if coolant leak is not from the expansion tank, then where else could it be from?

[AustinPorscheBill]

I have a 1998 Blue/Grey Boxster with 82,000 miles.

The other day I got a low coolant warning, I topped it up (always keep coolant in the trunk), when I pulled back into the garage later, there was a puddle of coolant on the passenger side and at or just in front of the rear wheel well.

I jacked it up to inspect from underneath and there was coolant along the rear fender-to-body pinch seam on that side, as well as a little on the rear suspension arm(s) and to the front near the underside valence cover below the thermostat-water pump.

The water pump and thermostat were replaced 18ish months ago, one of the radiators a couple of years before that, so I know my way around the cooling system reasonably well -- except for the expansion tank.

I checked the thermostat, water pump and the hose plumbing that goes toward the front and the radiators and the heater hoses.  No sign of any leakage

I checked the top of the expansion tank for signs of overflow:  none.  (As the overflow drain appears to go thru the tank, is a possible failure mode for this integrated-molded overflow channel to crack, allowing tank fluid directly into the overflow channel wihtout going thru the cap and onto the top of the expansion tank body?)

I checked the trunk floor around the expansion tank:  dry as a bone.  (But I didn't pul out the carpet.)

I checked the oil (in case a head gasket): golden clear (changed about 1,800 miles ago).

I then popped the engine cover and poked around toward the tank manifold with a borescope:  it appeared dry, though from the pics (I add as replies as there is a 2Mb limit), there appears to be moist(?) dirt all over the manifold -- but no sign of any actual liquid anywhere.

Before I dive into an expansion tank project, a couple of questions:

1. Can the tank manifold leak from the engine compartment side -- all the threads and videos I've seen only show leakage on the trunk side of the tank;  if so, how can I confirm it with a borescope or other method?

2. Are there other points in the engine compartment where a leak might be possible (not sure where the three lines on the expansion tank that are under pressure go) but it must be to the engine block?  If so, where else should I look?

Any help/advice would be much appreciated before I undertake a tank renewal that might not have been necessary.

Here's a pic of the manifold body between the rightmost pipe-hose and the other two hoses also under pressure.

 

Edited April 30 by AustinPorscheBill
change orietnation of the pic; add more information/details

[AustinPorscheBill]

Another pic of the engine side of the tank manifold, below the rightmost hose at the manifold (I think). . .

[AustinPorscheBill]

and another underneath all three hoses at the manifold (rotated to get the proper orientation -- I think)

[JFP in PA]

The easiest and probably cheapest way as well is to add a small amount of coolant compatible UV dye to the system and run the engine for a bit.  The dye will leave a highly visible (with a UV light source) trail to exactly where it is coming from.  We use this technique regularly in the shop to trace difficult leak sources.

[Bill Logan]

On 4/30/2026 at 3:51 PM, AustinPorscheBill said:

I have a 1998 Blue/Grey Boxster with 82,000 miles.

The other day I got a low coolant warning, I topped it up (always keep coolant in the trunk), when I pulled back into the garage later, there was a puddle of coolant on the passenger side and at or just in front of the rear wheel well.

I jacked it up to inspect from underneath and there was coolant along the rear fender-to-body pinch seam on that side, as well as a little on the rear suspension arm(s) and to the front near the underside valence cover below the thermostat-water pump.

The water pump and thermostat were replaced 18ish months ago, one of the radiators a couple of years before that, so I know my way around the cooling system reasonably well -- except for the expansion tank.

I checked the thermostat, water pump and the hose plumbing that goes toward the front and the radiators and the heater hoses.  No sign of any leakage

I checked the top of the expansion tank for signs of overflow:  none.  (As the overflow drain appears to go thru the tank, is a possible failure mode for this integrated-molded overflow channel to crack, allowing tank fluid directly into the overflow channel wihtout going thru the cap and onto the top of the expansion tank body?)

I checked the trunk floor around the expansion tank:  dry as a bone.  (But I didn't pul out the carpet.)

I checked the oil (in case a head gasket): golden clear (changed about 1,800 miles ago).

I then popped the engine cover and poked around toward the tank manifold with a borescope:  it appeared dry, though from the pics (I add as replies as there is a 2Mb limit), there appears to be moist(?) dirt all over the manifold -- but no sign of any actual liquid anywhere.

Before I dive into an expansion tank project, a couple of questions:

1. Can the tank manifold leak from the engine compartment side -- all the threads and videos I've seen only show leakage on the trunk side of the tank;  if so, how can I confirm it with a borescope or other method?

2. Are there other points in the engine compartment where a leak might be possible (not sure where the three lines on the expansion tank that are under pressure go) but it must be to the engine block?  If so, where else should I look?

Any help/advice would be much appreciated before I undertake a tank renewal that might not have been necessary.

Here's a pic of the manifold body between the rightmost pipe-hose and the other two hoses also under pressure.

 

j

 

[Bill Logan]

I have had the same leak, at the same place.  I just purchased a 2005 Boxster automatic 54k miles about a month ago. When I drove home and parked it overnight there was a small puddle passenger rear side. I looked up under the vehicle and the overflow hose was wet. I went to the trunk and there was a black cap on the expansion tank. I replaced it right away with the Blue .04 Cap. I wiped the residual coolant off the strut below the overflow hose and drove it around. I came back no leak.  I thought I solved the problem, not....I took it to the dealer in town and had some overdue service's performed. They were supposed to do a pressure test on the cooling system, drain and refill. I found out later after multiply leaks that he considered the vacuum a positive test.  That cost me and aggravated the hell out of me. I took it to another dealer in town. He preformed a pressure test and found it was the expansion tank leaking. I went back to the other dealer and talked to the owner. He said it was his mistake and will pay for the replacement of the expansion tank and the labor he charged me to empty and refill the system.  This comimg Tuesday after replacing the expansion tank they will preform an other pressure test to see if the rest of the closed loop is ok.