I have a '95 Pontiac Grand AM. The Heater Coil/Core is leaking onto the passenger side floor. Bad Core. In addition, the car keeps over heating intermittently, whether it’s stopped at a light or driving over 35mph out on the road. The gauge goes into the red, but then after a minute it goes back down. We replaced the Thermostat thinking that it was sticking. Drove it around a good 10 mile run. Didn’t over heat. Next day, it over heats. Just took it to the garage where they ran it and did compression check. For over 4 hours they tried to get it to overheat. It wouldn’t. Their opinion? They replaced an old radiator cap. They said if that doesn’t work, it could be the head gasket (no sign of any fluids mixing - oil cap is clean. No milky residue) or replace the thermostat, which we said we already did. Drove it 2 miles. Started to get hot again. Going to bypass the core to see if this might be the problem. Could there be air bubble from bad core? Not sure if this is the right direction, but I am at a total loss as to what it could be. Pump is running. Radiator fan running. Does anybody have any clues? Anyone have a similar story? Would greatly appreciate any suggestions.
It is imho a loss of coolant and pressure that will continue until you replace or bypass the heater core. There could be an air bubble but once the system is sealed that may autocorrect as long as the overflow cannister has the proper fluid level, check often.
I agree–overall–with Barkydog, but I have to add an additional thought.
Just because the water pump is “running”, that does not necessarily mean that it is effective.
It is not unusual in older cars ( yours is…apparently…17 years old, which places it in the senior citizen category ) for the vanes of the water pump to be severely eroded, and thus, to be…not very effective…at moving fluids.
In the case of a water pump with eroded vanes, the water pump is “running”, but it is actually pumping very little fluid through the cooling system. This is just something for you to think about.
There is a chemical tester that detects exhaust gas in the cooling system. It would be worthwhile to test for a failing head/head gasket. Is it a Quad 4 engine?
Four or six cylinder? If you have the six, did you open the bleeder located on the water pump housing to bleed the air out of the system when you refilled it with coolant? This is an absolute must with that engine or it will do exactly what your car is doing.
This is a 6 cyl and no flushing has taken place. I know that rust can build up and cause problems as well w/o regular maintenance. Do you think I should ask them to flush the system b4 bypassing the core? Just to make sure. They did run a pressure check to see if there was air getting in, but was told it was negative. Thanks all for responding. Don’t think the garage is trying to rip me, but they didn’t suggest much else but then a radiator cap and possible head gasket problem, which I am in no way going to entertain repairing for a 17 year old car! 
Before doing anything else, I would try opening the bleeder and add coolant until coolant comes out the bleeder. Do not do this with the engine running. You have not yet said whether or not you have done this, so I assume you have not. See if that solves your problem.
A leaky heater core does prevent the cooling system from pressurizing and can cause overheating. Your system rins at tempertures higher than boiling, and it’s the ability of the system to hold pressure that raises the boiling point and allows the system to operate at those temps.
But on a vehicle this old I wouldn’t assume the two are connected. I’d definitely bypass the heater core as a diagnostic step, but I’d also want to do some radiator testing and flow test the pump (see VDC’s post). Testing on the radiator would include a pressure test, a flow test, and perhaps even a “mapping” of the radiator with a simple infrared thermometer looking for cold and hot spots. A radiator that old probably has lots of gump in the core.
By the way, a radiator cap can be tested. And is isn;t rocket science to recognize that an engine can idle fine but overheat when under load. I’d look for a reputable raditor shop to get some real testting done.
I think the garage that replaced the radiator cap was on the right track, but if you have a known leak anywhere in the system, a new radiator cap is not going to stop it from overheating, even if the old cap is bad. If you aren’t holding pressure, you will overheat under load. The leak in the heater core will keep the pressure from holding.
Just wanted to report that we bypassed the core and had the system flushed. They were able to open the bleeder, which they weren’t able to do when they had checked it out previously due to being seized up. So far so good. Said the coolant was really clogged with crap. Doesn’t think the car has a head/head gasket problem. Did discover that the rac and pinion boot is ripped wide open, so when that goes out, the car will take a trip to the junk yard! Thanks for all your input. REALLY appreciate it! I only paid $300 for the car through a friend. Hopefully I’ll get all my money’s worth out of it! LOL!
Its not that big a deal to replace the boot on the rack. I’d think about it if that’s the thing that will send it to junk.
Cig’s right. As you can probably see from the attached drawings, the way these things are constructed the tierod end link assembly can be unscrewed, the old boot removed, and a new one slid on. The car then needs to be realigned.
I’d ask about the cost of replaceing the boot now, before the rack starts to leak, and try to prevent it.
Thanks! I’ll check into that. The mechanic said it would be expensive to fix and wasn’t sure how much money I wanted to put in the car. I never actually got a price. I do have a couple of people that I could probably ask to do this for me. Is this something you could or should get from a junk yard? Thinking about the savings.
Your mechanic seems correct about the expense, and I wrong about it being easy in this case. Many rack boots are quite easy because they are replaced as outlined by mountainbike above.
However, this rack is a center steer rack and has a one piece boot that spans both sides of the rack at once. It would necessitate rack removal and would be expensive. You might have a look at it and see if there isn’t something that could be done to at least slow the damage.
Ouch!
I tip my hat to Cig’s research. I was unaware that this rack was unique.
The rack on this car is not necessarily unique; GM used this design on many of their small and midsize cars from the late '80s to early to mid '90s, and they are a pain in the butt to deal with. Setting toe during an alignment is also very difficult to impossible unless all the tie rod ends are new.
Its possible that “unique” was an overstatement. But I’d go with “outside of the norm”
Same result, I missed that one. I sincerely appreciate the information from both of you. Hopefully, I’ll mever stop learning.
It was a new one on me too.
Not to discourage you from replacing the boot, but I’ve had cars with torn rack boots go for 10 years or more without the rack ever leaking, though lately, if I find a torn one, I replace it.