1990 7.4L overheat

[btye1]

I am running out of things to check on this, so any help is appreciated.

I have a 1990 Mercruiser 7.4L w/ a Bravo drive, raw water cooled. The exhaust manifolds are not getting any water to them and they are overheating, causing the spark plug boots to melt and ground themselves to the manifolds.

The risers are getting plenty of water and the thermostat works. The impeller is good. There is no blockage in the power steering/oil cooler. The oil level is also ok and looks fine, so i'm not thinking the block is cracked or anything like that. I am thinking it might be the circulating pump on the block, however I wanted to check here before I take it off.

 

[Fireman431]

Re: 1990 7.4L overheat

I would look first at the block mounted water pump. Cheap and easy enough to replace.

 

[HT32BSX115]

Re: 1990 7.4L overheat

Howdy,


In most raw water cooled engines, the water that gets to the manifolds and risers is not dependent on the circulating pump.

You're not CIRCULATING water through the manifolds, you are pumping water through the manifolds and then up through the risers(and then overboard).





You have some blockage somewhere that is not allowing water flow either to or through the manifolds.


In the 454, ALL the water that goes to the exhaust manifold exits out the top and then goes up through the riser and overboard. (UNLESS it's a DRY-JOINT manifold/riser system and closed cooled etc....)


It's also possible that you have a plumbing problem. But it sounds like your manifolds are partially clogged(with rust or other debris)

Do you have a picture that indicates how all the hoses are connected?

 

[TilliamWe]

Re: 1990 7.4L overheat

HT32's right on the money.

 

[btye1]

Re: 1990 7.4L overheat

I have it hooked up exactly as described in the service manual.

water is never getting to the thermostat to open it, so are we thinking that the block or intake manifold is clogged instead of the circulating pump then?

 

[Bondo]

Re: 1990 7.4L overheat

Ayuh,... You said the exhaust manifolds weren't gettin' water,...
Now ya say the Block is seein' no water,..??

The raw water comes into the t-stat housing where it goes to the circulating pump intake,+/ or the exhaust manifolds/ risers....

I'd be lookin' at the T-stat Housing myself...

 

[btye1]

Re: 1990 7.4L overheat

Ayuh,... You said the exhaust manifolds weren't gettin' water,...
Now ya say the Block is seein' no water,..??

The raw water comes into the t-stat housing where it goes to the circulating pump intake,+/ or the exhaust manifolds/ risers....

I'd be lookin' at the T-stat Housing myself...

I am just assuming no water is getting to the block, because according to that diagram, water comes from the engine to the thermostat and then out to the manifolds. and when i pulled the thermostat cover off, there is no water on top, AKA the thermostat has not opened and let water through.

 

[EddiePetty]

Re: 1990 7.4L overheat

.....ya' sure you don't have the gasket between the exhaust manifold and exhaust elbow 90 degrees out of sync.?

 

[HT32BSX115]

Re: 1990 7.4L overheat

My 454 t-stat housing had no separate connection to the riser. All the cooling water from that was supplied to exhaust manifold went to the riser........

You have the (infamous) t-stat housing "check-balls"........They're *supposed* to regulate flow to the riser and exhaust manifold (somehow) In theory it works when everything is nice and clean............but when the t-stat housing gets rusty those regulator "balls' sometimes get sticky or stick open/closed etc...... and they either allow too much flow to the risers or not enough......

and when i pulled the thermostat cover off, there is no water on top, AKA the thermostat has not opened and let water through.

Water is ALWAYS flowing to the exhaust system regardless of t-stat position)




The circulating pump circulates water through the engine ONLY (based on how open/closed the t-stat is).


From your first post, your engine is running at the "right" temp too?


You also may have the "restricting" type of riser gaskets......(I think there's a MC service bulletin to that effect)

There could be *some* rust flakes blocking flow out of the top of the exhaust manifolds (and into the bottom of the risers.)


You have 2 supply hoses to the exhaust system......any slight restriction anywhere in the manifold supply hose OR the exit point for manifold cooling water (the manifold-riser connection) would reduce or obstruct flow enough to cause a manifold overheat.............. but not affect the flow to the risers since they get water separately from the housing too.........

Since you still have "flow" all the water would then end up going through the risers and very little or no water would flow through the manifold.

Since you still have flow overall......... and as long as the t-stat is opening, the engine would still run at the "right" temp.......


If you haven't had the t-stat housing off yet, you should pull it and look for rust or blockage from debris etc.....

You should also pull the risers and inspect for rust and obstructions at the gaskets.

 

[cr2k]

Re: 1990 7.4L overheat

Also if you have melted you spark plug boots you have totally damaged the insides of your water hoses that go from the elbows to the Y-pipe. You wont be able to tell from the outside either.

 

[btye1]

Re: 1990 7.4L overheat

so i had time today to pull one of the risers and manifolds and the gasket was on the right way and it looked ok to me. I am going to put the riser and manifold back together this weekend w/ a new gasket and run water through the whole thing and see how flow is as I am not sure how old the manifolds are. I can tell you that they are aftermarket barr manifolds, which surprised me because the original owner I bought it from told me that nothing had ever been changed on the motor and it had low hours.

I am still struggling to understand how the water gets to the thermostat from the intake manifold, because looking at the diagram from the service manual that i posted, it looks as if water goes into the circulation pump, which goes into the block and then into the intake manifold and if the thermostat is not open it goes thru the bypass hose back into the circulation pump to do the same thing again until the thermostat opens, is this not correct?? So if the circulation pump is messed up, then it wouldn't pump water up through the intake manifold to open the thermostat. Or am i just not understanding this correctly?

 

[HT32BSX115]

Re: 1990 7.4L overheat

In just about every V-block engine the circulation pump pumps coolant into the block via the "side" discharge ports.

It then travels up though the block and into the heads.... exiting though the front of each head into the ( or front of the) intake manifold and out though the t-stat (if it's open)

This is why the temp-sense part of the t-stat MUST always be inside (toward) the intake manifold-side of the housing.

If the T-stat is closed, a small amount of coolant still flows around the t-stat through a bypass (depending on how large the bypass "hole" is) The t-stat housing is still hot since there's always a tiny bit of "flow" there....

More flows though the t-stat as it opens. And as it opens more, the hottest coolant flows by the "sense" part of the "stat" controlling how open/closed it becomes.


In a raw cooled marine engine, there's always excess cold water available for the circulating pump to push up though the engine as needed.


If the t-stat is completely closed nearly ALL the cold water (bypasses the housing/circ-pump) goes to the manifolds/risers and is dumped overboard.

If it's completely open (which probably never happens) the cold water is mixed with "hot" water from the engine and is then dumped overboard (through the manifolds/risers)...........

I.E. ALL the water from the raw water pump is dumped overboard (while some of it is metered though the engine by the t-stat.)

 

[btye1]

Re: 1990 7.4L overheat

ok, so assuming everything with the manifolds is ok, what else could be the problem w/ the spark plugs and the manifolds getting so hot? Timing? I can't think of anything else

 

[HT32BSX115]

Re: 1990 7.4L overheat

I have OEM manifolds on my 454.

You might pull your manifolds and see if there might be sand or other debris in the bottom of them around where the exhaust ports are. If there was a lot down there the cooling water might not get down low enough to completely cool them.



Mine do get pretty hot down there..........hot enough to burn the paint off.......and I don't think the water jacket extends all the way down to the flange area that bolts to the head. It really couldn't.

but it shouldn't get hot enough to burn the plug wires in the vicinity of the plug.

On another note, a regular automotive manifold gets at least as hot (and probably MUCH hotter) and doesn't burn up the plug wire hoods etc.....

 

[btye1]

Re: 1990 7.4L overheat

ok if thats the case with the manifolds, then what in the world is causing my plug boots to do that? timing? bad spark plugs?

 

[Bt Doctur]

Re: 1990 7.4L overheat

whats the spark plug number your using?

 

[btye1]

Re: 1990 7.4L overheat

NGK br6fs

 

[HT32BSX115]

Re: 1990 7.4L overheat

ok if thats the case with the manifolds, then what in the world is causing my plug boots to do that? timing? bad spark plugs?

Take a look at your complete plug wiring assy. Are they "premium" wires or are they the "cheapies"?

How long have they been on the engine?

 

[btye1]

Re: 1990 7.4L overheat

they are the OEM quicksilver wires, I'm going to pull all the plugs this weekend and see what they look like and check the timing..i sure hope thats not the problem though..one of my mechanic friends suggested i might have a blown headgasket...