dprantl

Ok, in ~90 deg F weather with the A/C blasting my GT likes to run just below the 2nd white line. If I'm cruising on the highway at ~80mph it will eventually cross the 2nd white line and almost hit the red. Today right after a drive I used an infrared temp sensor on the radiator hoses (temp gauge was on the 2nd white line) and found that the rad inlet was ~205 deg F and the outlet to the engine was ~195 deg F. A/C was on and both cooling fans were running on high. Is it normal for the rad to only drop the temp by 10 deg F? The car will reach these temps on the gauge even with the A/C off during highway cruising. Here's the status of the cooling system components:

- New 75 deg F thermostat installed along with the o-ring and rear seal
- New 13psi radiator cap
- Flushed and replaced coolant with Zerex G-05 and a bottle of water wetter
- Coolant level does not drop - no leaks
- Radiator and water pump were replaced ~3 years ago
- Cleaned out all the crap between the condenser and the radiator
- Heat works fine at idle and higher RPMs

A few weeks ago the car was over the 2nd white line on the highway, and I needed to get off to quickly get something from a store. Upon restart the oil pressure warning came on. My former '86 ran much cooler than this; with a 75 deg F thermostat it never got hotter than ~185 deg F even on the hardest fastest highway drives. The only thing I have not replaced yet is the oil thermostat above the oil pressure sender, but I have my doubts that this will help much. Since this only happens at high speed, the cooling fans are out of the equation. In fact, if I get off the highway and get stuck in traffic, the temp gauge drops back to just below the 2nd white line.

I've read many previous posts that this kind of running is normal, however I refuse to believe it. Or could it be that the S4 body style grille area is insufficient for the size of radiator in the 928 when compared to the earlier style? Or maybe it's because I have double water intercoolers in front of the condenser and radiator that could be restricting airflow enough at high speeds to cause this?

Dan
'91 928GT S/C

Marine Blue

My S4 ran very cool (almost too cool) but it had an aftermarket radiator so it's hard to say if it was normal.

My GT always runs below the second white line regardless of outside temps and how hard I'm driving it.

Wish I could offer more advice on the cause.

What percentage of coolant/water do you run?

ErnestSw

You've got an air flow problem. Could be your cooling fans, blocked air flow through the AC condenser, or dirty radiator fins. Look down behind the AC condenser to see how much crud has collected there.

IcemanG17

Okay a 91 does not have the stupid "air flaps" in the nose...so thats good

However it does appear you have a problem.....for normal driving that is too hot....my S4 auto never has gone over the 2nd white line...& thats only when driven extremely hard or on a very hot day with the AC on...even on the track after a 20 minute session it barely touches the 2nd white line....& when I measured the temps at the 2nd white line it also came back at 195F......

The problem you mentioned about the low oil pressure on a hot restart is a classic worn oil thermostat spring.....I had this problem too & I heard it can lead to a hot running engine too....so changing that might help...

Checking behind the AC condensor for junk is a good idea too.....

dr bob

Tampa928 reports similar problems after adding the SC to his car. His opinion is that the blower itself blocks a lot of airflow through the radiator.

dprantl

My car has an Eaton supercharger which takes the place of the intake so there is nothing between the radiator and engine to obstruct air flow. The only extra obstruction up front are the radiators of the charge-cooling system. Andy's stock kit had only one, but my car has two, which pretty much covers the entire area from left to right in the nose of the car.

Since I vacuumed out all the crap between the condenser and radiator and also the radiator fins themselves, the only other thing I could try other than removing one of the charge cooler radiators is to change that oil thermostat spring. If that's it, it's just my luck that the last thing I do fixes the problem

Is there a concensus on what kind of temperature drop I should expect from the radiator? It should be more than 10 deg F, right?

Thanks for the thoughts.

Dan
'91 928GT S/C

UKKid35

A 75deg thermostat makes no difference to the total cooling capacity of the system, it merely lowers the low load running temp, and therefore may increase fuel consumption and engine wear.

Schocki

My GTS runs on the 2nd white line all the time. Took it to Frankfurt and back lasts weekend (500 miles) and drove most of the time with the cruise set at 125 mph. The temperature went barely over the 2nd white line (outside temp 90 F).
I agree with Ernest, you hae an arflow problem. Did you remove the fans to clean the backside of the radiator too?

Charley B

My vote is the two water intercoolers are just not letting enough air flow through.

Bill Ball

10F is too small of a temp drop. It should be 25-30F. I think you have a combination of inherent marginal capacity, and relatively inadequate airflow when combined with high load.

The 928 radiator does not appear to have changed much if at all in capacity from 78-95 despite the big run up in HP. Nor did the water pump flow change. So, in the later years I think the cooling system became marginal. In 90, the oil cooler was moved out of the radiator. It's expensive to do, but I'd like to convert my 89 to the external oil cooler. Of course, you already have that feature.

With the AC on and an SC intercooler heat exchanger dumping in front of the radiator, you have very high load. On my 89, on hot days going up a long grade with the AC on (I have no intercooler), I will exceed the second white line. If it gets to 1-1/2 to 2 needle widths over that line, I kill the AC for a while. It makes a difference.

The cross-section of the air inlet to the radiator is pretty small. There is ducting and edge seals around it that, if intact, direct the flow through the radiator. Still, the over all shape of the air inlet feed is a reverse cone which really isn't the best. Some of the aftermarket fan systems do not have the shroud above the radiator that seals against the underside of the hood. That's not good.

I see you think airflow is not an issue since the car runs cooler off the highway in slow traffic, but under high load conditions, airflow may be inadequate despite the faster car speed. Some have suggested that at very high speed the fans are not contributing. Due to the shape of the air inlet, I'm not convinced the air ram effect is very good. Try driving on the highway with the fans disconnected - you won't get far.

205F inlet is high but not dangerous. 195 outlet is not great. If you want to improve this, I would relocate the SC heat exchanger to a wheel well if you can make it fit (where it will need a fan), and make sure all the radiator airflow ducting and seals are intact. Then I would consider adding a high-flow pusher fan in front of the radiator. Your SC is generating additional heat load (even with the heat exchanger relocated) and many SC'd cars have benefited from an additional fan.

When you stop a hot car, all water and airflow stops and the engine heat can superheat the coolant and oil inside the block. That may account for the oil pressure warning you saw.

Marine Blue

Still curious what percentage of coolant/water you're using, it will make a difference since water is technically better at transferring heat than the coolant. Water wetter also allows for a lower concentration of coolant and being in GA I'm not sure you see the super cool outside temps which dictate a higher coolant percentage.

dprantl

Right now I'm running 40/60 coolant/distilled water. I suppose I could go less coolant, but I doubt that would help much.

Bill, I tried driving on the highway without ever hitting boost going a constant 75mph hardly pressing the accelerator pedal at all with the A/C off all the way (cooking my *** off). Even this driving, after 10 minutes or so if it's like 90 degrees outside the temp gauge will start to go over the 2nd white line.

One thing I was thinking is the old viscous fan could blow up a huge sandstorm on my old '86; that thing had some serious CFM above 2k RPM. The electric fans even on high do not come anywhere near that kind of airflow. Maybe since I have two watercoolers up front I really do need some extra fan CFM. Haha, I could be the first in history to retrofit a viscous fan to a GT Too bad the S/C belt assembly is in the way.

Dan
'91 928GT S/C

Bill Ball

Ew, that's worse than I've seen for a generally healthy system. Hmmm... I still would do the same things, although I woulod make sure the stock fans are working well first. The low inlet versus outlet temperature differential argues airflow is a problem. Oh, it could be the radiator is coated internally with scale and has poor heat transfer while still flowing, but your radiator is new. You done so much, that airflow seems like the likely variable.

dprantl

Well, today the problem surfaced. The water pump decided to puke coolant and I can see into the timing belt hole the belt is starting to track towards the engine ~3mm. I can also see belt shaving dust in there. This on a 45k mile 3 year old water pump It's pretty pitiful when a water pump lasts this long, and such an important one that would take the engine out with it if I didn't catch it in time.

Where's that mad at your 928 thread? Actually, I should start a mad at your 928's water pump thread Well, the bright side is I caught it in time.

Dan
'91 928GT S/C 475hp/460lb.ft

jorj7

Dan,

After you replace the water pump and timing belt and before you start driving it again, check
the "partition" between the radiator intake and oil cooler intake. It's a piece of plastic sheeting
that clips in place behind the bumper air intake slots. There are a few clips that keep it in place,
but the sometimes will come loose or not seat right. As a results when the car is moving slowly
the air intake to both radiator and oil cooler are ok, but as you speed up the plastic partition will
rise up and block the air flow to the radiator. I had this happen on my car at an ORR race and
the car started running very hot. Resecured the partition and the temps went back to normal.

Just a thought....