Fuel pump sending unit cap keeps popping off

[Stainless]

I had a strange issue with one part of the new DPI fuel pump kit that I thought would be worth passing along. The kit is top notch and is working flawlessly, but the cap that goes in place of the original fuel sender would not stay on. I started to wonder if the tank's threads had worn out or if I was cross-threading the cap on the tank. It would seemingly go on tight, but the slightest nudge and it would pop off. Here's a quick clip showing how easy it would come off:


https://www.youtube.com/watch?v=VYWyni7Ra80&feature=youtu.be

This came to light after I filled up with fuel for the first time since the install and fuel came gushing out from under the car. It was a huge mess. I should have known something was up with how much fuel I was putting into the tank. When I was able to take apart the trunk to see what was up, the cap was just sitting there, not installed. After that is when I took the video. I snugged it down and left it overnight, only to discover that it was off the next morning, which means it was coming off just from the small pressure inside the tank too.

Upon inspecting the new cap with the fuel sender I had taken out, I discovered that the new cap had an extra o-ring that the former did not, and the o-ring did not seem to fit right or even belong there. This is what it looked like:

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I think the o-ring was preventing the cap from being tightened properly. Once I took the o-ring out, it more closely matched the cap from the fuel sender that I had taken out that had never had any leak issues. I snugged down the cap without the o-ring and I'll be topping it off with fuel to make sure that it fixed the issue.

[Helirich]

Wow, that's kind of scary. I have that setup and I don't think there's a problem, but I'm going to check. Sometimes I think I smell fuel. (I've never seen any) I was thinking of putting the original sender back in because I don't like that you have to have the car running to check the level.

Thanks for posting.

[WHO1DMC]

It looks like there's two seals in there or am I wrong? If there is I would eliminate one and make sure the seal your using is fully seated and try again.
I did this a couple years ago I had no problem with it so I don't know if it needs two seals to work.

Dave B.

[Delorean Industries]

The supplied viton O ring most definitely is required! If the O ring will no longer fit this is due to it swelling from being in contact with fuel. If you let the O ring dry of fuel contamination it should go back down to it's previous size. There is actually only one seal on this. The other white surface is a retaining membrane for holding the seal gasket in place. It is located at a depth of .135 below the inner contact point on the sealing cap. When installed in the groove, the O ring protrudes above this and contacts prior to the cap bottoming out and creating the desired seal. When we first started using this cap in our Gen 1 system in 2010 that featured a version of a stand alone fuellink module the company that manufactures the cap supplied an o ring that was rated for gas. When we moved over to our Gen 2 system with the integrated DM_ENG module system the caps were reordered and came with an o ring not rated for this. They are now subsequently swapped out for the proper rated O ring.

If you are seeing a pop off condition under tank expansion it is a safe bet it is not on all the way. The o ring contacting the surface can bunch up and bind on the orientation cut out for the factory sensor. This produces a false bottoming condition where the cap is not fully seated. Lubricating the O ring is the normal procedure for fixing this concern. I'm not sure how well installing a factory flat rectangular seal will work on this. The noted inner contact point is most likely pushing down vertically on the seal on the id of the factory port.

Attached is a reference picture of our cap (the black unit and o ring) next to the cap off of a failed dmch pump kit we replaced today. Note our component was copied one for one and is the only part that is identical between our system and the copy available.


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Quick edit- We were just asked how do I know what I have.

Gen 1 and Gen 2 sending unit modules pictured below.

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Additional edit after the previous edit:
Directions provided with this system have been updated to reflect taking off the tank sender closing cap, o ring swelling and proper installation techniques for this. They now also include a "dry sender dash gauge test" procedure that is recommended but not required. This allows for jumpering RPM relay, pump on module disconnected and cycling the float arm position/power to check your gauge resistance and accuracy.

[Bitsyncmaster]

My DMCH cap replacement had a problem sealing. My tank had a notch cut into the neck where the fuelZilla sender was placed. I guess that was done for Zilla sender. So that cap would not seal because of the notch. I ended up making my own closing plate for the stock cap.

[Stainless]

Updates:

I have tried several things to get this cap to seal properly. First, with the o-ring, I let it dry out for several days, then tried installing the cap, but experienced the same issue as before where the slightest bump would cause the cap to come off. If I tried to tighten the cap any more, it would act as if the threads were stripped and it would come loose, acting like I was starting over.

That led me to removing the o-ring. All issues with the cap tightening disappear without the o-ring. I can tighten it as tight as it will go with no thread stripping and it won't come off, no matter what I do to the cap, besides loosening it, of course. The problem, like DPI mentioned, is that the white membrane doesn't provide a tight enough seal, which I confirmed by topping off the tank and seeing fuel come out:
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I then looked at the fuel sender that I had removed to see what big differences were between it and the new cap, and the fuel sender had a rubber gasket that seemed to have sealed well:
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I then inspected the new cap to see what I was overlooking. It appears that the o-ring would work well as a seal if it were the right size. It seems like the diameter is too big for the job and interferes with the threads. This means that instead of providing a great seal between the top of the tank and the cap, it must be getting bound up in the threads, preventing it from being tightened down all the way and allowing the cap to pop off with such little force. When I try to press the o-ring into place, there is much more o-ring material than I need to make this work:
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I see only 3 options:

1) see if I can get a different o-ring from DPI
2) put back in my sender just to cap off the hole and not hook it up
3) try a different cap from another vendor

Thoughts? In the interim, I've got the cap on there without the o-ring as it only seems to leak gas if I overfill the tank, but not under normal driving conditions.

[Bitsyncmaster]

Will the old gasket fit in the new cap?

[Ron]

FWIW- I had one come in that did that. I didn't know about the non-fuel rated 0-rings and thought the wrong size was used. A smaller o-ring fixed it.
But then another one came along that had the notch. The smaller o-ring didn't work. A, much thinner, flat ring did.

[Stainless]

FWIW- I had one come in that did that. I didn't know about the non-fuel rated 0-rings and thought the wrong size was used. A smaller o-ring fixed it.
But then another one came along that had the notch. The smaller o-ring didn't work. A, much thinner, flat ring did.

Ron, where did you get the different sized gaskets?

[Ron]

Ron, where did you get the different sized gaskets?I have a box that I throw N/A o-rings left over from transmission rebuilds into...
Larger NAPA stores and machine shops that rebuild hydraulic cylinders should be able to hook you up.
If no luck, PM me an ID-OD.

[Michael]

I have the new style pump but with sensor delete which retains my OE sensor. I noticed that fuel would leak from the sensor retaining cap when the tank was full (no doubt from braking, movement, the force of 12-15 gallons just overwhelmed the cap seal). Even with a new seal(which didn't even fit right) it still leaked. I ended up going to the local parts store and picking up fuel proof sealant and it has not leaked a drop since. I even brimmed the tank trying to make it leak. In your case, I wonder if a smaller o-ring coupled with a fuel proof sealant or even a epoxy would do the trick? It's not like you would ever need to remove that cap again.

PS: Napa usually carries universal seals and o rings in stock that places like Advance and O'riley's would have to order.

[Stainless]

Well, rather than try to source an o-ring gasket locally, I chose to spend another $7 and buy an cap from DMCH. The o-ring was the correct size and my cap easily tightened down without issue. I'll be topping it off in the next couple of weeks (car is on jack stands for rear shock replacements) and am confident that I won't experience any leaks.

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[Delorean Industries]

We would have been more than happy to send you an additional o ring to go with our kit!

[MacStainless]

I have the new style pump but with sensor delete which retains my OE sensor. I noticed that fuel would leak from the sensor retaining cap when the tank was full (no doubt from braking, movement, the force of 12-15 gallons just overwhelmed the cap seal). Even with a new seal(which didn't even fit right) it still leaked. I ended up going to the local parts store and picking up fuel proof sealant and it has not leaked a drop since. I even brimmed the tank trying to make it leak. In your case, I wonder if a smaller o-ring coupled with a fuel proof sealant or even a epoxy would do the trick? It's not like you would ever need to remove that cap again.

PS: Napa usually carries universal seals and o rings in stock that places like Advance and O'riley's would have to order.

I?m going through this same problem now, on my third o-ring and the effing cap never stays on. I have a new o-ring from DPI but need to know what kind of sealant to buy. I asked in an email but never got an answer. Can you tell me what you bought? I?d like to be freaking done with this issue.

[FABombjoy]

A few years back Jimmy at Timeless Steel Garage recommended Gasoila E-Seal

I applied a pretty generous coat and have had zero trouble since.

[MacStainless]

A few years back Jimmy at Timeless Steel Garage recommended Gasoila E-Seal

I applied a pretty generous coat and have had zero trouble since.

Thanks. Did you just apply it to the threads on the cap or on the threads on the outside of the tank hole? Is there any concern of applying too much and then some overflowing into the tank?

That stuff doesn?t look like it?s sold in stores, so I?m going to keep looking as I?d prefer to get something from a local auto parts store. However if nobody has anything local, I?ll order some.

[FABombjoy]

Thanks. Did you just apply it to the threads on the cap or on the threads on the outside of the tank hole? Is there any concern of applying too much and then some overflowing into the tank?

I don't recall. It's a paste though, a lot thicker than you may be imagining. Overflowing is not a concern.