Prefacelift 320i Fuel Pump enough for a 3.5?

Post Mon Jun 09, 2014 1:50 pm

Under full load it seems the engine stutters completely, it doesn't cut out but the revs totally drop off, the engine then picks up again. It idles fine and doesn't stutter unless it's driving with my foot planted.

I've had loads of issues getting the car running so it could be anything, but it seems like it might be a lack of fuel. Is the standard 320i fuel pump man enough for a 3.5? I assumed so hence not replacing it.

Post Mon Jun 09, 2014 2:06 pm

It is, by a good margin, if it's working properly. Get a pressure gauge on the fuel rail, and see if it keeps up.

Post Mon Jun 09, 2014 2:19 pm

Have you changed the fuel filter, and are you using the 3.5 fuel pressure regulator, or the 2.0 one?

Post Tue Jun 10, 2014 12:39 pm

I tried a different FPR last night and no change.

After some testing I think it's a problem with the TPS. If you jam the throttle fully open the engine kangaroos big time, even out of gear/no load. If you build the revs up gradually it revs up fine, until the throttle goes past nearly fully open and the engine kangaroos again. The idle isn't perfect either so this could be a big indicator.

I read on the wiki that I should test it with a multimeter, so I'll do that next!

You may a good point with the fuel filter, it looks ancient, not sure it's causing this problem.

Post Tue Jun 10, 2014 1:02 pm

I had a similar problem with a Fiat twin-cam, which would kangaroo violently under certain load conditions at speed, or when going up steep hills - sometimes it would cut out completely. It turned out to be an aftermarket in-line fuel filter, causing fuel starvation/surging flow. Once removed, it ran fine.

Post Thu Jun 12, 2014 2:31 pm

I've found the solution.... In my motronic 1.3 conversion I used the matching TPS from the M20 engine loom donor, M20 and M30 TPS ARE NOT COMPATIBLE!!

On full throttle the M20 TPS was trying to make it idle, hence the cut out. A thorough googling suggested the signals are back to front. I bolted up a proper m1.3 M30 TPS and the engine was perfect.

M20 TPS part no ends in 321
M30 TPS part no ends in 320

Post Thu Jun 12, 2014 2:34 pm

Makes sense. Good to hear you licked it!