MSefi.com read only forum (Jan.15,2018)

Hi guys. I just got my MS install running yesterday and I have a really annoying issue with my fuel system.

The car was previously Bosch CIS - mechanical fuel injection - so I am re-using the original feed and return lines, as they will support MUCH higher pressure and flow rates than I will ever need. This includes a mechanical fuel accumulator that is supposed to hold pressure when the pump is not running.

On my fuel regulator's gauge, if I shut off the fuel pump, I can actually see the pressure drop from 45 psi to 0 in less than two minutes. Clearly my accumulator isn't working any more (that or the also-new fuel pump's check valve is failing). Thing is, it's almost brand new, horribly expensive, and reeeally hard to get to.

Anyone ever heard of a check valve that you can install on a fuel return line to hold line pressure? Where might I get such a device? The existing electronics prime the fuel pump for 1.5 seconds before starting the car, but that doesn't even raise 20 psi. Only using a jumper in the fuse box to run the fuel pump continuously generates the necessary pressure.

What's REALLY weird is that it generates FULL pressure - I had it up to 80 psi and holding fine, which is the original factory pressure rating on the CIS system. So it's not leaking anywhere. It just doesn't hold.

I searched for check valves online but didn't find much useful information. Any help would be appreciated. Thanks in advance!

What's REALLY weird is that it generates FULL pressure - I had it up to 80 psi and holding fine, which is the original factory pressure rating on the CIS system. So it's not leaking anywhere. It just doesn't hold.

For starters it sounds like you've built a good system. Just a bit too roomy (for your current fuel pump)

The existing electronics prime the fuel pump for 1.5 seconds before starting the car, but that doesn't even raise 20 psi

That would be one drawback to using the original plumbing. You've got more volume to pressurize than your fuel pump can handle with the 1.5 second prime at power up. Just curious, how many PSI do you get with two turns of the key (i.e. two 1.5 second primes)? And, as another consideration, is there any way you could shorten or minimize the run of plumbing from pump to injectors?

It sounds like your solution should be either to reduce the volume of the system or change fuel pumps to a higher gallon-per-minute model.

Clearly my accumulator isn't working any more

So you're saying that the original CIS system had a check valve in it somewhere that held pressure over long periods of non-use? That sounds right to me, and yes, it would have been on the fuel pump not on the accumulator (if I remember correctly the fuel pump included it's own fuel regulator function). You're talking about the accumulator as if that (holding pressure) is the accumulator's job. All an accumulator does is smooth out any pressure surges in a high pressure system. That's what an accumulator is (by definition).

mechanical fuel accumulator

All accumulators are "mechanical"

The fact that in many applications the system pressure gauge is mounted on the accumulator is just a convenience item because it's easier/safer to mount the gauge on the dry-gas side of an accumulator rather than a liquid side (fuel, hydraulic fluid, coolant, etc.) And that's also why you'll see system pressure guages that never read zero because when the fluid side pressure goes to zero the gauge will still read the accumulator pre-load pressure (unless the pre-load bleeds out of the dry-gas side of the accumulator).

I'm not saying your system is built like that, just running down the accumulator's job.

On my fuel regulator's gauge, if I shut off the fuel pump, I can actually see the pressure drop from 45 psi to 0 in less than two minutes.

My understanding is that a lot of electronic fuel injection systems bleed their pressure off between uses, so that's not unusual.

I worked with CIS in Volkswagen Sciroccos in the 1980's and I remember how critical maintaining the system's "prime" was. I also remember those systems would hold pressure indefinitely. You had to be really careful about releasing the system's pressure before working on anything, like injectors.

So, even if you found and installed a check valve in the system to hold pressure you'd still be left with a bit of a design flaw lurking in the background (that being your overall system volume from pump to injectors is apparently on the larger-than-necessary side). This would mean that your proposed check valve installation/fix would have to work flawlessly (i.e. holding system pressure) over any length of time that the car might sit or you'll be right back to where you are now (with the further complication that there's every chance the process of installing a check valve would add even more line volume to your existing situation, making it worse overall).

Once again, I'm no expert, but it sounds like the (simple/easily workable) solution lies in either to reducing the volume of the system or changing fuel pumps to a higher gallon-per-minute model, or a combination of both.

Admiral:

Your CIS system was plumbed from the pump thru its check valve to a system regulator (75 +/- psi) to the control pressure regulator (which was a controlled bleed to provide control pressure of about 35-45 psi) From the control pressure regulator, the bleed went back to the tank. The accumulator was tapped into the line somewhere. When you shut down, fuel is trapped between the check valve and the control pressure regulator with residual pressure provided by the accumulator as the control pressure regulator bled down -- very slowly -- to zero. When the pressure reached zero, it stopped bleeding fuel to the tank, so the sytem remained full of fuel between the check valve and the control pressure regulator. The system was thus "primed" -- full of fuel with little or no entrapped air -- so the the fuel pump immediately pressurizes the system on the turn of the key.

I suspect that you replumbed the fuel system to remove the control pressure regulator and insert a new fuel pressure regulator in its place. The new fuel pressure regulator maintains a system pressure by bleeding back to the tank. But, when you shut down, does it clamp down and trap fuel like the control pressure regulator did? If not, that could have caused the bleed from 45 psi to zero that you saw.

Or the check valve is slowly leaking. If it allows the system to fully drain, then EWFlyer is right that it takes time to refill the system -- you said it only got to 20 psi on key-on priming -- if is is "dry."

Or, the accumulator's diaphram is leaking which bleeds fuel back to the tank without a restriction in the line. All the pressure wil bleed in just a few minutes. You did not report that type of a fast leak.

You might try to plug the return from the new fuel pressure regulator, then switch on and let the pressure built to the 80 psi you reported. Switch off and see if it bleeds down. If yes, either the check valve or the accumulator is leaking.

Relieve the fuel pressure. Disconnect the bottom connection on the accumulator -- watch out for fuel leaking out! When the leaking stops, leave the connection open. Have an assistant switch on -- prepared to switch OFF immediately. If the accumulator leaks, SWITCH OFF! You have found your system pressure bleed point. Replace the accumulator -- or just replumb and remove it from the system since you don't need it.

If the accumulator did not leak, reconnect it and admit that your leak is the check valve on the fuel pump. Replace it.

Brian

Why do all of my cries for help relating to this car end in a "boy do I feel dumb"?

I had to replace the injector o-rings because I saw that one was leaking and when I checked the rest, two of them were cracked. Put it all back together, pressurized the system and found a big dripping gas leak on the rail that I swear wasn't there yesterday. Fixed that leak and left it for twenty minutes.

After twenty minutes it was at 38 psi and holding. After thirty minutes it was down to zero, but I tried just starting the car without re-pressurizing the system and it just fired right up, faster than my daily driver, actually (didn't even crank over all the way before it fired).

So, thanks for all the help! I guess that leak was the culprit, though I swear it wasn't there before. Problem solved, now I can get to tuning!