D jet fuel pressure question

[Graham]

Originally Posted by halw540

Graham, the 36 psi was when the motor was first started (and running) the day after the pressure had been set to 29. As I start and stop the motor, with it at near operating temp, I am also seeing around a plus or minus 2 psi change to the 29 psi pressure I set it to originally.

I am not having any problem now (after replacing intake manifold seals, injector seals, one injector, the MAP, and everything that looks like rubber tubing attached to the motor!! I am just trying to get this thing into tip top shape. The pressure variation just strike me as excessive, but I have no information to judge from.

Thanks, Hal

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.

[ABDow]

I am having a similar problem with my 1971 280 SE 3.5. Once the temperature gets above 180F (which it does on a hot day in Adelaide with the a/c on) the car will not restart after stopping until the temp drops to about 175 F. Then it starts and runs perfectly normally. If I remove the fuel pump fuse the car starts and runs for a few seconds until I put the fuse back in again and then the engine stalls. Sensor II is showing the correct resistance. I suspect a fuel pressure problem. I've checked the fuel pressure at 175F and it's 30psi when the car is running. I have yet to test the fuel pressure when the car is refusing to start, but I am starting to think that it might be that the fuel pressure is too high or too low. Is the fuel pressure regulator likely to be affected by heat?

[Graham]

Originally Posted by ABDow

I am having a similar problem with my 1971 280 SE 3.5. Once the temperature gets above 180F (which it does on a hot day in Adelaide with the a/c on) the car will not restart after stopping until the temp drops to about 175 F. Then it starts and runs perfectly normally. If I remove the fuel pump fuse the car starts and runs for a few seconds until I put the fuse back in again and then the engine stalls. Sensor II is showing the correct resistance. I suspect a fuel pressure problem. I've checked the fuel pressure at 175F and it's 30psi when the car is running. I have yet to test the fuel pressure when the car is refusing to start, but I am starting to think that it might be that the fuel pressure is too high or too low. Is the fuel pressure regulator likely to be affected by heat?

Usually when you have a hot start problem like that, it is because the engine is not getting enough fuel (vapor locking). So it is puzzling why the engine runs when you disable the fuel pump.

The attachment needs some work still, but is something I put together based on info from various sources on hot starts and vapor locking.

[pch2021]

On the 3.5 the engine will run off of residual pressure and fuel vapor through the fuel return network. It will die as soon as you put it in gear, but it will run.
the big issue on the 3.5 hot start seems to be with fuel volume delivery from the pump. the engine will run decently on 1 liter of fuel a minute, but when it gets hot, the fuel rail fills with vapor, and often an old pump can't deliver enough fuel to overcome the vapor lock for the car to actually start and run decently. I am wrangling with this issue on another 3-5. it seems the replacement pump I got can't take the heat.
fuel volume delivery, hot, should be a litre in 30 seconds.

[ABDow]

Thanks - it is strange that it starts when there is little or dropping fuel pressure when the f/p fuse is removed. I will test the fuel pressure and the fuel pump volume delivery when the car is hot and refusing to start and report back.
Graham, by 'leak down' do you mean loss of fuel pressure when the fuel pump has brought the system to full pressure and then has been turned off?
Alastair

[pch2021]

Feel free to call me if you have any issues...

[Graham]

Originally Posted by ABDow

Thanks - it is strange that it starts when there is little or dropping fuel pressure when the f/p fuse is removed. I will test the fuel pressure and the fuel pump volume delivery when the car is hot and refusing to start and report back.
Graham, by 'leak down' do you mean loss of fuel pressure when the fuel pump has brought the system to full pressure and then has been turned off?
Alastair

Yes - Leak down occurs after you have been running. It is due to small leaks through the injectors, cold start valve, pressure control valve and fuel pump check valve. Pressure should be 2bar +/-0.1 (about 29 psi) when you turn the key off. MB have a spec for the allowable leak down. It's a bit cryptic, but they say it may drop to 1.7 bar immediately. After another 5 min a drop to 1.5 bar is permitted (about 21.5 psig).

I have a fuel pressure gauge on the dash - If I stop for say 20 min, leakdown causes the pressure to drop to about 20 psig. At this pressure, summer fuel with RVP of 7psig will begin to vaporize at 165F. With heat soak, you would be way above that in the injectors and probably also in the fuel rails.

So, what it comes down to, is that you will have vapor in your fuel lines and injectors after a short stop. The question then is how to get the car going if you don't have time to let it cool down. The pdf I linked to provides some ideas.

[ABDow]

Dashboard f/p gauge - kewl! I'm in the process of working out an elegant way of connecting and disconnecting an under bonnet (hood) f/p gauge without having to undo the cold start injector line or other crude and messy arrangements. Will put a barbed T in the cold start injector line and a click fitting with a tap. Mate the pdf is a goldmine - thanks for sharing it.

[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.