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~ The Mystery Misfire ~

Hi, fellow Car Talk fans!

I would really appreciate any thoughtful advice. I’m in the process of moving to Austin texas and my car is acting up with all my belongings in it… limited cash… and my job prospect is pizza delivery!!! AAAAAAA!!! :S Anyway…


6th Gen Honda Civic 1998 DX Sedan (I believe the engine is a D16Y7, but it’s whatever the original engine is for that car). 240k miles. 5 speed Manual.

I’m not afraid of taking the engine apart as long as I know exactly what it is I’m trying to accomplish.


  1. The original and persistent problem (may be self contained problem): Coolant dissappears. Car eventually overheats if coolant isn’t added soon enough. This is about every 3 weeks / 500 miles (very approximate). The frequency of this has not really increased noticably since the problem began about a year ago. So, I’ve just been adding coolant periodically. Car has never gone “in the red.”

  2. Oil needs to be topped off occassionally… about every 3 weeks… but more and more frequently the older the oil gets… doesn’t seem out of the ordinary for a vehicle with this many miles.

  3. Coolant does not end up on the ground (no puddles).

  4. There is no smoke from the exhaust.

  5. There is a little bit of oil in the coolant… little isolated black gooey spots floating… and a dirty (dry) film on the inside surface of the reservoir. Assuming it’s oil and not some other kind of other grime or debris. This film has been a very gradual development over years.

  6. There appears to be no coolant in the oil. There is no “milky” oil on the dipstick… it’s either clean and transparent or eventually black (normal).

  7. If I remove the radiator cap to top off the oil, the level rises if I rev the engine and there are bubbles coming up through it periodically. I have read that this means that combustion fumes are leaking into the coolant, however see fact #11.

8> Check engine light comes on for a few days (occasional misfire code) and then goes out for a few weeks.

Ok so, the things I’ve mentioned so far I’ve just been dealing with… for about 1.5 years… BUT THEN THIS HAPPENED:

  1. The latest (scary) problem: RPM will drop BELOW NORMAL >ONLY< when idling (especially after driving a while and/or when temps are hot outside) and the idle is “rough” … the check engine light will go from solid to flashing after about 20 seconds of this low idling. The flashing will continue until the the car is turned off an back on again… then the light goes back to solid. Code reader reveals codes for misfire/s. P0301,P0302,P0303,P0304… even P0305 (305 makes no sense since it’s a 4 cylinder). P1399. It gives different codes at different times. For example, one day it just had a P0303. One day it just had a P0305. One day it had 01-04, and P1399.

  2. As long as the car doesn’t dip below that magic RPM threshold (about 600), IT FEELS NORMAL (no shaking or loss of power) and the light doesn’t flash and everything feels fine. In fact, if I drive on the highway long enough (just drove from Illinois to Texas), the light will actually GO OUT… until I get off the highway, then as soon as I hit a light, the check engine comes on, too low/rough idle, flashing happens again. <<<

Since this new misfire/low idle development, I’ve seeked the help of a couple mechanics (one con artist mobile mechanic, unfortunately, and one legit shop).

  1. The engine PASSES the block test (color stays blue). I saw this with my own eyes as the (legit) mechanic did it in front of me. <<<

  2. Legit mechanic detected low compression (50% of the other three) in cylinder 4.

  3. I got a new blower motor (because it’s hot in Austin and it was barely working), and since putting the new one in, running the cabin air on high seems to help the issue, and running the A/C helps even more (it raises the idle).

  4. If I’m at a light or parked and idling, it takes X amount of time before the CEL starts flashing. However, if I’m on the highway (moving fast) and I put the car in neutral (idle) for that same amount of time and coast, the problem will not occur!

Fuel filter, cap, rotor, wires, or plugs. (replaced) Mechanic stated that problem is in fact not ignition related at all… nor do they think it’s a head gasket. They also took the head cover off and verified that the car does NOT need a valve adjustment. I’m assuming they didn’t notice anything else amiss under there from what they could see. They said nothing was sticking. They also checked the engine ground. They suspected a problem with an exhaust valve for some reason, but said that whatever it is, it is internal (engine) and mechanical and that it would require removing the cylinder head to pursue further and that it would not be worth the expense considering the age of the car.

Thanks for reaing if you’re still here, and thanks for any help!

The bubbles in the coolant, and the coolant disappearing without leaking onto the ground are classic head gasket failure symptoms. The coolant being drawn into the cylinders could also potentially cause a misfire condition.

i agree, problem with that is it passed a “block test” meaning no combustion fumes were detected in the coolant. :S

Any water leaking inside the car from the heater core maybe?
Any water coming out of the tailpipe?
Maybe the iacv gasket is worn and a small steady stream of water is leaking into the intake where it is drawn into the cylinder diluting the air fuel mixture,causing misfire, and then being blown out the exhaust valve and dripping from the tailpipe.
3 weeks until the radiator overheats is a pretty small and slow leak, maybe there is a small slow leak in the heater core and the leaking water is evaporating before it accumulates enough to create a big puddle on the car floor.
since there`s no coolant on the ground the water has to be leaking into the engine or inside the car.

“do they think it’s a head gasket.”

Most mechanics would take a look at your list of symptoms . . . and very high on the list would be the head gasket

Who cares about the block test . . . you have all the other classic symptoms

Perhaps this mechanic you visited simply doesn’t want to work on your car

You can do all the tests you want, but you have a headgasket leak. Coolant is corrosive in the cylinders, it and if you don’t fix it the engine will self destruct.

And, since it appears you have an oil passage involved as well, you should know that coolant in the oil does not provide the protection to the bearings that they need.

The car is 17 years old with nearly a quarter of a million miles. Unfortunately, it may be time to part company with it if you have to pay someone to replace a head gasket and assuming that someone is willing to wade into it.

That’s not good news I know; just the sad reality.

It’s quite possible not only is the head gasket blown, but the head may be slightly warped by now

If somebody does remove the head, I highly suggest they lay a straightedge on it, and measure the warpage

There’s no mystery here. Disappearing coolant, overheating, oil in the coolant, misfires, bubbles…it’s a head gasket leak. The only mystery is why it would take a mechanic more than 5 minutes to reach that conclusion.

I’d guess that the mechanic probably didn’t want to end up with an old, engine-damaged Subie on his lot due to a mechanic’s lien… and zero compensation for his efforts. Can’t really blame him. He’s probably been burned before. The OP probably didn’t provide as clear a description up-front as he has to us. It’s probably been a learning process for him. He’s probably able to provide a lot more information now than he was able to before.


thankyou for your response!

no water/coolant leaking in car (unless it’s undetectable).

maybe there is water coming from the tail pipe sometimes… i don’t know… not much of the time, for sure, can’t remember the last time I’ve noticed that… but i have noticed it dripping before over the years, but isn’t that normal under certain weather conditions?

I do know that when I run the AC on hot days… there will be a dripping of what looks like pure water down onto the ground when the car is parked and turned off… and it’s dripping from x(left right: about the middle of the front passenger seat) and y(front back: just behind the right front tire). but i assume this was normal and had something to do with the AC system “relaxing and letting go.”

Never thought of the aicv gasket theory!


thanks for replying…

I watched him do the block test. he took it to the exhaust pipe and and showed me how it instantly turned yellow from those fumes.

I presumed that the block test detected microscopic and residual evidence of combnustion fumes from the past in the coolant… but is that not true? does it in fact only work if it “catches” a “live” air bubble… ? If the second is true then maybe that’s why it passed… if the engine wasn’t on long enough to start throwing bubbles… it oes take a couple mintues, i know that from topping off the coolant.


thanks for your response!

what do you think of blue devil head gasket sealer? can it restore compression in a cylinder?

same mountain bike:

hi! thanks for your response!!

I don’t have coolant in the oil! Isn’t that weird? What’s a mechanic’s lein? An I paid them $160 to do what they did… which wasn’t much.

Water dripping from a tailpipe is perfectly normal. Believe it or not, car engines manufacture water.

Gasoline is a hydrocarbon, hydrogen atoms bound to carbon atoms.
Air is about 77% nitrogen, 22% oxygen, and 1% argon and miscellaneous junk.

The combustion process is the carbon and hydrogen atoms separating from the hydrocarbon molecule and bonding with oxygen atoms.
Freed carbon atoms bond with oxygen atoms and form carbon dioxide (CO2) and carbon monoxide (CO).
Freed carbon atoms bond with hydrogen atoms and form H2O, water.

Since (ignoring atomic weights) gasoline is 50% hydrogen, and the optimum ratio is 14.7 parts air (22% of which is oxygen) to every 1 part gasoline (50% of which is hydrogen) you can see where the engine would create plenty of water. It is common for it to condense on the walls of the exhaust system and drip from the tailpipe.

The water from the A/C system is from a different source. Warm air can retain more water in suspension tan cool air. When the warm, moisture-laden cabin air passes over and contacts the chilled coils of eth A/C unit, its barrier layer loses its ability to hold the water and deposits it on the coil surfaces as condensation. It accumulates rapidly there and drips as water drops. That too is normal, and you’ll have a drain tube specifically designed to deposit the water under the car rather than letting it get on the metal chassis parts.

I’ve attempted to simplify these explanations to make them easily understandable, but if you have any questions please do not hesitate to ask. My explanations aren’t always award-winners. My kids point that out to me every time I try to explain things to them.

Everyone -

Thanks for your responses. Latest update: car would barely start… it was having a harder time starting that one would expect for a single cylinder misfire, though. It did eventually after some sputters an now it’s back to “normal” … or what has become normal.

The mechanic I took it to was the foreman of the shop and he actually insisted that it was not head gasket leak. I don’t know… I’m going to a new mechanic tomorrow to see if I can get a real full diagnostic for trade work or something.

For those of you saying there is a head gasket leak… I understand that seems to be the case (except for the block test)… but that doesn’t necessarily mean that that is causing a these more pressing misfire problems. it could be a vacuum leak, right? or a distributor core? fuel injector? o2 sensor? something is causing the car to idle at lower RPM than it is supposed to. It only misfires when this occurs, and only when it is not moving (idling/coasting it doesn’t happen). while i understand there may be a head gasket problem… I’m hoping that all the other symptoms combined should be red flags for someone out there who has experienced this same problem before.

I understood that you had oil in the coolant. Coolant in the oil may or may not show up on the dipstick until it gets severe. The oil is lighter than the coolant, and the coolant will separate out and sink to the bottom of the pan fairly quickly after the fluid turbulence stops. You’ll see it on the coolant as black gobs because it coalesces and floats to the highest point in the cooling system… which is the fill cap.

The block test simply is a lab test using chemicals that react to hydrocarbons and change color. Nothing more, nothing less. The hydrocarbons come from the combustion chamber and get blown through the head gasket breech and into the water jacket where the coolant is. They also migrate up through the cooling system to its highest point where they show up as bubbles, just as air does when you blow through a straw into a glass of water. The bubbles are the exhaust gasses from the combustion chamber being released.

Blown headgaskets cause misfire problems for a number of reasons, including loss of the cylinder to hold the combustion pressures (even compression pressures), and the fact that on the intake stroke a breech in the head gasket will allow coolant to be drawn into the cylinders (intake is a high-vacuum cylinder condition) contaminating the fuel mix. Long term, it can also cause spark plug damage and corrosion of aluminum parts.

All the things you mention can cause misfires. But they won’t cause most of your symptoms.

I strongly recommend a cylinder leakdown test. That’s a test where each cylinder is positioned where both valves are closed, air is pumped in through the spark plug hole using a special fixture, and the cylinder is monitored to see if it’ll hold the pressure. It’s inexpensive and definitive.

Oh, and the A/C water puddles are normal, as long as they’re under the car and not on the floor. If there were water on the floor I’d tell you to check the drain tube. It’s supposed to channel the dripping to under he car.

Again, my posts are long because I’m trying to provide explanations rather than just answers. Pleas, if you have any questions, feel free to ask.

thanks, yeah I have some questions:

Does there have to be a bubble actually happening at the time you do the block test or is all the coolant in the car “contaminated” already? Is it testing the coolant or testing the contents of the bubble?

If coolant is being drawn into the cylinders on the intake stroke then wouldn’t there be white smoke coming out the tail pipe? There are cars that have smoke and aren’t even misfiring yet.

I’ll see about the leakdown test. The mechanic I was referring to suggested that also.

I’m also thinking about trying a head gasket sealer product (coolant additive) such as blue devil since I have not much to lose at this point. I know there are cases where it has worked for people… but I have a feeling the coolant leak and oil cross contamination is not the root cause of the current problem of low idle an misfire.

No, it doesn’t have to be bubbling when you test. The coolant has already been acidified by exhaust gases.

No, you won’t necessarily see smoke.

I have a feeling the head gasket leak IS the root problem of the idle misfire. When you’re driving at high speed you’re running much more fuel/air through the cylinders, diluting the vaporized coolant enough to make the misfire go away. But at low speed the engine is not drawing enough fuel/air to dilute the vaporized coolant sufficiently, and a low speed misfire results.

You’re over-thinking this. You have a leaking head gasket that’s probably causing a misfire. If you decide it’s worth fixing, deal with the head gasket first. A new head gasket will likely make the “mystery” misfire disappear.

Or you could try the gasket sealer additive. The car probably isn’t worth the cost of a new head gasket.

It doesn’t matter if the mechanic was the shop foreman, shop owner, etc.

You have the classic symptoms of a bad head gasket

You appear to be focusing on the fact that the test fluid stayed blue

Are you at all handy?

Have any tools . . . such as a torque wrench and a straightedge?

Ever done a timing belt?

What I’m getting at is this . . . perhaps you could do the headgasket yourself, and save on labor

Because it may not be worth paying a shop to diagnose and repair your problems

Because the car’s not worth much