6.2L Mercruiser Cold Engine

[hakukamana]

I have a MX 6.2L MPI, it appears that I am running cold based upon my scan tool. My IAC follower is up on the counts almost double 80+ average @ idle. This means I am dumping a lot of fuel and the plugs verify the diagnosis. The problem appears at the water temp, raw water pickup, the engine will not warm up, I can run the engine for 10 minutes and it will still register below 120* after about 10-15 minutes the thermostat will pop and the gauge goes up to 160 in about 5 seconds, Then it closes and goes back down below 120. I have replaced the thermostat( twice) , I have pulled the water pump, it appears like there is no circulation in the block to heat the water? I think the thermostat opens periodically due to metal heat expansion from the heads or the block. No over heat problems, underheat problems????? Even the Mercruiser service center is at a loss. But the fuel consumption, is a disaster.
any thoughts would be appreciated.
Bill

 

[HT32BSX115]

Re: 6.2L Mercruiser Cold Engine

Howdy,

You might provide a little more info (serial number eng/drive).... Is the engine raw water cooled or closed cooled?

Inboard, Bravo, jet, etc? The EFI system is causing a cold rich condition or is it MUCH richer than that?

It sounds like the raw water plumbing might be connected very wrong and/or the t-stat & housing might be all wrong for the set up...

 

[JustJason]

Re: 6.2L Mercruiser Cold Engine

have you verified your temps yet with a pyrometer?

 

[rodbolt]

Re: 6.2L Mercruiser Cold Engine

while a pyrometer is nice a simple hand test will tell you.
at 160*F you can comfortably hold your hand on the intake water passage at the T-stat housing for about 5 seconds.
if you can then look for any water leakage past the T-stat, if its actually staying at 160 or so look for a faulty sensor.
they do fail occasionally and depending on how its rigged the engine temp guage on the dash may not be connected to the CTS on the block or T-stat housing.
the scan tool is going to use the CTS not the engine temp sender.
and yes with a faulty sensor you can dump amazing amounts of fuel while damaging your cylinder walls and diluting the engine oil with gas.

 

[newport dave]

Re: 6.2L Mercruiser Cold Engine

I agree with jason and rodbolt, the first step would be to verify that what your seeing on the gauge is actually happening. If it is, then there really is nothing other than an incorrect or incorrectly installed thermostat, or possibly debris in the thermo housing causing the thermostat to hangup, that could cause this problem.

As far as the IAC Duty Cycle, a cold engine (or a perceived cold engine) will cause a high Desired Idle Speed and therefore a high IAC position, but this can also be caused by a restricted IAC muffler.

 

[hakukamana]

Re: 6.2L Mercruiser Cold Engine

Raw water cooled, Bravo III, 0M036015, raw water routing is correct, thermostat & housing are correct.

 

[hakukamana]

Re: 6.2L Mercruiser Cold Engine

Not with a pyrometer but I have used the Rinda scan tool to read the water temps through the sender, via the ECM, I am reading initially low 80's at start up, when the thermostat pops, about 120.

 

[rodbolt]

Re: 6.2L Mercruiser Cold Engine

then we are back to debris in the T-stat or a defective sensor or wiring.
something is causing the scan tool to see a fluctuating temp that stays mostly to low.
when the engine first starts it should rev to about 1100 RPM then settle down within a few seconds to about 600.
however at 600 and the ECU thinks its cold it will add fuel and air and you may see 700 ROM.
as long as the ECU "thinks" the engine is cold it will act on the signal.
with no overheat problem I dont suspect water flow.
most these ECU's are rather stupid and if garbage information goes in guess what comes out?
so now we have to physically verify the engine temp and or carefully test the sensor resistance at the various operating temps,this can be done with a pan of water,a stove top and a candy thermometer and a digital multi meter coupled with the factory service manual.
like I say 160* is warm enough not to burn instantly but it will make you remove your fingers in about 5 seconds.

 

[hakukamana]

Re: 6.2L Mercruiser Cold Engine

Restricted IAC muffler, ??? The IAC is located on the throttle body and controls the air flow to the throttle body and the basic air fuel ratio , the pintel extends and contracts based upon information from the ECM, The ECM would be fooled if the ECT sensor sent cold engine start data, to the IAC follower, thinking it should dump more fuel into the system based upon start up. The thermostat is brand new and it is installed correctly. Air lock in the water system?, debris from saltwater usage???

 

[hakukamana]

Re: 6.2L Mercruiser Cold Engine

Initial engine starts at 600-650, living in Hawaii we really don't have cold starts, the engine RPM is right on the money, actually all sensor outputs are within spec with the exception of ECT, and IAC. I will move to the candy thermo, and verify sensor voltage. As I remember the notes in the manual reference 1.75 or something as voltage but there is a note indicating that it will vary with tempreture.

 

[JustJason]

Re: 6.2L Mercruiser Cold Engine

Holy cow dave agrees with me

hehe...
Hey hakuka.... is this your boat or are you a mechanic for a shop that can't figure out a customers engine??????

If the scan tool reads 120.... but a pyro says 90 (or anything drastically different than the tool reads as pointed right next to the CTS).... then you know you have a defective sensor or defective wiring between the sensor and the ECM. Unplug the sensor and meaure it's resistance. I don't have it handy but there's a spec for it. If im a little more lazy on this afternoon i'll look for it.

 

[newport dave]

Re: 6.2L Mercruiser Cold Engine

Restricted IAC muffler, ??? The IAC is located on the throttle body and controls the air flow to the throttle body and the basic air fuel ratio , the pintel extends and contracts based upon information from the ECM, The ECM would be fooled if the ECT sensor sent cold engine start data, to the IAC follower, thinking it should dump more fuel into the system based upon start up. The thermostat is brand new and it is installed correctly. Air lock in the water system?, debris from saltwater usage???

Thanks for the info, but I am fully aware of what an IAC valve does, how it works, ect. Your issue may be due to low engine coolant temp, but I just wanted to make you aware that high IAC Duty Cycle and blackened plugs can be a symptom of a very common problem with the ECM555 engines, and that is a restricted IAC muffler. In the throat of the throttle body where the IAC air passage begins there is a small block of foam intended to muffle the "hissing" noise of the IAC intake. This can get clogged and cause some of the symptoms you are having.

That said, if your engine temp is as unstable as you say, there really is no other possible cause other than a thermostat malfunction of some type.

 

[hakukamana]

Re: 6.2L Mercruiser Cold Engine

Mefi 3 Ecm

 

[hakukamana]

Re: 6.2L Mercruiser Cold Engine

Checked the ECT resistance, with the old candy thermo, 450 ohms @ 160* F

What about a blocked bypass from the block to the water pump? You need a pressure differential to create flow.

 

[hakukamana]

Re: 6.2L Mercruiser Cold Engine

Not a mechanic, but at my age and the number of boats over the years, I have seen it all, except this one. We are very limited when it comes to Mercruiser service on the island. "1" and he just got certified last year. So for a long time we have been required to get the manuals and the specialized tools and fend for ourselves. The problem is location, if I need to go to Oahu for a different dealer it would take 3 days by boat or longer based upon the water conditions. Range would also be a consideration, and access to fuel.

 

[Don S]

Re: 6.2L Mercruiser Cold Engine

The scan tool is still reading off the boats systems. That is why you need to verify if the engine is actually doing what it is or not. Can you actually feel the engine get 40? warmer, and then cooling back down?

Another question, do you have any accessories added to the cooling system? Such as a hot water heater, or a cabin heater (in Hawaii ??)?

 

[hakukamana]

Re: 6.2L Mercruiser Cold Engine

The engine is getting hot and there is hot water under the thermostat, but the thermostat housing never gets warm or hot to the touch. I installed today a fitting in the intake manifold next to the thermostat housing, ran a hose to the water pump suction side. The hose is very hot, the suction side of the water pump is not hot. I know there is hot water under the thermostat.

 

[Don S]

Re: 6.2L Mercruiser Cold Engine

If the thermostat is installed with the spring toward the hot water, then replace the thermostat. It's bad.