.........because I can't figure out this damn cooling system! 88 951 that my son bought had the radiator return plastic nipple 94410692700break off when driving. He bought a replacement and installed it. Topped up the coolant and everything was cool for a couple weeks.

Car starts overheats and dumps all of the coolant out through reservoir expansion hose. Pressure test the system at 10 PSI and it holds. I bleed the system using Clarks option 1 and all seems good. Drive the car and it starts dumping the coolant after 15 minutes again.

Is this a HG issue adding pressure to the system or am I a dummy and I am missing something easy?

Also, I actually love this car, I steal his keys and leave the 1973 RSR backdate in the driveway all the time.

 

How much boost you running? Maybe try 15 psi and see if that holds? Have you tried one of those kits to detect gases in the cooling system?

 

You need a new head gasket.

 

Also possible there is air in the cooling system from not being bled thoroughly after losing coolant from the system.

 

The coolant tank cap has a rating of 1.5 bar (21.75 psi, 150 KPa). If coolant is escaping through the reservoir expansion hose that means either the cap is faulty or the pressure in the system has actually exceeded 21.75 psi. Test or change your cap first and if the problem persists assume a head gasket failure.

 

Thanks guys, I will investigate further and let you all know. Not super excited about swapping the HG, I remember it being a bitch of a job.

 

Actually, I found it far less hassle than I expected. Cleaning the surfaces properly and getting the exhaust header back on the crossover pipe are two parts I recall giving me reason for foul language.. but otherwise, wouldn't mind doing another.

Be sure to have a torque angle dial tool

 

Do you have any other HG symptoms? Do you have any oil/coolant mixing? Wet plugs? Mocha sludge in oil cap? Coolant tank that looks like someone just shook a bottle of beer after driving? Rough start up? Wet exhaust? Bone stock or modified? I'd pressure test the system and cap up to the cap's limit, and let it sit for 5-10 minutes to make sure there are no external leaks (when coolant gets out, air gets in). Then vent it to the hilt, then put the overflow tube in a plastic bottle and see if the overflow corresponds to how hard you push the motor. But, yeah, I'd be worried about the HG too.... See my HG repair write-up linked below.

 

The water cooled system on the 944 is really not all that complicated. Usually the biggest issue guys run into is what you've described. With a fresh set of silicone hoses and keeping up with the water pump maintenance schedule, you shouldn't run into too many issues. Sometimes the 2 temp sensors on the head, and the one sneaky sensor on the rad, under the intercooler outlet pipe (throttle body side) can cause. So it's best to swap those out if they look a little worse for wear. They're cheap enough, and it's a pretty simple job. That sneaky sensor will likely be the trickiest to get at. Also, the odd time, you might hear of someone saying how coolant dumped all over the place while driving, and it turned out to be coming from the near the firewall near the brake booster. That's the heater control valve. They are plastic. After years of heat, the valve will tend to crack and finally explode and that's where the coolant gushes out of. The 928 is also a victim of this affliction. The valve itself is cheap to buy, but a little more challenging to get at until you remove the intake. Since it sounds like you're going to be doing a HG job anyways, I would say it's probably time to replace these parts 'while you're in there'. It's also a good time to look at vacuum lines, venturi delete, if have a stock '86, and maybe installing a steam vent line, if you want extra insurance for the next HG. I'll bet you anything the HG is blown at the back of the #4 cylinder. It's a known problem on the 951 that the head tends to trap very hot steam in a little pocket in the back of cylinder 4. The steam vent line helps alleviate this.

 

Thanks for all of the insight. I had to put this on the back burner for a time and plan to address tonight.

 

Okay, quick update. I bleed the system using the 8MM grease gun nipple with the hose connected tonight and let the car idle for an hour. Got up to operating temp and stayed there for the whole hour. I drove it around and was all excited that the pressure bleed was faulty and I had fixed it..........until I started driving hard and spooling the turbo. Shortly after that the temp creeped up all the way to turn on the light on. Failure, once again. What say you brain trust with this new info. Idle for an hour...fine, drive at low RPM with no turbo....fine, floor it with boost....very not fine.

Car is an 88, all stock.

 

That sounds like a blown HG to me.

 

After driving it hard and shutting off the engine does it stumble “misfire” right on startup? If not I wouldn’t condemn the head gasket just yet. Check your expansion tank caps mount (were the radiator cap mounts too). Also how’s the radiator cap? Also is there debris clogging the condenser or radiator? Is the radiator plugged up inside? Is the ducting mounted properly from the radiator to the condenser? Did you bleed the system with the front of the car in the air? Is the temp sending unit working properly? Is the water pump failing? Did you mix different types of coolant? A quality coolant pressure tester and dish soap in a spray bottle will help find any external leaks.

 

Pull your plugs ... one of them will have water on it !
Definitely sounds like a head gasket issue ...

Cheers
Phil

 

Hard to be 100% sure unless/until the issue is obvious, but what you describe is the single best indicator I've personally found to detect a HG that is starting to leak. You might poke around a bit more for yet more clues, but most likely the HG is leaking. In addition to overheating, I assume the coolant overflowed when you got on it? Is the coolant tank frothy-looking when it overflows, like it's been all churned up by high pressures? Wet plugs and/or rough start up 15 minutes after boosting? Compression test might show something. Mocha sludge in oil cap or coolant tank? But bottom line, I've replaced HGs with less evidence than you currently have, and they've always been blown.

p.s., I once routed an overflow tube extension under the wiper blade and could literally watch it spray out every time I floored it. That alone pretty much eliminated all doubt that time...