Diagnostic confirmation please

[Leedanger]

Mercruiser 5.7 MPI Bravo
Serial# 0w377395

Overheating issue. Replaced seawater impeller (was in pieces), replaced thermostat, and back flushed the cooling system (can't find any blockages)....still overheating. With my temp gun I can see that the engine (block, heads, manifolds) are running considerably cooler than the thermostat housing. Even with the thermostat out, the engine will slowly overheat but the hottest point is always at the therm housing. I have flush both risers so I'm stuck with blown head gasket. Agreed?

There is no water in the oil so is there any other way to verify this before the top end rebuild? Is there a way to determine which head is the issue? I thought of running some clear tubing from the t-stat to the risers to look for air bubbles, but I doubt it's accuracy. Thanks as always.

 

[tlewis1]

If your leaning towards a head gasket issue I would first do a compression test and then a cylinder leak down test. That will confirm for sure if there is a issue there before you go to all the work off tearing heads off.

Pretty common issue when an engine overheats that the heads warp slightly. If taking one head off I would be taking both off and have them gone through. Not that much more work involved.

 

[Rick Stephens]

Did you pull the feed line coming from the outdrive at the thermostat housing and fire up on muffs to make sure you're getting good flow at the thermostat of fresh water?

 

[Leedanger]

Yup did all that. Only the t-stat housing would get hot. I believe it's from exhaust gasses forming a bubble right there. I confirmed it by running a house out of the t-stat housing from the drain vent to overboard. This allowed the abnormally hot water a place to evacuate and the engine would run cool. Even though technically less water was then circulating through the engine, less heat was as well allowing it to run at norm temp. Half way through the head gaskets now...ill report back if all was a success

 

[Bondo]

Half way through the head gaskets now..

Ayuh,..... So did ya find the blown out area of the gasket when ya tore it down,..??

I too, woulda done a compression/ leak-down test to locate the issue,...

 

[Pete104]

Did you change just the impeller or put a complete kit in it? If not your probably gonna wish you did! How hot did this thing get? Impeller in pieces says pretty hot. Did you get ALL of the old pieces out of the cooling system? MOST usually won't work.

 

[alldodge]

Mercruiser 5.7 MPI Bravo
Serial# 0w377395

Is the an Alpha or Bravo and does it have the 7 or 3 point drain system?

 

[Leedanger]

I did a leak down test as I was reassembling the heads during valve adjustment. All is good so far, although now it seems like the manifold on one side isn't getting coolant flow as its running close to 200 yikes. Gonna pull it again and see if I missed some clog. Btw while everything was apart I cleaned out everything. Literally the whole cooling system has been flushed clear. But...assumption for human error.
This is a bravo drive and it has the 3 point manual drain system.

 

[alldodge]

With the 3 point system you don't have the check balls (7 point) in the lines feeding the exhaust, but you do have the dreaded single main drain and the Tee fittings. The pic below shows where issues have come up from other times which cause issues. You mentioned replacing everything so you probably don't have the exhaust manifold to riser issue. You did mention that cooling was normal when removing a line from the thermostat allowing less restrictive water flow. Check these points out for issues

 

[Leedanger]

Thanks so much...that is very helpful. Over heating issue seems to be solved. Whole new set of problems. Boat struggles to start and backfires if u rev it once running. I did a leak down test while adjusting the valves so I feel confident that it's not a head, valve, or compression issue although I never did an actual compression test... But in my mind a leak down is more accurate anyway (in theory). It seems as though the timing is outside of its curve. Will only start if u advance the throttle. Once started idles and runs beautifully, but if u rev it all types of **** goes funky. I must have made an error somewhere. Other than overheating non of these problems were existent before the head gasket repair.

 

[alldodge]

Back firing is always ignition happening to soon when intake valves are open, or from intake valves not sealing.

You have gone from over heating to this is not the issue and backfiring happening. This is a new issue and I'm going to need more info to be able to try to understand the issue. Will wait until I can hear/see more info

 

[Bondo]

Whole new set of problems. Boat struggles to start and backfires if u rev it once running. I did a leak down test while adjusting the valves so I feel confident that it's not a head, valve, or compression issue although I never did an actual compression test... But in my mind a leak down is more accurate anyway (in theory). It seems as though the timing is outside of its curve. Will only start if u advance the throttle. Once started idles and runs beautifully, but if u rev it all types of **** goes funky. I must have made an error somewhere.

Ayuh,.... Sounds like it's starvin' for gas,....

 

[Leedanger]

Starving for gas it was. The connector for the fuel pump was just not quite right. On one minute out the next... for now I just spliced and heat shrink the wires without any issues thus far. Running at a moderate 170 degrees and all is well at the moment. It hasnt been to the lake yet, only on the muffs so there is still room for failures but I feel pretty good about it. As a side note for anyone else, on the thermostat flange (the flat mating section) I drilled two 1/8th holes in it to help vent any other potential air pockets just in case. I've heard tell that you will find this on most Ilmor engines..Thanks again to all of you for contributing your knowledge.

 

[Leedanger]

Haha...so now it's running too cold on the water and won't get up on step. Pops and sputters under load. Never got over 120 degrees on the lake and won't exceed 2600 or so rpm.

 

[Bondo]

Pops and sputters under load.

Ayuh,..... Still starvin' for gas,...