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Discussion Starter #1
Hello all. I'll get right into it

2005 GLS 1.8T
105k miles

I just picked up this car for free because it was overheating and the owner couldn't afford chasing the problem. So it has an unknown service history except someone's "friend" did some work on it and you can tell.

Current symptoms
Gets hot but doesn't overheat, at idle at least. Got to 250F on my scan tool.
No heat until it gets to 250F then I get a tiny bit of heat.
Electric fan doesn't come on at any temperature unless I force it on by turning on AC switch.

List of things I already tried/replaced
Thermostat
Timing belt & tensioner
Water pump
Both belts
Air and cabin air filters
Attempted to bleed the coolant properly

Can anyone help me figure this out? Thanks!
 

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Discussion Starter #2
Oh, I meant to also say that it runs and drives great. It starts right up. It accelerates nicely. And there are no unusual sounds that I can hear. Thanks again!
 

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Discussion Starter #3
Sorry, keep thinking of more info.

No codes. And all the hoses are getting hot, including the ones to and from the heater core.
 

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It sounds like both the heater core and the radiator are clogged. With a/c off and the inop electric fan, my 1.8T never got that hot. Are you sure the thermostat was put in the right direction?
 

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Discussion Starter #5
Thanks for the reply. I was thinking about clogs but the coolant seems to be circulating. Maybe not very well?

Yes, I put it back in as it came out. Spring part in first, o ring, housing?
 

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Yes, spring in 1st. The radiator may be clogged enough to allow some coolant flow, but not a wide enough area to cool off the coolant. The temp sensor at the radiator end of the lower hose is the one that should trigger the fan.
 

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Discussion Starter #7
So that's not the same sensor that gives me the temperature reading to my scan tool?

Should I flush the core and radiator separately? I have seen a bunch of tutorials on heater core flush and it looks easy. A bunch of crud did come out when I first opened the drain on the radiator...
 

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The pink G12 coolant that should be in our cars is incompatible with the green stuff--it makes this clompy brown goo.

The coolant temperature sensor that your scan tool reads is on the back of the head, just forward of the battery. It is actually two sensors in one--one sensor is wired to the ECM and the other to the instrument cluster for the gauge. The first is readable in the ECM measuring blocks, the second in the instrument cluster measuring blocks.

The sensor at the radiator outlet is a dual temperature switch that controls the electric fan (low speed, high speed at a higher temperature). However, one aftermarket parts catalog mistakenly identified the analog sensor for this location for some years, and the analog sensor and switch will both fit and plug in either place. That would be a good thing to check--just measure the resistance between terminals of the lower radiator switch--it should be open circuit (unless engine is pretty hot, then closed circuit). If you measure a few thousand (k) Ohms, it's the wrong (analog) sensor.
 

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Discussion Starter #9
...it makes this clompy brown goo.

The sensor at the radiator outlet is a dual temperature switch that controls the electric fan
Yup, clumpy brown goo it is! Gonna try a more direct flush today. CLR through the heater core and water & flushing agent through the rest. The hose that goes nowhere capped off below the overflow bottle, is that where I should do the flush from? Is there a better place to drain more of the system from as the radiator drain only lets out a few quarts.

I'll be replacing that sensor as well. By the radiator outlet as in the little orange(?) drain plug behind the driver side fog light panel?
 

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So that's not the same sensor that gives me the temperature reading to my scan tool?

Should I flush the core and radiator separately? I have seen a bunch of tutorials on heater core flush and it looks easy. A bunch of crud did come out when I first opened the drain on the radiator...
Reverse flush procedures should be in that thread
 

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Discussion Starter #11
Thank you. I'll be following this procedure today. Could a clogged heater core contribute to my engine running on the hotter side or is the radiator likely clogged too?
 

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A clogged heater core won't make much difference to the engine cooling if everything else is OK but if there is brown glop in the coolant, everything is not OK.

I had a 1.8t that warped the head on a hot day due to a clogged radiator. I think the PO had put some sort of 'stop-leak' in it at some point but who knows? This was after I replaced the heater core and flushed everything, etc. It only over-heated when pushed hard but my wife likes to push cars hard ;) I ended up putting in a new radiator for about $100 IIRC (and a new head of course...) and after that the car was flawless.
 

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Discussion Starter #13
A clogged heater core won't make much difference to the engine cooling if everything else is OK but if there is brown glop in the coolant, everything is not OK.

I had a 1.8t that warped the head on a hot day due to a clogged radiator. I think the PO had put some sort of 'stop-leak' in it at some point but who knows? This was after I replaced the heater core and flushed everything, etc. It only over-heated when pushed hard but my wife likes to push cars hard ;) I ended up putting in a new radiator for about $100 IIRC (and a new head of course...) and after that the car was flawless.
I'm starting to wonder if a new radiator would be a good idea. Would have been a better idea while I had the front end apart doing the timing belt and stuff...
 

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Discussion Starter #14
A clogged heater core won't make much difference to the engine cooling if everything else is OK but if there is brown glop in the coolant, everything is not OK.

I had a 1.8t that warped the head on a hot day due to a clogged radiator. I think the PO had put some sort of 'stop-leak' in it at some point but who knows? This was after I replaced the heater core and flushed everything, etc. It only over-heated when pushed hard but my wife likes to push cars hard ;) I ended up putting in a new radiator for about $100 IIRC (and a new head of course...) and after that the car was flawless.
Is it possible the fan switch not working affects the cooling much at idle? When I turn on the AC to force the fan on I don't see much difference in the temperature, but revving the engine almost immediately drops the temperature an appreciable amount! Like 10-15F
 

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If you have restricted flow the fan won't make much difference. Higher rev's would increase the flow and then the fan may come into play.

If you have an AT you'll have to re-fill the trans if you swap radiators. It's not a bad job and it wouldn't hurt to do it anyway on a car with unknown history. Radiator replacement isn't a big job but the main radiator hoses have these plastic connectors that can be PITA.
 

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The engine driven fan can have a bad clutch. That would cause weak a/c and hotter temps at lower speeds. See if you can stop it from turning with a gloved hand. Either way, the electric fan temp sensor is a problem if the lower hose is hot near the radiator (unless the radiator is so gummed up there is not enough flow to get hot).
 

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Discussion Starter #17
Fan switch replaced, but not tested yet. Heater core is being back flushed on a loop with a 12v bilge pump and clr/water. Then will flush with water and air and water.

Will bypass heater core after and do a cooling flush with standard cooling flush stuff.

Drain, reconnect heater core, refill, bleed and test.
 

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Discussion Starter #18
Heater core flush pulled out a bunch of crap.

Flushing the rest now. Car is on and at around 225F but fan still isn't turning on. Suggestions?
 

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Discussion Starter #19
Ok after first flush, drain and water fill the car won't go above 210F. I know this is a good thing (?) But I really would like it to get hotter so I can see what's going on with my fan!

Still haven't reconnected my heater core yet.
 

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Ok now what? I drained the water and cleaning fluid. Reconnected the heater core. Refilled with coolant. Started the car and it warmed up nicely and I got heat!

Then it hovered around 235F. Still no fan. I tried a little drive to see what I could see and it immediately jumped up to overheat territory. Turned it off and I'm parked in the garage again.

Someone please give me something to work with. Should I bypass the heater core again? What's up with the fan? I'm so frustrated!

😪
 
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