D jet fuel pressure question

[ABDow]

I've developed a clickon/clickoff connector for the fuel pressure gauge - it comes off the fuel injection rail near the front of the engine. When the gauge is not connected it is replaced with a clickon blank. It runs at 32 psi and leaks down to 20 psi in about 20 minutes (when warm but not hot). Seems within the normal range? The next trick is to test the fuel pump volume. When I connected a hose to the fuel rail click on fitting and turned the key it produced a squirt of fuel but then the fuel pump cut out in the normal way it does, even though the system was not under pressure. Any tips on how to test the fuel pump volume?
Alastair

[Graham]

Originally Posted by ABDow

Any tips on how to test the fuel pump volume?
Alastair

You should use the hose that returns fuel to the tank - That is the one at the back of the engine after the pressure regulator and the damper.

To test the pump, you have to jumper the fuel pump relay so that it does not time out.

The manual describes how to do it. Flow should be 2l/min at 12+v. Check that the pump does have proper voltage.

[ABDow]

The manual I have describes a 6 point plug and that you should remove this plug and then bridge poles 1 and 3 to power the fuel pump while disabling the cut off. My fuel pump plug is under the CPU near the front RHS of the engine bay. It has a four pin plug (pic attached). Bridging poles 2 and 4 (the bottom two in this pic) powers up the fuel pump (with the ignition) attempting to bridge 1 and 3 creates a nasty shower of sparks. If I then turn on the ignition with 2 and 4 bridged, the fuel pump works, but the cut off is not disabled. Any idea what I am doing wrong, or what I should be doing to disable the fuel pump cut off? For interest I have also attached a pic of the fuel pressure gauge clickon/clickoff arrangement.
Alastair

[GGR]

Hi,

My fuel pump relay has the same set-up as yours and in my case I have to jumper the two connections most on the right on your picture to have the pump run continuously. To make sure, you have to study how your relay works. Usually there is a scheme on the cover with the plugs numbered.

[ABDow]

The relay is underneath the ECU and hard to get at - no cover that I can access. Clockwise from the big hole the numbers are 1,3, 4, 2. The two connections on the right are numbered 3 and 4. I'll give it a try. Thanks.

[Tomguy]

Originally Posted by Graham

The headers (exhaust manifolds) are on the other side of the heads, so not really close to the rails?? Did I misunderstand? The rails do get hot just from engine heat and being blocked in by the air cleaner.

I'm referring to where the rails are fed, by the front of the engine - not the rails themselves inside the V. The fuel comes into those rails by the distributor. That main rail runs along the frame rail on the left side (US Driver), relatively close to the headers, and heat soak can be bad in that area - made potentially worse by collapsed engine mounts or the lack of a heat shield/spreader on the exhaust manifold.

[ABDow]

the lack of a heat shield/spreader on the exhaust manifold

Should there be one?
Alastair

[ABDow]

Just to update on my hot start problem. I found that the fuel pressure was 33 PSi - above the recommended tolerance - and wound it back to 28 PSI. My hot start problem has vanished. Not sure of the reason, unless higher pressure makes the fuel more likely to vaporise. Intuitively I would have thought the opposite - that the boiling point would be higher under pressure.
Alastair

[Graham]

Originally Posted by ABDow

Just to update on my hot start problem. I found that the fuel pressure was 33 PSi - above the recommended tolerance - and wound it back to 28 PSI. My hot start problem has vanished. Not sure of the reason, unless higher pressure makes the fuel more likely to vaporise. Intuitively I would have thought the opposite - that the boiling point would be higher under pressure.
Alastair

At higher pressures, gasoline will be less likely to vaporize. Generally, increasing pressure will help.

The D-Jet fuel pumps have an internal relief valve that recycles fuel back to the pump suction if the downstream pressure is too high. I have know idea of what the original set pressure was, but perhaps higher pressure causes more recycle leakage?

It would be interesting to test the fuel flow from the fuel pressure regulator at different pressures. Being a positive displacement pump, there should be little change. But if fuel flow drops, that could explain why vapor locking occurs.

Only other possibility I can think of, is that higher pressure causes pump to draw more current. If there are bad connections, a voltage drop could occur. The pump output is strongly affected by voltage.

[ABDow]

The D-Jet fuel pumps have an internal relief valve that recycles fuel back to the pump suction if the downstream pressure is too high

Graham you may be on to something there. I noted that the leak down rates were well within Robert's specs, except occasionally when the engine was hot. Then the pressure would plummet - I thought it might be something sticking in the pressure regulator (I've no idea how the regulator actually works by the way). It is possible that when the fuel temp reached a certain point, combined with over spec fuel pressure, this caused the internal relief value was activated, and the fuel rails lost pressure, and the injectors were not getting enough to run properly. Just a thought.
With regard to testing the fuel flows, I've so far not been able to find model specific advice on how to override the fuel pump cutoff. The manual I have refers to a 6 pin relay, whereas my fuel pump is activated by a four pin relay under the ECU behind the front RH headlight.

[Graham]

Originally Posted by ABDow

Just a thought.
With regard to testing the fuel flows, I've so far not been able to find model specific advice on how to override the fuel pump cutoff. The manual I have refers to a 6 pin relay, whereas my fuel pump is activated by a four pin relay under the ECU behind the front RH headlight.

The way the relay works, is that it receives power to a solenoid switch from the ECU. This closes a switch and power is provided to the pump. So there should really be just 4 wires that come into this.

What you need to do, is jumper the switch. There should be a diagram on the relay. The power wire is 30 and the wire to the pump is 87 according to my wiring diagram. Jumpering 30 and 87 bypasses the relay. The fuel pump will then run whenever the ignition switch is turned on.

One of these days, I am going to bring jumper wires out to a switch so I can run the pump for a while after a short hot stop. This will get some cool fuel into the rails and hopefully flush out some vapour.

The fuel pressure regulator is really just a spring loaded relief valve. You set the pressure with the adjusting screw and it will maintain that pressure , This is a pic of one. (It includes a line to the intake manifold, but D-jets don't have this, because the MPS already knows what the intake pressure is)

[230/8]

Originally Posted by Graham

The 36 is surprising - maybe the pressure control valve sticks until it gets hot?

Regarding the variation - I need to get my gauge hooked up to see what is happening under way. Most people wouldn't know it varies - maybe it is normal

With my pressure at 30 psig or more, I am probably running rich and need to adjust the MPS to bring the CO back to 2% - But, I am having no luck finding a reasonably priced meter.

Graham:

If you are still looking for a good CO meter you might try this model:

http://www.eastwood.com/digital-exhaust-co-analyzer-with-pulse-pump.html

I have one and believe it to be reliable and fairly inexpensive for what it does.

230/8

[Graham]

Originally Posted by 230/8

Graham:

If you are still looking for a good CO meter you might try this model:

http://www.eastwood.com/digital-exhaust-co-analyzer-with-pulse-pump.html

I have one and believe it to be reliable and fairly inexpensive for what it does.

230/8

Thanks - I had read about that one and it would have been my choice folr a CO meter.

But, what did, was install a full time Air-Fuel-Ratio meter that gets it's signal from a wideband oxygen sensor. The AFR can be converted to %CO. I have gauge mounted on dash. This makes it very easy to set up the MPS and the ECU.

I would highly recommend the AFR meter for D-Jet owners. There are also portable units, but the permanent one lets you know how the engine is running all the time.

[ABDow]

There should be a diagram on the relay

Thanks for the explanations re the relay and the fuel pressure regulator. Graham the relay is bolted under the ECU and 'upside down' - that is when I get under the car to pull down on the female part of the relay, it comes away exposing the four fixed 'male' pins which point downwards. I can also pull a plastic cap off the back of the female part of the relay which exposes the four wires leading into the four holes, which BTW are numbered 1 - 4. At the risk of asking a very dumb question, where should I be looking for the diagram?

[Graham]

Originally Posted by ABDow

Thanks for the explanations re the relay and the fuel pressure regulator. Graham the relay is bolted under the ECU and 'upside down' - that is when I get under the car to pull down on the female part of the relay, it comes away exposing the four fixed 'male' pins which point downwards. I can also pull a plastic cap off the back of the female part of the relay which exposes the four wires leading into the four holes, which BTW are numbered 1 - 4. At the risk of asking a very dumb question, where should I be looking for the diagram?

Your relay may be different. On my '72 350SL, the diagram is imprinted onto the casing. Like the one below. Sounds like you have one like GGR - He said to short 1 and 3, I think. He may be able to help more than I can for your car.