Engine temp rising rapidly to operating level after various changes to engine.

I have a one owner vw mk4 golf, low mileage (140,000km) 2.0 8v. Pre changes, the rise to operating temp when I start the car was gradual. Recently I made changes , being a sports cam, HD valve springs, new hydraulic lifters, heavier valve stem seals, new timing belt, waterpump, thermostat. The car is back in service and running well. However, there is definitely something sus with it's running temps / cooling. I only have the factory dash guage to go by (I believe reality can be 10% either side of the 90% it shows as normal). The temp guage never exceeds the 90% marker, never has and even now with this issue it doesn't, but the speed at which it gets to that point is a few minutes of idling on the drive after startup from cold, is concerning. The fans run now after every trip for a minute or so after shutdown, after normal driving, that would previously only occur on hot days after a track session.

The engine is definitely getting hot although I have not used a second temp guage to confirm that, I just know from owning the car 20 years how warm the water pipes should be when warming up. I have removed and checked fitment of the thermostat, all seems ok and I dont think there are any air pockets in the cooling system, although that is based on me going through some youtube guides on how to remove any air if present. The coolant level is stable and no leaks. Unfortunately to take a closer look at the new waterpump I have to take off the timing cover assembly, which means removing mounts etc.

I'm throwing this out there in case anyone has any suggestions on where I should look, I cant leave it like this as I will be taking to track days and if it's even running slightly hotter than normal now, it will be a problem when it's being pushed. I guess if I can't find anything I could remove the thermostat and see how it goes the?

Any suggestions on what I should be checking first or anyone who had similar issues I'd appreciate any feeback, thanks for reading my post.

Mark,

Some of my suggestions are going to be more universal in case it's helpful to others as well.

Air in the system is super common. I swear by kits like these to help get it out without making a mess:

https://www.napaonline.com/en/p/SER24680

Beyond that, some engines have bleed valves so bleed them if present.

Putting a hole in the top ring of the tstat where a jiggle valve would be can help promote circulation as well.

If both hot and cold hoses are hot, then you likely have circulation so thermostat related stuff likely isn't your issue.

Did you raise idle target speed based on the new cam?

Did you retard idle timing at all to help idle stability with the new cam?

Both of those things will make the engine heat up far quicker than it used to.

It sounds like verifying whether the engine is getting too hot or not is vital at this point.

If the stock ECU is fitted can you use an OBD2 scanner to view actual coolant temp, or if you're tuning the car I would log it with your datalogging software.

Was the thermostat the same value, and design as was fitted? Many modern-ish vehicles are designed to recirculate the cooland within the engine while warming up, and if it had a non-stock design fitted before your ownership that might have affected warm-up? If so, they're usually designed to pass coolant around the valve assemply and any bleed hole may have reduced the internal circulation?

With a more "sporty" camshaft the engine will normally be a little less efficient at idle, requiring it to draw in a little more air and burn a little fuel, which may shorten the warm-up time.

Similarly with "normal driving", you may also be driving it a little harder and hitting the switch temperature for the fans.

If you're quite sure it has been bled properly, I wouldn't worry about it unless the increase in power, and heat rejection into the coolant, has brought the temp's into an area of concern.

Hi Mike and Gord,

Thank you for taking the time to put down some thoughts on this, I really apprecite this and hope it also helps others.

Thanks for that funnel product link, I'm getting one.

Mike I made a few comments below on your points:

Did you raise idle target speed based on the new cam? - No, nothing yet I let the stock ecu setup alone, it idles around the same rpm, same as pre cam , just lumpy. I'm having a standalone kit with harness made up, I left all the fuel iginition etc all alone to get the car back running and do the big changes in separate steps.

Did you retard idle timing at all to help idle stability with the new cam? No, I read some guys advancing cam timing 4 degress but I left all of this to stock. I reinstalled everything back to where it was with a factory cam. The install was a first cam instal so didn't want to change too much, the manufacturer said the stock ecu tune will cope, and it did, actually the car is running better on fuel.

Both of those things will make the engine heat up far quicker than it used to. Thanks, noted on the above I will keep that in mind.

It sounds like verifying whether the engine is getting too hot or not is vital at this point. Yep I really need to sort this. First step is to plug it into VCDS and monitor the temp.

If the stock ECU is fitted can you use an OBD2 scanner to view actual coolant temp. I have the obd2 software for the car, I'll just have to find the measuring block. Yeah I should have done this already, good idea.

Hey Gord, some comments below,

Was the thermostat the same value, and design as was fitted? Many modern-ish vehicles are designed to recirculate the cooland within the engine while warming up, and if it had a non-stock design fitted before your ownership that might have affected warm-up? If so, they're usually designed to pass coolant around the valve assemply and any bleed hole may have reduced the internal circulation?My wife bought the car for a daily driver right off the vw showroom floor, so 20 years back. So we have had the waterpump / thermostat changed once before as per the service schedule and that was done by VW. There was no weird temp rises back then. The part I put in and the previous part seemed identical, however I did not know about this so I'll do some research. Many modern-ish vehicles are designed to recirculate the cooland within the engine while warming up, and if it had a non-stock design fitted before your ownership that might have affected warm-up? If so, they're usually designed to pass coolant around the valve assemply and any bleed hole may have reduced the internal circulation?

With a more "sporty" camshaft the engine will normally be a little less efficient at idle, requiring it to draw in a little more air and burn a little fuel, which may shorten the warm-up time. Ok noted, I guess my paranioa being a noob is that I torqued th cam bolts incorrectly or the waterpump I put in was a dud, but will go back and do some simpler checks up front.

Similarly with "normal driving", you may also be driving it a little harder and hitting the switch temperature for the fans. Good point, and on reflection, I am running a higher rev range everywhere, just feels sweet under the poedal. Yeah could be a factor.

If you're quite sure it has been bled properly, I wouldn't worry about it unless the increase in power, and heat rejection into the coolant, has brought the temp's into an area of concern. Cheers for the feedback, yeah I'll check the temps from the diagnostics, then if it does look hot I'll check the thermostat again and re-add the coolant with the bleeder kit.

I'll drop a line when I have something to share, really appreciate the time put into feeback, many thanks.

Sounds good. Let us know what you find when you get temp readings from the ECU.

I logged coolant temp from the ecu, first session was just sitting still no driving. Logs show 5 mins from 30 to 99 degrees, fans went on at about 97 or so, so I kept it idling, temp never went past 99. Second run was moving, just normal speeds/load. The engine was warmer to start off, approx 40 deg and got to 90 in 4 mins. But the temps did not exceed that level, so normal operation when moving so I guess the water pump is moving the coolant around ok.

Couple of learnings.

VW do program the ecu to show 90 on dash temp to indicate temp is ok. It’s rarely correct. So even if it’s spiking it’s going to read 90 and nothing to worry about. It’s within engine tolerances.I had pre upgrade logs with temp. The car was never more than 72 degrees coolant temp after 5 mins from any starting temp I had logs for. Just to be totally ok with it I’m thinking of draining the coolant and removing the new thermostat, refill with the proper funnel system to avoid air, and see what temps do then.

I logged coolant temp from the ecu, first session was just sitting still no driving. Logs show 5 mins from 30 to 99 degrees, fans went on at about 97 or so, so I kept it idling, temp never went past 99. Second run was moving, just normal speeds/load. The engine was warmer to start off, approx 40 deg and got to 90 in 4 mins. But the temps did not exceed that level, so normal operation when moving so I guess the water pump is moving the coolant around ok.

Couple of learnings.

VW do program the ecu to show 90 on dash temp to indicate temp is ok. It’s rarely correct. So even if it’s spiking it’s going to read 90 and nothing to worry about. It’s within engine tolerances.I had pre upgrade logs with temp. The car was never more than 72 degrees coolant temp after 5 mins from any starting temp I had logs for. Just to be totally ok with it I’m thinking of draining the coolant and removing the new thermostat, refill with the proper funnel system to avoid air, and see what temps do then.

Good work Mark. Yes most manufacturers purpose do that to avoid customers noticing variation in temps until they are severely overheated, to avoid unnecessary customer concern. It's not ideal for enthusiasts who want to see the real data, but at least you can log it.