I replaced the glow plugs this week on my 99 turbodiesel with BERU plugs. I had two that were bad with the glow light on. Removal was relatively easy with no broken plugs! The glow light/plugs are now working perfectly, but I have a slightly rough idle on cold starts for about 5 seconds. Then, the engine idles like a V8! I have no codes. I tested all of the plugs after installation;they are fine. Any idea what could cause this skip/roughness for a few seconds (even with new plugs)?

Thanks!

A slightly rough idle is common for a few seconds on a diesel especially as they age. Lower but still good enough compression on one or two cylinders could be making those cylinders harder to ignite when cold. You could experiment by plugging in the coolant heater. Make sure the cord is in good safe condition, they can get to the point of being a fire hazard with age.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Check if any air bubbles in the clear fuel pipes when it’s started , most likely the pre filter connection is letting some air in.
When you did the work on glow plugs you may have disturbed it by resting a hand on it etc , doesn’t take much effort to get the pipe fittings to move slightly and the pre filter is the highest point in the fuel system so any air ingress is noticed as rough idle upon start up normally.

On my odder model you can keep the key on the pre-glow position after the glow plug light goes off and that keeps the glow plugs on till the timer shuts them off. And if you liked you can turn the key of and back to pre-glow and start another glow plug timed cycle.

Did you use something to ream the carbon out of the glow plug holes?

Some sort of an anti-size type compound on the glow plug threads aids in removal the next time.

What make of glow plugs did you use?

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84 300D, 82 Volvo 244Gl Diesel