Mercruiser 7.4L MPI slowing down

[Scottd21]

Originally Posted by wingless

Is the hit-or-miss mechanic doing better than you can do? Is he charging hourly to run your boat for this effort?

The reported effort / results are at the DIY level, unless I'm missing some "magic".

The mechanic "progress" would make me INSANE (more insane than usual).

The boat needs to be operated under the problem conditions, then those conditions changed to discover when the problem fails. Doing things like changing throttle position is just one example.

Opening the hatch once the problem occurs, to cool the engine so the problem clears, is not very useful. Soaking the engine in the problem condition, so it doesn't clear once the hatch opens would permit using freeze spray on a component to better understand the problem. The systems that could be targeted are: the ECM; the distributor base and the exhaust temperature sensor.

Has it been confirmed that the operational exhaust may be continuously hand-held? Has the raw water impeller been confirmed to be good? Have the raw water passages been back flushed? Has the raw water intake screen / strainer been cleaned?

Did the mechanic operate w/ the DDT connected, or connect afterwards to determine if fault codes existed? If so, what faults were detected. Note the DDT tool has been discontinued although used examples pop up on eBay.

The correct FSM is invaluable when analyzing the problem. As was stated, there are diagnostic tables to pinpoint the possible causes.

All this beats the heck out of the current "shotgun" process.

Your boat should be capable of proper operation w/o extra cooling added. IMO adding the proposed extra cooling modification is just another shotgun repair attempt. I would guess the implementation by this "mechanic" would result in totally unacceptable audible noise in the cockpit. First locate the problem, then determine the solution.

Note, my southern Florida boat now has a custom thermostatically-controlled heat exhaust system that I designed, fabricated and installed to resolve a vapor lock problem that occurred after stopping for an hour, then being unable to restart until the fuel rail cooled. Now my heat exhaust blowers kick on and off, maintaining my engine room temperature, I now never get vapor lock and can always restart my engines. My implementation adds ZERO extra audible noise in the cockpit. This heat soak vapor lock was never a problem when operating in New England.

I agree this hit or miss method isn’t helping me it’s only helping him rack up charges by the hour. I wouldn’t have lasted this long with this mechanic only that if the issue is the ECM to find the right part, replace it and apparently it needs to be programmed to the engines fuel curve (I don’t even know what that means) so since it looked like we were dealing with computers, sensors and mechanical parts but if it’s just something getting warm how about putting a wireless thermometer under the hatch to see how those conditions are changing when it does occur.

In regards to the hatch that’s a scary thought about it making the cockpit noisy and I’m glad you mentioned that. Also I would think that the boat wouldn’t have been designed that way if it were a hatch venting issue and maybe I should contact Sea Ray for some advice on that. That’s interesting about your vapor lock solution that does not introduce noise.

As far as doing things and and making changes to see where problem occurs I am simply running the boat at its optimal 3100 rpm for a half hour to an hour and then it slows down to 2500 on its own so there is no change. The only possible differences are the gas tank level and outside temperature. I still think opening the hatch is an important piece of information because it’s the first thing that has actually fixed it (temporarily) whereas before it was all guessing. That to me pointed to a ventilation or heating issue but because it didn’t clear it by opening the hatch one time when it was warm outside - that threw out the ventilation theory in my opinion.

Also I don’t understand how you can get to the ECM, temperature sensor, and distributor and test them without opening the hatch? Opening the hatch would create a false positive making it look like cooling the temperature sensor fixed it when in fact it was opening the hatch that cooled off the components.

I should be getting the boat back for good now while I mentioned that I did not notice any hoses in the front of the engine heating up I was driving and switching it off so couldn’t hold it the whole time. I don’t know about the raw intake I had boat winterized and they said they checked all major components and I had engine compression test done.

The mechanic originally tested with the DDT and it threw no codes. This is what I don’t get. If some part is heating up and limiting the engine wouldn’t the ECM record the rpm drop? Wouldn’t it be helpful to know if the ECM thinks that it’s going 3100 rpm when it’s only really going 2500? I thought the purpose of the ECM is to help show where issue is so if there’s no issue with ECM and engine power is being reduced shouldn’t it definitely show that even if it doesn’t say why it’s reduced.

I asked about running without engine cover and my mechanic said my engine doesn’t have a real cover (it’s just plastic). I do have a fireport access to the engine and could try running it with that cover removed. If opening the hatch is fixing issue I would think putting in wireless thermometer to monitor the conditions when it occurs would help.

I am not looking for a bandaid (fixing what is heating up is my main goal) rather than cooling off whatever shouldn’t be heating up in the first place - and eventually that part is going to fail anyway if that’s not figured out. Are there lots of possibilities as to what could be heating up under the hatch to cause this issue and if so since the ECM doesn’t tell us anything how would you know unless you felt the part heating up, cooled it off and it fixed it, or replaced it?

Thank you

Scott

[Scottd21]

Also I found West Marine has a small 12v engine blower - maybe that would be better than installing a vent:

https://www.westmarine.com/buy/west-marine--engine-blower-4--P013855028?pCode=13855036&mrkgcl=481&mrkgadid=3252 241798&cm_mmc=PS-_-Google-_-GSC%3ENonB%3EProduct%2520Type-_-13855036&product_id=13855036&adpos=1o1&creative=10 8421551724&device=m&matchtype=&network=g&gclid=Cj0 KCQjwkK_qBRD8ARIsAOteukC5P0LYqNxAp4lC3VtEYY57uFaTw UNnwArt7XSmcVbNLnCkz7ZNoo4aAj3FEALw_wcB

[wingless]

Originally Posted by wingless

Has it been confirmed that the operational exhaust may be continuously hand-held?

What is the answer to this inquiry?

[Scottd21]

No I mentioned that I did not notice any hoses in the front of the engine heating up I was driving and switching it off so couldn’t hold it the whole time. I don’t know about the raw intake I had boat winterized and they said they checked all major components and I had engine compression test done.

I should be getting boat back for good now though so will check that

[wingless]

Don't need to hold the whole time.

Again, bring the engine up to operating temperature, with the engine running at idle, with the boat stationary, is it possible to keep a hand on the cast iron exhaust manifold, riser and elbow, or are those parts too hot to hold?

[wingless]

It looks like the boat already includes a pair of high volume exhaust blowers, one sucking from down low, the other sucking from up high.

If correct, if the exhaust passes the hand-hold test, then the problem might be resolved by continuous usage of the blowers when the exterior ambient temperature is hot.

It is not possible to overstate the importance of passing the hand-hold test. That is a fast / easy check of raw water flow and exhaust corrosion.

It does not eliminate the requirement for annual inspection / periodic replacement of the internal exhaust passages or periodic impeller inspection / replacement.

[Scottd21]

Originally Posted by wingless

Don't need to hold the whole time.

Again, bring the engine up to operating temperature, with the engine running at idle, with the boat stationary, is it possible to keep a hand on the cast iron exhaust manifold, riser and elbow, or are those parts too hot to hold?

I am going to test that again Sunday/Monday. Are those three parts (the manifold, riser and elbow) the hoses in the front of engine that you see in the picture you posted? Want to make sure Im not missing something there

[Scottd21]

[QUOTE=wingless;2949936]It looks like the boat already includes a pair of high volume exhaust blowers, one sucking from down low, the other sucking from up high.

If correct, if the exhaust passes the hand-hold test, then the problem might be resolved by continuous usage of the blowers when the exterior ambient temperature is hot.

It is not possible to overstate the importance of passing the hand-hold test. That is a fast / easy check of raw water flow and exhaust corrosion.

It does not eliminate the requirement for annual inspection / periodic replacement of the internal exhaust passages or periodic impeller inspection / replacement.



[/QUOTE

wouldn’t an exhaust blower work without the bilge blower on? Are you saying this could possibly be resolved by using the bilge blower?

Again I will try handheld test again.

[wingless]

These cast iron parts pass the hot exhaust gas through the center and pass cooling raw water through the cooling water jacket.

There should be sufficient water flow and there should not be excessive internal water jacket corrosion so those parts are cool enough to continuously hold the exterior while the engine is running at idle while at operating temperature.

[Scottd21]

Also I just found a good article about testing for ventilation and adding exhaust blower:

https://www.google.com/amp/s/www.pow...ts-engine-room

[wingless]

Originally Posted by Scottd21

wouldn’t an exhaust blower work without the bilge blower on? Are you saying this could possibly be resolved by using the bilge blower?

Again I will try handheld test again.

Yes, if the exhaust doesn't have excessive internal corrosion and if the raw water impeller is pushing sufficient water, then that existing blower system, as-shown in the image, has one hose sucking down low (as-required by ABYC to remove potential heavier-than-air gasoline fumes from the engine compartment) plus a second hose up high, that would remove heat from the engine compartment.

Sure, it is possible to add a third blower, like the existing up high blower, to make it possible to run only one instead of running two, but why bother?

[Scottd21]

Okay I thought the bilge blower was a separate pump used just to pump fumes out of the bilge area and thought if this is the engine exhaust that it runs all of the time to keep engine from overheating.

Also mine doesn’t appear to have a higher and lower one (both look lower) but there are 2 pairs.

That would be great if just running the bilge blower fixed this!

[wingless]

The ABYC standards require a blower to suck from low to extract heavier-than-air gasoline fumes (that shouldn't exist) from the bottom of the engine compartment.

Mine has one per engine, sucking from under each engine.

Run this for 10 minutes prior to starting, shut off while fueling, and run 10 minutes prior to starting after fueling.

My custom-designed heat exhaust system automatically kicks on and off to maintain the engine room temperature at the setpoint under thermostatic control. There is not an detriment for continuous operation.

[Scottd21]

Originally Posted by wingless

Yes, if the exhaust doesn't have excessive internal corrosion and if the raw water impeller is pushing sufficient water, then that existing blower system, as-shown in the image, has one hose sucking down low (as-required by ABYC to remove potential heavier-than-air gasoline fumes from the engine compartment) plus a second hose up high, that would remove heat from the engine compartment.

Sure, it is possible to add a third blower, like the existing up high blower, to make it possible to run only one instead of running two, but why bother?

Maybe that’s the issue - they mounted my exhaust hoses (all four hoses) to the bottom so it’s not cooling it as well as if it were on the top.

[wingless]

From the image, it looks like the boat has two blowers, mounted up high, out of the image frame, with the two intake hoses on the port side and the two exhaust hoses poking through the transom on the starboard side.