I have a 96 4Runner with 5VZ-FE engine with 270k miles. About 2k miles ago I started getting a misfire on cylinder 2 (P302), which is when I noticed the coolant was low. I replaced the spark plugs and filled the coolant. I kept getting P302s and lost about 5 cups of coolant in 3,000 miles. No obvious steam or leaks. Recently it started to idle rough. As I was investigating the coolant loss, I noticed the lower radiator hose is always cool, regardless of coolant temperature, which never seems to go above 195°. That’s a lot of coolant and the tailpipe seems like the only possibility to me.

What doesn’t add up

Why is the lower radiator hose always cold - replaced the thermostat, but old one tested fine.
I haven’t seen any bubbles in the coolant - but some say the thermostat must be open for that.
I did a radiator pressure test while cold. At 30m it only lost 0.5psi and at 4hr it lost 4psi. Fine?
Boroscoped the pistons - they all loook the same - none looked scoured cleaned by coolant
Leakdown Test showed a range of 2.7% to 8.9% and cylinder 2 was only 5.4%. That seemed pretty good for a car this old.

Items I eliminated

Ignition coils, Spark plugs, high tension cords (to spark plugs 2,4,6), Thermostat
Camshaft sensor w/in spec
MAF temperature w/in spec

I not sure what else to test and I’m (gasp) considering visiting the dealership.

Any ideas?

It looks like you have done an awesome job of trying to diagnose this frustrating issue. I am curious if you had a chance to do a radiator coolant check for combustion gases? If that coolant is disappearing, and it's not obviously dripping all over the ground, makes me think it's getting burnt inside the engine then spitting out of the tailpipe. I'm not sure if there is a device that can sniff that tail pipe exhaust and tell you if coolant is inside that gas!

I am dying to know what you find out!

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You shouldn’t loose any pressure.

The lower hose is the cooled antifreeze. It will /should be colder than the top depending on heat load it may be only a few degrees above ambient.

Get a borescope for a smartphone and look inside the cylinder for antifreeze


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My first guess would be there's a split in the HG around the passage closest to the exhaust port and coolant is being drawn out during the exhaust stroke, but as you pointed out that would be extremely weird to not see anything in the cylinder. You would also think it would burn off by the time it reached the tailpipe, especially if it has to travel around that last bend. It would also make more sense for exhaust gasses to be pushed into the cooling system instead of drawing coolant out
Anybody know if our valve seats are dry or wet? I'm assuming dry as that's much more normal for gasoline engines, but if they're wet they could be leaking and being drawn out that way. I've seen it countless times on diesels (in fact most of my job was addressing that issue). When that happened gasses wouldn't enter the cooling system as the pressure is blocked when the valve is sealed, and if it leaked on the underside of the seat it would hardly make it onto the fire deck, let alone the pistons or cylinder walls. That sounds like it could be the culprit, but like I said, only if the seats are water cooled. I've never worked on a 5VZ head so I wouldn't know

That's been done already:

Thanks.

I was going to do that coolant combustion check, but a couple of the videos I watched said the thermostat needed to be open for that to work. At that point I suspected the thermostat wasn't working. (The lower hose on my subaru is way warmer than the 96 4R.)

Chris Fix had a video that said if you have a no spill coolant funnel, you can use that to look for combustion gases, in lieu of the combustion gas fluid tool. I followed his method and didn't see any bubbles.

Isn't the leakdown test superior?
Or would the combustion gas test find things the leakdown test would not?

Its possible u could have a leak somewhere else, like the fitting underneath the intake manifold or perhaps the intake manifold itself
Good luck

MikeM

After seeing your post, I spent the weekend and retested my other cars more extensively. After an initial minimal loss the pressure held steady on both my Subaru and 15 4R.

I decided to retry the 96 4R, and started to loose pressure a lot faster than the previous time I tested it. I walked around to the passenger side and there was a puddle of antifreeze on the found, leaking from the rusted hard line running to the rear heater.

I clamped that off those lines and re-pressurized and the pressure help stable for a several hours.

I'm not exactly sure what to make of it.

Perhaps I did the test incorrectly the first time or pressure leaked somehow w/o a lot of coolant. How?

I'm going to shunt the rear lines and retest. Hopefully those rusted lines account for the coolant loss and the misfire is a separate issue.

This is true of both of my 4Runners. The delta on my 15 4R was 100° and the lower was practically ambient. The Subaru lower hose was a lot warmer. I need to be more careful applying some of the YouTube general car advice.

I've changed my mind. I don't think it is a head gasket issue. It now looks like I had two separate problems.

I removed the rear heater lines from the coolant system. Now, the pressure holds overnight, even with the #2 spark plug removed. I saw no coolant in the cylinder the next day (borescope) and turned it over with a rag over the cylinder. Seemed like only fuel came out - no coolant.

The P302 has become increasingly regular. After a new front O2 (seemed biased lean), cleaning the IAC valve & throttle body, I saw no changes to the misfire - though it does run better.

Compression is: (1,3,5) 203, 205, 210 and (2,4,6) 205, 210, 220. My current thought is that it is a fuel related misfire, probably a dirty fuel injector.

If it were fuel pressure, I'd think the misfire would move cylinders and perhaps be towards the return of the fuel rail on the passenger side. Thoughts?

I'm just started a tank with Lucas Fuel System cleaner. We'll see.

Your diagnosis is sound. You know that it's not a compression loss so you can rule out the valves being out of spec or broken. You know it is not spark. It's not a head gasket. Only thing left is a bad injector.

Stick will a well known seller and avoid places like Ebay/Amazon for injectors, others like Five-O, Motorwest, etc that are mentioned on here are fine. Toyota sells new injectors, everyone else sells remanufactured FYI. If the injector is just clogged the Lucas will help it out a little bit. If it's the electrical component that has failed in the injector the Lucas will do nothing.

You could swap injectors to see if the missfire follows. With your numbers it should. 4-6oz of acetone will help also.


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I went though a somewhat similar debacle, 8 or 9 years ago. I was certain in my mind that I had a blown head gasket, as many symptoms pointed to that. But I learned, yet once again, that things very often are not what they seem.

I never did a compression or leakdown test, but after checking for exhaust gas in my coolant, and finding none, I tried swapping 2 injectors, and the error code followed it to the other cylinder. I am extremely glad I did that, because I would have been far beyond pissed if I had either replaced the HG myself, or paid someone about $2,500 to do it, and the problem would have still persisted.

I bought a whole set of rebuilt injectors from a vendor recommended on here (I forgot their name, but possible Motorwest). It turned out that one was faulty, but they quickly made it right, and that particular problem was fixed.

Thanks, that's a good point. I'll see if I can pull forward the timing cover to access the connector for the #2 injector without taking the air intake off. Then test the coil's resistance.

I had been planing on just getting my injectors cleaned/serviced either locally or at a recommend shop (assuming the coils test w/in spec). Would you recommend replacing all 6 as a set w/ re-manufactured?

So far Motorwest disappointed - they never replied to my email. Maybe I'll ping them again when I look into Five-O.

Are the machines/tools/knowledge/replacement parts at those places any better than my local options: for example this video half way down page seemed good to me: Feul Injector Cleaning | Kirkland, WA | Kirkland Tranmission (They are linked to this local company: Services - Doctor Injector)

Do you think I could successfully carry out this experiment by re-using the relevant gaskets in the fuel line and air intake manifold?

No need to replace all 6 as that gets a bit costly. Having them services though is a great option, places normally only charge $25-$40 an injector which includes flow matching, cleaning, and servicing the injectors. They also test the internal resistances (we have high impedance injectors) and will advise if it should be replaced. I have had it done and the local place did an amazing job. They even numbered the injectors and had me put the slightly higher flow injectors (we are talking like 1-5cc's) in the center cylinders as those see the most heat and help protect against head gasket failure and the lower ones in the front as those cylinders are cooler due to the fan and airflow. Top notch stuff.

You could even just have it done now and skip the rest of the diagnosis. Having all 6 serviced > replacing the one in the misfire cylinder, and it will cost about the same.

It’s not hard servicing the injectors yourself. Only thing you can’t do at home is flow measurement without a graduated cylinder and a fuel pump and some mineral spirits.


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