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Mercruiser 3.7L - turns over but will not run, fuel, spark, distributor all check ok.

J

James Kay

Member
Frustrated with this one - twin 470/3.7 engines and both were running well, had been at idle for a while when one motor started to splutter and then died.
Started back up but it was running rough and so went back to base on on one engine.
Have check compression 135 (as has been normal with this engine each check), spark okay on spark tester, timing set okay approx 5 degrees btdc (have checked it is on correct stroke), fuel okay and swapped carby over to one that works on the other engine.
Someone thought it maybe a worn cam or cam gear jumped but not sure how to check if this is the issue - removed rocker cover and valves all seem to open and shut.
Hope someone has come across this before and can assist.
 
Does it do anything when you try to start it?

You might simply have seriously fouled plugs. Some ether might help here. Strange.

Jeff
 
Look at spark plugs as installed,
Look in each pocket and see if there is any dried or moist antifreeze traces where liquid may be pooling at spark plug.

This would indicate a leak at radiator cap.

And Yes I have found this as the cause of rough running.
Not saying its your issue but check anyway.
 
fastjeff said:
Does it do anything when you try to start it?

You might simply have seriously fouled plugs. Some ether might help here. Strange.

Jeff
Click to expand...

Thanks Jeff - when it first started running a bit rough we had been idling for a while, turned off. Checked plugs the next day and they all seemed fairly clean. Since then have tried to get it going and yes the plugs went black, cleaned but still cannot get it to run, goes for max 10 seconds and sounds like firing on 2 cylinders. There does seem to be a lot of fuel in the inlet manifold, have tried using a squirt of ether/starter
 
Thanks - have tried all these but maybe need to re check. For some reason had thought it was a head gasket as it blew head gasket before but last time it still ran so this time i find it hard to pin point what it is as the compression seems okay.
When it first started running a bit rough we had been idling for a while, turned off. Checked plugs the next day and they all seemed fairly clean. Since then have tried to get it going and yes the plugs went black, cleaned but still cannot get it to run, goes for max 10 seconds and sounds like firing on 2 cylinders. There does seem to be a lot of fuel in the inlet manifold, have tried using a squirt of ether/starter
 
Thanks - just the one tank for both motors. Checked plugs the next day and they all seemed fairly clean. Since then have tried to get it going and yes the plugs went black, cleaned but still cannot get it to run, goes for max 10 seconds and sounds like firing on 2 cylinders. There does seem to be a lot of fuel in the inlet manifold so maybe it is just a severe flooding issue?
 
Thanks - have checked this and no sign of moisture, just trying to work out why it would have started running rough after going for approx 4 hours fine and now cannot get it going. Would a head gasket and/or a cam gear slipping a cog possibly cause this?
Any idea most welcome as have spent hours trying to find out the reason - they are a basic motor and understand they have issues but this does not seem to be one of the standard issues ie there seems no issues with coolant in motor from header or pump.
 
Look for horses not unicorns
If it was running then conked out then the cause is something like a clogged fuel feed or a bad ignition component. probably not something internal like cam or timing.
 
o2batsea said:
Look for horses not unicorns
If it was running then conked out then the cause is something like a clogged fuel feed or a bad ignition component. probably not something internal like cam or timing.
Click to expand...
Thanks - maybe i will go back again and re check spark is really running correct and strong first, appreciate your help.
 
You've eliminated the carb as a potential issue
You've eliminated the fuel tank as a potential issue
Next on my list would be checking the fuel pump pressure. After that youou should try adjusting the idle mix screw. Black plugs means running rich, so turn the needle inward until rpm begins to drop due to the lean mixture, then turn the screws outward and adjust for maximum engine rpm and smoothness. If rpm start to drop as your turning outward, then you've gone too far

If none of this solves the problem I would inspect the intake manifold for vacumn leaks. After that start ignition troubleshooting. I don't have any real experience with mercruiser so I'll let others help you with ignition troubleshooting as it pertains to your engine
 
andrewdouv said:
You've eliminated the carb as a potential issue
You've eliminated the fuel tank as a potential issue
Next on my list would be checking the fuel pump pressure. After that youou should try adjusting the idle mix screw. Black plugs means running rich, so turn the needle inward until rpm begins to drop due to the lean mixture, then turn the screws outward and adjust for maximum engine rpm and smoothness. If rpm start to drop as your turning outward, then you've gone too far

If none of this solves the problem I would inspect the intake manifold for vacumn leaks. After that start ignition troubleshooting. I don't have any real experience with mercruiser so I'll let others help you with ignition troubleshooting as it pertains to your engine
Click to expand...

Thanks - appreciate the advice, there is plenty of gas pressure there maybe too much but definitely enough. As for the mixture, yes will turn it right in and see if we can actually get it to run at all...
 
o2batsea said:
Look for horses not unicorns
If it was running then conked out then the cause is something like a clogged fuel feed or a bad ignition component. probably not something internal like cam or timing.
Click to expand...

OUTCOME:
Thanks all and appreciate the feedback - we worked back through fuel, electrics and really appreciate the quote 'look for horses not unicorns'; we went back through swapping condenser, coil, checked distributor rotor etc and then found the base issue was one little wire to the coil was corroded at the lug on the motor (solenoid I think it is) and while we seemed to have spark on re check it was not strong enough and also faulted at times. Put in a jumper wire to coil and had great spark and put the plugs back in and it ran like a bought one!
Also wonder if there was some water in the fuel as when we took it for a run it maxed out at 2000rpm, going for a while and it seemed to clear and up to 3400rpm which is about max we ever take it out to. Think this may have been some water in carburettor that cleared out as it warmed up?

Appreciate the input from all and hope this comment may help others in future.
 
Sometimes it's something that your brain has already ruled out, which is why troubleshooting can be tricky. You'll of course be checking ALL your connections now...
 
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