Overheating 1.8 dx

A while ago my gti's thermostat got stuck open, needle on the gauge barely moved asides sitting in traffic. So I changed the thermostat a few weekends ago for a circoli one from ecp.

So then the temp gauge moved but was a bit irratic going up and down and back up again and so on, I thought perhaps the sender was possibly at fault, I've not changed it as yet but have tried cleaning it see if dirty electrics are to blame, no difference but wasn't expecting much anyway.

Anyway, I've not driven the car much recently, bad weather, time off work (only really get to drive it to work) but I drove it today, got a few miles into a 8-9 mile drive to work, temp gauge only gone up a bit and the light starts to flash at me, and the gauge starts going up at quite a rate. So quite carefully carry on seeing if it's just a blip, no, it goes up to around the middle of the light and stays there, the light flashes, flickers goes off then on solid etc.. I've now got the heater cracked up to max to try get the temp down a bit, might've helped a bit but still telling me more Hot than I was comfortable with. Get to work and it doesn't smell any more hot than usual so go about my day dreading the drive home.

The drive home was much the same, except the temp went up faster but more or less stayed between the light again, flashing most of the time, but didn't smell hot, seemed to run fine (asides a warm up issue which I've been trying to get to the bottom of) get home and have a poke around, loads of coolant, the bottom rad hose was warm but not hot, and the top hose was hot. I turned it off squeezed bottom rad hose a bit then started it again, it got hot after that and the fan cut in just the other side of the light.

Would an airlock cause the hose to be warm rather than hot? I can't imagine it should only be warm at normal operation?

Have I bought a duff thermostat?

Is sender or wiring from sender to gauge not good?

I should mention my clocks do play up rev counter and back lights the main culprits, never had an issue with temp or fuel gauges mind so I'm not sure it would be the clocks.

Any pointers as to best ways to test to rule things out would be much appreciated, thanks.

Apart from the red flashing light it sounds normal. As you warm up, drive fast/slow, power up hill, coast down hill, sit in traffic, manoeuvre in parking spaces etc the needle should move slowly and be around the central position either side of the LED. The fan cuts in and you haven't mentioned it boiling up and losing water.

The LED is also coolant level warning and like the oil pressure light can play up but check the level and the connection on top of the expansion bottle.

It just seems a bit high, usually I'd expect it to be around halfway between cold and the led normal running and get to about the led sitting in traffic etc, not middle of the led whilst driving. They're mfa clocks with the mpg if that makes any odds.

I don't have a connection on the expansion bottle, just a plain cap 🤔

It does seem peculiar that since I posted about my temp guage a few weeks back I've seen several posts about temp guage issues!
It is possible that you have a guage issue, but equally possible that you have another issue - maybe both.
If you don't have the level sender on the expansion tank it makes things a bit easier to check. with ignition on, If you pull the wire off the temp sender the guage should not move at all, and the light shouldn't flash. If you ground the sender wire then you should see the guage start to move (don't ground it for too long as you can damage the guage - just enough that you see it start to move). In normal operation the light should flash briefly when you turn the ignition on, then go off. If it keeps flashing, even with the sender disconnected then the capacitor in the guage circuit has had it. You can replace it, but it means the whole cluster out careful use of a soldering iron. I found this useful: TheSamba.com :: Vanagon - View topic - blinking coolant temperature LED
The guages are slightly different in terms of build but the circuit is the same. I used a Tantalum bead (https://www.ebay.co.uk/itm/121655307631) capacitor in min einstead of an electrolytic - you can use either as they are both polarised capacitors.
It might also be worth looking at some of Briano1234 posts regarding dodgy earths on the dash cluster as that could be the cause of some of your issues (rev counter, lights, etc) and would be an easier place to start that taking the whole thing apart!

I'm pretty sure the issue regarding the clocks is an earth, I've tried to investigate a couple of times, but I'm hopeless with electrics. It's something I really need to get better at as some of the electrics in my mk1 are not great.

I did think just change the temp sender, but rather do some tests  and actually problem solve rather than just change parts needlessly

So, grounding the sender presumably that need to be done with a warm engine? Maybe a stupid question tbh

Defo have a look at some of Briano1234 posts - he's the earth guru!
No, grounding the sender wire (just touch it to something metal on the car) will make the guage think it is getting a very hot reading from the sender (basically the hotter the sender gets, the lower it's resistance, which the guage is effectively measuring). Just pull the sender wire off, turn ignition on guage shouldn't move and light shouldn't flash. Earthing the sender wire should make the needle start to move.
If you have a multi meter a cold sender should measure about 200 ohms (one probe on the top, one on the metal coolant pipe it's screwed into).

I will check out some of his posts, I want to get a bit better at some simple electrical stuff as currently a lot of it means very little to me.

Thanks for the advice, hopefully if I can get out a few minutes early I'll do both of those checks before I go to work, and I'll report back with my findings

Just had a quick look as had a spare 10 minutes, grounded wire to cylinder head, nothing. Although the light didn't flash with the wire connected to the temp sender either.

Same for testing the sender with the multimeter, black in the com, red in the ohms, set to 2k ohms. It went to about 135 for a split second randomly but only did it once and not straight away and got nothing else out of it. The car hasn't been started today if that makes any difference.

I could easily be doing something silly though in all fairness

OK so I tried again this morning, same thing with grounding the wire, absolutely nothing

But, the multimeter said that with the connector disconnected from temp sender around 1500 ohms, and then around the 200 mark with the connector connected and multimeter on top of the connector.

So with the ohms reading between 195-205 I'd say that the sender isn't the issue but then why would it read 1500 without the connector on? That was both ignition on and then off

First thing to remember is that not all new parts are good.  Did you check the t-stat by placing in a pan of water and raise the temp of the pan to make sure that your t-stat "popped" open?

How did you fill the radiator?  From the upper hose to the radiator, then the Upper hose to the block?  This prevents the air bubble, and if you drill a 1/8 " hole in the lip of the t-stat then you don't get that bubble.

Next I would ask you to verify that your Fan on the radiator cycles on and off.

Do you have the upper radiator card on the radiator to the valance?

Is there a ring of oil in your expansion tank if fitted?

Have you checked your grounds for a voltage drop?  if the cables are OEM (bare wire that may be cruddy green or Brown, you might want to think on replacing them with new from battery to frame and frame to engine.

Is your fuel gauge rock hard and correct or does it do wonky things as well?

Verify that your grounds to the clocks is good, and that your gauge is correct if you place a 9v DC battery to the back side ofthe gauge as it should go to more than 3/4 full or full hot, and stay there for about 4 minutes to verify that the gauge doesn't have cold solder joints.

From your original description it appears the thermostat,  cooling, fan switch and fan are working has intended.

It warms up and the needle moves slighly and slowly depending how much work the engine is doing and how much air flow its getting (cruising vs parking etc). The fan kicks in and it doesn't boil up.

Don't get in to an expenive and time consuming wild goose chase swaping parts becasue you think the needle is too high and/or the light is flashing when its not meant too.




 

Early-1800, I'm not going to get into spending on unnecessary parts, don't really have the funds to change parts willy-nilly.

However, the car has consistently run around 1/3 up the gauge whilst driving and around halfway when sat in traffic etc the whole time I've owned it, now it's going to between 2/3- 3/4 when moving which is unusual for my car, perhaps others run at that temp happily but mine has never got that hot before (according to the gauge anyway) so I think It's worth at least a bit of investigation. I'm fairly sure that whatever/if there's a problem that it's some electrical gremlin as I don't think the car is actually overheating

Briano1234 I didn't test the new thermostat to be honest, in hindsight probably should've. Filled from just into expansion bottle, had no issues with this way when I did waterpump (should probably have done stat then but was working then) but taking rad hoses off to fill makes sense.

Fan is working and cutting in, the gauge didn't move much between driving and sitting idling until the fan cut in.

Car has never had a radiator card in the time I've owned it, never really thought they'd make any difference other than aesthetics?

Didn't look like oil in water, I only really checked level but I'm sure I'd notice oil in it.

Grounds need looking at, the one from alternator to rocker cover isn't great, and needs replacing. The earth to the clocks is definitely not good, the rev counter and back lights are the main culprits, though shift light has been on constant for a while. But otherwise the temperature and fuel gauge have always been reliable, though the fuel doesn't read completely full when it's been filled up, reads just under full.

You can use a lower temp t-stat, I do that over here as the air temps in the summer on the road hit 140F.

It could also be the Pump, but if you have a good flow on the return to the expansion tank and it increases with RPM, then you are usually good.

The upper radiator card block the radiator from re-cycling engine compartment air over the radiator.

Read the 1st half explaining the radiator cards,,, the upper is Critical.

Radiator Fan operation explained? :) | Volkswagen Owners Club Forum

If I remember correctly the thermostat I fitted opens around 85°c.

Pretty sure coolant was returning into the expansion tank (I can't remember was a few days ago now!) And yes would increase as rpm increased, it did stop start when I first started the car after refilling but I'd assume that's normal as its getting air out of the system.

Ah right,  I'll look into that for sure.

I have briefly looked into clock earth a couple of times but I've no real idea unless blatantly obvious what to look for

Did you bleed the system after doing the thermostat?

I ran it up to temp with expansion cap off heaters blowing full blast on hot until the fan cut in, then topped it up and drove it. The gauge was being a bit irratic after that but never read particularly hot until the other day

So, I think I've found the culprit, or at least a part of it.. last night ran the car, checked the coolant etc etc waterpump working, no oil in coolant, bottom rad hose got hot suggesting thermostat opening as it should and fan cutting in. Meanwhile light still flashing so I give the connector block on the back of the clocks a bit of a wiggle, and the light goes out. Drive it a couple of miles and temperature gauge read around halfway then about centre of led when fan cut in, same this morning on the way to work. But did flash and needle started to climb on the way home, another wiggle of the connector block (I have no dash undertrays) and it goes off and temp goes back down. So a bad/loose connection to the clocks as well as a bad earth (which read around 0.9- 4ohms) feel a bit dumb but there we are.