My 1990 300zx Twin Turbo build by megaDan

6 minute(s) of a 731 minute read

Issue 1: I hesitate to say this given all of my seemingly poor Z-related luck this month; but I think issue 1 was a fluke and just a loose connection in my PTU sub-harness. I do plan on making this more robust in the future; but a dozen or so drives later and I haven't had anything like this happen again.

Issue 2: The only thing that made sense to me about why it wouldn't suck coolant back on it's own is the inability for the system to hold pressure; thus not creating a vacuum as the system cooled down and not pulling coolant from the overflow tank. I initially pressurized the system with my compressor and didn't hear or see any leaks, but it's not a great static test.

What was expected to be a simple cooling system pressure test turned into a rube-goldberg like chain of events. I rented a coolant pressure tester from Advance on Friday and got home to find out that the adapter I needed was missing. Ugh. After 20 minutes of futzing I got the universal adapter to work. I pressurized the system to 15psi and heard the telltale sign of a tiny leak "pss pss pss". Unfortunately I traced this down to my heater core bypass hoses. During my bi-yearly hose clamp check it looks like the hose clamp was now off the hard line and now squeezing on just the soft heater hose itself. I purchased fresh hose and replaced both splices. Everything is locked up tight.

Old crusty hose crushed by migrating hose clamp

Both hoses replaced, and I marked the end of the hard pipe to make sure the clamp doesn't walk off it over time

I've almost convinced myself that if I run into heater core issues again the dash is coming out and I'll do it the right way. I'm just very scared of creating squeaks, rattles, and other NVH issues. I've never had that problem with other heater cores I've done, but none of those were on 30 year old cars either.

Thanks to the coolant weeping that I was unaware of, I got out the carpet cleaner and cleaned the floor carpet as well as the floormats. I generally keep the interior in tip top shape, so there's almost no difference in before an after pictures that's visible. The carpets FEEL much better though.

Before

After. Disappointing


With the cooling system finally nice and tight to the best of my knowledge, I got the pressure tester installed again and set to 15psi and didn't see the needle move at all. After an hour, roughly a 1psi loss. After 12 hours, roughly 7psi loss. I would think this should be enough to maintain pressure after a cooling cycle to suck coolant back from the reservoir but I'm still seeing 2-3" of missing coolant from the radiator. I'm starting to get a little frustrated/worried.

With the cooling system finally nice and tight to the best of my knowledge, I got the pressure tester installed again and set to 15psi and didn't see the needle move at all. After an hour, roughly a 1psi loss. After 12 hours, roughly 7psi loss. I would think this should be enough to maintain pressure after a cooling cycle to suck coolant back from the reservoir but I'm still seeing 2-3" of missing coolant from the radiator. I'm starting to get a little frustrated/worried.

Ready for testing

Almost by accident I discovered the source of the very slow coolant pressure drop. I found a smaller-than-pinhole leak where a bar meets the endtank. I started brainstorming repair options and on a whim googled "mishimoto warranty". Turns out they have a lifetime warranty (it says so right on top of the radiator, D'OH). I had to pay shipping, but they sent out a new one no questions asked. It showed up last week and I installed it Wednesday night in under an hour. I continue to wonder why these cars get a bad rap for wrenching. Why is having to take the engine out for turbo replacement where most people draw the line? I had a discussion with a buddy with a C5Z and he is firmly in my camp. I can do a clutch myself in an afternoon. He needs to plan a weekend around it. I can do spark plugs in 15 minutes, he needs an array of tools and a yoga warmup. In the grand scheme of things, who cares; but I find it comical that this car has the reputation it does (says the guy posting all his troubleshooting issues).

It's almost difficult to tell which is new, which is old

Enough of that. I brought the Z to work today, new radiator installed. I'll probably go pick up the pressure tester again to confirm there are no issues; and I'll check coolant levels tonight but I'm not super optimistic. I've got the nagging feeling its consuming coolant without telltale signs. Either way, I'm still having lots of fun with it; and I'm still managing to take care of it even with a 6 month old at home. I apologize for the lack of excitement this year. No roadtrips, no cars and coffee. Nothing to write home about aside from commutes and minor issues. Thanks for sticking around.