Had my car (97 Z28) in the shop three times in the past month, and now a brand new problem has appeared. I'm preying somebody here is gonna tell me this isn't what I think it is. (A head gasket)

Driving home (from the dang shop) the other day, my low coolant light came on. Check it later on, was a little low, maybe a quarter inch, nothing drastic. Saw no puddles, no smoke, didn't smell any anti, car stayed rock-solid cool, etc... Wasn't really sure what to make of it.

Since then, the low coolant light is on every time I start the car, with the same story every time. No puddles, no visible leaking, nothing in the oil that I can see, no overheating, no real bubbling in the radiator from what I can hear/see, etc...

But, I started getting a bit of a rough start intermittently, with a check engine light that would come on every now and then, until it finally just started coming on every time I start the car. After some googlin, I read that it might be the coolant temp sensor, so I changed that, didn't do anything to help.

Just ran the codes, got: p0100 (MAF), p0171 (Bank 1 too lean), p0174 (Bank 2 too lean).

Now, the MAF may just have been that I didn't secure the harness or air intake well enough when I pulled everything out to replace the coolant sensor. I just re-seated everything, and I don't think it was as tight before as it should have been. So, now that I've cleared the codes, I'll have to see whether that one comes back or not...

That said, I just started the car, ran it for like 30 seconds, and then realized I hadn't checked the coolant level today, so I turned it back off, and went to pop the rad cap. (It wasn't hot)

There was significant pressure on the cap, and when I cracked it, about a cup of it came blowing out of there. So, clearly some serious pressure had built up in just the 30 seconds the thing was running.

Once when I first got the car, I saw it spray fluid out of the reservoir, so there may well be a leak in that tank or hose somewhere if it fills up too far, which would explain where the fluid is going (I believe)...

But the question is, if my radiator is building up that much pressure that fast, and Im getting misfires, doesn't that point to there being a leak in the head gasket between the cylinders?:

 

Do a hydrocarbon test on your coolant, check some plugs...

 

Try bleeding the coolant system to expunge any air pockets, and make sure the coolant tank is not overfilled.

The coolant level sensors are finicky. On my 96 sometimes it turns on the dash light if the coolant system hasn't been bled correctly or if the coolant system is overfilled.