Dropping fuel pressure while standing

Post Sat Jan 27, 2018 1:09 pm

Hi all,

A while ago, while troubleshooting something else, I noticed that while my fpr was working just fine with the engine running, the fuel pressure would quickly drop to 0 with the engine stopped. This makes starting quite slow, as the pressure has to build before it starts.

Two things: I've got a crap eBay fuel pump that I need to replace. I'm not sure if this is causing the problem? Should they be a no return valve in the original pump, and this is missing it?

Going through a cupboard, looking for something else, I found an 8mm non-return valve. Could I put this in line with the fpr inlet safely to stop the fuel pressure dropping? Or is this a stupid idea, and I should just put up with slow starting until the fuel pump is replaced?

Post Sat Jan 27, 2018 1:42 pm

I had a similar problem and it turned out to be a tiny split in a fuel hose that was letting out no more than a 'mist' of fuel. Replaced the pipe and everything was fine.

Post Sat Jan 27, 2018 1:55 pm

Thanks - all pipes/hardlines are new though. Not to say they can't be damaged, but would be unlikely. I'll check them out though.

Post Sat Jan 27, 2018 2:18 pm

Clamp off the supply and return hoses in turn and both at the same time, to find out where the pressure is going.
If clamping the supply line stops the pressure dropping, then the pump's built in non return valve or a supply pipe or filter is leaking.
If clamping off the return line stops the drop, then the pressure regulator has an internal leak.
If clamping both doesn't stop it, then it'll be one or more leaking injectors or a leak in the pressure regular to it's vacuum line.
BMW actually sell a non return valve to be used as you describe.

Post Sat Jan 27, 2018 2:42 pm

Excellent suggestions as always Brian - thanks! I'll refit the pressure gauge and dig out a g-clamp after checking the lines for leaks. I don't think it's injectors (or I hope not, as they've just been rebuilt a couple of thousand miles ago, so that would be a pain!).

My money is on the rubbishy pump, but I'll test and find out.

Post Mon Jan 29, 2018 7:40 pm

mine takes a while to start a cold engine, but noticed the other day, following fully charging the battery as it had not been used for a while , it started up a lot quicker.
Apparently on the e30 the f/p only runs when the engine is actually cranking ,(no priming like modern cars.) therefore if the battery is not 100% when your cranking the engine that take a lot of power, with little left for the fuel pump to operate fully. meaning it takes longer than it should to get the pressure up to 3 bar. well that's my theory anyway.

Post Mon Jan 29, 2018 10:44 pm

Starting quicker after standing for a while usually indicates one or more leaking injectors.
Any fuel in the inlet manifold makes the mixture too rich to fire, so the engine doesn't start until it has been through the engine and out of the exhaust. When the engine has stood for a while, the leaked fuel has had time to evaporate, and the engine fires as soon as fuel pressure is built up.
An E30 should start on the first firing stroke after the missing tooth on the crank nose wheel has passed the crank position sensor.

Post Tue Jan 30, 2018 9:40 am

Thanks for that info. I appreciate your vast knowledge on E30 and am sure everyone else on here does too.
I don,t think i,ve got an issue with the injectors leaking as . i,ve recently fitted newer one,s.
I was try to say more about if the battery , if its not 100% . that could affect the operation of the fuel pump while cranking the engine. As when i left my car a few weeks it took longer to start , then following another 2/3 weeks of non-use ( rest ), I fully charged the battery overnight and the car started a lot quicker with a fully charged BATTERY. its interesting to know about the sensor and crank nose wheel, thanks
noted , I need to get SPARE as you previously stated it can,t be by passed and caN leave you stranded on the Rd. just call the AA. LOL NO not alcoholics anonymous.