JayPoorJay

1986 Manual 928

Hello all. I'm still (slowly) working out the kinks and something unusual happened and I need some help/feedback.

It had been really hot here. Heat wave. Was the day this all started. Since, traveling some, working on the XJS and XK8, and have let her sit with the idea of coming back to this.

That day I took her for a drive and when I came back and parked --- on a very slight nose down incline, tipped a little towards the passenger (RH) side --- I noticed drips of Dex coming from what appeared to be the lower front RH corner of the Rad. The best I could, with shroud in place, I peeked at the back side of the Rad to see what I THOUGHT was a small, super small even when hot, wet spot 1/4 of the way up on the passenger side bottom corner of the Rad. I have a great radiator guy, I wasn't toooooo worried, and had a plan to bring it over when I got to it.

Fast forward to yesterday. I removed the shroud, it was about 85degrees here. In the 2 weeks or so since this began it seems that the reservoir had completely (slowly) drained and I don't know how much more than that. I topped the reservoir off and started her. Let her run for a good while to well hot. So hot the second e-fan kicked in,,, NO LEAK. I got in and drove a bit, cam back, parked, no leak. Let her sit overnight with a white sheet of paper under the front end, NO LEAK. No leak there or anywhere I can see. Ok, what's going on here?

I know that I had seen this leak, the entire reservoir drained. How can that be? One thing I did do BEFORE filling and heating her up was detach the small line that goes from the top of the Rad, thru a small switch, to the reservoir, and pumped just under 20lbs of air into the system to try to chase the leak. No leak found with this approach either so I reattached the line at that switched spot in the hose just forward of the reservoir and tightened things back up then started.

Are or is there a problem point(s) anyone can suggest? Any usual suspects? I looked all around the area I thought the leak was, and others, and didn't find anything.

Any help, guidance and suggestion is greatly appreciated.

Jason89s4

That is odd. My only thought is that the leak you had during the heat wave was actually the overflow tube on your expansion tank, due to the heat. I know you said the leak was further to the front, but water can take funny paths on the way to the ground.
Then, when you pressurized the system you may have pushed fluid into areas that had bubbles and the fluid properly displaced it thereafter?

In any event, don't believe that just because you have run it in the driveway until it was hot that your problem is gone. You are going to have to give it a proper hot test drive(s) under load to confirm.
-Jason

Mrmerlin

Most likely the pass side rad tank has failed,
they crack at the top edge where the flange is due to an air pocket that inside the tank ,
over time the hydrocarbons and heat cycles make the tank brittle .

Buy a new side tank and seal have a competent rad shop do the swap
NOTE also get a new top radiator outlet port a new drain plug
inspect all the hoses at the water bridge where they clamp inside for damage
replace the T stat and O ring and rear seal
get a new coolant cap and heater control valve and short hose
NOTE its wise to replace all of the hoses on the front of the engine
NOTE also inspect the pressure valve in the return line,
as this part can also crack and leak and be very hard to find

JayPoorJay

Hi! Thank you.

Forgive my ignorance.
When you say passenger side rad tank do you mean the white reservoir up in the passenger side corner near the FW,,, or is it a section of the main tank? The plug came to mind for me after I wrote the post. I'm really really hoping that it is not a thing that is going to cost me a ton.

Edit - what is this pressure valve? Is it the little wired switch-ish looking thing on the right side (up by the hood line 10inches down from the reservoir)...? I think it might have been the place/connect I disconnected to pump some air into the system.

FredR

The "pressure valve" Stan refers is a pressure switch- the thing delaminates with age. Some like myself have deleted such for the limited value it has.

The radiator tank is the plastic fitment on both sides of the stock radiator

JayPoorJay

Got ya. Do you think that valve is the one that I described? Little mushroom shaped thing, two wires coming off one end, black, interrupts that hose that goes from the LH top of the Rad over and across, up the passengers side fender to the reservoir tank? If so, I'll do away with it.

Understood on the tanks. I'll go take a good long slow look. I can't just go get on the road knowing it's like this.

I've changed some of the hoses, the two main right off a while back. Everything under the hood was dusty due to the car sitting a while,,, but since I've been messing around a while it seems some of the many bits and pieces had been kept up pretty well in the past. This leak problem sucks tho!

Schocki

There are a couple of weak spots in the cooling system that like to fail.
1. Radiator on the passenger side where the side tank connects to the radiator itself. Check the area around the upper big coolant hose. This is where it leaks normally. The hot coolant from the engine enters the radiator here and therefore this part sees the biggest temperature changes.
2. Coolant pressure switch: likes to deteriorate and leak
3. Cap on the expansion tank has a weak spring and fails to pressurize the system properly. As soon as the engine is hot, coolant is pushed outside via the overflow hose on top of the tank where the cap is located.
4. Coolant bridge where it attaches to cylinder heads and the big o-ring at the bottom of the bridge.

The leak location that you describe, leads to a faulty radiator. Call Roger and get a new CSF, don't even think about repairing the old Behr unit. The Behr fails, because the gasket between the radiator and side tank fail and the plastic tanks deform over the years.

FredR

'That is the item but I doubt that is your specific problem.

If you still the have the original stock radiator, although a fine item in its own right, is well past its sell by date for the reasons mentioned. I migrated to an alloy radiator years ago and even those have been known to have their problems. The CSF item sold by Roger or Mark appear to be the best value replacements. Carl also does one and I have one of those I inherited- a bit more expensive I believe but nonetheless appear to be a well made item.

I have no idea how much a replacement end tank is but if one tank has gone one might expect the other to be not far behind and if the cost of repair is factored in one is probably well on the way to the cost of a replacement by the CSF unit.

The 928 is a fine automobile but cheap to run it is not. That being said there are ways to keep the costs minimal and that is where this list kicks in.

JayPoorJay

It is definitely a Behr unit. Original? I have no idea...

Sad but happy to say (strange combo with these amazing machines) looks like spend on a new rad may be in my future... By 1000 cuts, as they say

928 DesMoines

I had a similar problem where the seal between the radiator and left tank failed. Roger had the seals and plastic “tank ends” in stock. I took my original radiator and parts to a local shop and they made the repair. It was not so expensive and let me hold onto the original radiator.

This was a few years ago, so not sure if new old stock still exists.

Short answer: call Roger

The Forgotten On

If anyone hasn't already, replace your rad mounts on both the top and bottom. They harden over time and cause the side tanks to crack.

They are very inexpensive to get ahold of anyways.