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

I should probably read through this thread in its entirety once again but I think I'm having more of a cold start issue with the new pump/sender unit. I know my entire cold start system is working 100% and I do have Dave McKeen's RPM relay which primes the fuel system for one second just before startup. I usually don't have a hot start problem that I've realized.

There are times where I'll start my car and it fires up just fine when it's cold. Other times, it takes about 3-4 cranks at about 5 seconds each. It acts exactly like a fuel problem...kind of like when you go to start the car up right after putting the intake manifold back on. It's a bit hard to start, finally catches, sputters a bit for around 5 seconds, and then all's well. For me, I can only imagine that there are times where the check valve functions correctly and other times where it sticks open. And it makes sense: if the valve sticks open, the entire fuel loop would be open and it would be difficult for even the upgraded RPM relay to prime the system and develop enough fuel pressure at the injectors. Almost like the system would backfeed if the check valve were open, if I'm correctly understanding the system design.

I'm guessing the check valve would solve the problem and it looks like they might've just gotten them back in stock. I just ordered one off the website even though it says they're not in stock. I had called them earlier today and the girl on the phone told me they should be in stock in about a week or so, but then later in the day there was the option to order one on the website. So we'll see!

Sounds similar to the problem that DeLorean03 was having with his car, other than the sputtering once it catches. Unfortunately, the addition of the external inline check valve didn't help his problem any.

However, I've experienced the sputtering around issue prior to my installation of the external inline check valve, during both cold and hot starts. Although the sputtering wasn't consistent, when it would occur on my car, I'd have to play with the gas pedal to prevent the car from stalling out before a normal idle finally kicked-in.

Since installing the check valve, roughly 700 miles ago, I haven't experienced the sputtering around problem once, thus it appears to have solved that issue for me. I also haven't had a single hot start issue since installing the valve. Thus I've concluded that even though my pump sender combo unit shouldn't have had the faulty check valve, as according to DMCH my unit came from their revised batch of a different design, it most certainly did have a bad check valve.

It's also Dave M.'s (Bitsyncmaster) and mine theory that the internal check valves in the pump/sender combo unit aren't rated to hold the kind of back-pressures that our fuel system require, as it was probably originally designed for a standard EFI system that operates at around 50psi. So while some may have lucked out with their units for now and into the near future, I honestly don't see any of the internal check valves on any of the combo units lasting longer than 4 years.

Best of luck!

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Originally Posted by NightFlyer

Sounds similar to the problem that DeLorean03 was having with his car, other than the sputtering once it catches. Unfortunately, the addition of the external inline check valve didn't help his problem any.

However, I've experienced the sputtering around issue prior to my installation of the external inline check valve, during both cold and hot starts. Although the sputtering wasn't consistent, when it would occur on my car, I'd have to play with the gas pedal to prevent the car from stalling out before a normal idle finally kicked-in.

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Thanks Josh. My problem definitely sounds like the same one that you were having and it only started about a few hundred miles after I installed the new pump/sender unit. At times, I have to do the exact same thing with the gas pedal. And the more I think about it, the likely I've been also experiencing a hot start issue but just didn't realize it. There must've been a few times when my coolant temp wasn't less than 95 degrees, so it was difficult to start without an extra shot of fuel from the cold start system. For me, the problem is less likely to happen during cold starts. I'm not sure what else it could be, particularly with Dave's RPM relay and its priming feature. I never had a SINGLE issue like this up until I added the new pump/sender.

And don't get me wrong. I love the new design (taking out the screen and pickup tube), the new fuel sender (and an awesome fuel gauge), etc. If all it takes is $20 worth of parts to get everything back in order for an indefinite amount of time, then I'm fine with that. And I'm hearing it might become standard practice to include an external check valve as a precaution with the new units. I think that'd be a good thing to do.

I'll let you know what I find out! Thanks again for your research on the CV and thanks to Michael for getting this very useful thread going!

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.

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Originally Posted by DeLorean03

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.

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Hmm... these results just may prove interesting.

We'll see what happens. I want to know once and for all if it is either the pump or the cold start system. Drove the car here to work tonight so I won't be testing the cold start system for at least 2 days. With that being said, I'm more than ready to swap the pumps first and see what happens.

On that note, does anyone know the size of the threads for that threaded stud inside the fuel tank?

If the pump check valve did stick open, that would be a problem but it could be something in the CSV circuit that is not working for cold starts. There are a lot of wire connections in that CSV circuit and that increased the chance for problems with it. My car had a bad bulkhead connection when I got it so the CSV never worked. It's a tough circuit to trace since it does not match the circuit in the schematic. There is a jumper on the back side of two bulkhead connectors that was used to make one of the connections. Wire colors were also not matching. I had to unwrap the harness to trace the circuit.

What I have found:
My car can be parked up to 4 to 6 days in the summer and it will not need the CSV to start it. Longer parked times will need the CSV to get it started. My guess is the injector lines leak down after 6 days. In the very cold winter, most always it will use the CSV to get it started.

How I know when the CSV is used:
I don't have the thermal time switch (TTS) wired into my engine. So if my engine cranks for one second, my hot start relay activates the CSV. It's easy to know that one second of cranking with no engine fire.

Ever since I did my water pump/VOD touch up, this has been the case. Problem is, I introduced the new DMCH combo at nearly the exact same time, so it's always been a "is it this one or that one?" problem for me. What I have noticed is if I unhook my CSV, my car starts MUCH more easily. Today with the CSV hooked up after a 10 hour overnight shift, my car nearly didn't start in the parking lot. I haven't done this level of diagnosis with John's pump - hence - my desire to try it.

This may not be the right thread to ask this in, but I'm hoping it will reach a lot of people who have the unit by posting it here. I've been having an issue with mine pretty consistently since I installed it where if the gauge drops below about 1/4 tank, I start getting the same sputtering symptoms I used to have with the OEM setup and a collapsing pickup hose.

It happened today on the way home from an event - 260 miles on the clock, a hair below 1/4 on the gauge and I got on the throttle to merge onto the freeway and bam - bucking and shuddering. Pulled off at the next exit and filled up - the tank took 11 gallons. So the gauge is pretty accurate - it goes precisely to the 4/4 mark when full - but what's the deal with the delivery at 1/4 tank? Symptoms were totally gone after the fillup and only occur when the tank is "low"

It is also worth mentioning that I've got a few thousand miles and a couple months on the pump and haven't had any issues with the check valve - hopefully it stays that way.

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Originally Posted by jawn101

This may not be the right thread to ask this in, but I'm hoping it will reach a lot of people who have the unit by posting it here. I've been having an issue with mine pretty consistently since I installed it where if the gauge drops below about 1/4 tank, I start getting the same sputtering symptoms I used to have with the OEM setup and a collapsing pickup hose.

It happened today on the way home from an event - 260 miles on the clock, a hair below 1/4 on the gauge and I got on the throttle to merge onto the freeway and bam - bucking and shuddering. Pulled off at the next exit and filled up - the tank took 11 gallons. So the gauge is pretty accurate - it goes precisely to the 4/4 mark when full - but what's the deal with the delivery at 1/4 tank? Symptoms were totally gone after the fillup and only occur when the tank is "low"

It is also worth mentioning that I've got a few thousand miles and a couple months on the pump and haven't had any issues with the check valve - hopefully it stays that way.

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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!