Mercruiser 260hp misfiring!

[ulyden]

I was out today to a guy with problems with his 87 Mercruiser 5.7litre 260hp engine!

The engine had carburetors and thunderbolt ignition system.

I was there for adjusting the timing! It was 20deg at 3000rpm an I increased it to 30deg.

We went out for a test run. The engine was performing much better according to owner!

But at 3800rpm according to tachometer it shut down!Just like a overspeed protection on a petrol car maybye slower frequensy! Everytime it passes 3800rpm the ignition cuts for a sek or less!

The tachometer did not look reliable

At stop it sometimes showed 2000rpm and at idling it was always showing sorting else than my stroboscope! Always higher!

When the engine was running it looked ok!

I was checking the gear protection switch and the engine reacted exactly in the same way !

When we tried to accelerate with full throttle it was accelerating up to the same engine speed! The engine was running perfect as long as the speed was below 3800!

So ?

I have to cheeck that the speed is stopping at 3800rpm when the engine is in neutral

I had problems with my stroboscope! Spark voltage was to low i think!

I have to remove the possibility that the propeller is to small and that the engine is ovespeeding!


Is there any over speed protection on this system?

Could it be the tachometer making the trouble?

Any suggestion?

 

[Bilgediver]

I was out today to a guy with problems with his 87 Mercruiser 5.7litre 260hp engine!

The engine had carburetors and thunderbolt ignition system.

I was there for adjusting the timing! It was 20deg at 3000rpm an I increased it to 30deg.

We went out for a test run. The engine was performing much better according to owner!

But at 3800rpm according to tachometer it shut down!Just like a overspeed protection on a petrol car maybye slower frequensy! Everytime it passes 3800rpm the ignition cuts for a sek or less!

The tachometer did not look reliable

At stop it sometimes showed 2000rpm and at idling it was always showing sorting else than my stroboscope! Always higher!

When the engine was running it looked ok!

I was checking the gear protection switch and the engine reacted exactly in the same way !

When we tried to accelerate with full throttle it was accelerating up to the same engine speed! The engine was running perfect as long as the speed was below 3800!

So ?

I have to cheeck that the speed is stopping at 3800rpm when the engine is in neutral

I had problems with my stroboscope! Spark voltage was to low i think!

I have to remove the possibility that the propeller is to small and that the engine is ovespeeding!


Is there any over speed protection on this system?

Could it be the tachometer making the trouble?

Any suggestion?

Mercruisers do have overspeed moreover they sometimes have two settings. One for neutral running and or engine guardian mode and a higher setting when in gear. It might be worth getting the Thunderbolt manual.

 

[ulyden]

Thanks Bilgediver

I have to check the tachometer next time! I take a optical one! I think the previous owner have had a lot of problems with this boat! I could see that on the wiring!

 

[Bilgediver]

Thanks Bilgediver

I have to check the tachometer next time! I take a optical one! I think the previous owner have had a lot of problems with this boat! I could see that on the wiring!

Thunderbolt manual

http://www.boatfix.com/merc/Techbk/95/95HGB4.PDF


It seems the normal overspeed is around 5000RPM however there can be a lower overspeed setting for when the engine is in neutral or guardian mode and also it seems that low sea water pump pressure can trip to the lower setting.

Googling brings up all sorts... Have fun.... I enjoyed playing with these systems on a boat belonging to one of our forumites in the USA. It seemed very easy to get missfires due to plug fouling if doing too much idling. A good blast across the lake would clean things up



One thing I would remind you of. In that engine it is normal to set the ignition IE time the engine on the distributor setting which is around 8 deg BTDC at about 1800RPM. You do not set the timing at 30 BTDC.

The reason for this is that the timing is derived from two sources. The distributor gives the initial mechanical advance of 8 degrees and then the Thunderbolt module and depending on which one is fitted gives additional advance based on the engine requirements. There are curves in the manual showing how it vries the advance depending on load RPM and acceleration. Because of all these variables if using the strobe then you might be miss timing the engine if the Thunderbolt is not giving the full 22 degrees of advance.

This engine has knock detection so if you over advance then this might kick in and this COULD be your problem.

I would suggest setting the timing to 8 BTDC at 1800 RPM and then see what the Thunderbolt does at higher revs however do not expect to get good strobe readings unless at a steady LOADED speed so as to achieve the correct manifold depression.

You should find that at 4800 RPM the timing is around 28 to 30 BTDC when the Thunderbolt is doing its job and adding the 22 degrees to the 8 that the distributor provides.

 

[spannerman]

I have the same engine in my boat and you cannot make any adjustments i.e. idle, mixture,timing without first putting it into 'service mode', as the engine will compensate via the timing to maintain 700rpm idle.
You do this by connecting the white/purple wire in the harness by the distributor to earth with a jumper wire, this locks out the timing module so you can set the timing to 8 degrees at idle, adjust the mixture and set the idle to 650 rpm 'in gear'.
When you remove the jumper wire and it will hold 700 rpm at idle.
This system has a an optimum timing function so the module increases the timing to obtain the optimum timing point, it does this in stages and monitors the rpm increase after increasing the timing a few degrees at high rpms.
It only does this until it encounters cylinder knock then the knock sensor reduces the timing rapidly to avoid piston damge.
It sounds like you have too much timing and the knock sensor is retarding the timing and therefore you have no performance.
I know this as I have just been through the entire setup on my motor after my first trip this year last week, engine ran perfectly last year when I bought the boat so put her into storage for the winter, put her back in the water and she fired up immediately and ran smoother than any V8 I have worked on, drove out for about 5 mins and noticed the speed falling off, gave a bit more throttle with no increase in speed, so I stopped and looked in the engine room, there was a bit of smoke from the engine breather hoses at the carb which didn't look promising. When I went above 1200rpm I got knocking which suprised me seeing as it has a knock sensor!
Drove back at idle and tested the knock sensor which was working perfectly, put it into service mode and straight away the idle was lumpy, pulled off the plug leads individaully and found there was no work being done by cyls 3 and 5, checked the compressions and there was none on those two cyls, pulled the head and found the head gasket blown between the to cylinders and worse it had burnt the metal away between the two combustion chambers in the head so it was now scrap, £900 for a new one here in Norway, but I know a guy who builds hot rods and has a pile of GM heads so got a good used one for £300.
The cause was that the previous owner had a Toyota dealership and fitted Nippondenso car plugs and the center electrode on plug 5 had fallen out and had bounced around the cylinder marking the piston top before vanishing out of the exhaust! This caused predetonation which the module couldn't compensate for and blew the gasket then the hot gases blew between the cylinders and burnt the head, the scary thing is it happened in only 5 mins and with no warning whatsoever, as I closely watched temps and pressures as it was the years first run after 6 months in storage.
So do the set up again with it in service mode and it will be a lot different.

 

[Bilgediver]

I have the same engine in my boat and you cannot make any adjustments i.e. idle, mixture,timing without first putting it into 'service mode', as the engine will compensate via the timing to maintain 700rpm idle.
.

I wasn t sure if I d be teaching granny to suck eggs however this is covered in the manual in my link


I was concerned the OP might have got into the position he was over advancing and causing knock(detonation).

This procedure is well covered in many Mercruiser and Mercury forums though most are repeats of this manual.

 

[ulyden]

Hi and thanks!

I was a bit confused because the timing was not steady at idling!

That’s why I asked. I was expecting a mechanical distributor and it was an electronic devise on it! I could see that the timing was following the speed as described in the manual!

I work with high bmep otto engines so I know knocking. He engine was not knocking!

But sea water pressure is more interesting. The owner had a lot of trouble with this engine. He bought a new engine block and cylinder heads! He also mounted fresh water cooling. So now the water pressure might be different. Problem: I don’t se the pressure switch/gauge on the drawings!

The timing seamed to be steady at speeds above 3000rpm. The engine was running fine!

I read yesterday evening that this ignition system had a service mode! So maybe I use that next time!

The spark plugs were black of soot so I se the point of cleaning it up by running the speed up for a wile!

But any way the ignition cuts at 3800rpm regardless full throttle or not!

 

[Bilgediver]

Hi and thanks!

I was a bit confused because the timing was not steady at idling!

That’s why I asked. I was expecting a mechanical distributor and it was an electronic devise on it! I could see that the timing was following the speed as described in the manual!

I work with high bmep otto engines so I know knocking. He engine was not knocking!

But sea water pressure is more interesting. The owner had a lot of trouble with this engine. He bought a new engine block and cylinder heads! He also mounted fresh water cooling. So now the water pressure might be different. Problem: I don’t se the pressure switch/gauge on the drawings!

The timing seamed to be steady at speeds above 3000rpm. The engine was running fine!

I read yesterday evening that this ignition system had a service mode! So maybe I use that next time!

The spark plugs were black of soot so I se the point of cleaning it up by running the speed up for a wile!

But any way the ignition cuts at 3800rpm regardless full throttle or not!

If you read the manual on the link I gave it tells how to go to Service mode. You need to do this to get stable setting conditions.

The cut out at 3800 RPM is what happens when the Igniition unit believes ther is a problem and so cuts in at lower RPM to protect the engine.

It may be worth getting a lead and software and downloading the fault history. However you will see from web discussions that some faults don t show up in the history and one of these is the block cooling pressure!!!!!


The unit obviously thinks there is an engine fault so you need to determine what is causing this.

 

[ulyden]

Tanks a lot for your support!

I think the current owner have a manual in his boat! Have to check the wireing and sensors!

 

[spannerman]

If this is an '87 carb motor then there is no water pressure sensor, this is only on the latest injection engines.
This engine only has the ignition and knock sensor module mounted on the distributor and there are no download functions or fault codes, these only come from engines with fuel injection.
It just needs everything setting up while in service mode.

Idle - timing-idle-mixture-idle in gear. Maybe you will have to go through it twice as everything effects each other.

 

[ulyden]

Update.

I asked the owner to go to the dealer. I was quite sure that the engine was working well but the electronics was cutting the ignition at 3800rpm regardless if the engine was under acseleration, stedy state or spinning in neutral.

He called the dealer and the dealer told him that they had tols for trubbelshoting this cind of trouble. The owner went 60nm and dealer spent 4 hours
trouble shooting the engine. The result: The dealer could not find the reason and the owner was told to by a dirtrubutor witout fancy elektronics.

The owner bought one that had no knock sensing and idle speed adjustment.
After some weeks he got it and i was asked if i could help him to install.

The new one had only two wires and a new coil. When i finished at least 15 wires was not connected. The hi voltage leads was a different type and we had to replase theese. (Strange fiering order) We had some problems with mounting because of the oil pump drive.

After 2 hours we started up and adjusted the timing to 30deg at 3000rpm.
Aprox 8 at idle. The maximum speed went up to 4800rev and boat speed
above 40knots.(Sea ray 25ft) Never seen a so pleased owner.