620i Running Rough

[RedBarnNate]

Hello,

We have a 620i with ~300 hours on it. It runs very rough. It sometimes idles fine, but when you press on the throttle it bogs down, chokes, sputters, occasionally backfires, and won't move forward. After coughing and sputtering for 3-7 seconds, it starts moving forward, usually lurching as engine rpms quickly pick up. After moving a short distance, it will cough sputter, choke, and begin losing rpm/power/speed. Then, it will lurch back to speed and rpms will go back up.

Here's what I have done so far to try to fix the problem.

1. Checked spark plugs - OK.
2. Noted smell of gas in engine oil - changed oil and filter.
3. Remove throttle body and clean - re-install with new gaskets (found mounting screws were loose when removing).
4. Replace fuel pump (old one was very loud and whining while running) (new pump started making weak whining noises shortly after changing).

Immediately after this service, the gator was running great! Less than 1 day after these services were performed, it was back to running terrible again. What should I do from here?

Thanks,
Nate

[idaho04]

For me, it was the fuel supply. I am not sure if it is the same thing but a generic fuel filter fixed it for me.

[whoscook]

for me it was siphon old fuel out and add 93 octane fuel.

[berryday38]

When my 620i started acting up and running rough dealer gave me instructions on removing the throttle body and spray fuel injection assembly with carburetor cleaner. That got the engine running smooth again. Then I started putting about 1/2 pint Sea Foam in each 5 gal of gasoline. Engine been running smooth now for over 2 years. Still gutting gas in oil and having to change oil every 30-40 hours.