Page 1 of 1
Cleaning oil-sludge from engine
Posted: Thu Jun 18, 2026 8:31 am
by don58van
Engine experts
I am recommissioning a 948 engine after about 15 years of sitting unused. It is still in the car. I have checked the condition of the oil and found that it is this disgusting heavy, tar-like goo. No exaggeration, it was like tar. It is a blend of dark green and black in colour. I wouldn't consider trying to start the engine with that as 'lubricant'. So, I have removed the oil filter, sump and oil strainer and cleaned them out (what a horrible job that was).
My question: I don't want to remove and tear down the engine if I can avoid it. So do you think there is more I could do to get this muck out of the engine before an attempt at getting it running. I am thinking of putting fresh oil in (maybe heated) with a flushing additive and turning the engine on the starter for a few lengthy sessions (plugs out, with battery charging interspersed), then changing the oil with another fill of fresh oil. Only after this, I am intending to try to start it.
I will appreciate your suggestions.
Don
Re: Cleaning oil-sludge from engine
Posted: Thu Jun 18, 2026 8:56 am
by philthehill
You can run a engine on flushing oil for several miles without concern but I would (and have) run the engine on diesel engine oil which has a high detergent content.
The flushing oil works better if the engine is hot.
You may have to change the oil filter on a more regular basis but the inside of the engine will be kept gunge free.
Re: Cleaning oil-sludge from engine
Posted: Thu Jun 18, 2026 11:12 am
by svenedin
I used flushing additives on my engine when it was old and worn out before it was rebuilt and I found them to be very effective at releasing filth from the engine. Bits I could see like the rocker gear were extremely clean afterwards.
I bet you don't want to hear this, but I would be inclined to drop the sump first. If the engine hasn't been used in 15 years the sump will be absolutely full of crud and even the best flushing agent won't get all of that out.
I used Wynn's engine flush. Added the flushing additive then ran the engine up to temperature. I actually took the car out for a short drive (less than a mile) and I let it idle for about 20-30 minutes. Then I drained oil and filter. The oil was absolutely black and filthy. Then I refilled with flushing oil, more flushing additive, a new filter and repeated the cycle but this time no driving. More rubbish came out. Finally, another new filter and decent engine oil. Then used the car normally but did an oil and filter change very early at about 200 miles.
All told rather expensive in oil but it did clean the engine up very well.
Just one warning, the detergents and solvents in the flush additive will dissolve crud that might just be the only thing holding oil in so be prepared to find leaks.
Stephen
Re: Cleaning oil-sludge from engine
Posted: Thu Jun 18, 2026 11:43 am
by Bill_qaz
svenedin wrote: ↑Thu Jun 18, 2026 11:12 am
I used flushing additives on my engine when it was old and worn out before it was rebuilt and I found them to be very effective at releasing filth from the engine. Bits I could see like the rocker gear were extremely clean afterwards.
I bet you don't want to hear this, but I would be inclined to drop the sump first. If the engine hasn't been used in 15 years the sump will be absolutely full of crud and even the best flushing agent won't get all of that out.
I used Wynn's engine flush. Added the flushing additive then ran the engine up to temperature. I actually took the car out for a short drive (less than a mile) and I let it idle for about 20-30 minutes. Then I drained oil and filter. The oil was absolutely black and filthy. Then I refilled with flushing oil, more flushing additive, a new filter and repeated the cycle but this time no driving. More rubbish came out. Finally, another new filter and decent engine oil. Then used the car normally but did an oil and filter change very early at about 200 miles.
All told rather expensive in oil but it did clean the engine up very well.
Just one warning, the detergents and solvents in the flush additive will dissolve crud that might just be the only thing holding oil in so be prepared to find leaks.
Stephen
The Op has removed sump etc
Catch tray under engine bucket of diesel over pick up pipe. Plugs out and crank engine to flush out oil ways, repeat until clean.
Reassemble and follow advice on flushing oil.
Re: Cleaning oil-sludge from engine
Posted: Thu Jun 18, 2026 4:13 pm
by Pete Bags
svenedin wrote: ↑Thu Jun 18, 2026 11:12 am
Just one warning, the detergents and solvents in the flush additive will dissolve crud that might just be the only thing holding oil in so be prepared to find leaks.
Stephen
In the early 1960's, my uncle worked for Veedol, an American oil company, wishing to enter the UK market with a revolutionary new motor oil. It was a semi synthetic multi-grade (I believe it was the world's first) and had a degree of detergent properties in it.
What could possibly go wrong? Unfortunately, rather a lot!
Cars that had previously used normal oils started to have engine issues when using the new oil - the cause was old sludge which had sat harmlessly for years, suddenly breaking free and blocking oilways etc. leading to engine failures and leaks. Of course, had the new oil only been used in brand new cars, the issue would never have arisen, but the bad publicity plus the difficulty of trying to persuade the motor trade away from the traditional big British brands of the day spelled disaster for the product and it was pulled from the British market.
Re: Cleaning oil-sludge from engine
Posted: Thu Jun 18, 2026 4:22 pm
by svenedin
Pete Bags wrote: ↑Thu Jun 18, 2026 4:13 pm
svenedin wrote: ↑Thu Jun 18, 2026 11:12 am
Just one warning, the detergents and solvents in the flush additive will dissolve crud that might just be the only thing holding oil in so be prepared to find leaks.
Stephen
In the early 1960's, my uncle worked for Veedol, an American oil company, wishing to enter the UK market with a revolutionary new motor oil. It was a semi synthetic multi-grade (I believe it was the world's first) and had a degree of detergent properties in it.
What could possibly go wrong? Unfortunately, rather a lot!
Cars that had previously used normal oils started to have engine issues when using the new oil - the cause was old sludge which had sat harmlessly for years, suddenly breaking free and blocking oilways etc. leading to engine failures and leaks. Of course, had the new oil only been used in brand new cars, the issue would never have arisen, but the bad publicity plus the difficulty of trying to persuade the motor trade away from the traditional big British brands of the day spelled disaster for the product and it was pulled from the British market.
Very interesting. This does not surprise me at all. When my engine was totally disassembled, cleaned, machined and rebuilt I decided that I would move on to a modern synthetic oil designed for classic cars (high zinc etc). This is more expensive than standard classic car oils but it has stayed remarkably clean and at oil change time it almost seems a shame to be throwing it away! Had I done this when the engine was old and worn it would probably have been a disaster without meticulous pre-flushing or certainly a waste of money.
Stephen
Re: Cleaning oil-sludge from engine
Posted: Fri Jun 19, 2026 2:46 am
by don58van
Thanks all for your advice.
Bill. I want to make sure I haven't misunderstood your suggestion. My understanding is that with the sump still off, I should position a container of diesel fuel so that it envelops the oil pickup strainer. Then spin the motor on the starter to pump diesel throughout the oil passages. The idea appeals to me because the old oil is so gooey that just using fresh engine oil with flush additive may not be very effective (bearing in mind that that as I don't want to run the engine yet, the engine and fresh oil will be cold).
It occurs to me that I could loosely refit the sump and fill it with diesel. That would be less messy because the diesel would recirculate into the sump rather than dripping from the open crankcase.
Don
Re: Cleaning oil-sludge from engine
Posted: Fri Jun 19, 2026 10:41 am
by Bill_qaz
That correct but the idea of the catch tray although a bit messy is to ensure the diesel for cleaning only circulates once so is always clean and all the crap ends up in the tray. You can also top up the bucket to repeat . Put an old T shirt over the bucket to stop crap falling in.
If you have access to compressed air, whilst no sump take out oil relief valve, filter head, and oil pressure switch and blow down the ports.
Re: Cleaning oil-sludge from engine
Posted: Fri Jun 19, 2026 10:48 am
by Bill_qaz
PS, diesel is good because its a lubricant, penetrant and not flammable in those conditions.
Re: Cleaning oil-sludge from engine
Posted: Fri Jun 19, 2026 11:52 am
by michael4
Might seem obvious but the longer diesel sits in contact with old deposits of junk the better it softens them.
I did this job on a Fiat 600 a little bit at a time.
When you've done the oil it is time to think about the water, flushing the cooling system is an excellent way of finding weak points...
Re: Cleaning oil-sludge from engine
Posted: Sat Jun 20, 2026 4:39 am
by don58van
Bill and Michael
Thank you both for your further advice on how to proceed.
I'm not new to maintaining Minors, but I have never tackled an engine in this state before.
Don