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Porous engine block causing oil in coolant?

1998 Honda Accord Coupe, 4 cyl ULEV V-TEC / SOHC. 170 k.

My friend’s Honda had oil in the coolant. Her regular mechanic, a very good friend of hers, is a Honda dealership mechanic but does some of her repairs as a favor. He suspected the head gasket, but when he pulled the head, he didn’t find any obvious explanation for the contaminated coolant. Now, with a new head gasket, the coolant seems to show the same problem, though there’s some speculation that what has reappeared may be residual contamination after having the cooling system drained and refilled. Or not. I don’t have details on how much effort was put into cleaning the cooling system, or how much effort it takes to get the system clear of oil if that’s even the problem. But it has at least been drained and refilled, maybe “flushed”…I just don’t know yet.

Some internet research that I didn’t conduct revealed a pattern - many reports of exactly the same problem, that is, 98 and 99 4 cylinder Honda Accords with oil in coolant and no problem with HG. The explanation given in most of those cases was what evidently has a name: “porous engine block”. That sounds bogus to me, but all these people reported the same thing - ie, NO head gasket problem. Is this porous engine really another way of saying cracked head, or cracked block? The mechanic has not yet examined the car after the head gasket replacement, and has not yet rendered an opinion.

The oil and the transmission fluid each seem ok based on dipstick observations, though that may not be a convincing way to judge. The coolant in the overflow reservoir evidently leaves a black coating on the plastic surface of the bottle. Tomorrow we’ll put drops of each fluid on separate pieces of white paper towel to better see the color. The car runs fine.

One potential explanation is contamination from a damaged transmission cooler in the radiator if it has one. But wouldn’t a breech there only result in coolant entering the transmission coolant line due to the pressure differential, or would it cross contaminate? And if they did cross contaminate, would the contaminated coolant look more red than black if it were transmission fluid in the coolant? Maybe not - what color do you get when you mix red trans fluid with green coolant? I’d guess an indistinct ugly brownish. I hope it turns out to be that simple, but I don’t like it.

So, back to oil…It seems to me that if the coolant is contaminated but the oil is not, then it must be combustion pressure exceeding the relatively lower pressure in the cooling system, driving oil toward the cooling system, making me think the problem is in the head. That would be relatively good news. But I would imagine the mechanic looked hard at the head when he had it off. Would a compression test be any use at this point?

So what’s the best next diagnostic step? Is there some reliable test to determine if it really is oil - not tranny fluid - in the coolant? Or does the color say it all? If it is oil, then what? Begin looking for a donor car with a sweet running engine?

Finally, I gotta ask: has anyone else ever heard of “porous engine block” problem in Hondas? Is it really possible that there’s a metallurgical problem with the casting of these engines?

Thanks for any advice on this one.

My friend just reported that tonight the coolant overflow reservoir is nearly empty, “way below the min line”. And btw, it’s been about a week since the head gasket was replaced, with one road trip of a couple hundred miles total and some elevation change, plus usual around town low speed driving.

It takes awhile for all the air in the system to be expelled and the tank level stabilizes…

You don’t spend a lot of time and money on 14 year old cars with 170K miles on them…Drive it until it will drive no more…

I’m not sold on the porous engine block theory. This must be an automatic transmission so there could be a transmission cooler intergrated with the radiator, I’d check out that next.

All it takes is a small crack in either the head or block to leak coolant. It mght not be evident on visual inspection, but it still might have been repaired with the new gasket on the head.

Keep topping off the resoirvor until the level stabilizes. A “coating” on the overflow tank isn’t really unusual in an old car. Rather than spend a bunch more money on this older high mileage car, you might consider changing the coolant more frequently - ie every 2 years and just run it out.

I agree with Uncle T - that’s not a good explanation, 14-year-old castings don’t usually suddenly become ‘porous’. I’d go back to square one and try and track down the source of the problem. A cooling system pressure test, a compression test, a check of the transimssion system cooler, etc. No more new parts/work until more checking is done.

I agree. Porosity IMHO is a bogus theory. A cracked cylinder wall would be far more likely than porosity, and even far more likely than that is a breech in the radiator betweene the tranny cooler portion and the coolant portion. And while logic would suggest that coolant would go to the tranny rather than the other way around, tranny fluid could also get pulled into the cooling system when the coolant cools and contracts after the engine is shut off.

Such a leak should show up with a cooling system pressure test. The pressure should push the coolant through the breech and into the tranny. This won’t show up as a visable leak, just a pressure drop.

Texases is right. You need to start over and take a complete look at everything.

If an engine block was porous enough to allow engine oil into the coolant then it would never have made it to 170K. A porous block would be a very weak block.

Thanks for these helpful responses. I’m glad I’m not the only one doubting the porous block theory. Hopefully this will turn out to be in the trans cooler/radiator, that would be a cheaper repair. @mountainbike…I like your explanation for how trans fluid might be sucked into the coolant, sounds plausible I think. The cooling system pressure test sounds like the place to start. My friend will pass all this info on to her mechanic.

I did have a quick look at the car this morning. What’s in the coolant overflow reservoir looks dark and oily. The other fluids looked ok, though I was just a bit skeptical about the purity of the trans fluid based on color. Not sure though, I don’t know how long since the last fluid change.

Any further thoughts greatly appreciated! Thanks everyone.

I probably should have mentioned…if you find the fluids mixing, and I suspect you will, you should have the tranny drained and flushed with fresh fluid once the repair is complete.

Yup, thanks. I’d already thought of that. I really hope this is where the problem lies. Grateful for your input.

You might do some digging on this issue. I think there may be a Honda technical service bulletin out about porous engine castings on both the 4 and 6 cylinder engines. Porous castings are rare but do happen from time to time.

In the past I’ve seen probably half a dozen carburetor castings leaking fuel internally (mostly Ford), a Subaru extension housing on a manual transmission, and not one, but two, Nissan rear differential leaks. That’s the ones from the top of my head.

Regarding the latter, a Nissan from an out of town owner was towed in with a trashed rear differential. One could detect a tiny pinhole where gear oil leakage over time had washed away the paint and which eventually wiped the differential out. A new from Nissan diff was ordered and installed.
A week later the customer called and said the new diff was leaving spots on the garage floor. On the surface, this would point to me as having left a filler plug loose or something like that but I was certain I did not screw up. The car was towed in just to be safe and the entire rear diff was covered in oil with the gear oil level being down a bit.

I thoroughly cleaned the diff, filled it with fresh oil, and allowed it to sit while I went on with other things. About an hour or so later a fresh dab of gear oil had appeared on the housing and by the end of the day it had turned into a very slow drip. The new differential also had a porous housing; just not in the same spot as the old one. (Housings are cast iron.)
Of course, in operation and with hot oil the leak would have been worse due to the oil thinning and possiby the pore in the casting enlarging from heat.

I know Saab’s had an issue with porous blocks, as well as Honda’s… From what I have been told it has to do with the hardening process. After casting, the blocks are put in giant ovens and baked to harden them. The problem is if the heat is uneven the blocks in the center of the oven do not harden as much as they should, and become porous… At least that is what I was told by a Saab Engineer…

@ok4450 and @gsragtop:

Thanks gentlemen. I can accept that porous castings do occur as your posts suggest. But would this porous block condition develop over time? I’m certainly no metalurgist, but I would think that if the block (or head) were porous, that would have been obvious in this car long ago. Like in the first year.

In fact I had that problem with a clutch master cylinder installed by my regular mechanic - but that problem was evident a day after the work was done after I’d driven about 100 miies. The mechanic determined that brake fluid was seeping right through the casting and newly peeled paint on the firewall right below seemed to confirm that.

You might do some digging on this issue. I think there may be a Honda technical service bulletin out about porous engine castings on both the 4 and 6 cylinder engines.

How do I look for TSBs on this? Is it just a google search, or do we need to contact the Honda dealer, or something else?

Not likely. It’d be that way from day one. So, like others have said, it’s probably something else.

The TSB might be 01-092 although I did not spend a ton of time digging details out.

It would seem to me that if engine oil is getting into the coolant then the reverse should also be true and the engine oil should be contaminated.

Do you know if they have done a proper cooling system pressure test? I would pressure that thing up and allow it to sit for half an hour at a minimum just to see what happens with the gauge.
It would be a good idea (if there is a pressure drop) to double check hoses, radiator, etc, etc and make sure that drop is not caused by a minute leak.

I would not blame the block unless it’s known with 100% certainty that the block is the problem.

@ok4450. Thanks for the help on the TSB. I’ll check that shortly.

No cooling system pressure test yet, but I hope that’s next. Others on this forum have advised the pressure test as well. The woman who owns the car is out of town visiting family, and I don’t know quite what she has worked out with her mechanic friend. Mountainbike suspects a breech with the transmission oil cooler in the radiator (if it has one - and yes, it is an automatic trans). He has made a good case for cross contamination which I would not have expected to be possible based on pressure differential, but I do agree with his explanation. I sure hope he’s right, that would obviously be a better scenario than a cracked block. But it’s between my friend and the mechanic to decide how they will proceed. FWIW, the radiator hoses were changed last week when the head gasket was replaced, not sure about heater hoses but I’m guessing they were also. But if there’s a leak there, that would explain only a pressure drop, not oily contamination in the coolant.

@texases. Thank you. It just didn’t make any sense that porosity would develop at 170k. I am curious to see what the mechanic will find. It must be “something else.”

I greatly appreciate all the help. Thanks everyone for sharing your insight.

If the transmission cooler has a breach, the transmission fluid would likely get into the coolant when the car is running, due to the transmission line pressure being at least 10X what the pressure in the cooling system is. Once the car is shut off, then the reverse could happen–the transmission cooler will have no pressure in it, while the radiator has over 15 PSI. But coolant is unlikely to enter a pinhole leak in the cooler, while under high pressure, transmission fluid could certainly get out. It would have to be a pretty large leak I’d think for coolant to get into the transmission.

If you think the cooler is leaking, you could install an aftermarket trans cooler and just bypass the in-radiator one. The aftermarket coolers do a much better job anyway.

Interesting, thanks. I was missing the fact that trans fluid would be pressurized. That makes the cooler breach scenario more plausible I think. The coolant is obviously contaminated with a dark oily fluid, but the trans fluid looks reasonably good. If there’s any contamination, it’s less evident in the trans fluid. It’s pink but maybe slightly darkened. Is there any way to analyze the trans fluid chemically to check for presence of coolant (or vice versa)?

Just to clarify: You say “coolant is unlikely to enter a pinhole leak in the cooler”, by which you are just restating that this is only when engine is running, correct? (You also said, that when “the car is shut off, the reverse could happen”.) I just want to be sure that a pressure test would reveal a pinhole leak at that location - if one did exist.

The aftermarket trans cooler sounds good, though if there is a breach, the radiator would still have a leak that would have to be resolved.

If coolant was going to enter the transmission cooler, it would most likely happen when the car is hot, but not running, and the cooling system has a lot of pressure. The highest pressure in the cooling system is probably a minute or so after the car is shut off, when the coolant isn’t circulating, but the engine continues to heat it and build up pressure. But I have no idea if your transmission cooler is leaking, it was just something to throw out there.

Any place that does oil analysis, such as Blackstone Labs, for one, can analyze an oil sample and tell you what it contains. Not sure if they can analyze coolant–might be worth calling or emailing them. I’ve used Blackstone before and they will send you sample containers for free–you pay when you send the sample to them for analysis. I haven’t done it in a couple years, but the cost used to be about $25.

@oblivion: Thanks for the tip on Blackstone, that may be useful.

I get your point about the coolant temp being highest just after the engine shuts down - engine releasing heat, no circulation or airflow to carry it away. And that’s right at the time where trans fluid pressure drops, so cross contamination seems possible if there’s indeed a leak. It’s the ugly black oily mess in the coolant overflow reservoir that is the most obvious sign of the the problem, at least so far. Sure hope we find the problem there, rather than in the engine.