Weird overheat question, 740 turbo
 

Hey all,

I just picked up a '91 740 turbo wagon for $500. The PO's wife was driving it when it had a catastrophic coolant casualty, dumping all of the coolant through the broken plastic radiator fitting. Her husband came and rescued her before she drove it any further, but she didn't notice anything until vapor was coming from under the hood.

The car when to a mechanic who replaced the radiator and water pump, so that's new, and the engine starts up and idles well, with the temp gauge settling out to about midpoint at idle. Once underway, the gauge shoots up and pegs out at max temp, but the car seems like it's driving fine. When I stop to idle, the gauge goes back down slowly.

I'm going to check for a blown head gasket and have the coolant analyzed for exhaust gases and probably do a leakdown test on each cylinder. But I'm also curious as to why, if the car is still overheating, does the gauge start reading lower at idle?

Should I check the thermostat or the temp sender? None of the wires look ruined on the sender, and I haven't pulled the thermostat, just wanted to check here first and see if anyone has had a similar problem. There are some other strange issues with the car, but the overheating indicator is my first priority.

Thanks in advance--oh the car has 114,XXX miles

I had that happen once (radiator blows apart), but I never had the funny temp thing like that. My sender became erratic years later, but not like yours. The only thing I can think of is making sure that you have all of the air out of the system and no blockages. I would still think that it would get worse at an idle, but maybe the sender has gone haywire. I'd wait for a few more responses.

I'd say you have a bad HG. At idle the system is maintaining but under load it lets go. Keep us posted.

Sure will. I'm pretty curious about this. Also might possibly be the voltage regulator in the instrument panel going haywire with alternator output. The heater core is leaking so I bypassed that right off.

Having the coolant analyzed with tell me for sure whether the HG is ok, hope it is, but either way, not a big deal.

1st check the temp gauge. Find the temp sender on the motor and with someone in the car and the key on touch the sender wire to ground and see if the temp needle goes over to max. Only do this for a second. If it does then the gauge is OK. Also are you loosing water when you drive it. Is there still water in the fill tank? When the car is cold remove the radiator reserve tank cap and start the engine. Is there a lot of bubbles in the water? This would be a sign of a HG leak. Is there any antifreeze on the outside of the motor or running on the ground. We just need some more data.

Yes, there is fluid in the fill tank, but after I run the car, I can hear a hissing sound coming from the fill tank, like there is compressed air in the tank. When I open the fill tank, it sounds like opening a soda bottle and I hear fluid draining to the radiator...

I'll check the sending unit today and drain the radiator/oil and who knows what else...

Thanks guys, I'll keep at it.

Did they replace the thermostat when they replaced all the other parts?

I had a eerily similar problem on my 740. Ended up replacing multiple things at once due to paranoia, but definitely check the thermostat (mine was literally in pieces).

Definitely the head gasket. The exhaust gases created a bubble in the cooling system that stopped when the engine was at idle, giving me a false "cooling" reading, but then the bubbles would circulate once I was on the go, and the engine would overheat.

I opened the coolant fill reservoir with the engine running and could smell the exhaust gases. The good news is that the oil doesn't look cloudy so I don't think coolant made it's way to the sump.

Gonna to a head gasket change and send the head off to a machine shop.

Found a spare head from a volvo here in town that apparently spun a main bearing. I've never what an engine sounds like so the guy tried to start it and the engine just made a knocking sound and wouldn't fire.

He's parting out the car and some of the stuff is ok for spares. Is there any way that a spun bearing (assuming that is really the problem) could wreck the head--since that's the part that I want?

I just want to make sure that I don't end up with two bad heads in case my existing one can't be machined back into service.

Any thoughts?

It seems like you are jumping off of the deep end before verifying what the problem is. I would verify that there are exhaust gases in the cooling system 1st. I would also do a compression test and maybe a leak down test. I assume you looked for visible leaks and found some/none? If it is a head gasket it doesn't necessarily mean you need to replace the head. You would have to remove the head and verify the leak. Then check the head and block to verify it's flatness. If they are OK then just replace the gasket. If the head is warped then you would need to replace it. Oh a spun bearing does not hurt the head. This job is mostly labor so if you are a DIY type it is only time.

Yeah you're right. I didn't get the head just because I wanted a chance to tear mine down and see what's up. I have verified exhaust gases in the coolant so I guess the next step is taking off the head.

I'll get to work....

Yeah, it's the HG. Exhaust gas pressurizing the coolant system is what blew the heater core too. Generally speaking, a spun bearing should have no effect on a cylinder head.

Got word from the machine shop that the head is ready. I'll post some pics of what it looks like. Apparently they had to resurface the head, but it should be good to go. Now I'm waiting on my head bolts and timing belt from IPD to put everything back together.

I'm pumped! Now I just need to borrow a torque wrench...:D