Failing Belt tensioner leading to running hot?

Hey guys,

My main question is in the title: Could a failing belt tensioner lead to your car running warmer than it should? 

Context: I bought my 1995 BMW 318ti several months back. It ran a little warm on the way home, but I cranked the heat and it was no problem.

I preventatively changed the radiator, hoses, thermostat, and water pump. Bled the system multiple times. No leaks. Aside from the water pump, every other component should be ~OE quality. 

The car still wants to run warm, from traffic to freeway cruising.

The electric fan doesn't work (needs resistor replacement), but that's not new, and in my opinion, wouldn't be the cause of running warm on the freeway.

I noticed that the belt tension seems a little light  (by feel not by measurements). Before buying an OE part for $80, can anyone confirm that there's a chance poor belt tension could lead to the Water Pump slipping/not moving enough fluid to keep the engine cool?

New Thermostat stuck shut? I know I put it in with the correct orientation..

Not likely.  Doesn’t take much force to rotate the water pump if the pump is good.  If the bearings are shot, that another matter, but then you would probably hear the belt squealing.

What is "ran a bit warm"?  The factory gauge 'o random colors was high?  A proper gauge with accurate numbers?  Steam from out of the hood? Scan tool data?

What about O2 sensor getting a little wonky and letting you lean out a little too much at cruise?

Streetwiseguy said:

What is "ran a bit warm"?  The factory gauge 'o random colors was high?  A proper gauge with accurate numbers?  Steam from out of the hood? Scan tool data?

Factory gauge showing between "1/2" and "3/4", up to the start of the red at "HOT," but coming back down to the middle when I blast the heat

wae said:

What about O2 sensor getting a little wonky and letting you lean out a little too much at cruise?

I should clarify, it happens in freeway driving and around-town driving. No real discrimination on when.

In reply to classicJackets :

when PO bought it, the needle stayed dead nuts vertical.   after mucking about with "the octopus" to get the intake off to replace the plastic heater hose manifold thingy on drivers side of block, it never settled back to center.  i'm not 100% certain that all wires went back where they belong, as someone else finished that job.  i also don't know when the electric fan quit working.   perhaps they are related.

AngryCorvair said:

In reply to classicJackets :

when PO bought it, the needle stayed dead nuts vertical.   after mucking about with "the octopus" to get the intake off to replace the plastic heater hose manifold thingy on drivers side of block, it never settled back to center.  i'm not 100% certain that all wires went back where they belong, as someone else finished that job.  i also don't know when the electric fan quit working.   perhaps they are related.

I did change the temp sensor for the fan - maybe I need to double check the temp sender/wire for the gauge itself. It definitely works to some extent though, as the car starts "cold", and then climbs.

classicJackets said:

The electric fan doesn't work (needs resistor replacement), but that's not new, and in my opinion, wouldn't be the cause of running warm on the freeway.

 

 

I don't agree with this statement.  If you can bring the temp down by running the heater fan why wouldn't you want to run the radiator fan?

 Fix the obvious problem, then see what happens. 

Radiator fans do nothing at highway speeds. I'm guessing you've still got air in the system. I would certainly function-test the thermostat and check the water pump for free movement as well.

02Pilot said:

Radiator fans do nothing at highway speeds.

I disagree with this statement, too.

Cars should basically never run hot enough that turning on the heater makes a difference on the temp gauge. Cars shouldnt need their fans to work at highway speed. You've definitely got an issue, but I doubt it's with the belt drive itself. I'm not familiar with the particular heater hose manifold thingy you mention but i can definitely picture a scenario where you take 4 coolant hoses off something, reattach some back to the wrong barbs, and cause a coolant flow problem. 

In reply to Vigo :

assembly of heater hose manifold thingy really is idiot-proof.   molded hoses that can only be installed one way, etc.

02Pilot said:

Radiator fans do nothing at highway speeds. I'm guessing you've still got air in the system. I would certainly function-test the thermostat and check the water pump for free movement as well.

I disagree a bit here depending on the fan.  One of the engineering debacles with electric fans is that when they aren't on, they block a significant amount of flow through the radiator.  Old-school clutch/belt fans still move even when the clutch is disengaged and they are usually spaced a few inches away from the radiator.

In reply to Streetwiseguy :

The heater acts as an extra radiator.   The fan just moves the air through it since there is little air flow without.

iceracer said:

In reply to Streetwiseguy :

The heater acts as an extra radiator.   The fan just moves the air through it since there is little air flow without.

Exactly.  So, if you have too much heat in the system, and you don't have enough airflow through the radiator to liberate that heat, turn the cooling fan on to draw more air through the radiator.  The heater core is adding cooling capacity, to compensate for the inoperative engine fan.

I'm not guaranteeing that the cooling fan is the problem, but I sure would fix it before I wasted a bunch of time and money on stuff that isn't obviously broken, when there is an obviously broken component.

Went and changed the oil/filter last night, and left it on the ramps while I ran it for a while with the hood open. It came up to temp and ran perfectly cool - not even getting hot enough to open the thermostat with me holding the throttle open, and adding quick revs to try and bring it up hotter. I went for a drive, and the temp climbed pretty quickly. Thermostat is definitely working, as both hoses were hot by the time I stopped.

Lots of fun. I'll probably skip the autox I was thinking of running in this weekend and wait for a resistor to come in in the meantime, and take it from there.

I had a similar issue with my Ti.  The temperature was fine until it reached highway speed and then it would overheat.  I eventually found that several of the radiator passages were clogged.  New radiator fixed the issue.  I see that you have already changed the radiator but this may support an air in the system hypothesis. 

If there is poor air flow through the rad you will need a fan at speed.  You are also under constant higher load at higher speed.  You may actually need a rad fan at speed.  Also, airflow over the nose and around the car will change with increased speed at slow speed the air may be going in the rad while at higher speeds allot more air may be going around and over the car and less going into the rad.  Is there an undertray on these cars?  If so it may also be blocking the air from exiting the engine compartment at highway speeds while at slow speeds enough can get out to allow enough airflow.

The short of it is fix your fan and report back even if you just hot wire it on full and take it for a test run.   

I would also second that it could be a radiator as well.  I have seen "new" and rebuilt rads that the passages were clogged.  An IR camera would be helpful.  Watch the rad as the car heats up and see if it is even across the whole rad.