Small coolant leak on driver side leaking down to slave cylinder (see photos) where is this coming from?

Kingralph88 · Apr 11, 2026

1985 F-250 6.9 IDI

Been driving truck around with nicer weather and a trailer, been getting up to temp for sure & pushing coolant around.

have a small coolant leak coming down on the back side of the engine driver side, thought it was oil or even break fluid from the slave cylinder but it is coolant (green & hard to smell but I want to say a little sweet smell).

In the photos you can see some old deposit of something I’m guessing that is oil coming back from the turbo right up top for a while, see the newer fresh fluid on side of block.

ihc1470 · Apr 11, 2026

Coolant pressure tester would be your friend.

TahoeTom · Apr 11, 2026

Could be head gasket. Are they original? 6.9s have the coolant ports in rear corners. I have experienced this myself. Also 6.9 have open coolant ports on head that are sealed by the valley pan and some sort of sealant. The 7.3 eliminated those by using cup plugs. The head/valley pan ports would more likely leak into crankcase than externally. Still a poor design corrected with 7.3 IDI. I had the machine shop bore and thread those ports with pipe plugs when my heads were getting a valve job.

IDIBRONCO · Apr 11, 2026

I'd vote along with Tom. I'd try to retorque the head bolts and see what that does.

rreegg · Apr 11, 2026

Everything above plus good time to double check all coolant line clamps and overflow reservoir

Kingralph88 · Apr 12, 2026

TahoeTom said:
Could be head gasket. Are they original? 6.9s have the coolant ports in rear corners. I have experienced this myself. Also 6.9 have open coolant ports on head that are sealed by the valley pan and some sort of sealant. The 7.3 eliminated those by using cup plugs. The head/valley pan ports would more likely leak into crankcase than externally. Still a poor design corrected with 7.3 IDI. I had the machine shop bore and thread those ports with pipe plugs when my heads were getting a valve job.

I purchased this truck from the owners son as it was his dads truck & it sat for a little while (10ish years) with small drives just to keep fuel ok, I’m pretty sure it’s the original head gasket, what’s the best way to tell for the head gasket? I dont see any white smoke or bubbles out of the reservoir. Any tell tale signs?

IDIBRONCO · Apr 12, 2026

You can buy a leak detection kit. You put some dye in the radiator and drive it a while. Then you use a UV light to see where the leak is.

quickster · Apr 13, 2026

I had a similar problem with my 87. I pulled the valve covers and found a snapped head bolt . that one thing is all it takes.

Small coolant leak on driver side leaking down to slave cylinder (see photos) where is this coming from?

Kingralph88

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