D jet fuel pressure question
 

I set my 74 450 SL fuel pressure to 29 psi when the engine was at operating temp. The next morning with everything cool the pressure was 36 psi @ 80 degrees ambient. After driving it to operating temp the pressure had fallen to 28. There appears to be a plus or minus 2 psi variation between starts with the vehicle at operating temp.

I do not know what to expect with this fuel system - any thoughts/experience/facts?

Thanks,
Hal

It went to 36psig? Was pump running?

On my '72 SL, I adjusted the pressure from 27 psig to 30 psig. (This got rid of the hot start problem I was having, presumably due to vapor locking).

This is the pressure without engine running. Once I start engine, it goes to 32 psig. I assume this is because fuel is flowing back to the tank via the return system and that there is a pressure drop in this line.

I removed the gauge, but have a permanent one to mount, once I decide on a good spot for the readout!

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.

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.

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.

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

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

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.

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.

Sounds like a good plan.

I have seen fuel pressure regulators with an integral gauge that look good.

I mounted the sender for my gauge on the hose that runs across the front of the engine and added a bracket to hold it in position. See pic.

http://www.peachparts.com/shopforum/...4&d=1253999368

Good idea. That position also reduces the likelihood of fuel splashing onto hot parts of the engine.

Just a theory here but the metal fuel rail runs really close to the headers. Maybe the fuel in that rail is more vapor than fuel so plugging the fuel pump in forces that to the injectors vs the fuel in the cooler fuel rails atop the engine.

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.

MB changed the routing of the fuel feed line at one time. In picture above, it comes up front of engine but on later cars it comes up back and parallels the driver side rail and connects in same place. Maybe they were trying to reduce heat pick up.

Even later cars used the A/C refrigerant to help cool the fuel supply.

I am thinking of covering the exposed part of my rails with some split 50# 5/16" fuel hose and then wrapping with aluminum tape so as to reduce heat pickup above the engine. Should also look at feed tubing below and insulate it where it is near to the manifolds.