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Old 08-07-2022, 10:20 AM
BogfordGarage BogfordGarage is offline
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Location: Devon, UK
Vehicle: 1990 Volvo 940 TD & 1989 Volvo 760 TD
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Default Burning oil and blue smoke

Hi I was wondering if anyone had any suggestions for what might be causing my problem. On my 940 TD, the one that had the cam sprocket on the pump belt spin. Recently while cold it has been emitting a worrying amount of blue/white smoke when under heavy throttle. When I say cold I mean it's not until I've been down the daul-carriageway at 70 mph for 30 minutes that it goes away and back to a puff of black smoke on acceleration with no sign of blue or white smoke at all! The pump timing is set to 0.95mm and the car starts instantly with no smoke at all. It's not using any coolant and the oil drops around 5mm on the dip stick every month this is the same as before the smoke started happening. I drive the car every weekend and it gets good long trips up the motorway for road trips/parts runs for other cars. I'm fairly confident it's oil that's being burnt as with the side exit exhaust having the window open it stinks of burning oil but only when the smoke occurs. I'm suspecting it's the turbo but I had this rebuilt in September 2020 and only put about 22,000 miles on it since then. It has had a boost controller on it in that time running 15 psi at full throttle but its not often I run at that pressure. My thinking is once the turbo gets properly hot it closes whatever gap is created between the turbo housing and the seals but I'm no turbo expert so not sure if that is even plausible. Sorry for the long explanation.
I hope someone can help, thanks in advance.
Tristan
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Old 08-08-2022, 10:51 PM
v8volvo v8volvo is offline
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Based on the fact that oil consumption has not increased at all, I would be looking at fuel injection related causes in addition to burning of oil. With diesel fuel it can be a little hard to tell oil from fuel smoke by smell (both are oils really ).

But your turbo explanation sounds plausible also. Turbo failures on the D24T are very rare even at high miles. Just don't see them often. Rare enough that it would normally be way down the bottom of the list for troubleshooting of any issue under most circumstances. But believe it or not, the fact that yours has been rebuilt actually makes it more of a suspect for potential trouble vs even a well used original turbo, rather than less, in my mind. So perhaps worth further investigation in your particular case.

I would expect oil usage from an internal leak to get worse rather than improve as the engine warms up and oil gets thinner. But it's also possible to imagine a clearance closing with higher temp as you describe.

Another explanation is that oil is being burned in the engine cylinders due to leakage from the turbo on the inlet side, or from valve guides, etc. When the engine is cold it's harder to fully combust the oil (or fuel) leading to smoke. A hot engine can burn any kind of fuel more cleanly so that could be why the smoke disappears when hot. The cause of the smoke might not have gone away, but it's being masked by the more efficient combustion in a hot cylinder.

All that said, with no noted increase in oil consumption, another possibility is worn fuel injectors resulting in smoke that could then also clear up on a fully hot engine for the reason mentioned above.

Unfortunately I think the most effective way to make progress on pinpointing this issue would be to start doing a little bit of invasive exploration. To rule out turbo issues, you could disconnect the exhaust downpipe from the turbo outlet and look for wetness from oil. Could do the same on intake side of turbo (although this is hard to do conclusively since oil will always be there in some quantity from the crankcase vent tube, unless you clean everything up then reroute the CCV into a temporary catch can for a while and see if oil still appears in the inlet tract even with the CCV gases absent).

Then in terms of injectors, pulling them out and bringing to a Bosch shop to have them pop-tested and assess pop pressures and spray pattern is usually not too costly. Sometimes even just doing this with their testing fluid will have the effect of cleaning and improving the injectors. If needed they can rebuild them with fresh nozzles.

One other possibly relevant question. Is your engine equipped with EGR and if so, do you know if the system is still functional?
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Old 08-09-2022, 04:06 AM
BogfordGarage BogfordGarage is offline
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Thank you for taking the time to give us an in-depth answer V8volvo.

I have to say the fuel injectors are the only thing left on my list of big maintenance jobs that I haven't yet done since buying the car so if in getting them rebuilt sorts the issue I would be happy.

In terms of the turbo I have run a homemade catch can for some time and having recently pulled the outlet pipe off for inspection the vapour is oil free at least at idle and under no load throttle. There is some end play on the turbo intake side when cold but I keep forgetting to check when warm as from what I gather it is better to check this when warm due to the expansion of the metal. It would be worth mentioning that this turbo is not an original it's an exchange unit from a very highly respected company, however from what you said about it being rare for a D24 turbo to fail, it might be worth changing it back to an original unit. I have had very bad experience with cheap turbos in the past so my suspicion keeps leading me back to it (despite costing over £300 for the exchange unit). Following this trail of thought the one I sent off to get exchanged that was passing oil when I got the car was also not an original but was a remanufactured Garrett unit.

In regards to an internal issue, one of my recent maintenance tasks was the valve shims and almost all the intake side had no clearance before I changed the shims so perhaps this could have had an effect on the valve stem seals?

Finally I have no EGR on this engine as it is an early type with mechanical lifters and such.

I hope this gives you a bit more information and I will try removing the exhaust for evidence of oil on that side of the turbo.
For interest here's the company I used for the exchange unit: http://www.universal-turbos.com/
And I've attached some pictures one is the turbo I took off the car (red tag) the other is the exchange unit.
Thanks again for your help
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