intermittent knock at high rpm under load

I have a knock at 4600rpm max throttle every 30sec or so. If I back off the throttle off just a bit to 4500rpm it goes away. It sounds like a rod knock. My guess is that there is detonation.

2004 merc 5.0L alpha one genII 2bbl carb w/TKS, thunderbolt V ignition. Engine only has 160hrs. Compression check last year showed >205 in all cylinders.

I just replace thermostat as it was running too high at about 170F. Now it runs at 160F right where it should be. I replaced all spark plugs and found two on starboard side had cracked insulators. The tips where tan but there was some soot lower down in insulator. This helped reduce the frequency of knock/detonation. I have confirmed the spark plug wires are in correct firing order. When I replace the plugs I left them gapped at 0.040". The spec for my engine is 0.045". So I need to fix that. I plan to check timing is 10deg BTDC next. I confirmed oil level is below max. I confirmed rev limiter kicks in at 4850rpm so that's not the problem. The thunderbolt V has a knock sensor but it does not seem to stop this.

So what should the next steps be? Are there any common issues with this platform?

Re: intermittent knock at high rpm under load

Check your timing.
Run better octane fuel.
My boat may not require it but she gets premium every fill,best and cleanest fuel you can buy,little to no ethanol as well which is a good thing.
Never put crap in something that you plan on keeping.

Re: intermittent knock at high rpm under load

OK timing was off. Now off for a test ride.

Re: intermittent knock at high rpm under load

So I must have not done the timing correctly. On the test ride I could only get 4000rpm full throttle. It should be around 4800rpm. So while on the water I just arbitrarily rotated the distributor counter clockwise, then went to full throttle untill I got to about 4600rpm max. But I still seem to have the knocking.....

Re: intermittent knock at high rpm under load

Set the timing with a light.
This is the procedure for setting base timing:
Mercruiser Small V8 Wiring

V6 and V8 Carbureted Models (Thunderbolt V Ignition)

1. Connect timing light (91-99379 or similar) to No. 1 spark plug wire. Connect power supply leads, if applicable, on light to 12 volt battery. Refer to Specifications – “Engine Rotation and Firing Order” for cylinder numbering and location.
2. Connect a shop tachometer to engine.
3. Using a jumper wire, connect the ignition system timing lead (PUR/WHT wire) to a good engine ground (-). This locks the ignition module into the “Base Timing Mode”.

NOTE: Before starting engine make sure the timing tab and marks on damper are clean. Chalk or white paint on timing marks may help visibility.

4. Start engine and run at normal idle speed. Allow engine to reach normal operating temperature.
5. Aim timing light at timing tab, located on the timing gear cover and crankshaft torsional damper.
6. If adjustment is required, adjust timing by loosening distributor clamp and rotating distributor body as required until timing mark on damper or pulley lines up with the mark on tab specified in “Specifications”. Tighten clamp and recheck location of timing mark.
7. Make sure that the distributor has been tightened. Remove the jumper wire from the timing terminal.
8. Remove jumper wire between the timing lead (PUR/WHT wire) and ground (-).

IMPORTANT: Be sure to disconnect the jumper wire from between the ignition system timing lead and ground (-) before attempting to resume normal operations. If the jumper wire is left in place, the ignition module will operate in the “Base Timing Mode”. This means that the additional timing advance features would not function.

9. Stop engine and remove timing light.

Re: intermittent knock at high rpm under load

bungholio said:

So I must have not done the timing correctly. On the test ride I could only get 4000rpm full throttle. It should be around 4800rpm. So while on the water I just arbitrarily rotated the distributor counter clockwise, then went to full throttle untill I got to about 4600rpm max. But I still seem to have the knocking.....

Click to expand...

Ayuh,.... Welcome Aboard,... Check the contents of yer fuel filter for water, 'n crud, then replace it,...

She's goin' Lean, 'n Detonatin'....

Set the Timin', per Spec...

Re: intermittent knock at high rpm under load

Bond-o said:

Ayuh,.... Welcome Aboard,... Check the contents of yer fuel filter for water, 'n crud, then replace it,...

She's goin' Lean, 'n Detonatin'....

Set the Timin', per Spec...

Click to expand...

Good call but his timing is also set incorrectly causing the detonation, not that it's lean.
It may also have bad fuel contributing to the problem.
As I was saying earlier, run some premium through, detonation gone.

Re: intermittent knock at high rpm under load

Previously, I put premium in and replaced the fuel filter. I did not ground the ignition timing lead so this was most likely my problem. The timing was probably set correctly before I changed it.

I ordered a new timing light with inductive pickup and accurate rpm read out.

Re: intermittent knock at high rpm under load

Went to regap my plugs and cylinder 2 and 6 on starboard side broke in half. After only one time out with new plugs! So this must be some serious detonation. I am now wondering if there is a water leak in the head on 2, and 6? What do you guys think?

Re: intermittent knock at high rpm under load

update So 2,4,6 had cracked plugs. Which I just put in. So I did a compression check and get 200, 195, 200, 205 on cylinders 2,4,6,8. Does this mean I do not need to pull the head? I am now suspecting that something may be clogging the cooling on the starboard side (2,4,6,8). Pulled upper exhaust bellows. Do not see anything blocking so far. I will pull exhaust manifold next.

Re: intermittent knock at high rpm under load

I am not sure but the restrictor plate may have been in backwards on the exhaust bellows. I did not look at that very closely when I took it off. The small hole side of the gaskets is suposed to go to the elbow tube. The engine does seem to stay at a steady 160F now. I was seeing 170F before. So the theory would be that cylinders 2,4,6,8 where running hot and detonating. Also the plugs being set with a smaller gap would also hurt.

I regapped all the plugs to 0.045", set the timing to 10BTDC @650rpm with the timing wire shorted to ground (base timing) Thanks Levithan! I actually was not all that far off with my hand tune job. I put a new cap and rotor on as well. There was about 1/32" of oxide build up on the rotor contacts. Measured the spark plug wires and they where all in the range of 8k to 13k ohms. Time for a test run.

Re: intermittent knock at high rpm under load

It's easy to crack those spark plugs too. So tighten them carefully, you can crack them installing.
The gasket between your exh manifold and riser (what you are calling a restrictor), can allow water into the exhaust tract so check it well. Use the silver grey ones, they have a metal core and graphite coating, they seal best. Don't bother with those green paper coated gaskets, they are hopeless.

Re: intermittent knock at high rpm under load



yeah I used the graphite one

Re: intermittent knock at high rpm under load

took the boat out and get to about 4500rpm and still have a knock. Does not seem as bad as before. If I just drop the throttle slightly from WOT it goes away. So I am now wondering if this is just the way it is with a 2bbl on a merc5.0 pushing a 5000lb 22' sey ray weekender???? The fuel injected motor is able to produce 260hp while the 2bbl only does 220hp. Fuel injection tachs out 200rpm higher but will burn clean and can deliver the right mixture at full throttle. I wonder if the 2bbl is running lean at WOT?

Re: intermittent knock at high rpm under load

Tore the cooling system apart looking for any blockage. So i put in a new water pump. Old one seemed good. Another test ride.....

Re: intermittent knock at high rpm under load

bungholio said:

took the boat out and get to about 4500rpm and still have a knock. Does not seem as bad as before. If I just drop the throttle slightly from WOT it goes away. So I am now wondering if this is just the way it is with a 2bbl on a merc5.0 pushing a 5000lb 22' sey ray weekender???? The fuel injected motor is able to produce 260hp while the 2bbl only does 220hp. Fuel injection tachs out 200rpm higher but will burn clean and can deliver the right mixture at full throttle. I wonder if the 2bbl is running lean at WOT?

Click to expand...

Ayuh,... That's what I've been sayin',.... It ain't that a 2bbl. carb Can't feed yer motor,...
It's that Yer Carb Ain't doin' it's Job,...

Ya got a Fuel Delivery Issue,.... It could be a Carb Problem, or fuel pump, or tank, or,......
It needs to be Diagnosed to see what needs fixin',... Was there water in the filter ya changed,..??
Keep runnin' it, 'n yer gonna Hole a piston, or worse...

Every little Click that you hear is another little piece of yer motor, goin' out the exhaust pipes....

NHGuy said:

It's easy to crack those spark plugs too. So tighten them carefully, you can crack them installing.

Click to expand...

Ayuh,.... Agreed,... The broken spark plugs are a Mechanic problem, not a Motor problem,...

Re: intermittent knock at high rpm under load

I did not find any water in the fuel filter. I checked the fuel line from the fuel pump to carb for blockage. It was clear.

For the spark plug issue, to make sure its not a mechanic problem I put anti-seize compound on and only tighten a 16th of a turn past hand tight. But that's good feedback that you think it is not breaking plugs from detonation.

Re: intermittent knock at high rpm under load

ok built a fuel pressure tester for the gas tank side. Will start there.

My fuel tank vent looks to be open and inside the gas cap opening. Yet the gas cap looks like it is sealed with a rubber ring. I don't see how air gets to the fuel tank vent with the gas cap on.

Re: intermittent knock at high rpm under load

0 psi on gas tank side, so there is no restriction from gas tank to fuel filter

Re: intermittent knock at high rpm under load

https://www.youtube.com/watch?v=Iw3tM9IYhoU

OK here is the fuel flow test. It shows 0psi of back pressure. So no restriction. Should the fuel hose be full of fuel? Does this point to a problem with fuel pump?