2G 2G GST smoking after 6 bolt swap

S'tan · Jun 5, 2026

Hello,
I was just wondering if anyone could provide some guidance to help me resolve an issue I'm having with my 97 GST.

I recently did the 6 bolt swap on my engine, with a few upgrades along the way. I've rebuilt the engine before but this was my first time using parts that were not stock, so I was fairly sure I knew what I was doing, generally speaking, and I was able to find the information I needed on the things I wasn't sure about. When I finished, I felt everything went well and the engine started right up on the first try. It ran rough at first because of the spark plug cables and ECU configuration had to match the new components and after a bit of tinkering the engine sounded closer to what it should. However, it was immediately obvious that the car was smoking excessively (white smoke, indicating coolant in the oil) and something was wrong.

To get to the point, the cause was not equipment malfunction, it was user error. I had connected one of the oil lines (oil filter housing) to a coolant line, because I had mistakenly believed that it was an oil feed line. Rookie mistake, and now I had to fix it. I ended up doing around 3-4 oil flushes, and even added a couple of those oil flush treatment bottles, and also removed the everything downstream from the turbo so I could clean out all the residue that was left on the exhaust system, and cleaned out the oil catch can. Once everything was clean, I added some fresh oil but the problem is that the car still starts smoking up as the engine warms up. I figured there would be a bit of smoke at the beginning but had expected that eventually all the residue would burn up and the smoke would stop, but the car keeps smoking.

My question is what else could be causing the car to keep smoking and how do I stop it?
(Just a few more notes, in case they matter

The smoke comes out of the muffler, not anywhere else. It is also not very thick, but it is noticeable, and it starts to smoke up as the engine warms up, not right when the engine starts. It has a brand new MLS head gasket, ARP head studs, head and block were resurfaced, and when I check the dipstick the oil is clear and honey- colored so I don't think there is any more coolant getting mixed with the oil. When doing the flushes, I used some cheaper oil that was thinner than normal (0w-20) because it was on sale and it was just going to go to waste anyway, so I didn't think it would be a big deal, at first. I have now put in the oil I would normally use (Castrol full synthetic 10w-30))

My brother also has a modified DSM, but his works fine and he mentioned that he uses Rotella oil for diesel engines, and also read that that oil is generally favored by the DSM community, so I'm wondering if perhaps using a thicker oil could help solve the issue (in the even that the oil is still too thin and some of it is getting past the piston rings. The spark plugs look dry but the pistons look a bit moist)
I am not an expert, nor do I pretend to be. I just like working on cars, so any help/guidance is appreciated. Hope I included enough info, but sorry for long post.

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pauleyman · Jun 5, 2026

Backup, you hooked up what coolant and oil line?
Even if you did no lines would feed coolant into the combustion chamber. Elaborate please.

S'tan · Jun 5, 2026

I took off one of the plugs on the head, thinking it lead to an oil gallery (I was tired and didn't think about it as much as I should have). I connected that line to the oil filter housing. My brother later told me that the plug I removed on the head lead to a coolant gallery, not an oil one.

I tried to show the connection I had made in the picture. The red circle is where the plug that I removed was, and the green arrows follow the line I connected to the oil filter housing. Everything else had been going smoothly, until I realized that the 6 bolt block does not have a plug on the block that the 7 bolt does. I don't remember what I was thinking but I do remember that after checking to make sure that the turbo had a feed and return line for oil, and coolant, I still had one more line left over to connect and at some point

I got confused and decided I needed to connect the oil filter housing to an oil gallery. Once I had finished everything else I had turned the engine on 3 times, and had it running for around 20-30 seconds total, when I realized my mistake. And unfortunately that was long enough for the oil to get mixed with the coolant. That was the only mistake I did and because of that mistake my car has been parked since, and I haven't been able to enjoy the upgrades I put in.

Here is where I had connected the line that caused the issue. The green arrow follows the hose, and the circles are the connection points I had used

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chrisman287 · Jun 5, 2026

Those plugs look pretty fouled. Looks like it’s running pig rich which might be why it’s smoking. What’s afr at idle?

pauleyman · Jun 5, 2026

S'tan said:
Here is where I had connected the line that caused the issue. The green arrow follows the hose, and the circles are the connection points I had used

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That still wouldn't put coolant in to the combustion chamber but it would certainly contaminate both cooling and oiling systems. Did you not notice those are coolant lines?

Your brother is wrong. The line at the head is oil. Both lines at the oil filter housing are coolant. One in one out.

DSMPT · Jun 5, 2026

S'tan said:
My question is what else could be causing the car to keep smoking and how do I stop it?

Anyways you connected a oil feed line to a coolant line. I don't know how long exactly you ran the engine being like this and how much coolant got into the oil passage and vice versa, but I imagine that the oiling system got a lot of coolant, and the cooling system got a lot of oil in it since at least you ran the engine until the engine gets hot. I hope it hasn't damaged the engine bearings, new cylinder wall/piston rings and the turbo.

The coolant/moisture might still remain everywhere inside crank case, intake manifold, lifters, exhaust pipe etc. And even flushing the engine a couple of times I don't think you can take all the remaining coolant/moisture out from crank case.

If it fortunately didn't damage anything and the smoke is really related to the remaining coolant, then probably the smoke will stop once all the remaining coolant gets evaporated, if you go driving the car (not only idling) it would make it evaporated way quicker.

Another thing came up to mind is since you occupied the factory oil feed for the turbo by connecting to the oil cooler coolant line, did the turbo has a oil feed while you were running the engine? If not, maybe the turbo got shot and that is causing the smoke now.

pauleyman · Jun 6, 2026

S'tan said:
(I pressed 'post' by accident)
- Continued:
... I got confused and decided I needed to connect the oil filter housing to an oil gallery. Once I had finished everything else I had turned the engine on 3 times, and had it running for around 20-30 seconds total, when I realized my mistake. And unfortunately that was long enough for the oil to get mixed with the coolant. That was the only mistake I did and because of that mistake my car has been parked since, and I haven't been able to enjoy the upgrades I put in.

If you post by accident the three dots at the bottom open some options. The edit button can be found here.

S'tan · Jun 6, 2026

Thank you. I will keep that in mind in the future

chrisman287 said:
Those plugs look pretty fouled. Looks like it’s running pig rich which might be why it’s smoking. What’s afr at idle?

They are brand new. They only have about 20-25 mins run time total, at idle. I hadn't considered the afr as a cause for the smoke, I thought it was all because of me. I haven't checked the afr yet but it does have a bit of a rough idle at first. It's pretty hard to start, for some reason, but when it eventually does, the idle is a bit high at around 2.5k rpms for around 30 seconds, then it hovers around 1.5k but it dips down and sounds like it's about to stall, but it doesn't if i tap the gas lightly once or twice. After about 1-2 mins the idle hovers around 1.5k-2k rpms. I had assumed that all the oil+coolant mix that had been expelled out through the exhaust and then the smoke was messing with the 02 sensors. I had hoped that the idle issue would go away once I resolved the smoke issue.

pauleyman said:
That still wouldn't put coolant i to the combustion chamber but it would certainly contaminate both cooling and oiling systems. Did you not notice those are coolant lines?

Your brother is wrong. The line at the head is oil. Both lines at the oil filter housing are coolant. One in one out.

Yeah, it did contaminate everything, and at the time I did not realize what I had done. I have fixed the connections now, but still have to deal with the consequences of assuming I was right without checking or thinking about it... I did pretty much the whole job from memory (except for the torque values, I did make sure to follow the specs on those) without any major issues, so I thought I knew what I was doing and got a bit careless right at the end... But well, at least now I get to learn how to fix another issue I hadn't dealt with before.

pauleyman · Jun 6, 2026

I assume both the coolant system and oiling system look like a chocolate milkshake?

S'tan · Jun 6, 2026

DSMPT said:
Anyways you connected a oil feed line to a coolant line. I don't know how long exactly you ran the engine being like this and how much coolant got into the oil passage and vice versa, but I imagine that the oiling system got a lot of coolant, and the cooling system got a lot of oil in it since at least you ran the engine until the engine gets hot. I hope it hasn't damaged the engine bearings, new cylinder wall/piston rings and the turbo.

That is a good point I hadn't considered. I knew the coolant and oil were both mixed and got into places where they should never be. I knew the crankcase was contaminated but hadn't thought about the bearings, lifters, and the intake piping. I'm hoping that the internal components were able to withstand it.

I didn't see any metal flakes when doing the oil flushes so I think the bearings are fine. I will check the lifters and intake piping next. And yeah, I'm hoping that it is all residual moisture and that all it takes is simply letting the car burn it all off on its own, but I haven't tried it because of the HOA where I live. I've been trying to come up with a way conceal the smoke somehow so that it's not so noticeable. I'd consider going for a drive but I lost my job recently and so I had to suspend the insurance, and the thought of getting pulled over makes me feel like it's not worth it. I start a new job soon, but until then, I'm pretty much limited to solutions that I can perform with no money using only what I have in my garage.

And yes, I did make sure that the turbo had its oil feed and return lines connected. I had spent too much money upgrading all the other parts to risk running the car with no oil.

One other question I had is would running a thicker oil help? I've been using Castrol 10w-30 since I got the car but from what I've read recently, that is not the best oil. My brother mentioned the Shell Rotella, which is what he uses, and even Google said it was well-regarded in the DSM community.

Could anyone enlighten me on this? I would like rid myself of some more ignorance. Have a bit too much of that already.

By the way, I appreciate all the input and feedback I've been getting. Every time I post a question on this site, I feel humbled by the amount of knowledge available and the willingness to help from the community. Just wanted to say thanks.

pauleyman · Jun 6, 2026

You also have the turbo, oil cooler, throttle body. All these would have coolant or oil.

S'tan · Wednesday at 9:15 AM

pauleyman said:
I assume both the coolant system and oiling system look like a chocolate milkshake?

Not anymore. I did something like a vanilla shake at first, but I guess I didn't run the engine long enough to get the chocolate flavor. I didn't really get any in the coolant though, weirdly enough, only on the oil. Or maybe I just haven't let the engine run long enough to turn the coolant into shake?

pauleyman said:
You also have the turbo, oil cooler, throttle body. All these would have coolant or oil.

Well, I ended up disconnecting the coolant lines off the throttle body so that I could connect the the (coolant) feed line to the oil filter housing. There are a few more things that could be mentioned that are important to explain how things got here, but ultimately I need to resolve the main issue I have now. Suffice it to say, that I'm hoping that nothing else got damaged, particularly with the oil lubrication(bearings, turbo).

But before I get too sidetracked, I'm hoping to find out how to get the engine to stop smoking, and I'm hoping that all I need to do is simply let the engine warm up long enough to burn off the remaining moisture that could still be left in places where it shouldn't be. How likely could that be? Is there perhaps one of those engine treatment bottles that could work?

Now, a few of the details to explain a bit the current state of some components:
The coolant was surprisingly oil-free (I did do a flush as well, and it seems like that was enough).
I did get a bit of "ice cream mix" but it wasn't quite chocolate shake, only a bit of creamy white.
The intake and intercooler pipes were not covered in any of the oil/coolant mix.
The exhaust pipes were initially covered in a thick mixture of oil/coolant, which was black, from the hot side of the turbo all the way to the muffler, which I tried to clean as much as I could (I'm starting to think there may still be some residue left, so I'll probably have to clean it again).

I'm willing to provide more information that I may have missed to mention.

One last note: I tried to respond to all the comments, but it seems like they didn't post, for some reason. I am reading all the comments though, I'm not sure what happened.

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diambo4life · Wednesday at 9:25 AM

What does the bottom of the oil cap look like? Do you have access to a leakdown tester?

S'tan · Friday at 7:49 PM

diambo4life said:
What does the bottom of the oil cap look like? Do you have access to a leakdown tester?

The cap is clear. And unfortunately I don't. But what would the tester show exactly? I haven't used one before, but is it the one that you connect to the inlet of the turbo and then pressurize to 30 psi to check for leaks? I heard of a tool that is meant to be used that way and assumed that was what it's called, but I may be mistaken. But let's assume that I had access to the tester you mentioned and used it, why would it check and how would the results help explain the smoking issue, or help figure out the solution? I'm just trying to understand. I was thinking the causes could be some some residual moisture somewhere in the engine, the exhaust pipes being dirty still, and I'm really hoping it's not the piston rings letting too much oil into the combustion chambers. If the cause could be something else, then I have no idea what it could be and that's why I came to this site. I figured someone else would know more than me and help me narrow the cause.

Edit 1: After I posted my response, I was taking a look at the engine and just happened to look at the oil filter and then realized I forgot to replace it after doing the last flush. I have been changing it after every flush but I don't recall if I did it the last time, so I'll just assume that I didn't to be safe. I will change it and then post an update with the results, but in the meantime any other suggestions are welcome.

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1990TSIAWDTALON · Saturday at 9:47 AM

I caught 2 things.
1 - Was the block decked (or checked for true flatness) and was the head surfaced? Both are needed for an MLS Headgasket otherwise you would use a composite, as they are more forgiving to a non-flat surface to seal. (You say white smoke = coolant)
2 - Did you fire the new engine up with synthetic oil? Your rings will not want to seal with synthetic oil. Use conventional oil for the breakin to seat the rings.
Marty

S'tan · Saturday at 11:00 PM

1990TSIAWDTALON said:
I caught 2 things.
1 - Was the block decked (or checked for true flatness) and was the head surfaced? Both are needed for an MLS Headgasket otherwise you would use a composite, as they are more forgiving to a non-flat surface to seal. (You say white smoke = coolant)
2 - Did you fire the new engine up with synthetic oil? Your rings will not want to seal with synthetic oil. Use conventional oil for the breakin to seat the rings.
Marty

1. Yes, both the block and the head were surfaced prior to installing. I used an MLS head gasket with ARP head studs, so I doubt that's the issue, thankfully.
2. Yes, I did use synthetic. I thought it was what I was supposed to use. I didn't know about the oil issue with the piston rings, thanks for letting me know. How long should the oil be left in there before switching back to synthetic?

My guess is that once the oil and the coolant were mixed inside the block, the coolant made its way to all the components, and being less viscous than the oil, made its way past the rings and into the combustion chambers, along with some of the oil due to me using a thinner oil than I would normally use. The mix then made its way down the lines coating all the components with a nasty black compound (due to the soot from combustion) and exiting the exhaust in smoke.
I have done multiple oil flushes now to clear out everything inside the block and the oil now seems clean. The coolant also seems clean. The intake piping doesn't appear to show any build up beyond the normal. I cleaned out the exhaust pipes (as much as I could), including the O2 sensors. I cleaned out the catch can as well. My questions are: is there something I missed? Or do I simply need to let the engine idle, and smoke, long enough to clear out any remaining moisture? I haven't let the engine warm up to temp because I keep turning it off once it starts smoking to avoid the neighbors complaining to the HOA.
Any suggestions?

1990TSIAWDTALON · 2026-06-14T10:47:43-0500

The oil may have collected in the muffler also. I'd go drive it 5-10 miles and see if it subsides. My son bought a 1987 Honda Accord Hatchback that sat for over 12 years. When we first got it started, I could not even see the car. I told him to go drive it to reseat the rings and that got rid of most of the smoking issue. A long drive gets the whole exhaust HOT so it burns out any residual oil.
I leave my breakin oil in for 300-500 miles and its straight weight Lucas 30 (they also sell 20w50 breakin oil, I posted both). After 500, you can do what you want but I always run another batch of breakin oil. Its always worked out great

. With the methods I use, I don't even have any smoke on first startup.
THIS engine had maybe 5-10 miles on it and maybe 2 hours total of run time. It has Lucas 30wt Breakin Oil in it.
Marty

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