Take me to school on fuel pump ology 85 325E - Long Crank after sitting overnight

[tomstin]

First thing in the morning, the car began very long cranking before starting. Once it started, it ran fine, started fine for the rest of the day. Even after sitting for up to six hours.

I check the resistance on both crank sensor and they meet spec.

I installed a fuel pressure gauge, in line, at the cold start valve. I noticed very little pressure in the fuel line as I disconnected the line. Again long crank before pressure built to 35 psi, and the car started. It then started fine after that.

Tough to check the cold start valve as the garage was already at 90 degrees.

Fuel pressure regulator worked according to Mr Bentley with the vacuum hose removed and crimping the output hose. I think the pressure regular is good (about a year old replaced as a part of new fuel injectors).

Let the car sit overnight and the fuel pressure was down to 10 pounds. Long crank before starting as the pressure built.

I then reinstalled the fuel pressure gauge before the fuel injectors and capped off the output in order to remove the possibility of a leaky injector or fuel pressure regulator.

I briefly cranked the engine and the fuel pressure gauge jumped above 60 psi which caused my plug to leak. I quickly stopped cranking and by the time I tightened the plug in the line the pressure was down to 40 psi. I think the high pressure pump is good! A few hours later and the pressure is down to 30 psi and I’ll let it sit overnight again.

I’m betting the pressure will bleed off tonight, but I want to verify to be sure. I’ll pull the seat and open up the access panel in the morning and see if the low pressure pump is actually working. The previous owner replaced the low pressure pump several years ago with an Airtex pump.

As I understand, fuel pumps like to pump versus suck. My theory is the low pressure pump is either not working or the check valve is bad. Once the high pressure pump manages to pull some fuel, the siphon effect keeps the fuel coming. But the long crank in the AM is the high pressure pump sucking enough fuel to build pressure to get going.

Is it correct to assume the in-tank, low pressure pump:

First, contains the check valve to maintain pressure in the system at shutdown?

Second, provides necessary volume to supply the high pressure pump?

Thanks for the insight!


Edit 8/3 14:50 Eastern
In tank pump appears to be working. Output roughly 4 PSI. For the next guy, the Bentley picture shows the line with the curve being the output. On mine its the return. The output is the larger of the two lines (which makes sense being the low pressure high volume line).

Also clarified the title.

Questions remains, how far should the fuel pressure drop overnight? And should the in-tank pump hold the pressure?

[downforce22]

My thought would be a leaky or old fuel pressure regulator. The fpr holds the rail pressure at 43.5 psi and with the return fuel system, all excess is sent back to the tank. So with the car off, the fuel pumps dont run and the only thing holding rail pressure is injectors, fittings, or fpr. with no leaks, and a car running well, I would think the fpr could start failing.

This shows fuel rail hold pressure should be 2.1 bar. I suspect the same on my 318is that takes longer to crank after sitting for a couple of days.

https://www.e30zone.net/forum/viewtopic.php?t=279003

The 325e m20b27 uses a 2.5 bar fpr while the 325i m20b25 uses a 3 bar fpr

[tomstin]

A bit of an update. Pressure on the pump side of the fuel rail dropped fairly quickly and was down to zero overnight. Moving the gauge to the fuel rail side, pressurized, and pinched off the input side, held 20 psi overnight.

I'm assuming from all this that the fuel injectors are not leaking down and the fuel pressure regulator is good.

There seems to be conflicting information as to the check valve (to hold pressure) is in the high pressure pump or the in tank pump.

The pressure regulator and high pressure pumps are both Bosh and 2 years old. I hate to throw parts out it but the only item left in the fuel system that I haven't replaced is the in-tank pump. That's an expensive guess!

[roguetoaster]

Seems like you should test for flow from the lift pump, and if it is flowing just add an in line check valve as close to the outlet as you can. I've used something similar to this before to solve a bad check valve on an otherwise good pump. https://www.usplastic.com/catalog/it...x?itemid=85394

[tomstin]

To close this out. The root cause was a bad Thermo-Time switch. Many thanks to terminale30 for this "cold start problem resolved (cold start valve and thermo-time switch troubleshooting)":
https://www.r3vlimited.com/board/for...roubleshooting

Also, the connector basically fell apart in my hand. I replaced the connectors for the Cold Start Valve and the Thermo-Time switch. BTW, when you replace those connectors, make sure you wire them correctly...I fought that self inflected problem for two days before totally backing up and VERIFYING everything with a multi-meter. Always hard to check your own work as you tend to assume!