Offenhauser Overheating

Stu E · Post by **Stu E** » Tue Jun 14, 2011 10:01 am

Hi All
I wonder if anyone can shed some light on an overheating problem.
It's a 3.5 with an Offenhauser 360 dual port manifold.
Car is fine in town and traffic and at lower speeds, but at motorway speeds it sometimes gets very hot before opening the themostat, I don't lose any water and the fans don't come on. I've read about the mod you do to a edelbrock manifold to aid circulation, has anyone had to do the same to an offy, or can anyone offer another suggestion?
Thanks Stu.

Stu E · Post by **Stu E** » Mon Aug 01, 2011 2:05 pm

Incase anyone else has a similar problem. I bit the bullet and drilled my manifold as per the Eddy modifications, it did stabilise the temp a bit, but eventually traced the overheating problem to a duff gauge.
Nothing like trying to find a fault that doesn't exist.

DaveEFI · Post by **DaveEFI** » Mon Aug 01, 2011 3:11 pm

Stu E wrote:Incase anyone else has a similar problem. I bit the bullet and drilled my manifold as per the Eddy modifications, it did stabilise the temp a bit, but eventually traced the overheating problem to a duff gauge.
Nothing like trying to find a fault that doesn't exist.

Even with an accurate gauge temperatures will vary according to conditions.

My BMW has a gauge that goes to exactly half way and stays there - unless the computer sees temps outside its working range. So really just a version of an idiot light. But they must have a reason for doing things this way.

Stu E · Post by **Stu E** » Mon Aug 01, 2011 5:05 pm

I understand temps will vary with conditions, but my gauge was going down to 70 and the stat would shut then not open until 120, 74 degree stat.
I have owned the car for several years and not had this happen before. It appears that once it reached just over 80, it would almost immediately read over by a minimum of 20 degrees. The manufacturers supplied me with the details of what resistance there is at various temps.
Don't know how a gauge can go so far out of calibration so quickly, but at least the problem has been found.

DaveEFI · Post by **DaveEFI** » Mon Aug 01, 2011 5:31 pm

The temp sensors can and do fail - I've been through several. Less likely the gauge itself. Other possibility is a voltage regulator, or a iffy connection.

Stu E · Post by **Stu E** » Mon Aug 01, 2011 6:07 pm

The makers supplied me with the ohms values for various temps, and tell me I don't need a voltage regulator. All I did was connect up a variable resistor and meter to see what readings the gauge came up with, so with the correct resistance the gauge is way wrong.
I doubt it's a wiring issue as the higher the reading on the gauge the lower the resistance, so if it was a bad connection it would read lower. If it was the sender I would put in a new one, but I can't be bothered to remove the dash to replace a faulty gauge at the minute, maybe in the Winter though.
Just glad to get to the bottom of it.

DEVONMAN · Post by **DEVONMAN** » Tue Aug 02, 2011 9:47 am

Hi Stu.
Given that you have not removed the gauge from the dash (Due to access problems) I assume you are testing it via the wires connected to it behind the dash. If so you have not eliminated a possible fault in the wiring at the back of the gauge. If the sender wire is shorting partially to earth then the gauge will read high.

Worth a check before you buy a new gauge.

Cheers Denis

DaveEFI · Post by **DaveEFI** » Tue Aug 02, 2011 10:02 am

It does seem an odd fault to me - the gauge itself going out of calibration. I'd be looking further before replacing it.

Stu E · Post by **Stu E** » Tue Aug 02, 2011 4:42 pm

It is a bit of an odd fault. The temp would register 120 degrees before the stat opens, then when it opens the fans wouldn't kick in, the fan sender is in the top hose straight after the stat and set to 90 degrees, and I can't see as there's a 45 degree difference between where the temp sender is and the stat. I haven't checked what resistance the sender has at various temps.
I guess I could remove the sender and put it in a pan of water, heat it to see what the resistance is compared to the values supplied by the manufacturer. I have tested the tempersture on the outside of the manifold by the sender and have readings up to 40 degrees lower than the gauge
I don't think it's a short circuit in the wiring as it happens at certain resistances and does so each time I've tried, the gauge reads as it should til it reaches about 80 degrees, then goes out of calibration.
I wont be fitting a new gauge right now, perhaps in the Winter or when I sell the car.
Thanks for the input.
Stu

If anyone can come up with a different idea as to what's wrong I will listen and check it out.