Help!! Overheating

[91 Dancer]

The motor is a 91' 7.4 Bravo Mercruiser. It ran hot at the end of last season. I replaced the impellar and t-stat over winter. It idles at the correct temp (140 degrees), but gradually creeps up to 190-200 at cruising rpm. I have backflushed everything and can't find any restrictions. I replaced the circulating pump to no avail. I even did the shop manual test for head gasket (clear hose from t-stat to manifold to check for bubbles at higher rpm's, I only saw a few small bubbles and they didn't change at any rpm). I reset the the timing and checked the plugs too, and they look fine. By the way, it's raw water cooled in fresh water only, and the old impeller was intact so there isn't rubber downline. The only thing I haven't tried is to pull manifolds. One has two bolts with the heads rusted off (any suggestions on getting those out?) The manifolds, risers, and elbows are the same temps on both sides (measured with a temp gun) I NEED HELP! :smt100

 

[PlayDate]

Please post the temps for the manifolds, risers, elbows, tstat housing and the engine temp gauge when the engine is warm and then when the engine is hot. I'm suspecting based on your comments about the manifold bolts that the manifolds are probably shot. Let me know if this is a salt water boat and how many hours are on the engine.

If you enter your engine number at the mercruiser web site you can get a diagram how your engine is cooled by the raw water pump.

I appreciate how difficult it is to get rusted and round off bolts out of a manifold. However, if the heads of the bolts are rusted off it is far easier to deal with. Remove the other bolts and remove the manifold then use a stud remover or pipewrench to unscrew the bolt.

Before you pull the manifolds, check the water flow to each manifold at idle and 2500 rpm just to make sure that water is flowing correctly.

-John

 

[brimanst]

I will wait for details on the temperature readings, but have you checked the sea strainers and water input. Even if all your water pumps work great, if there is no water, then it can circulate. As far as he rusted bolts, I have a couple of suggestions.

1. Use PB at least 3 times for a week prior to removing. If you can get a steel bruch in between that helps as well.
2. If there is still some shape to the hex head, use a smaller size metric socket and hammer them on the head of the bolt.
3. If there is not much hex shape to the head, then go to Sears and buy the "easy outs" for hex heads. They are desigined to grip stipped heads and work fairly well.

I have used these methods before I ever had to use heat, since I really did not want open flames in my bilge and had great results. Good luck.

briman

 

[91 Dancer]

Here are the temps at idle/after 10 minute run at 3200 rpm.
T-stat housing 142/203
Port and Strbd manifold 147/210
" risers 154/214
" elbows 145/206
The gauge shows 150/210
This boat is freshwater only and the motor has 492 hrs.
To check the flow at stated rpms, do you mean just pull the hoses going into the manifold and check there.

As far as strainers and inputs, no strainer (pickup from outdrive). A new inlet hose delivered today (thought it might be collapsing). Will install hose this week.

 

[PlayDate]

Here are the temps at idle/after 10 minute run at 3200 rpm.
T-stat housing 142/203
Port and Strbd manifold 147/210
" risers 154/214
" elbows 145/206
The gauge shows 150/210
This boat is freshwater only and the motor has 492 hrs.
To check the flow at stated rpms, do you mean just pull the hoses going into the manifold and check there.

As far as strainers and inputs, no strainer (pickup from outdrive). A new inlet hose delivered today (thought it might be collapsing). Will install hose this week.

Obviously the entire engine is getting too hot not just a component. Please check the water flow to each manifold/ elbow at the manifold/elbow with the following procedure:

The engine can be cool/warm for this test and you need another person.

1) release the hose clamp on a manifold/elbow hose and remove it.

2) Have your friend start the engine and observe the flow. Increase rpms to 2500 and observe the flow. Water flow (it should flow like a garden hose) should increase slightly with higher rpms....it should not decrease.

3) Stop engine and check the next hose.

If we have a blockage, it will show up on this test. You have a fresh water boat with low hours so I am having you focus on engine cooling. Your idle temps are too hot for a 140 degree thermostat.

If water flow is ok (which I doubt) then we need to pull the thermostat out and recheck idle and hot temps. Did the boat just start having this problem last year?

-John

 

[91 Dancer]

The only temp difference with t-stat removed is eventually the idle temps will go a little lower. Same temps on hot end. Yes, the problem started last summer. This is the third summer I have owned the boat. I see no reasons to doubt the previous owner when he told me it was a freshwater boat only. I'll do the flow test after lunch and post results. Thanks

 

[91 Dancer]

I did the flow test w/muffs and garden hose. Both manifolds had pretty good flow, and increased w/rpm. You said like a garden hose, but this wasn't the full capacity of the hose, if that's what you mean. It started at about 1/3 the capacity or diameter of the hose and went up to about 1/2. It also pulsed or surged, not a steady flow. I started on the strbd side, and when it was cold, I was getting a lot of water out of the manifold itself and the hose. After checking the rest and the motor was warm, I went back to that one again, and I got no water out of the manifold, only the hose. Don't know if that means anything.

The elbows had not as much flow as man's, and the port had maybe a little less than the strbd. Flow pulsed also. Both increased w/rpm's.

 

[PlayDate]

I did the flow test w/muffs and garden hose. Both manifolds had pretty good flow, and increased w/rpm. You said like a garden hose, but this wasn't the full capacity of the hose, if that's what you mean. It started at about 1/3 the capacity or diameter of the hose and went up to about 1/2. It also pulsed or surged, not a steady flow. I started on the strbd side, and when it was cold, I was getting a lot of water out of the manifold itself and the hose. After checking the rest and the motor was warm, I went back to that one again, and I got no water out of the manifold, only the hose. Don't know if that means anything.

The elbows had not as much flow as man's, and the port had maybe a little less than the strbd. Flow pulsed also. Both increased w/rpm's.

Stan,

Something doesn't seem right with the impeller/pump. The only way to do this test is in the water with the impeller providing the pressure. A garden hose and earmuffs provide positive pressure which may be giving you a false impression that the impeller is working properly. The flow at idle should be consistent with a garden hose at 1/2 or 2/3 flow. It should be a steady flow on all hoses.

The fact that the water stopped flowing from the manifold leads me to think the impeller/pump is the problem. Have you grounded the boat since replacing the impeller? That's where I would start and I appreciate the pain that goes with it. We need to get the pressure up which will probably solve the problem. Thanks for pulling the tstat, that just puts more spotlight on the impeller/pump.

-John

 

[91 Dancer]

Thanks John. I figured it needed to be in the water, but I thought I would go ahead and try. I haven't grounded the boat, and I replaced the impeller again two weeks ago. Is there anything else that could cause the pump to go bad? The shop manual explains how to do a volume test, I'll try that and the flow test next weekend on the water. I gather from the direction we have gone that you don't think it's the head gaskets?

 

[PlayDate]

Thanks John. I figured it needed to be in the water, but I thought I would go ahead and try. I haven't grounded the boat, and I replaced the impeller again two weeks ago. Is there anything else that could cause the pump to go bad? The shop manual explains how to do a volume test, I'll try that and the flow test next weekend on the water. I gather from the direction we have gone that you don't think it's the head gaskets?

Stan,

When head gaskets go they show one of two problems: 1) if they blow between cylinders, the engine runs unevenly and depending on the cylinders...backfires through the intake and exhaust. 2) if they blow to a water passage, the oil in the engine turns milky brown and it steams like an old style freight train. A cylinder compression test would completely rule it out.

However, I think there is a problem with flow either with the pump casing itself or a blockage. Your engine is getting hot and not enough cooling water is flowing through the components. When you pulled the tstat the temps should have really dropped and the engine should not have even come close to 160 degrees.

Let's see what the volume test shows.

-John

 

[91 Dancer]

I found the problem. The tapered insert that holds the water inlet hose in the transom plate was almost completely collapsed. Replaced and took a 50 mile cruise over the weekend and temp never got above 165. Thanks John, for keeping me on the right end of the motor and problem.

 

[PlayDate]

I found the problem. The tapered insert that holds the water inlet hose in the transom plate was almost completely collapsed. Replaced and took a 50 mile cruise over the weekend and temp never got above 165. Thanks John, for keeping on the right end of the motor and problem.

Stan,

Great news! I'm glad I could help.

-John

 

[Turtlesboat]

As my boat gets older, i'm going to move closer and closer to where playdate lives :lol: