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Hesitating engine revs after start up

Hi Experts, why is it that the old Ford Focus 03 has a cold starting problem. It starts okay and then the engine revs drop as if it is going to stall and then pick up again, worst on cold mornings and this problem started after having a new air filter and oil change.

Any brilliant ideas out there?


Inspect the air induction tube from the air cleaner to the throttle body, if it is loose or cracked induction air will bypass the mass air flow sensor and cause drivability problems.

Thanks very much for your quick response. My husband has checked the air induction tube and it looked fine visually. However, after the air filter change, he noticed the air box was missing two bolts and he replaced those but the problem persisted.
Any other ideas?

When this happened with a neighbor’s Ford pickup, it was finally solved with a new fuel pump.

Thanks for your input, we will look into it.

First off, look at the dipstick, make sure the oil level is correct. If that’s not the problem , next up, as already mentioned, there may be some problem with the air filter replacement. It’s possible to damage the mass airflow sensor if not done properly for example. More likely a vacuum hose has been left off somewhere, and the engine is leaking vacuum. that would cause poor idle performance with a cold engine. Experienced shops know how to do a complete vacuum system test to make sure the whole vacuum system is intact and not leaking, something they do all the time.

If this happened to me I’d probably just take all the air intake stuff apart from where the air comes in by the grill all the way to the throttle body input, make sure there’s nothing blocking any of the paths. Sometimes a shop will put a rag somewhere to protect the air intake from getting dirt in it while under maintenance, and forget to remove it.

Less likely, the idle air control function is not working for some reason. could have been inadvertently damaged during the other work, but not likely. More likely in that case is that there is no relationship to the previous work, coincidence. Idle air control malfunctions is a common problem reported here btw. Not only the focus, but in general.

Thanks very much for your thorough response. We will check what we can and
if it persists, we will take it to a mechanic/dealer to check the more
complicated possibilities. I am learning so much from all the feedback. Big
thank you to all!