Carbon fouled O2 Sensor:'92 VX - Perplexed
 

After initial success in trying to eliminate my "miss under load/ bucking under light load" issue, the situation has gone south rather badly.

My O2 sensor (new, by previous owner-not more than a few months old) has caused a CEL for the LAF (code 48) and is now stuck in open-loop.

There is heavy carbon sooting on the sensor tip and I strongly suspect it being from coolant as there is a mysterious loss of it from the radiator. NO leaks can be found. Anywhere.

The perplexing thing is tho, that there is absolutely no fouling of the spark plugs. I would expect there to be some if the engine is burning coolant/oil, yes?

How could contaminates bypass the combustion chamber? I have to assume this contaminate is being burned by the hot exhaust gasses and then depositing carbon on the sensor.

I noticed carbon on the sensor before I did an IM rebuild but just cleaned it off.

Things that are and are not:
1) Can't see or smell anything out of the ordinary from the tailpipe. When first started or when warm.
2) There is no evidence of oil/coolant cross contamination.
3) No overflow in the coolant reservoir.
4) Suspect head gasket to be recent -area around it very clean and it just 'looks new'.
5) Have recently rebuilt the IM and systems [See the thread :"Air Intake System rebuild questions" for more details of what was done]
6) I rechecked everything I did in #5 and can't imagine what I could have screwed up but am open to suggestions.
7) Don't suspect oil leak - oil level stable
8) Suspect Coolant leak but WHERE???

{whimper} help?

Possible cause: Pinhole leak in one of the exhaust passages in cylinderhead, remove exhaust manifold and see if one port has been super cleaned as if it was steam cleaned which is what a pinhole leak does. Low pressure zone created in exhaust manifold at port syphons coolant out of the high pressure 13-16 psi in the water jacket. The only fix for this is to weld up the hole or replace the head. I would suspect that the previous owner had this problem, ergo the replaced head gasket. Good luck.

Ugh. Sounds serious. If this is the problem, is it possible to do a spot weld without pulling the head? Seems like access for someone to weld it should be decent with the head in place with the EM removed.

Maybe drive it to a weld shop, pull the exhaust manifold and spot weld? Or...?

When I've found coolant going into a motor, everything would be super clean.

It depends on how close to the exhaust manifold mounting surface the hole that [U]may[U] exist would be. If there is a hole, it would have to be TIG welded, a MIG (even with an aluminum spool gun) would not be capable of getting the penetration required. Also, if it is welded in the car, disconnect your ECU to protect it.

Would I have to drain the coolant? Would localized vaporization of the coolant occur around the weld site and could that cause problems or poor weld?

Now if it was an intake coolant leak, you might get away with JB Welding it, but exhaust heat will eat up the JB weld in a couple of weeks, perhaps sooner. However, there are high temperature putties that would hold up to the heat, but I don't fancy their chances of holding the coolant pressure. If it's accessible through the port without removing the head, I'd be inclined to shove JB Weld down inside it, then cover over the top with the high temp putty...

A small headgasket leak could be it.

If the headgasket was replaced recently, and they didn't make sure the head was straight, it could be leaking because it was slightly warped and needed to be machined.

I would take off the exhaust and see if one of the ports looks cleaner. You could also take out the spark plugs and see which sees different.

A compression tester will probably tell you which cylinder is leaking, but some headgasket leaks are so minor they will only occur when the car is either warmed up or cold. Another thing you could do is a leak-down test, where you pump air into the cylinders. This would be the best test to do, but the most expensive tester to buy.

GL fixing your z1.

When you say "carbon fouled"- do you mean a fluffy dry black soot? If so, I wouldn't assume that its residue from burning coolant- it sounds more like its just running too rich- or, if its an oily soot, maybe you need to replace you exhaust valve stem seals.

Its possible that the coolant loss is a separate issue- no antifreeze leaks from the heater core? ie. Do your windows fog up inside the car more than other cars do during this time of the year?

A quick and dirty way to check for leaks is to slightly pressurize your cooling system with compressed air and then let it sit overnight. If you suspect the exhaust ports of the cylinder head, you can take off the EM.

To pressurize- take off the overflow tube of the radiator fill neck. When the engine is stone cold, use a compressed air blow gun to inject a small amount of air into the overflow fitting (small bursts- you don't want to over pressurize it) until you feel a decent amount of pressure when you squeeze the upper rad hose. Next, see how long it takes for the upper rad hose to get soft again and look for coolant on the ground, or use a mirror to look up in those exhaust ports.

When you start the car up after this test, listen for a miss on one cylinder for the first 4-5 seconds, this might indicate a little coolant getting into a cylinder and fouling the plug for a few seconds.

Warning- if you have a big cylinder coolant leak- pressuring the cooling system could put enough coolant into a cylinder to hydro lock the engine. If you see that the system will not hold pressure then the safest thing to do is take out all of the spark plugs and crank it over to get any excess coolant out of the engine before you try to start it with the plugs in.

... and clear the spectators, a cylinder full will hose anyone within 20ft with hot coolant.