Blocked Cooling System — Diagnosis & Repair

Guide

Blocked cooling system — diagnosis & repair

Difficulty: Advanced Time: varies Applies to: Gas boards Last updated: Sep 2026

Watch the walkthrough

Watch on YouTube ↗ · Jetboard Australia — "Avoiding Disaster: Why Cleaning Your Jetsurf is a Must After Salt Water Rides" (~9 min). Timestamps below follow the video's own time.

A real repair job: a board that came in with cooling problems. This walks the diagnosis — telltale to exhaust branch to telltale branch to cylinder head — and ends with a cylinder that salt corrosion had eaten past the O-ring groove. It's the case for flushing properly made in metal.

Symptoms — 0:32

0:43 — The telltale wasn't working; cooling water was only dribbling through the exhaust, not enough to keep things cool.
0:55 — Running on the hose, the cylinder was getting very hot.

How the cooling circuit works — 1:08

1:13 — The blue line is the main cooling feed into the board (same push-clip as the fuel tank — pull the collar back, pull it off).
1:23 — Water enters the tube to the cylinder; a portion splits off to the exhaust; the rest runs through the cylinder → cylinder head → out the telltale.

Test each branch — 1:39

1:44 — Prop the cooling line up, turn the hose on. Plenty of water through the main tube.
1:56 — Pull the exhaust tube off and try to block it by hand — flow is reasonable, so the exhaust branch is OK.
2:16 — Run a 2 mm drill down the exhaust tube — it's packed with muck. Clear it to the right depth, refit the tube to the cylinder and the exhaust.
3:02 — Pull the telltale tube — badly corroded. Pull the elbow off. Run the drill through: it's full of salt, right down into the cylinder head. If it's this bad, the head has to come off.

Pull the head — 4:07

4:09 — The head cooling fitting was blocked solid and seized — it'll need heat to come out. Pull the engine and the cylinder head. (See engine removal and Piston removal & installation.)
4:32 — Warning sign spotted on the way in: the side of the cylinder was lifting — pushed out by corrosion building behind it.
4:57 — Head off: the O-ring had popped through, with corrosion built up around it. The cylinder bore itself wasn't badly corroded and had little carbon — a low-hours board.

Why the cylinder is scrap — 5:42

5:49 — The cylinder had broken away at the top — the O-ring groove was gone, so the O-ring squashes out the side instead of sealing.
6:06 — Same at the bottom — corrosion had already started pushing the O-ring out there too. It will not seal. The cylinder is done.
6:29 — Cause: the board sat with salt water in it. Visible salt build-up throughout the water jacket.

The fix — 6:59

7:02 — Remove the cylinder — 10 mm open-ender plus a thin-wall ground-down 10 mm ring spanner to reach the nuts. Wriggle it off.
7:53 — Inspect: crankshaft lovely; piston not badly scored; pull the ring and check the end gap in the bore. The rest of the engine is fine.
8:38 — New cylinder, new piston rings, cooling system reassembled. Good run — fixed.

Tip: diagnose the cooling circuit one branch at a time — main feed, exhaust branch, telltale branch — with the flusher on. A 2 mm drill clears a blocked tube; if the telltale branch is salted right into the head, the head has to come off. A cylinder side that's lifting is corrosion pressure behind it — that cylinder is already lost.

Caution: this is what a board that sat with salt water in it looks like inside — a scrapped cylinder on a low-hours board. Flush after every salt-water ride (see Flush & clean your JetSurf like a pro), and use a salt-dissolving product (Salt-Away, Salty Captain) that films the water jackets and breaks down the crust. Skipping the flush costs a cylinder.

Blocked cooling system — diagnosis & repair