Temp gauge testing

[Tom]

Would my only option at this point be to pull the block and get it resurfaced at a machine shop? Everywhere I read online said I just had to clean the surface of the block and it’d be fine, but i was skeptical that it would seal well seeing the condition of mine. The surface felt smooth, but there was definitely some corrosion that I couldn’t remove even with a brass brush.

Did you have the head skimmed when it was off? Warped heads are pretty common, especially if it's been overheating, so if you did not have the head machined when it was off, that would be worth checking. For the block, check it with a precision straight edge for flatness. Put the flat edge corner to corner and use a feeler gauge to check for low spots -- there's an official procedure for that in the factory manual... Also have the machine shop pressure test the head for cracks and inspect the block closely for same. That's unlikely, but worth checking. Widefire head gasket can sometimes help on the margin too, along with an extra 5-10 ft lbs on the head nuts (especially if using an older or cheaper torque wrench that hasn't been calibrated in a long time). I use Raceware head studs torqued to 95 ft lbs. Raceware insists 65 is enough, but I have a collection of used head gaskets that says otherwise...

[BennSport]

Yes I did have the head skimmed, I took it to a machine shop who took 14 thousandths off. They did also pressure test the valves and they were good. I showed the photos to a friend who brought up a good point that the water is clear, not bright blue like the coolant I used. Could this be condensation and not a failing head gasket? If so, that wouldn’t explain the overheating issues I’m still having.

[spacecad3t]

I didn't catch this in the prior posts, but are you seeing white smoke from the exhaust? Sweet smell? Sample the oil, any coolant in it?

Blue coolant can suck, blue is low contrast and it may just be the amount/photo quality... I used Pink G40 from Zerex, as I saw most of the supplies sold this and my records had shown that my car was already running with it.

Follow Tom's advice for checking flatness across the block... My only other thought would be a hairline crack in a sleeve, also possible from overheating cycles... I see alot of oil in those photos? So it feels like Tom is correct and it may be the head.

I have not done a head job on a 944, so I'm unfamiliar, but in the VW motors I know, there is a top ring that acts like a crush waster between the sleeve and the combustion chamber, does the 944 use similar?

[BennSport]

Im not getting a sweet smell or seeing any white smoke. I’d really like to do everything I can to test before pulling the head off again, as it was quite labor intensive. The head gasket did have metal rings around the cylinder so I would assume they act the same purpose as in the VW’s. While blue can be quite low contrast, I’ve noticed with the one I’m using even a few drops have a blue tint which is something I’m not seeing here. Even the droplets on the spark plugs were completely clear. For sampling the oil, would just checking my dipstick and looking for water/blue drops suffice?

[Tom]

Im not getting a sweet smell or seeing any white smoke. I’d really like to do everything I can to test before pulling the head off again, as it was quite labor intensive. The head gasket did have metal rings around the cylinder so I would assume they act the same purpose as in the VW’s. While blue can be quite low contrast, I’ve noticed with the one I’m using even a few drops have a blue tint which is something I’m not seeing here. Even the droplets on the spark plugs were completely clear. For sampling the oil, would just checking my dipstick and looking for water/blue drops suffice?

I have blue coolant too and can vouch for it looking clear unless it's pooled up enough. One trick that can sometimes work is to dab it on a pure white tissue or white printer paper-- often you can get a hint of the blue that way. Pretty unlikely that you would see droplets of condensation on every plug, especially if you've had it up to temp recently. Best way to check the oil is to drain it. If there is coolant in it, it'll be pretty obvious. Oil seals don't normally pass coolant in the cylinders, but if you get enough coolant in the oil, it can steam up into the cylinders (and probably lose some of its color in the distillation process). Seems unlikely unless you saw a lots of coolant in the oil when you changed the gasket, but its another possibility to be aware of. Can't tell when all this pics were taken and if that oil got in when you pulled the head, but as others have said, you shouldn't have puddled oil in the cylinders either -- that would make me think bad valve guides/seals. Are the plugs oily (and watery)? Did you ever do a compression or leak down test? And does it hold pressure if you pressure test the cooling system?

[BennSport]

The plugs were just watery, not oily though. I’m going to to do another compression and leakdown test, although the problem is that I’m using crummy autozone rental tools that I don’t entirely trust. More information hopefully later today.

[BennSport]

I just tested my engine block temp sensor with the wire disconnected and I’m reading about 130 ohms of resistance. I also pointed my infrared thermometer at a spot near the sensor and it’s reading ~142 degrees Fahrenheit. Following Tom’s chart at the beginning I’m about 70 ohms off from what I should be reading. That would definitely explain part of my issue. Just for fun I tested the resistance between ground and the wire itself disconnected to the sensor and that was giving me about 122 ohms. Is it just me or does that seem high?

[Tom]

I just tested my engine block temp sensor with the wire disconnected and I’m reading about 130 ohms of resistance. I also pointed my infrared thermometer at a spot near the sensor and it’s reading ~142 degrees Fahrenheit. Following Tom’s chart at the beginning I’m about 70 ohms off from what I should be reading. That would definitely explain part of my issue. Just for fun I tested the resistance between ground and the wire itself disconnected to the sensor and that was giving me about 122 ohms. Is it just me or does that seem high?

I'm not entirely clear how you measured. The only way that yields a useful value is to remove the wire, then with the wire disconnected, measure the resistance from the sensor itself to ground. Is that what you did? Your last sentence about removing the wire just for fun makes me wonder if the 130 reading was with the wire connected? If so, that value is of no diagnostic value for this purpose. Similarly, if you took the wire off and then measured from the wire to ground, that 120 reading isn't useful.

Even assuming you measured correctly, I'd say those tests are inconclusive (and not directly related to head gasket problems). If the engine block was 142F, I assume the car was either in the process or warming up or cooling down. If so, I wouldn't necessarily think the coolant and block were the same temp. Worth keeping in mind also that the sole purpose of that sensor is to display the temp on the dash -- which is something of a sun dial by design. So even if your gauge isn't very accurate (are any?), I'm not sure how that relates to the HG issues?

[BennSport]

Yes, my reading of 130 ohms was from the disconnected sensor to ground. I was just wondering if my measurement from the wire to ground was important enough to mention. This all started because I had a slightly leaking head gasket, causing combustion gasses to leak into the coolant system causing me to overheat due to all of the air pockets.

[Tom]

Yes, my reading of 130 ohms was from the disconnected sensor to ground. I was just wondering if my measurement from the wire to ground was important enough to mention. This all started because I had a slightly leaking head gasket, causing combustion gasses to leak into the coolant system causing me to overheat due to all of the air pockets.

You might try it again in the morning after the car sits all night, and compare your results to the ambient temperature.