Engine runs cold

[hmclaughlin67]

I recently got a 2005 Yukon XL. The engine runs cold all the time, which signals a stuck open thermostat. I’ve replaced it twice, so a total of 3 thermostats, and the problem still happens. It will reach operating temp while in park, but will eventually overheat and the fans never turn on. Replaced the fans too, and they still don’t turn on. It’s as if the thermostat is stuck closed when parked, but open when driving. I assume it must be an electrical issue or something, but not sure on where to start. Any ideas?

 

[OR VietVet]

Welcome to the forum.

First thing to remember is that the gauge is just a reference and not completely accurate, some times. Need a laser temp gun to check all readings. Might want to get one of these to know that you have a chance of accurate readings being sent: https://www.rockauto.com/en/moreinf...1BXBeqi9UL+Ra983xch91eva1pzmX0odU+EviSbuMJK6x

When you say it overheats, what temp is showing and how do you know it is overheating? Stick with Genuine GM parts, when you can.

 

[hmclaughlin67]

Welcome to the forum.

First thing to remember is that the gauge is just a reference and not completely accurate, some times. Need a laser temp gun to check all readings. Might want to get one of these to know that you have a chance of accurate readings being sent: https://www.rockauto.com/en/moreinfo.php?pk=1145874&cc=1504410&pt=4748&jsn=991&_nck=ZExt1ysfqhqWnq+EvUNdS3Txyey60IDtnjPWF5UU9J0BWQv7pvqZbh+YrdIIoJ2QMq9prrkIerbFMqky5nGcn4teOg9r2Sw3+Pr1CNOXFqQFzxO1xi2naIVVDqptp38yXkE5m0eM5QCA+y15xTcreiyKbrkDUxw8hiDTRm3mksJ9AP1HFO4K5TmAzi6MNBaPQ7CtV+Xav4ITw/GiJhmqXXHpB+pYHri3LjUtnTR9mmP9YASRZK1NHNJpma46wxvz8j6SGthmzgVHsthVCok1BXBeqi9UL+Ra983xch91eva1pzmX0odU+EviSbuMJK6x

When you say it overheats, what temp is showing and how do you know it is overheating? Stick with Genuine GM parts, when you can.

OBD-2 says it reaches above 210, I shut it off before it reaches 215. And the gauge is correct, both OBD and the gauge cluster are the same, and when they both say the engine is hot, the heat is hot as well as the upper radiator hose indicating that it is actually hot when it says it is and cool when it does.

 

[Breadman03]

I’m his shop laborer/dad. Here’s what I was going to post:

My kid’s “new” Yukon is having some odd symptoms. It slowly warms up while idling and will eventually get up to 210-215 (read with my OBD2 live data, dash gauge matches pretty well) without the fans ever turning on. We replaced the fan assembly and the new one doesn’t turn on. I plan on pulling out my multimeter to see if any power is being sent to them and trace it back as the fuses look good, but are hard to see with a brace over the fan fuse box. Any pointers?

Secondly, it cools off to the 140’s while driving. Thermostat, right? Well, we replaced it twice and it still does the same thing. We also used a thermometer in boiling water to watch it open up, then fully close by about 186°. This is what really didn’t make sense to me. The thermostat seems to be working but the temperature still drops??? The upper radiator hose warms up as the engine does, suggesting some level of coolant flow before the thermostat should be open. Is the system supposed to weep some flow through, or did we somehow replace a bad thermostat with two more new/bad t-stats? I can’t imagine the odds of getting 2 new bad thermostats from different sources (Autozone and NAPA).

Relevant info: Both heater cores were clogged. We gently backflushed them and got the front one clear with good flow using our garden hose, but the rear core still isn’t flowing based on line temperature. We’re going to pull the rear lines and flush them separately but didn’t want to risk breaking a part we can’t replace before work and school tomorrow.

 

[Fless]

You'll want to thoroughly check the fan relays to make sure they're operating correctly. If I'm not mistaken, the fans should (may?) come on automatically if the A/C is turned on. Also, pull the two connectors to the PCM and look for water migration and pin fitment.

Having said that, the slow warmup is an issue. The stock radiator has a restrictor in the fitting for the small hose that runs from the right (passenger) side of the radiator to the surge tank. If the radiator's been replaced at some point and it no longer has that restrictor, the engine might never warm up, or could take a long time. Too much flow into the surge tank causes that problem. You can test the theory by pinching off -- mostly but not completely -- the hose to the surge tank to see if you get a quicker warmup.

There are several threads on here about the restrictor, and I'll post a short one below. If you're interested in others, search or yell and we'll see about finding a couple more.

07 Tahoe engine hot ac turned off

You are on to something there, with the radiators. Struggled with the hot, AC off bs for a while. I made a number of cooling system improvements at the same time, so it took a while to figure it out. But I think I have it solved on my machine. Source of the problem: Aftermarket radiator...

www.tahoeyukonforum.com

 

[hmclaughlin67]

You'll want to thoroughly check the fan relays to make sure they're operating correctly. If I'm not mistaken, the fans should (may?) come on automatically if the A/C is turned on. Also, pull the two connectors to the PCM and look for water migration and pin fitment.

Having said that, the slow warmup is an issue. The stock radiator has a restrictor in the fitting for the small hose that runs from the right (passenger) side of the radiator to the surge tank. If the radiator's been replaced at some point and it no longer has that restrictor, the engine might never warm up, or could take a long time. Too much flow into the surge tank causes that problem. You can test the theory by pinching off -- mostly but not completely -- the hose to the surge tank to see if you get a quicker warmup.

There are several threads on here about the restrictor, and I'll post a short one below. If you're interested in others, search or yell and we'll see about finding a couple more.

07 Tahoe engine hot ac turned off

You are on to something there, with the radiators. Struggled with the hot, AC off bs for a while. I made a number of cooling system improvements at the same time, so it took a while to figure it out. But I think I have it solved on my machine. Source of the problem: Aftermarket radiator...

www.tahoeyukonforum.com

It does take longer to heat up, but it also heats up and then proceeds to overheat if just parked and idling. Any ideas for that?

 

[Breadman03]

Thank you. I’ll give that clamp idea a try now.

 

[Fless]

Fan operation. But I'd work on both issues at the same time. You likely have two separate problems.

 

[OR VietVet]

You'll want to thoroughly check the fan relays to make sure they're operating correctly. If I'm not mistaken, the fans should (may?) come on automatically if the A/C is turned on. Also, pull the two connectors to the PCM and look for water migration and pin fitment.

Having said that, the slow warmup is an issue. The stock radiator has a restrictor in the fitting for the small hose that runs from the right (passenger) side of the radiator to the surge tank. If the radiator's been replaced at some point and it no longer has that restrictor, the engine might never warm up, or could take a long time. Too much flow into the surge tank causes that problem. You can test the theory by pinching off -- mostly but not completely -- the hose to the surge tank to see if you get a quicker warmup.

There are several threads on here about the restrictor, and I'll post a short one below. If you're interested in others, search or yell and we'll see about finding a couple more.

07 Tahoe engine hot ac turned off

You are on to something there, with the radiators. Struggled with the hot, AC off bs for a while. I made a number of cooling system improvements at the same time, so it took a while to figure it out. But I think I have it solved on my machine. Source of the problem: Aftermarket radiator...

www.tahoeyukonforum.com

IIRC, besides the PCM, isn't there a fan module that kicks them on at the correct temp, when a/c is not on.

 

[Fless]

IIRC, besides the PCM, isn't there a fan module that kicks them on at the correct temp, when a/c is not on.

I don't think so, it's the PCM that controls the fan relays up behind the main fusebox.