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5.7 mercruiser cuts out when warm

W

wmclare

New member
I have a Silverton 310 Express with 2 x 5.7 mercruisers (1996) We had both engines out and did a full rebuild ( pistons, rings, bearings, new Edilbrock carbs etc etc.)on them as they had been submerged. Both engines run very well at start up, but when warm (10 mins) one engine looses power and eventually cuts out. Fuel has no water in it but there is no spark.
I took it to a Mercruiser dealer who checked it over and said the ECU was the problem. I bought 2 of these and put 1 on each engine. - problem still exists so I replaced the coils, and 1 ignition switch which was a bit iffy. Problem still exists!! I am running out of electric parts to replace!!!
Any tips on where to go next? I have thought of coil leads and so while fishing and floating for 2 hours I swapped leads from 1 to the other and then the faulty engine started. But it then cut out after about 15 minutes and the other was still ok.
The tacho's are both faulty and thought that that could be where my problem could lie but it appears to be more heat related as the engine cuts out when it warms up.
HELP!!!
 
The next thing I would do is replace the tach's; since eventually you will want to do this anyhow.
A shorted tach can kill ignition.

Rod
 
I spoke to the Australian Distributor for Mercruiser yesterday, and they recommended changing the module within the distributor, replace upper and lower loom as well as tacho's. So I then rang the dealer they recommended ( who has had my 3.3hp tender outboard for 4 1/2 months - for an annual service) and ordered the 2 modules, 2 tacho's and an engine loom and he will meet me at the boat ramp on Friday when I need to take the boat out again and get the fault to manifest itself again. He has a remote start loom which he can connect in - replacing the upper loom and that should narrow down the fault. Once I have this guy in my boat he will have to fix it to get back on land! The frustrating thing is that if I run the boat here at work for 90 minutes and it doesn't fault. It needs to be under load to fault. I either need a dynamometer or a test tank to do it here. We are 130km from the sea.
 
An update. - I replaced the module / sensor inside the distributor and took it for a spin this morning. No Difference - it has power for a bit then you can feel a surging and then nothing. There was a bit of a knocking sound when reved up a bit to about 1500 rpm but no knock at idle. When I backed off the revs again to idle it stalled and then there was no spark again. I have trailered it home and put the muffs on and it started immediately but the knock is still there when you rev it up a bit.
Any suggestions? Could it be the knock sensor and is that why I am low on power? Is it also why the problem manifests itself when warmed up and not when cold? Is there a way to test this sensor or should I just buy 2 of those as well? I think I have pretty well replaced all the other electronic parts on the motor!
 
How warm is warm?

Is the motor overheating?
Is the knocking sound related to the tempurature?
Is the knocking sound there when you first start the motor and it is cold?

How are you checking for spark? at the plug or the coil?

I would suggest the assistance of a merc technician with a merc scan tool on the boat to see why your no spark condition exists and it will give you the temp also when this happens.
 
Hi and thanks for the reply. Firstly new gauges / senders have been fitted and the temp is only getting into the normal zone. I don't think the knocking is related to temperature because after I trailered it and brought it home (2.5 Hours) I put on the muffs and it started straight away. When I reved it though, I could hear the knocking sound.
I am checking for spark at the plug firstly, and then I checked at the coil. Both coil and lead set have beed renewed and it still happens.

Your suggestion that I get a mercruiser tech to check the spark is fine but that is where I started. I got a bill for his time of 2.25 hours of his time and he said the ECM was the problem. It wasn't and nor was the coil, leads, ignition switch, tacho or distributor position sensor that he suggested. Unfortunately there are only 2 Mercruiser service techs within a 150 mile radius and 1 of them says he hasn't dealt with anything this big and really doesn't have the time. That is why we have done it ourselves. I have our own truck mechanic who is doing the work, but marine is a bit different than a car / truck. Before he came along, if I had a problem with a truck I would go to the Mack service sentre and they would start replacing parts until the problem disappeared. I could do that and save some money and so I employed my own mechanic 3 days per week.
Really the only 2 things left electrically are the knock sensor and the loom. We might have a go at them tomorrow! But how can I check to see if the knock sensor is working?
 
The knock sensor retards the timing when a knock is detected. It does not stop the spark.

You have EFI correct? You say ECU. If not EFI then you are referring to the ignition control miodule??
With a merc scan tool if it is efi than a GOOD technician will have it plugged in when this condition occurrs and can then diagnose why you lost spark.

The cost of replacing everything is very high vs having someone who can troubleshoot properly in the boat.

You are asking questions from very very far away and maybe some more info is needed..........

Is it EFI or carburated?

Is the coil new or not?

You have a shift interuptor switch at the shift linkage, it stalles the motor when shifting OUT OF gear only but if the switch defective it can short the ignition to ground. Or if your cables are out of adjustment the same thing can happen. one of the switch wires goes to ground, if the engine has no spark one of the first things to check is the interuptor switch, remove the BLACK wire from ground and recheck for spark. if no change then the switch can assumed to be good.

Loss of spark in my opinion is related to several basic things,

1. bad connection(s)
2. bad interuptor switch
3. bad ignition coil
4. bad lanyard switch
5. bad igition sensor in distributor
6. bad ignition control module or ecm if efi.

As far as efi systems go I am not up to date so I can not help with that system.
 
Motor has a carb, (also brand new Edelbrock 1409) and electronic ignition. The 2 control levers are brand new and were replaced after this motor first manifested the problem. I have just restarted it this morning and the timing is way out. It runs very rough and cannot get it to rev very high at all. We have had a look at the rusty looking knock sensor but don't know how to check it. I will probably just get a new 1 to be on the safe side.
 
Just a thought. Do these engines have Thunderbolt ignition? Did you change trigger module inside the distribtor directly under the rotor? Some of these will corrode on the underside and cause the problems you are having.
 
Are you talking of the sensor with 3 wires that fits under the roror? If so I have replaced that and the problem still exists. I think we have narrowed it down to the knock sensor. By swapping from 1 motor to the other we are getting it to run a hell of a lot better.
Will advise
 
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