous: about 6 months ago and put them in last night. Another 15 min job, if that. I was a little nervous when I got the old o-rings out because there wasn't anything obviously wrong with them. Once I took the old small o-ring off and put it next to the new small o-ring, the old one was noticeably larger than the new one. That was the only difference. But Hervey's new o-rings were a perfect, snug fit (thankfully and with intense relief) and I didn't have any problems getting them back in and damaging them along the way. I put a very light coat of silicone paste on the o-rings to help facilitate sliding everything back in, just in case.

And that was it. The DeLorean starts up just as fast as my 2013 Passat, whether hot or cold.

Doc Brown in reference to the PPR O-rings: "Unbelievable that this little piece of junk could be such a big problem".


Thanks for the help!

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Glad to hear that you were finally able to figure out the problem! It's great to finally have a car that you know will start every time, isn't it?

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Originally Posted by Bitsyncmaster

Don't worry about the larger O-ring. It really stays dry and does not need to seal.

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Great, thanks Dave.

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Originally Posted by NightFlyer

Glad to hear that you were finally able to figure out the problem! It's great to finally have a car that you know will start every time, isn't it?

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There's hardly anything worse than trying to start the car and nothing's happening! And it always happens in front of a group of people. Then you're just waiting for the jokes to start flying.... :)

It'll be nice to get the Viton check valve and PPR o-ring in place and be done!

So for anyone who's interested, it's been about two months since I put a regular (assuming) Buna-N o-ring in my PPR plunger and my car just started doing the same thing as I described earlier. So I guess one could say the average life span before Buna-N will deform is about two months. Dastardly ethanol.

Today I installed the nitrile o-ring with the size Dave M mentioned and I'll report back if there are any problems. If I pass the two month mark, I'll take it as a good sign.