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

[jawn101]

1) PLEASE - install an external inline check valve. Trust me, you'll be glad that you did. It's $18 and all of 20 minutes of time. It's definitely worth it IMHO!

2) When you installed the pump, are you sure that you had the bottom of the pump as close to touching the bottom of the tank as possible? The unit, as you probably know, is spring loaded and isn't very smooth in it's action when decompressing after being compressed (it's easy to get it stuck in a compressed state). It also doesn't fit the tank when fully decompressed, thus some level of compression is necessary. I test fit mine about 4 times before I was able to get it where the bottom of the pump was just barely off the bottom of the tank. I've only been running mine for the last 3 months and about 4,000 miles, but have always run it down low enough to trip the low fuel light. No issues such as those you've described thus far.

That's about the only thing that I can think of.

You may also want to check the RPM relay - if that's opening/closing while you're driving, it could potentially cause what you're describing as it's stopping and re-engaging the pump.

Best of luck!

Yes, an external check valve is on the list for the rainy season when it starts in a month or so. Peace of mind is worth everything to me.

I don't actually know for sure about the pump being compressed fully. That might be a factor, but would the gauge be off if it wasn't fully compressed in the tank? Or is the float attached at the top? I can't remember.

RPM relay is already Dave's solid state one and works fine under all circumstances - this issue only crops up when the tank drops low-ish.

[NightFlyer]

Yes, an external check valve is on the list for the rainy season when it starts in a month or so. Peace of mind is worth everything to me.

I don't actually know for sure about the pump being compressed fully. That might be a factor, but would the gauge be off if it wasn't fully compressed in the tank? Or is the float attached at the top? I can't remember.

RPM relay is already Dave's solid state one and works fine under all circumstances - this issue only crops up when the tank drops low-ish.

If I'm recalling correctly, the float is attached near the middle of the pump body. However, the arm is long and contains a near 90 degree bend, thus allowing it to extend above the top and below the bottom of the pump. So, the float will still read properly even if the pump is over-compressed in the installation.

Remember, you want the pump as fully DE-compressed as possible in the installation.

[NightFlyer]

If I'm recalling correctly, the float is attached near the middle of the pump body. However, the arm is long and contains a near 90 degree bend, thus allowing it to extend above the top and below the bottom of the pump. So, the float will still read properly even if the pump is over-compressed in the installation.

Remember, you want the pump as fully DE-compressed as possible in the installation.

OK, the mount/pivot point is actually towards the top, but the angle of the rod is a full 90 degrees. Recalling my pump, I distinctly remember the float bottoming just slightly below the bottom of the pump.

I'd also check for cracks in the plastic housing - maybe air is being introduced that way...???

[jawn101]

OK, the mount/pivot point is actually towards the top, but the angle of the rod is a full 90 degrees. Recalling my pump, I distinctly remember the float bottoming just slightly below the bottom of the pump.

I'd also check for cracks in the plastic housing - maybe air is being introduced that way...???

Interesting. So I should remove the pump assembly and make sure it's fully extended to the bottom of the tank. That makes sense, but I'm not looking forward to it. Maybe I'll do it when I splice in that check valve.

[NightFlyer]

I am considering re-installing John's power pump setup and seeing what happens. I'm getting a bit tired of this situation. I will prolly do that over the weekend and let everyone know my status if it changes anything.

I'm curious as to whether or not you tried this yet, and if so, what the results were...

[andy blackmon]

Today I installed the check valve in my fuel line. I had an on and off hot start issue and I figured what the heck the valve can't hurt. For anyone on the fence with this, hey the cost was around $15 and it took me around 15 minutes to put it in.
Andy B 03513

[DeLorean03]

I'm curious as to whether or not you tried this yet, and if so, what the results were...

Didn't get to it. Here's why. I have unhooked my CSV, and ever since then, my car fires nearly flawlessly all the time. It's something in the cold start system, and since Eddie is about to fix that permanently, I said "heck with it".

Car heads to him a week from Monday, and it's 55 here for the high, so yeah, the days of driving the DeLorean for 2013 are pretty much over.

[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.

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 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!

[opethmike]

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

[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.