Overheating advice

[project 92z]

So I recently saved a 92 Z34 from being crushed. The yard said the guy drive it in but thought it had a blown head gasket. If it does, it's unlike any I've ever seen. No unusual white smoke, not even after sitting all night, will idle without overheating, I can even drive it at highway speeds or WOT. There's also no oil/water mixing. Overheating only happens if I don't let the car get to operating temp before pulling off then it seems to level out with no further issues.

 I did a tune-up: plugs, wires, valve cover and plenum gaskets, because there's a stumble and slight "sputtering" feel sometimes when starting off or quick acceleration. I've checked fuses and relays and burped it also confirmed no leaks.

At this point, I'm not sure what else to check other than the thermostat. I was planning on that today but had to order it. Any other ideas? Also concerning the thermostat, I always thought lower temp options were best, especially for that engine, but the salesman said no.

[rich_e777]

IIRC a basic off the shelf water pump (Duralast) that has the non-factory plastic impeller was said to cause overheating issues. Clogged up cat converter will as well.

 

[White93z34]

I guess may as well do the thermostat first as its relatively easy.

Unless you're getting the fan turn on times adjusted for a lower temp thermostat there is no point. Maybe put a tiny hole in the flange of the thermostat to assist with air bleeding.

Even my z34 with a chip in it I have the factory 195* thermostat yet, but have the fans adjusted to keep it more consistent.

[GTP091]

If I understand your post correctly it overheats briefly then is fine if you keep driving it?  I’m not quite sure what you mean by pulling off without letting it get to operating temp.

-Have you tried bleeding the air out? Your lq1 can’t do this on its own.  That’s an easy win for sure. There’s two brass air bleed screws up top, 8mm I think, they break easy so be gentle.  Turn them out and bring up the rpm with the throttle body and when it starts to piss a steady stream tighten it up. Do the heater core last as it’s higher up.

-Did you do a complete flush and fill when you did the intake gaskets?

-if somebody before you mixed coolant types incorrectly it can gum up the rad quickly. Hopefully not as the heater core will be clogged first and that would be  a bitch.

 

Your issue doesn’t sound too severe so could just be a lack of maintenance from the previous owner.  Like Chris said, water pump or Tstat if the simple things don’t help.

Edited September 15, 2021 by GTP091
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[Schurkey]

On 9/13/2021 at 12:38 AM, project 92z said:

So I recently saved a 92 Z34 from being crushed. The yard said the guy drive it in but thought it had a blown head gasket. If it does, it's unlike any I've ever seen. No unusual white smoke, not even after sitting all night, will idle without overheating, I can even drive it at highway speeds or WOT. There's also no oil/water mixing. Overheating only happens if I don't let the car get to operating temp before pulling off then it seems to level out with no further issues.

 I did a tune-up: plugs, wires, valve cover and plenum gaskets, because there's a stumble and slight "sputtering" feel sometimes when starting off or quick acceleration. I've checked fuses and relays and burped it also confirmed no leaks.

At this point, I'm not sure what else to check other than the thermostat. I was planning on that today but had to order it. Any other ideas? Also concerning the thermostat, I always thought lower temp options were best, especially for that engine, but the salesman said no.

1.  Thanks for saving a rare and wonderful vehicle.  You know the previous owner neglected the vehicle, it's going to need substantial repair and preventative maintenance to set it right.

2.  I had a '93 3.4L with blown head gaskets.  One head gasket actually failed and leaked, when I pulled the other head I found identical damage in the identical place--it just hadn't started leaking yet.  It's as if there's a "soft spot" in the head casting; the head distorts, loses gasket-clamping in a certain area (near #1, kinda towards the lower part of 1 and 3; and the same spot on #6, towards the lower area of #6 and #4.  Very near a coolant hole in the gasket, and a bigass coolant passage in the block.)  The gasket wiggles free until it fails from fatigue.  Had to mill the heads .015, and I wasn't happy about the wear on the block surface--but I took a chance and it seems to have worked.  Got the engine running beautifully, and then the trans failed.  Haven't driven the car in almost ten years.

3.  Some photos of the two head gaskets stacked on each other, and scarred deck surface attached.

4.  It's two steps away from hopeless, trying to diagnose driveability issues without a scan tool to give you insight into the sensor readings, and the computer outputs.  Sure, codes can be helpful, but the data stream is ESSENTIAL.

5.  It's probably worth your time to conduct a compression test, and then a cylinder leakdown test of the best and worst cylinders, if not all six.  Do this BEFORE you take things apart.

6.  Stay with the OEM-temperature thermostat, and don't bother drilling any holes.  The vehicle has bleeder screws to bleed air from the cooling system, the thermostat doesn't need to pass air when closed.

7.  Plenum gaskets are one thing.  The actual lower intake gaskets--where the intake meets the head--are a known trouble area leading to unstable idle and off-idle problems.

8.  Don't forget about the timing belt.  Previous owner almost certainly neglected it.

Edited September 16, 2021 by Schurkey

[project 92z]

So basically it sounds to you like it is the head gasket? I think if I'm gonna go that deep into it, it might be worth looking into a total rebuild. The one shop who I called that had any experience with that motor said aside from the parts, he might also run into a tool issue. Anyone have access to and or wanna part with the Kent-Moore kit for the timing? I was able to find a JDM rebuild kit and while JDM obviously isn't my first choice they had some decent reviews. I couldn't find the tools though and without the tool kit, it wouldn't matter how good it is.

[Schurkey]

 

On 9/21/2021 at 1:45 AM, project 92z said:

So basically it sounds to you like it is the head gasket? I think if I'm gonna go that deep into it, it might be worth looking into a total rebuild. The one shop who I called that had any experience with that motor said aside from the parts, he might also run into a tool issue. Anyone have access to and or wanna part with the Kent-Moore kit for the timing? I was able to find a JDM rebuild kit and while JDM obviously isn't my first choice they had some decent reviews. I couldn't find the tools though and without the tool kit, it wouldn't matter how good it is.

Head gaskets, valve job, and timing belt is far less work than a total rebuild.  The short-block is pretty reliable.

How many miles on the engine?  Did it have good oil pressure?  Have you performed a cranking compression test?  Any knocking noises, including piston slap which is totally common, GM says it's "harmless" but I say it might trigger the knock sensor leading to loss of fuel economy and power.  Oil leaks other than what can be easily fixed with the heads off?  (That is, all the gaskets above the head gasket, plus the "usual" oil leak at the oil pump drive.)

 

I would start with the thermostat, bleed the system according to the service manual, and maybe verify the water pump and belt.

You could perform a cylinder leakdown test; but when my head gasket(s) failed, there really wasn't much leakage with the tester--it took combustion pressure to get past the failed gasket(s).  I was kinda surprised that the leakdown tester didn't show indication of gasket failure.

Or pressurize the cooling system over night with all the spark plugs removed.  Next morning, see how much the pressure dropped, and have someone crank the engine while you look for geysers at the plug holes.

Edited September 27, 2021 by Schurkey