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240 with fuel pressure problem.

So, swapping in a known good Bosch fuel pressure regulator made no difference.

What did make a difference was, once the tank was empty and I filled it with 50 litres of cold fuel, all of a sudden the fuel pressure stopped dropping.

It’s evident that the fuel temperature is directly responsible for the loss of fuel pressure.

Now, if only I could find out why.
 
Well, process of elimination makes it sound as though you have a bad fuel pump, despite being new. My guess is that as the fuel temp rises through constant recirculation, the in-tank pump is getting hot and either causing it to stop or slow down, reducing supply to the main pump and lowering pressure.

Measuring voltage at the pumps confirms the wiring and relay are ok, but without knowing current draw (amperage), you don't know how hard that little motor is working, if at all.

That's my dart from the other side of the world.
 
Does warm fuel cavitate more easily than cold fuel?

Is the rubber hose between the sender and the in-tank pump known good? Supposedly the kits these days include a piece of hose that isn't submersible rated.
 
Valid points.

Yes, both pumps and filters are six months old. Bosch main pump, continental/VDO tank pump. New gates submersible 5/16” hose and clamps were installed too.

It is 100% heat that is the issue. I haven’t had a chance to measure current draw yet.

The other test I wanted to run was to put the pressure gauge right after the tank pump and take measurements with the fuel cold and hot.

That way, it would be clear which pump is causing the pressure loss.

Noise doesn’t seem to change coming from the pumps, and the main pump isn’t getting hot to the point that I’d be concerned. I think the problem is starting at the tank pump.
 
im gonna go on a hunch and say the fuel pumps work correctly.


OP how much % of ethanol is in your fuel?

Do you still have the wierd roll over valve in your car? if so remove it.

Smog check time is coming up on my 1991 245. It blows clean every time but the evap seems to be the weakest point on my car. I?m thinking of preemptively replacing the brittle plastic line that goes from the charcoal canister back to the gas tank since that?s how they got me last time. Is there... https://turbobricks.com/index.php?t...ng-from-charcoal-canister-to-gas-tank.362932/

Post in thread 'Replacing EVAP line going from charcoal canister to gas tank' https://turbobricks.com/index.php?t...coal-canister-to-gas-tank.362932/post-6279076
 
Last edited:
Tell us more about this initial observation:
hessam69 said:
What I found was that shutting the engine down and restarting it immediately resulted in a perfect running engine with correct fuel pressure, for about five minutes, until the fuel pressure starts dropping again.
Click to expand...

Later, you wrote:
hessam69 said:
So I stopped, removed the gas cap, found no change. Stopped again, disconnected the charcoal canister vacuum lines at the throttle body and joined them together, no change detected again.
Click to expand...
Did you make these changes with the engine still running, or does shutting off the engine only work sometimes?

Recovering almost immediately after shutting off the engine suggests that either something is obstructing the flow, which releases once pressure/vacuum drop, or that the pump bearings are seizing/squealing [I don't know the proper term] but recover temporarily once rotation stops.
 
Ethanol content is 10%.

I will examine the rollover valve.

I think I shut down the engine to remove the fuel cap and the throttle body vacuum lines, but I remember this not having any effect.

When I refueled, the engine ran well due to the cold fuel mixing with the ten litres of hot fuel still in the tank.

The more fuel in the tank, the longer it takes for the fuel pressure to drop.

In testing, the warmer the day and fuel temperature, the faster the fuel pressure would drop.
 
Could a rubber fuel line that’s old collapse internally causing a partial blockage with heat?

Similar to a brake line.

I was thinking of replacing the line from the tank pump to the main pump just because it’s most likely original and could possibly start a leak one day.
 
240 240 said:
im gonna go on a hunch and say the fuel pumps work correctly.


OP how much % of ethanol is in your fuel?

Do you still have the wierd roll over valve in your car? if so remove it.

Smog check time is coming up on my 1991 245. It blows clean every time but the evap seems to be the weakest point on my car. I?m thinking of preemptively replacing the brittle plastic line that goes from the charcoal canister back to the gas tank since that?s how they got me last time. Is there... https://turbobricks.com/index.php?t...ng-from-charcoal-canister-to-gas-tank.362932/

Post in thread 'Replacing EVAP line going from charcoal canister to gas tank' https://turbobricks.com/index.php?t...coal-canister-to-gas-tank.362932/post-6279076
Click to expand...
IMG_3708a.jpg
 
I found the fuel supply line from the filter to the engine was wet where it passes over the transmission crossmember.

It softened the rustproofing and was easy to get it off to expose the nylon line.

This is where I have a tiny crack causing a leak.

Anyone know where I can buy new 6mm nylon fuel line? I need three metres.
 
shoestring said:
belmetric.com

Cohline Series 3011 Polyamide Hard Plastic Hose

This hose plastic connects lines with a leak-tight, corrosion-resistant seal built for automotive brake and fuel systems. Available now from BelMetric.
belmetric.com belmetric.com

This is the OEM line.
Click to expand...
That’s great.

Many thanks.

I still need to do more checking to see if this fuel leak is responsible for the 10-20psi loss of fuel pressure, which it seems to me to be too much for a small crack to be responsible for. But it’s during the heat increase that the pressure loss shows itself.

Need more testing time to be 100% sure.
 
The issue has been rectified.

The cause was a blocked in-tank pump strainer.

Even though I had already replaced it twice, it still managed to become blocked enough that under high load situations it would starve the pump thus lowering the pressure at the rail.

Thank you to all who contributed.
 
240 240 said:
does debris clogging look like rust(brown) ? or gelled up varnish from old gas (yellowish brown)
Click to expand...
Brown rusty dirty fuel came out of the strainer and the inlet of the main filter.

No gel or varnish in the tank or anywhere else. It’s just old.

The tank is clean, but, there is surface rust forming on the inner surfaces.

Should have taken a picture.
 
Glad you found it. During all the searching for what was causing this issue, had you ever re-tested the flow from the in-tank pump by disconnecting the hose at the main pump and jumping the in-tank? Just curious if you never ran that test because the pump was running, so it wasn't likely to show anything useful.

I tried it when dealing with my dead in-tank, only I wasn't jumping the in-tank, but rather the main :pow: and then I realized the in-tank wasn't running afterall :e-shrug: so no use in having pulled the hose off the main, so I put it back together and went inside the tank to find my issue.
 
towerymt said:
Glad you found it. During all the searching for what was causing this issue, had you ever re-tested the flow from the in-tank pump by disconnecting the hose at the main pump and jumping the in-tank? Just curious if you never ran that test because the pump was running, so it wasn't likely to show anything useful.

I tried it when dealing with my dead in-tank, only I wasn't jumping the in-tank, but rather the main :pow: and then I realized the in-tank wasn't running afterall :e-shrug: so no use in having pulled the hose off the main, so I put it back together and went inside the tank to find my issue.
Click to expand...
No, never did that test but did run the engine with just the main pump and found there was even less fuel pressure.

That’s when I decided to investigate further.
 
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