S50 swap idles rough and engine bogs when warm

[NipplesTheEnchilada]

How's it going guys, project car problems.

The problem:
Car starts and runs great for the first 15-20 minutes once it warms up however, it idles rough at stops, and the engine bogs hard when driving, progressively getting worse the longer you drive it.

I should mention my car has no thermostat, coolant is free flowing. Car sat for two weeks and this problem is brand new with the new fuel pumps.

My car:
1987 325is with an 1995 s50 swap.

Fuel pump went out, replaced my in-tank pump with a walbro 255, and my external pump with a bosch 044, as well as a new fuel filter.

Messed with swapping fuel pump relays, but can't diagnose the problem, car feels like it isn't getting enough gas, but I've learned a rich mixture causes this too.


Any thoughts

[M3PO]

Coils?

[ian e30 318is]

I would check fuel pressure. A fuel pressure regulator controls your pressure at the rail.

Also look into an wideband AFR gauge. It will allow you to see if you are running rich or lean.

It also sounds like it could be an issue after the warm up cycle which could indicate a bad 02 sensor.

[faffierwaffle]

Possibly O2 sensor.i had an m42 engine (yes different but still from the same era) that would idle hunt HARD and bog down when I came to a stop but only after it had been running for about 20 minutes. Replaced O2 sensor and runs like a charm now.

I remember reading somewhere that the dme ignores the O2 sensor during the start of your drive (first 15-20 minutes) while the engine warms up. So the car will run fine at first but then once it starts monitoring O2 it bogs and idle hunts because it cannot make the correct mixture to make the exhaust on target for emissions (or something along those lines). So that's where I started and it fixed my problem. Good luck.

[S38Mike]

First of all, if it runs great cold but crappy when it's warm you may be running rich which would explain the bogging. The lack of a thermostat will not allow the engine warm up sufficiently to enter closed loop where the o2 sensor begins to function. All engines like a rich mixture cold but at not rich once the engine warms up. A rich engine will run horribly, make less power, foul plugs, get poor fuel economy and have excessive wear. The no thermostat thing is a common "fix" by v8 morons but it doesnt work on modern cars with FI.
I'd put a good thermostat in it first then remove the air filter housing with mass air flow sensor and smoke test the intake for leaks.