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BMW M328i, Fuel Pressure Regulator

There was one thing left that I hadn’t tested for in my quest to sort the idle issue, and that was the fuel pressure.

Way back when I had Tess, and she had some issues, I picked up a fuel pressure testing kit from China. It turns out this came in handy for the BMW, even if I didn’t use it on Tess.

Too much fuel pressure can lead to too much fuel being injected into the engine, leading to a rich condition. This could account for the misfire and fuel smell at idle.

I found the easiest way to test fuel pressure on the S50B30 was down under the car, after the fuel filter. The fuel lines at the rail are hard up against the firewall, with little access, and there is no Schrader valve like on other models. The drop from after the filter to the rail would be minimal, so testing here is accurate.

I teed into the fuel hose and fired the car up. At idle I had almost 50psi.

The car has a 3Bar regulator, which is about 43psi, but at idle the vacuum should pull that down about another 5-10psi less. I had too much pressure.

I ordered a replacement regulator off RockAuto. It wasn’t genuine, but it crossed over and was cheap.

To access the regulator, you need to remove the intake. Again. Sigh. It lives here, down the back of the throttles.

Removal was a pain. Undo the clamp around it, remove the clamp completely (and there is a washer under it too), and then use some vice grips and a screwdriver to twist and pry the thing out of the housing. The two O-rings will be holding it in, tight.

The new one is slightly thinner than the old genuine one, so to fix that, when refitting the washer, flip it over so the concave side is facing away from the flat, not against it like previously.


Concave against flat (wrong)

Concave away from flat (right)

This takes up the extra space and allows it to be held in properly.

Fitted. Some rubber grease on the o-rings allows you to just pop it in by hand.

A quick leak test with the intake off showed we were all good, so on went the intake, and it was time to test the pressure again.

This time its much closer to what it should be. Still maybe a couple of psi higher than expected at idle.

Results are average. Less fuel odour from the exhaust, but the idle did not change at all. Another item struck off the list, but still no change.

And in saying that, this will be the final post from this project. After a couple of calls earlier this week, the BMW has gone off to its new home this morning, to be turned into something of a weekend toy.

Now to find my next project.

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