P38 diesel sometimes knocks when cold

Hi guys, hope all is well!

Since I'm sitting on my last day of a ski holiday sipping an expresso my thoughts turn to all things home, the p38 being one of them!

So some times from cold I get diesel knock over 2.8k revs and it's a bit lower on power than when it doesn't happen and hence having to go over 3k. The throttle seems even longer if that was possible.

Once warm it's the same and it doesn't always happen. I think I notice it more on early 'colder' mornings but couldn't be sure.

I was thinking maybe the ecu temp sensor?

Any ideas guys

Cheers

If its a diesel its probably injector rattle
Does it sound like its a light knock or heavy

Hi mate, Yep it's a diesel, quite light knocking, as in diesel knock not a metallic or physical knock. It's just the fact it doesn't happen some times makes me think I can fix it, if you know what I mean!

Yea mime has developed the same thing about a month ago
rattle whilst hard accelerating from cold? Goes when you back off or when engine has warmed up?
At the time I also had cooling problems so thought it might be the waterpump impeller braking up but the mechanic at work said its injector rattle and its common it older engines
He recommended some injector cleaner might help but havent tryed yet

Normally on older Diesel engines they have a kind of choke to alter timing to help with this. As the bmw engine has fancy electronics I am blaming those. May try to get some diagnostics if I can

gfromleeds said:

Yea mime has developed the same thing about a month ago
rattle whilst hard accelerating from cold? Goes when you back off or when engine has warmed up?
At the time I also had cooling problems so thought it might be the waterpump impeller braking up but the mechanic at work said its injector rattle and its common it older engines
He recommended some injector cleaner might help but havent tryed yet

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Yes sounds similar !

roccotune said:

Normally on older Diesel engines they have a kind of choke to alter timing to help with this. As the bmw engine has fancy electronics I am blaming those. May try to get some diagnostics if I can

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Never seen a diesel engine with a choke, they just run a rich mixture when cold. One of my old engines had a manual cold start button on the FIP, these days it's done electronically.

Datatek said:

Never seen a diesel engine with a choke, they just run a rich mixture when cold. One of my old engines had a manual cold start button on the FIP, these days it's done electronically.

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My 1982 Vw camper diesel has a manual choke, it just pulls a lever on the side of the fip to advance timing.

But yes you are correct it's done electronically and so got me thinking could well be fip temp sensor or ecu temp sensor?

roccotune said:

My 1982 Vw camper diesel has a manual choke, it just pulls a lever on the side of the fip to advance timing.

But yes you are correct it's done electronically and so got me thinking could well be fip temp sensor or ecu temp sensor?

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That is not a choke, a choke restricts the air inlet. Your lever is just a manual fuel enrichment device.

Datatek said:

That is not a choke, a choke restricts the air inlet. Your lever is just a manual fuel enrichment device.

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You know what I mean, Was just referring to a similar device people would be familiar with

Datatek, I take it that your p38 diesel does not do this so this can be described as a fault?

roccotune said:

Datatek, I take it that your p38 diesel does not do this so this can be described as a fault?

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All diesels knock more when cold, mine sounds diffent according to temperature and I think humidity and atmospheric pressure, never bothered to actually quantify exactly what has the most effect other than temperature. I always allow it a minute or two to warm up before I move off and treat it very gently until the temperature gauge starts to rise.

Datatek said:

All diesels knock more when cold, mine sounds diffent according to temperature and I think humidity and atmospheric pressure, never bothered to actually quantify exactly what has the most effect other than temperature. I always allow it a minute or two to warm up before I move off and treat it very gently until the temperature gauge starts to rise.

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Yes I agree that's the best way to treat it. Just aiming to get it perfect really.

roccotune said:

Yes I agree that's the best way to treat it. Just aiming to get it perfect really.

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It's a BMW engine, don't expect perfection.