Best Way to Clean out an Engine After a Blown Head Gasket?

May 16, 2021
Hey all,

Just yesterday I bought a project car, a '92 Mitsubishi 3000GT SL with 120k miles, with a blown head gasket. Car has not been driven around since the blown head gasket was detected (which was very early on), however there is still coolant mixed with the oil. Car has been sitting like this for about 3 months. I will be tackling this job later this week but am wondering what products you all recommend to completely flush the engine of the "milkshake" residue? I've heard about mixing ATF in with the oil due to its inherent detergent properties, and have also heard great things about Liqui Moly engine flush. Any tips or suggestions would be greatly appreciated!
No reason to use any special flush chemicals, just drain as much as you can out, clean the heads up real good while you have them out, if you want to go above and beyond and you feel like moving the exhaust and the transfer case out of the way to drop the oil pan and clean it up real good and it's probably not a bad idea to reseal an oil pan that's been on a car 30 years anyways. If you don't want to drop the oil pan just drain the oil as good as you can and run some cheap oil for a short period and then change it again pretty quickly and then keep an eye on it and maybe change it again in a few hundred miles if nothing looks unusual.
What michaelluscher suggested for the oil. If you have oil in the cooling system, take the thermostat out and do a flush with Cascade dish washing detergent followed by a full back flush with a Prestone flush kit.
Awesome thanks for all the feedback. I'll take a look at the oil pan as I've heard a dented oil pan for some reason or another can cause the engine to spin a bearing. If the oil pan does look ok as its had the 60k and 120k service I'll probably just leave it as is for now and flush the engine and radiator as best I can
Since I'm in the middle of a BHG repair right now, I feel your pain. The gasket in my '88 Supra went again. I figure it ingested maybe 4 cups of coolant. Most of that went out the tailpipe. There was very little mayo in the system.

What I will do is, when it's running again, drive it to get the oil fully hot, then change the oil and filter. Then, I'll run a short OCI, like 500 miles, before going back to my usual 6000. Any water left in the oil system will boil off quickly and go through the crankcase vent system and out the tailpipe.

I'm putting in new coolant anyway, but I am not planning on flushing it at all. A little oil in the coolant won't hurt anything, and I figure it will end up in the overflow bottle in the end.
For the oil system, I’d run Seafoam or a similar snake oil with the cheapest oil you can find - the isopropyl alcohol and 2-butoxyethanol are good to help emulsify water-based contaminants. Run it for a quick flush. An old tip suggested by Detroit Diesel after a head gasket failure was to add some butyl cellosolve to the oil, run the engine at idle for up to 20-30 minutes, and then drain.

for the cooling system - you need a phosphated detergent ideally but dishwasher detergent can work as a degreaser followed by a thorough back flush until no detergent remains. Skip Blue Devil or Prestone unless it’s the Command line.
Butyl cellosolve is an old recommendation from 20+ years ago. If you value the car, do a couple of oil analysis samples maybe 3 months apart to monitor for coolant contamination and more importantly, wear.
I'd do 2 short runs /with an hdeo , more detergents.
Maybe one 10 minute get it hot run then dump.
Then a 500 mile run.
I'd pick a reasonable thin dino / cheap oil (i.e. 5w20) and run it with short OCI (500 miles, 1k, etc) for a few times and then go back to usual.
Hey guys. I'm in a similar situation. Blown head gasket caused coolant to mix in with my oil. Unfortunately, the gasket job isn't something I could tackle on my own, so I had the car towed to an AAMCO shop for an estimate. The guy there is trying to BS me into buying a new engine for my 98 Buick Century on the grounds that the sludge can't be 100% cleaned out of the engine, and it will ruin the bearings because of this. The car hasn't been sitting but a week since it blew the gaskets, and I just drained the sludge myself the afternoon before having it towed to the shop. Because it had sat long enough for some separation to occur, the first thing to flow out was coolant water, about 1/3 gallon, followed by the milky oil. I let it drain as completely as possible before replacing the drain plug. I originally told him that as well as the head gaskets, I was willing to have the intake manifold gaskets replaced as well. But this guy has been trying to rip me off at every turn! Actually tried telling me that in order to do an estimate he was going to need to put clean oil and new coolant into it and wanted to know if I was willing to pay $260 for him to do this and have him then be able to give me an estimate. Lol, needless to say, I declined! Whatever happened to honest shops?!!
I asked him to give me some time to think about it when he broke the news that I needed a new $4,225.00 engine, knowing he was lying. Now I'm going to see if I can speak with a manager regarding his unethical employee, and get the car taken care of right!
Unethical. Lying. Dishonest.


I wouldn’t give you an estimate, either, until I could tell the extent of the damage. You might have driven it farther than you claim with water in the oil, causing extensive damage.

What if he replaces just the gaskets, and the engine is knocking because the bearings were already damaged?

Then you blame him again, claiming he’s unethical, lying, and what, incompetent?

Since you’re not willing to pay for any diagnosis, you get the safe estimate: engine replacement.

By the way, that price isn’t for a new engine, it’s for a new-to-you engine, no doubt.
Yet another falsehood that needs to die. Compared to engine oil, ATF has WAY less cleaning ability. I know @MolaKule has preached this for ages.
Yup. This myth got started because even with old burnt fluid transmissions stay pretty clean, but that's mainly because transmissions don't deal with combustion byproducts, water, and a constant inflow of oxygen, they're mostly sealed.
Yup. This myth got started because even with old burnt fluid transmissions stay pretty clean, but that's mainly because transmissions don't deal with combustion byproducts, water, and a constant inflow of oxygen, they're mostly sealed.
I think it's old type A ATF had naphthenic oil and sperm whale oil in it, which gave the oil some solvency, so like 60 years ago ATF had some efficacy as a flush agent, but modern ATF is essentially nothing but thin very poorly additized motor oil.