Overheating Mercruiser 7.4L MPI

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DracoDan

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Well guys, back to the drawing board :-(

Swapped back in the old thermostat that works properly and while it idled at 170? (like the starboard engine) as soon as I increased the RPM it started overheating again. I removed the thermostat completely so I could go out and enjoy memorial day (only took it about 2 miles from the marina). During the trip the temperatures quickly climbed to over 210?, so I stopped and decided to remove the thermostat completely, since that worked when I was trying to determine if it was bad. After this it dropped to about 170?, then bounced around a lot, everything from 100? to 170? (not overheating). I thought that everything was going to be okay, but I realized I couldn't get over about 2400RPM, any higher and it would misfire, have preignition or some other symptom that caused the RPMs to instantly drop about 200 ever time I tried pushing it. Most of the time I had to keep it around 2200RPM. sometimes when it happened it would sound like a loud pop was coming from the engine bay, but not every time

New suspicion:
Could this be the TPS? In other words, could the engine think that I'm at idle throttle and be giving it the idle throttle amount of air even when I raise the RPM (by increasing fuel)? This wouldn't cause overheating would it? Could the TPS also cause the timing to be off, causing preignition?

I'm about ready to take it to a mechanic to have it positively identified, but I would really like to figure this out myself... I really wish the thing had OBDII :-/
 

DracoDan

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any chance you have dry joint manifolds

I had to look this up, I don't think I have this, it was used on engines manufactured in 2001 and later. Mine has the original manifolds and elbows from 1999. If it helps (and I haven't clarified this already) the engine serial number is 0L347000.

Here are the things I have changed to each engine (that are different from the factory items):
  • K&N air filters (just installed a brand new set)
  • Alternators replaced with https://www.amazon.com/gp/product/B008Y3OPUQ - I also removed the little metal box with two wires that was connected to the old alternators. I never figured out what that black box did, it has no part number on it and it's not shown in any of the diagrams from Mercury, maybe tpenfield knows what I'm talking about? I don't know if it's a formula thing or what...
  • Plugs replaced with NGK BPR6EFS Spark Plug
All other parts on the engines are OEM as far as I know, but I only bought the boat in 2012.

tpenfield: What year is your 330? What engines does it have?
 

tpenfield

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tpenfield: What year is your 330? What engines does it have?

My boat is a 1996 . . . has the 7.4 LX MPI Engines. Similar to yours, but I think the intake plenum is a bit different on the 1999 engines.

I had cooling issues on the Port engine all of last season. It turned out to be some marine life ( a shell) that got stuck at the inlet hose at the bell housing. I got the boat in 2012 from lake Winnepesaukee (NH) and put full closed cooling systems on the engines before going into the ocean.

I ended up buying the Diacom software to analyze the engine. I think you still have a cooling system issue rather than a sensor issue.
 

DracoDan

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My boat is a 1996 . . . has the 7.4 LX MPI Engines. Similar to yours, but I think the intake plenum is a bit different on the 1999 engines.

I had cooling issues on the Port engine all of last season. It turned out to be some marine life ( a shell) that got stuck at the inlet hose at the bell housing. I got the boat in 2012 from lake Winnepesaukee (NH) and put full closed cooling systems on the engines before going into the ocean.

I ended up buying the Diacom software to analyze the engine. I think you still have a cooling system issue rather than a sensor issue.

I'm going to try the LED code trick to see what codes the engine is throwing, since that's such an easy way to rule out TPS and detect knock. I'm not getting any alarms though, so you may be right.

Here are the temperatures I was getting with the old thermostat that I still suspect to be bad (spec'ed at 160?, starts opening at 175?, full open at 192?) at idle:
Starboard engine (good)
Temp (F)
Port engine (bad/problems)
Temp (F)
Starboard-side exhaust elbow
108
Starboard-side exhaust elbow
94
Port-side exhaust elbow
91.5
Port-side exhaust elbow
83
Thermostat housing front
105
Thermostat housing front
135.5
Thermostat housing back
101.5
Thermostat housing back
88
Return hose*
140.5
Return hose*
124
Sensor (on housing)
160
Sensor (on housing)
180
* - Hose from circulating pump to thermostat housing

I back flushed the power steering and fuel cooler line as best as I could. There was absolutely no debris or sign of restriction and the water used came out crystal clear. I can try the engine flush connection again to see if maybe it was the bad thermostat AND restricted cooling inlet, but other than that I don't know where else there could be something slowing the coolant flow other than exhaust (but I feel like this is unlikely with my cool running elbows). Should I remove the elbows and inspect them? I don't want to waste money and time on the gaskets unless someone points to a good reason that this could be the culprit.

What's the best way to check the oil system to see if it's having problems circulating oil through the system? I was thinking this might cause one or more cylinders to be much hotter than the others, resulting in preiginition/knock. I should know after tomorrow since there are knock sensors and it should report repeated knock.

One of the reasons I started leaning away from cooling as a culprit is that even when the engine is running at the correct temperatures (170?, no thermostat) I can't push above 2400RPM before it starts stuttering/losing power.
 

tpenfield

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On the port engine . . .I think you have more water going directly out the exhaust than through the engine. Do you have separate hoses to the bottom of the exhaust manifold and to the elbow? or just a single hose to the bottom of the exhaust manifold?

The top of the elbows should be 95-100 F. So while the port engine block is hotter, the elbows are cooler.

If you have a hose that goes directly to the elbows . . . See if you can pinch the hose(s) about 50% with a clamp and see if it makes the engine temp go down.

Often, there is an insert in the thermostat housing that allows the proper flow of water . . . not sure if you have that type though. The parts diagram that I found for your engine shows closed cooling.
 

DracoDan

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The hose just goes to the bottom of the manifold, not to the elbow.

Check here for all the diagrams you could ask for https://www.mercurymarine.com/en/us/parts-and-accessories/parts-catalog Click "Launch Parts Catelog" and then enter serial number 0L347000 and click the engine on the right. The information about the cooling system is under "STANDARD COOLING SYSTEM". This is a great resource for anything Mercury Marine related. I'm sure you'll find it as useful as I have!

I was thinking about something today... What are the symptoms of a failing oil pump? Could my original problem (impeller failure) have clogged my oil cooler enough that my oil hot so hot that it "cooked" the pump? I know it seems like I'm just taking shots in the dark here, but there are a few reasons I think this could be the problem:
  • When I changed the oil cooler, both it and the lines had hardly any oil in them.
  • The water is leaving the engine much hotter than it seems like it should.
  • I haven't mentioned this, but I've noticed my oil pressure gauge has been showing higher than usual oil pressure, around 50 PSI as opposed to the usual 41-42.
  • From what I've read, lack of oil circulation can cause hot spots that lead to preignition even when the engine seems to be running cool.
Thoughts?
 

jmarines

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Have you tried a 140* thermostat? I had a similar issue.. it would idle at 170..180 degrees and almost reach 200 during a pull. After messing with it for weeks i finally bit the bullet and took in to the mechanic. After a new impeller and flushing out the cooling system with no change in temps he installed a 140 thermostat and it solved the issue. Something about having a restrictor gasket between the manifold and elbow was causing a air pocket in the thermostat housing causing alot of fluctuations.
 

tpenfield

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Here are the temperatures I was getting with the old thermostat that I still suspect to be bad (spec'ed at 160?, starts opening at 175?, full open at 192?) at idle:
Starboard engine (good)
Temp (F)
Port engine (bad/problems)
Temp (F)
Starboard-side exhaust elbow
108
Starboard-side exhaust elbow
94
Port-side exhaust elbow
91.5
Port-side exhaust elbow
83
Thermostat housing front
105
Thermostat housing front
135.5
Thermostat housing back
101.5
Thermostat housing back
88
Return hose*
140.5
Return hose*
124
Sensor (on housing)
160
Sensor (on housing)
180
* - Hose from circulating pump to thermostat housing

Using your temperature chart . . .

The areas in RED are where I think you have the issues. The thermostat should start to open at 160 F and be fully open at about 170 F. So, if that is still not happening, get a new one and test it on the stove in a pot of water / thermometer.

I also noted that the thermostat housing and the exhaust elbows on the 'hotter' engine are actually cooler that the 'good' engine. To me that says that you are not getting enough flow through the engine block, much of it is just going out the exhaust and not even through the engine.
 

tpenfield

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A couple of other ideas, if you have not already checked . . .

Check the large hose on the recirculating pump for clog or debris.

See if the recirculating pump has any restrictions in the area where it mates to the engine block.

Does you boat have the water heater tank, and is that run off of the port engine? It should only help, but just curious.
 

DracoDan

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Thanks tpenfield
  • The circulating pump is brand new (although I've already witnessed one failure in a brand new part so who knows). I guess there's also a chance that part of the gasket blocked one of the holes during install, but if it did I would expect to see a leak around that area (there isn't).
  • Large hose was checked when replacing the circulating pump and thermostat housing, the only thing I found was rusted iron impacted into the rubber on one end from the old thermostat housing (it had pretty extreme penetrative rusting on it).
  • Water heater runs off the starboard engine.
  • Regarding the thermostat that was opening too high, it's already been removed and is no longer part of the problem.
  • While cruising I see the temperatures fluctuate quite a bit (with no thermostat installed). It jumps around from about 90? to about 170?; I wouldn't expect it to suddenly get that hot with no thermostat!

I saw something last week that leads me to believe my revving problems are nothing more than a failing tachometer! This seemed like a crazy answer at first, but after reading more about how and why it could happen, a lot started to make sense. I replaced all of my gauges the first year I had the boat (the original gauges were in very poor condition and most didn't even work). Ever since doing that, the port side gauge would randomly start showing weird readings, even while cruising with no affect on performance/handling. When this would happen I could hit the front of the tach and it would randomly jump around but usually settle on the correct RPM. From what I've read in other posts, this is a tail-tale sign of a shorting tachometer. I think the problem is internal to the tach since hitting it causes wild fluctuations, so my next test (planned for tomorrow) will be to swap the starboard side tach to the port engine and see if that changes the revving problem.

Of course that wouldn't affect my temperature... I think I have a restriction somewhere INSIDE my engine block, and the absence of a thermostat allows enough pressure/volume to keep the engine cool in spite of this. I'm starting to think a high pressure block flush is in order. Is there a flow rate of water that should be able to pass through the block? Like something I could measure to see if there is in fact a restriction? I've seen on youtube where people did a flush using a drain hole under the exhaust on a similar engine, is that the way to do it? Should I just take it to a mechanic since I have so many questions (lol)?
 

tpenfield

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Looking back through this thread, I'm just wondering what the sequence of events were that led up to the overheating situation. Essentially, when was the engine running at proper temperature and were there any significant events or maintenance that happened leading up to the point where the engine started to over heat. did the overheat come on suddenly or was it gradual?

FWIW - I had those fresh water flush kits on my engine, just before the sea water pump, like yours. That was the first thing that I threw away and went with a sea water strainer and just straight hose to the pump.

Of course, now that I think of the flush kits . . . I am wondering if it is sucking air on the 'bad' engine?
 

DracoDan

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Okay, here's the story, sorry it's so long!

Late last year I had an impeller fail on me that caused the engine to overheat. I limped back to my dock on the starboard engine and replaced the impeller and removed what I thought was all of the old impeller pieces from the line. During the impeller replacement I realized the housing was also broken and would need to be replaced. I went out one time after that (last year) and didn't have any problems.

I went to de-winterize the engines at the beginning of the year (just running them really and making sure everything was okay) while we were cleaning the boat to get ready for the season. This is when it first overheated. My first thought was "that impeller probably failed due to the damaged housing" so I rebuilt the entire seawater pump using a new housing/plate/gaskets/impeller.

When this failed to solve the cooling issue I started looking at other potential causes. First up was the circulating water pump and thermostat housing since both had very deep rust, especially the circulating pump. I also decided to replace the thermostat and the plastic sleeve since I was doing the new thermostat housing anyway. The problem persisted after this and I went to the oil cooler as the next culprit. I found a very large amount of debris blocking it, including pieces of the impeller and the housing. There were also a lot of rocks stuck in the paths so I decided to just get a new oil cooler... I still had a problem but to a lesser degree (no pun intended). That's when I discovered I was sent a faulty thermostat. I went back to the original thermostat that opens at the correct temperature (tested with a pot, stove and thermometer). But even still the engine would get too hot (about 200? IIRC). I took out the thermostat completely and it stopped overheating, so we took the boat out (this was memorial day after all).

During our trip out it was fluctuating wildly between the lowest temperature on the gauge (so <90?) and about 180?. This is when I noticed something wasn't right with the tachometer. No matter how many times I hit it (my normal fix), it would not show over about 2200RPM. The weird thing was the engines seemed to be running perfectly smooth and acted like they were at the same speed (starboard was at my cruise RPM of 3200). I'm basing this on the fact that the boat was easy to control, unlike how things are when one engine is spinning much faster than the other. I was getting the occasional "pop" sound from the engine compartment, but it didn't seem to affect anything. I now think this was a backfire either from the bad tachometer or from the engine running too cool and affecting the ability to burn all the fuel. During the trip back (just a few miles) I noticed the port engine was now staying <90? but now wouldn't get above ~2400RPM (the tach now seemed to be reading correctly) without popping and dropping ~200RPM. The boat was also noticeably harder to control, as if one engine really was running slower.

I'm wondering if maybe running without a thermostat managed to flush something out of the block and the low temperatures were what caused the backfire and power drop. I think it's that or the tachometer, which like I said has acted erratically for a long time.
 

tpenfield

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Did you have the outdrives off over the winter?
Was there any marine growth noticed inside the pick-up holes on the lower unit, etc.?
Was the fresh water flushing system used as part of the winterization or de-winterization?

I am assuming that since the boat ran fine at the end of last season, the debris at the oil cooler, although a problem, was not the primary issue. Since you have changed just about everything at the thermostat housing, then that is probably not the primary issue.

The tachometer issue may be a distraction, yet have something to do with the engine not running quite right. Maybe you can disconnect the tach for the time being. I am not sure if the tach runs off of the ignition coil or if it gets its 'data' from the ECM. I have seen situations where a bad tachometer will kill the ignition coil and stop the engine or limit its rev's.

I think the engine will run OK if the ECT sees a colder temperature, might just be a bit rich due to a longer fuel injector pulse from the ECM.
 

tpenfield

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I did not make progress on the overheating issue on my boat until I was able to do a comparison test of the sea water flow between the engines. At idle speed (600 ish RPM's) the 'bad' engine had 20% less flow. At higher speeds that would equate to much greater than 20% less flow relative to the speed of the engine.

Like you, I spent all kinds of time thinking it was the engine itself, only to find it was the insufficient flow of cooling water.

Can you set up a test of each engine's raw water flow? I did my engines by taking the input hose off at the thermostat housing , hooking up a bit of extra hose and run the engine at idle and timed how long it took to fill a 5 gallon pail (which was about 30 seconds). The 'bad' engine took 20% longer to fill the bucket . . .

I figured the engines would survive for 30 seconds (or so) without a supply of cooling water. They did.
 
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DracoDan

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i'
I did not make progress on the overheating issue on my boat until I was able to do a comparison test of the sea water flow between the engines. At idle speed (600 ish RPM's) the 'bad' engine had 20% less flow. At higher speeds that would equate to much greater than 20% less flow relative to the speed of the engine.

Like you, I spent all kinds of time thinking it was the engine itself, only to find it was the insufficient flow of cooling water.

Can you set up a test of each engine's raw water flow? I did my engines by taking the input hose off at the thermostat housing , hooking up a bit of extra hose and run the engine at idle and timed how long it took to fill a 5 gallon pail (which was about 30 seconds). The 'bad' engine took 20% longer to fill the bucket . . .

I figured the engines would survive for 30 seconds (or so) without a supply of cooling water. They did.

I thought I had ruled that out as an issue by running the engines with the coolant tank hooked to the engine flush valve, therefore bypassing the intake on the out drive completely. I had no changes when I did this.

I did not have the outdrives off for the winter, my boat stays in the water. I did do a flush with banfrost2000 just as I do every year, plus it didn't even get very cold this winter.

I'm 99% sure the tach is run off the ignition coil, I had to set the number of cylinders on the tach when I installed it (telling me it reads the pulses and divides by 8 to determine RPM).
 

tpenfield

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I thought I had ruled that out as an issue by running the engines with the coolant tank hooked to the engine flush valve, therefore bypassing the intake on the out drive completely. I had no changes when I did this.

My only comment on this is if you used a large enough hose from the 'coolant tank' . What is the coolant tank that you are referring to ?

just wondering if you can get some sort of measurement of the flow through the engines and compare them . . .
 

DracoDan

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the coolant tank is the one that's used to winterize the engine, I hook a tank full of antifreeze up to the line and open a valve to flood the engine with antifreeze. This is drawn in just before the seawater pump.

I ruled the tach out as a potential problem today, but I'm no closer to an answer. I recorded a video to demonstrate what it's doing at this point, tell me what you think.

https://www.youtube.com/watch?v=fXRRXR1Ka6g
 

tpenfield

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Probably will have to read the codes from the ECM . . . seems like ignition is cutting out.
 
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