fwiw... hre is an opinion from CHAT GPT
Q: what causes a 1995 Johnson 150 shuts down after 5 minutes of full throttle
ChatGPT said :
Good question — a
1995 Johnson 150 (2-stroke, V6, carbureted or EFI depending on model) shutting down after about 5 minutes at full throttle is a
classic symptom of a few possible issues related to
fuel delivery, heat, or ignition protection systems.
Let’s go through the
most common causes, ranked by likelihood
1. Fuel Starvation (most common)
At wide-open throttle (WOT), fuel demand is at maximum. If the engine runs fine for a few minutes then dies or bogs and restarts after a short wait, this usually points to
restricted or insufficient fuel flow.
Check these:
Clogged fuel filter / water separator — replace or inspect for debris or water.
Anti-siphon valve at the fuel tank outlet — can restrict flow under high demand.
Fuel line collapsing internally (especially if ethanol has softened old hoses).
Vapor lock — caused by heat soaking in lines or VST (if EFI version).
Fuel pump diaphragm — weak or leaking, can’t maintain volume at WOT.
Fuel tank vent — closed or obstructed; test by running with fuel cap slightly loose.
Symptom clue:
If the engine
restarts after idling for a bit, fuel starvation is almost certain.
2. Overheat or S.L.O.W. Mode
Johnson/Evinrude motors have a
S.L.O.W. (Speed Limiting Overheat Warning) system that limits RPM or can trigger shutdown if:
- The engine temperature sensor trips,
- Or oil/fuel sensors detect a problem.
Check:
- Proper tell-tale water flow (strong and steady stream).
- Thermostats and pressure relief valve condition.
- Overheat horn sounding before shutdown?
- Use an infrared thermometer at the heads — shouldn’t exceed ~160–170°F.
Symptom clue:
If you can restart right away but RPMs are limited (~2500), it’s in S.L.O.W. mode, not a total stall.
3. Ignition Breakdown (heat-related)
After several minutes at WOT, ignition components can fail when hot.
Suspect:
- Power pack (CDI) failing when hot.
- Coils or stator with heat-related internal shorts.
- Timer base wiring with cracked insulation.
Test idea:
Run it until shutdown, then check for
spark immediately.
If no spark → ignition issue.
If spark present → look back to fuel.
4. Fuel Tank Pickup or Debris
Sometimes debris in the tank floats up and
blocks the pickup screen under high suction, then drops away when suction stops — giving you a “runs fine again after a minute” pattern.
Diagnostic Steps
- Run with a portable tank (clean 50:1 mix) directly to the motor → isolates boat fuel system.
- Monitor spark immediately after shutdown.
- Check fuel bulb — is it collapsed or empty? (means restriction or vent issue)
- Check for overheat horn or RPM limiting.
- Inspect plugs — if they look lean (white) = fuel problem; oily or wet = ignition.
If you tell me:
- Whether it’s carbureted or EFI,
- Whether it restarts right away,
- And if there’s a warning horn or light,
…I can narrow it down to 1–2 exact components to test first.
Would you like to give me those details so I can pinpoint the cause more exactly?
REPLY
Carbed engine .... after a period of 10-30 min it will run ok all day .Problem is hard to replicate unless run wot for 5 minutes
ChatGPT said :
Perfect — that detail helps
a lot.
So:
- 1995 Johnson 150 carbureted,
- Shuts down (dies completely) after 5 min at full throttle,
- Then after cooling or resting 10–30 minutes, runs fine the rest of the day,
- And it’s hard to reproduce unless you hold WOT for a sustained period.
That pattern points to something that
fails with heat or fuel draw, but then
recovers once cooled or refilled.
Let’s break it down now that we know it’s carbureted
MOST LIKELY CAUSES (ranked)
Fuel Starvation — Main Culprit
This is still
#1 suspect on a carbed Johnson V6.
At full throttle, the engine drains the carb bowls quickly; if the fuel pump or delivery system can’t keep up, it runs lean and dies.
After it sits, the bowls refill — and it’s fine again.
Check these first:
- Primer bulb condition:
- When it dies, immediately squeeze it — if it’s collapsed → tank vent or line restriction.
- If it’s empty/soft → pump not keeping up or line restriction upstream.
- Fuel tank vent:
- Try running with the fuel cap slightly loose next time.
- If problem disappears, the vent is blocked.
- Pulse fuel pump:
- If diaphragm is weak or leaking air, it’ll starve under sustained load.
- Rebuild kits are cheap (<$30) and often solve this issue.
- Anti-siphon valve(at tank outlet):
- Common restriction — replace or temporarily bypass for test.
- Fuel pickup tube or debris in tank:
- A floating flake or plastic film can intermittently block the pickup during heavy suction, then fall away.
Hint: Since it runs fine for the rest of the day afterward, that’s consistent with
air in fuel system working itself out or
vapor lock dissipating.
Vapor Lock / Heat Soak in Fuel Lines
After long WOT, the fuel pump and lines near the powerhead can get hot.
If there’s
ethanol fuel, vapor bubbles can form → loss of prime → stall.
After sitting 10–30 min, fuel recondenses, and engine restarts fine.
Fix / Test ideas:
- Route fuel lines away from exhaust cover.
- Use ethanol-free gas if available.
- Ensure fuel pump check valves are sealing tightly.
Power Pack or Stator Heat Failure
A bit less likely because the engine runs fine all day afterward, but worth mentioning.
A stator or power pack that fails when hot can cause a complete shutdown (no spark), then recover after cooling.
Test:
- When it stalls, check for spark immediately on one plug.
- No spark = ignition side issue.
- Spark present = fuel side.
If it’s ignition heat failure, it will usually recur more quickly each time, not disappear for the rest of the day.
Fuel Bowl Venting
Each carburetor has a bowl vent — if one is clogged, you get
vacuum lock in the bowl, limiting fuel refill at high demand. After shutdown, the bowl slowly equalizes, and all’s fine again.
Clean all carb vent passages with compressed air.
PRACTICAL TEST PLAN
Here’s how I’d troubleshoot it systematically:
- Next time it stalls,
- Pump primer bulb:
- If motor restarts instantly → fuel starvation confirmed.
- Check spark right away if no restart.
- Run on a portable fuel tankwith short new hose (bypasses boat’s tank & lines).
- If problem disappears → restriction or vent issue in boat fuel system.
- Rebuild or replace fuel pump (it’s cheap and a frequent culprit).
- Inspect carb bowls and vents for debris or varnish.
- Use fresh, non-ethanol fuel and check fuel filter for air bubbles.
If I had to bet:
It’s either a weak mechanical fuel pump or a restricted fuel vent/anti-siphon valve causing starvation at WOT.
