Mercruiser 470 overhearing.

[Crewent]

1986 Mercruiser 3.7L 470, 170 HP, alpha 1 Gen 1 drive, Mercarb, 4” heat exchanger, prestolite distributor with pertronix igniter.

Engine overheats well into the red within 3-5 minutes at full power, running in gear and seems to lack power compared to the other identical engine. Checked 200+ degrees at manifold with a temp gun. However, wide open throttle in gear, tied to the dock and I get no more than 170 degrees checking the same area of manifold with temp gun. You’d think the issue would be worse with engine ‘overloaded’ tied to the dock.

Engine bogs down a little from 3/4 to full throttle and plugs were a little white. I suspected fuel pressure or carb issue so I cleaned out carb and checked fuel pressure. Pressure was 5 psi at idle, 7 at wide open throttle. With cleaned and adjusted carb plugs are more brown to black but still overheats and still bogs at WO throttle.

As for ignition timing there is no pinging, does not kick back at start up or shut down and idles very smooth. However it is set about 20 degrees off from what it should be. I suspect the flywheel slipped on the crank at some point with a sheared or missing key?

Compression checked at 135-143.

Vacuum test does not indicate any type of blockage in exhaust.

thermostat pulled and checked good in a pot of hot water, using a temp gun.

No exhaust gasses present in cooling system, no coolant loss.

I’m a little stumped as to why I’m overheating in one scenario but not the other and why, at the same time I’m seeing less power from this engine? The timing issue is definitely unusual. I’m hoping maybe all this is indicative of an issue I haven’t considered. Thanks in advance!!

 

[nola mike]

20' off with your timing is plenty to do many bad things,including overheating, blowing headgaskets, putting holes in pistons, etc. Assuming we're talking about ignition timing here. Turn the motor to TDC and make sure the crank timing mark lines up with engine's. Fix that first. I'll save Scott Danforth the trouble of telling you how easy it is to blow a hg with an overheat on these engines, and to buy a lottery ticket if yours is still intact.

edit: missed that you checked for exhaust gasses. How? Did you change the impeller yet? Checked/cleaned the hx?

 

[Crewent]

I know the timing being so far off needs to be addressed further. But the engine, in all other respects seems quite happy where it’s at. Years ago I had set timing at 8’ instead of the 4’ The prestolite distributors want not knowing I had Prestolites and instead of Delcos. And this wreaked all kinds of havoc with kicking back, dieseling, etc. I have none of this going on presently. However I do need to get to the bottom of why it’s so far of, just trying to pick off one thing at a time. And I don’t understand why it would overheat rapidly while running but full throttle tied to the dock in gear for an hour and I can’t get anywhere near overheating. Next chance I get I will find TDC and compare the marks on the front of the motor.

checked for exhaust gasses in coolant with a coolant die test, multiple times. Had engine out of the boat this this spring and cleaned out heat exchanger, all new bellows, installed new drive with new water pump, etc.

Overheating started when I played with timing. But doing so the engine started pulling its weight. 24 knots vs 14. But why won’t it overheat when tied to the dock? That seems bizarre.

Will do more troubleshooting on timing and let you know.

 

[Scott Danforth]

first, do you have the upgraded HX? if no, suggest you get it

second, no load on engine when tied to the dock

third, drain the coolant and do a cooling jacket pressure test

4th, have the HX cleaned

 

[Scott Danforth]

and yes, open deck aluminum blocks with iron heads will spit the head gasket if overheated by as little as 10 degrees.

 

[dubs283]

you need to stop running your engine with the timing that far off, stop asking questions here and TDC your motor, check the balancer timing marks. this MUST be done bofore you move onto any other troubleshooting

there is a wealth of knowledge about these particular engines here, a lot of love for em and even more hate. personally i think there are great engines if everything is up to date, sound and they are cared for

one thing to note is you didn't mention replacing the water pump base and gasket in the drive, corrosion/bad gasket/faulty base assy can cause exhaust gases to be pulled into the cooling water under the pump and through the heat exchanger at higher rpm, especially when the boat is on plane and the drive housing is above the water line

 

[Crewent]

Thanks for all the advice! While I can’t rule out anything I did install a brand new drive which included a new water pump this spring. And I do have a good flow of sea water all the way through to the exhaust.

Yesterday I found tdc and noted that the timing marks on the front of the motor and the crank do line up. So I reset ignition timing on my prestolite back to 4’ btdc at 1000 rpm. (I’m seeing about 25’ at 2800 rpm). Changing the timing back may well make the overheat issue go away as it was not present prior to messing with timing but I was unable to take the boat out and test it and have been unable to duplicate the over heat while tied to the dock in gear.

I did not drain the cooling system and pressure test. However I did do another die test checking for exhaust gasses present in cooling system which came up negative. And I noted that the vacuum bulb on top the die test assembly held a vacuum (stayed partially compressed) for nearly five minutes while conducting the test on the running engine which to a certain extent tells me the cooling system and head gasket are intact.

Next I ran the engine in gear at the dock and noted 2200 rpm at somewhere between 1/2 and 3/4 throttle and about 1700 at wide open throttle. My other engine with the same propellor will make about 2800 rpm in gear tied to the dock at full throttle.

Back to suspecting the mercarb I went through it again once again with a fine tooth comb. Other than not being able to verify what main jets should be installed everything is clean, all passageways are open, and everything is adjusted correctly to spec with the solid type float needle installed and 6-7 psi fuel pressure at WO throttle, 5 psi at idle. As for the main jets, there was a time when this engine ran great with this carb and the 165 jets currently installed so I don’t think they are the issue but haven’t been able to find information to confirm.

I began to question whether or not the power valve was functioning correctly even though the actuator in the air horn moves freely and spring pressure snaps it right back, the air inlet hole is clean and you can see the actuator move up and down through the hole, and the power valve itself in the bottom of the carb is clean and the needle moves freely. And, with the power valve removed I can blow air through both orifices into the holes under the Venturi cluster. Just to see I took an old power valve apart removed the spring, put it back together, installed it and ran the engine. Effectively ‘sticking’ the power valve open. This made zero difference in rpm.

I feel like I have beaten the fuel system to death. Ignition timing and advance mechanism seem to be functioning as they should, I have good compression, no vacuum leaks present on the intake side, vacuum test did not indicate presence of any blockage in exhaust. Engine runs good, sounds healthy and pulls hard up to about half throttle than you can hear it flatten out and finally decelerates for the last third to WO.

Still suspecting fuel the only thing left I can think of is to install the carb off my other good engine on this one and see if the problem goes away. I ran out of time to do this yesterday. This engine has had me tearing my hair out for the better part of a year now. Early on it would sometimes drop power, often forcing us to finish a trip at 8-10 knots in stead of 20-25 knots. Now it seems to be consistently lacking. Any other ideas?