small idle jump when cold...

[skr]

ok, so this thing is minor, but it keeps bugging me for some time. i know this ain't exactly a diesel forum, but i'll give it a try. when i first start her up in the morning it does this random idle jump of about 60-80 rpm . at first i thought that it's getting a "moving" signal from the diff sensor and it wants to raise the idle to the "moving" idle. (It idles about 100 rpm higher if the weels are turning, but this is normal) i disconected the diff sensor and it still does it. i checked the signal injector, cleaned the contacts , still does it.
it seams temperature related because lately the temps have gone up and it does it more seldom. also on cold mornings i get that "injection system problem" idiot light for about 5-10 seconds. might be related, i don't know.
i allways had the light come on when cold but the idle jump is recent (2-3 months). i'm out of ideas, searching the net again for some good diesel resources... i know i probably won't get an answer from here, but i had to try, this is by far the best online car forum and i hope someone knows a good diesel forum or something...

thanks

[Ferret]

ok, so this thing is minor, but it keeps bugging me for some time. i know this ain't exactly a diesel forum, but i'll give it a try. when i first start her up in the morning it does this random idle jump of about 60-80 rpm . at first i thought that it's getting a "moving" signal from the diff sensor and it wants to raise the idle to the "moving" idle. (It idles about 100 rpm higher if the weels are turning, but this is normal) i disconected the diff sensor and it still does it. i checked the signal injector, cleaned the contacts , still does it.
it seams temperature related because lately the temps have gone up and it does it more seldom. also on cold mornings i get that "injection system problem" idiot light for about 5-10 seconds. might be related, i don't know.
i allways had the light come on when cold but the idle jump is recent (2-3 months). i'm out of ideas, searching the net again for some good diesel resources... i know i probably won't get an answer from here, but i had to try, this is by far the best online car forum and i hope someone knows a good diesel forum or something...

thanks

Mine does this when cold and it's because I've got air in the fuel lines - does it stop doing it after you've run the car for a bit?

Apparently it's something to do with the tank fuel pump... but mine clears the air after a few minutes running.

If you've got clear tubing between the fuel filter and injection pump, check for air bubbles after startup...

EDIT : Something else to check -
Does it get hard to start near an empty tank?

[skr]

ok, first off all, thanks for the reply, nice to see other diesel owners out there... totaly forgot about that clear hose between the pump and filter, i had it but it cracked and i replaced it with a black one and then forgot about it. gonna get one to see if there are any bubbles. air in the lines seams very likely. but how exactly can it get there? i got no fuel leaks, new nozzles, new return lines, the pump in the tank works and it does start quick even if i'm "on the light". I'll try to fill the tank and see f it still does it. an get that clear hose allso. i'll let you know...

thanks

[Ferret]

ok, first off all, thanks for the reply, nice to see other diesel owners out there... totaly forgot about that clear hose between the pump and filter, i had it but it cracked and i replaced it with a black one and then forgot about it. gonna get one to see if there are any bubbles. air in the lines seams very likely. but how exactly can it get there? i got no fuel leaks, new nozzles, new return lines, the pump in the tank works and it does start quick even if i'm "on the light". I'll try to fill the tank and see f it still does it. an get that clear hose allso. i'll let you know...

thanks

Something that's very common in high mileage diesels is that the fuel filter housing header fails. Or more specifically the two non-return valves in it fail - which causes cranky starting and bad idle for the first couple of minutes running...

If they've failed, usually gravity will pull the fuel back through the system and introduce air all over the place through vaccuum leaks etc in the fuel system. A good indication of this happening is to shut down the engine after a few minutes of running and see if air bubbles start to move backwards from the injection pump to the fuel filter housing.

[e34.535i.sport]

I have the same thing on my '90 535i, even though its petrol - i wouldn't worry too much i dont think its a problem.

[Ferret]

I have the same thing on my '90 535i, even though its petrol - i wouldn't worry too much i dont think its a problem.

It's not a huge problem, no, but the differences between a diesel idle and a petrol idle are immense...

Petrol idles tend to waver very slightly anyway,and a slight jump doesnt grab your attention. Diesels idle so solidly that when they jump unexpectedly, they really grab your attention - and it distracts you while you're on the road. Anyone that's owned a diesel will know what I mean... the crackling noise they make at idle means you become really sensitive to their speed.