Mercruiser 5.7 MCM carb low idling

[mnjjansen]

Hi guys,

Always awesome here, the knowhow everyone shares. I have a minor question. My 2007 merc 5.7 TKS carb 2 barrel engine runs great but (tends to) stalls on me when hot and gets back to idle speed. It starts "stamping (galoping?)", oil pressure up and down, even low voltage 11V. RPMs 650 down to 500. This only happens after LONGER takeouts. However, when the engine is just on temp 175F and I shutdown and restart the rpm sticks to 500. If I then throttle up a bit and back to neutral it stays at 650 which is stock. Oil pressure 40+ at all times then.

As it comes back to 650 I don't consider to increase the carb idle screw.
Oil pressurre between 40 - 50 psi in neutral.
When engaged idle 40+.

Carb acceleration pump reacts extremely well. In general the engine reacts and sounds very properly.

What might cause this going up and down of the rpm or the low rpms ? My goal is to ensure the rpms stay above 650 in all conditions.

Carb idle circuit affected (dirt) ?
Fuel lock ?
Throttle installation (though very firm, no movement at all).

Any suggestions where to look at much welcome. It isn't a biggie technically but in terms of driving safety is when docking for instance and the engine stalling ...

Regards, have a great WE.
Martijn

 

[jimmbo]

Unless it has a Closed Cooling System, 175 is too Hot.

What have you Fiddled with, so far?

 

[mnjjansen]

Unless it has a Closed Cooling System, 175 is too Hot.

What have you Fiddled with, so far?

Hi, thanks for your answer. No alterations have been made by me. I have this boat and this engine for a couple of months now. The engine has since my ownership always hit 175F and no warning buzz. I checked your statement and indeed seems hot (open cooling system). Manual states 160F.

Since the last two take outs my alarm goes off, irregularly. Temp is steady at 175F, oil 45 psi. When I rev up or down the alarm immediately goes off.

So what has changed in the meantime, as I haven't changed anything.

Perhaps it correlates to something else. My temp SWITCH for keeping the solenoid TKS enrichment valve is defect I believe. This is a separate electrical circuit from the alarm circuit. The alarm circuit uses the temperature SENSOR as it grounds at higher temperatures. The temp switch for keeping the solenoid activated grounds above 110, totally different purpose. I mention it here to be complete in my observations.

Back to the higher temperature of 175F. So the cooling water is hot, my manifolds / risers so far holding up, why would that give the stalling (rpms up and down) behavior of the engine ? What would be a proper place to investigate first and/or disclose from the failure mode experienced ?

Nevertheless I will check my thermstat installed and see if I need more block/manifold passage / balance. So surely thankfull for pointing out to me (had severe cooling water issues in the past ... other engine, I remember the issues more then well).

Many, many, many thanks Jimmbo !

Regards,
Martijn

 

[ROY WILLIAMS]

YOU was 2-3yrs of the outdrive water impeller . changed it ...
I was the temp gun ...
the 140-160F engine boat temps ...
the land of that boat engine running and the prop is a lot of that water or a bit of the water lower issue ..

 

[alldodge]

YOU was 2-3yrs of the outdrive water impeller . changed it ...
I was the temp gun ...
the 140-160F engine boat temps ...
the land of that boat engine running and the prop is a lot of that water or a bit of the water lower issue ..

Hey Roy your real hard to understand, maybe take some more time typing

I'll try to translate what I think Roy means

Your impeller needs to be change every 2 to 3 years, if it has not been changed do so now

Did you use a IR temp gun to measure thermostat housing to verify 175* on the gauge?

A open cooled motor should be 140 to 160*

the land of that boat engine running and the prop is a lot of that water or a bit of the water lower issue ..

No clue what this is

 

[Scott06]

Hi, thanks for your answer. No alterations have been made by me. I have this boat and this engine for a couple of months now. The engine has since my ownership always hit 175F and no warning buzz. I checked your statement and indeed seems hot (open cooling system). Manual states 160F.

Since the last two take outs my alarm goes off, irregularly. Temp is steady at 175F, oil 45 psi. When I rev up or down the alarm immediately goes off.

So what has changed in the meantime, as I haven't changed anything.

Perhaps it correlates to something else. My temp SWITCH for keeping the solenoid TKS enrichment valve is defect I believe. This is a separate electrical circuit from the alarm circuit. The alarm circuit uses the temperature SENSOR as it grounds at higher temperatures. The temp switch for keeping the solenoid activated grounds above 110, totally different purpose. I mention it here to be complete in my observations.

Back to the higher temperature of 175F. So the cooling water is hot, my manifolds / risers so far holding up, why would that give the stalling (rpms up and down) behavior of the engine ? What would be a proper place to investigate first and/or disclose from the failure mode experienced ?

Nevertheless I will check my thermstat installed and see if I need more block/manifold passage / balance. So surely thankfull for pointing out to me (had severe cooling water issues in the past ... other engine, I remember the issues more then well).

Many, many, many thanks Jimmbo !

Regards,
Martijn

My raw water cooled engine with a 160 stat runs 170-175 also I don't think this is a contributing factor.

Does the TKS module shut off enrichment if this is enriching the mixture at warm idle this could cause it to stall.

Is the throttle pushed fully back against the idle speed screw thinking cable may be holding it back sometimes. Have you tried to turn idle speed screw up a hair ? Have you tried adjusting the mixture screw? if you turn it in it should stumble if not you are likely getting fuel from TKS module or elsewhere.

When the voltage is low is that when rpms are really low ?

the alarm can be over temp , low oil pressure, or low drive lube level you can disconnect these to see which one is tripping it.

 

[Lou C]

The stalling/surging issue and the higher than typical temps are likely to be 2 separate issues.
1) the boat is 16 years old. The age affects everything that affects reliability, the ignition system, the fuel system and the exhaust system.
Even in freshwater, you can have clogged exhaust elbows and manifolds which will cause elevated running temps, it may be time to inspect (at least) the exhaust system, to determine of you have rust clogging the water exit ports inside.
2) the cooling system of a boat needs regular maintenance, the impeller is definitely a wear item, they usually get replaced every 3 seasons or so, if you run in shallow sandy water maybe even sooner, when they wear you get the same symptom, elevated temps. Besides impellers and the exhaust, the thermostat can get sticky after a few seasons, they need replacement as well.
So unlike an automotive application, a boat cooling system does not run for 10 years with no maintenance.
3) the stalling/surging can be a dirty carb, a vacuum leak, mis-adjusted float, defective carb needle, etc. Boats often sit unused for a long time each year, that is not good for the fuel system, I seem to have to clean and rebuild the 4 bbl Quadrajet on my boat every 6 seasons or so.

So it sounds like some normal maintenance needs doing, nothing out of the ordinary.

 

[mnjjansen]

The stalling/surging issue and the higher than typical temps are likely to be 2 separate issues.
1) the boat is 16 years old. The age affects everything that affects reliability, the ignition system, the fuel system and the exhaust system.
Even in freshwater, you can have clogged exhaust elbows and manifolds which will cause elevated running temps, it may be time to inspect (at least) the exhaust system, to determine of you have rust clogging the water exit ports inside.
2) the cooling system of a boat needs regular maintenance, the impeller is definitely a wear item, they usually get replaced every 3 seasons or so, if you run in shallow sandy water maybe even sooner, when they wear you get the same symptom, elevated temps. Besides impellers and the exhaust, the thermostat can get sticky after a few seasons, they need replacement as well.
So unlike an automotive application, a boat cooling system does not run for 10 years with no maintenance.
3) the stalling/surging can be a dirty carb, a vacuum leak, mis-adjusted float, defective carb needle, etc. Boats often sit unused for a long time each year, that is not good for the fuel system, I seem to have to clean and rebuild the 4 bbl Quadrajet on my boat every 6 seasons or so.

So it sounds like some normal maintenance needs doing, nothing out of the ordinary.

Hi Lou, thanks for your extensive reply. Totally correct remarks. Fortunately the engine has been overhauled by a professional mechanic just before I bought it. Exhaust manifolds, risers and impellor have been replaced by new OEM items. As you stated I am not necessarily concerned about the 175F but it might trip the alarm and will check when my sensor grounds. Nevertheless I want this to be resolved but aside from the stalling issue that concerns me more now.

I do not believe the carb has been overhauled. The TKS module is not faulty, I tested. Secondly, enrichment stops as it is supposed to be because of the oil pressure switch. The temp switch is an issue but not in the described situation as the engine is simply running when the stalling occurs obviously. The carb might contain polution, for sure I will overhaul it and clean it out ultrasonically.

I will also check seperately all trip sensors / switches, wiring and connections to be sure what is causing the trips and go from there.

Many thanks for your advice, much appreciated.

Regards,
Martijn

 

[Lou C]

The main thing with a carb overhaul is the cleaning all the jets, passages & air bleeds must be open. The float had to be adjusted to manufacturers specs and gaskets all replaced.

 

[mnjjansen]

My raw water cooled engine with a 160 stat runs 170-175 also I don't think this is a contributing factor.

Does the TKS module shut off enrichment if this is enriching the mixture at warm idle this could cause it to stall.

Is the throttle pushed fully back against the idle speed screw thinking cable may be holding it back sometimes. Have you tried to turn idle speed screw up a hair ? Have you tried adjusting the mixture screw? if you turn it in it should stumble if not you are likely getting fuel from TKS module or elsewhere.

When the voltage is low is that when rpms are really low ?

the alarm can be over temp , low oil pressure, or low drive lube level you can disconnect these to see which one is tripping it.

Hi Scott,

Thanks for your extensive reply. The TKS solenoid switches off as the oil pressure switch is operating well. This is easy to check based on carb sound and solenoid temperature itself. Thus, as long the engine is running the solenoid receives current, closes within 8-10 minutes and keeps closed. The problem is here, when I shut down the engine, the solenoid is immediately starting to open again since the temp switch is not proving current while the engine is hot. That causes a too rich mixture when restarting hot and, when I thinking about it right here and now, affects my spark plugs I guess. That can also cause issues at lower rpms perhaps.

The voltage drops to 11V at very low rpms, 500 or so. As stated, when I then throttle up one or twice a bit, back to 650rpm and Voltage between 12 - 13V normal. When I shut down completely and restart a few secs after that, the rpm starts at 500 and can repeat the whole thing. Engine is hot then (else the TKS enrichment bumps up the rpm). When docking or doing sluice entries I cannot fiddle around with rpms, so it is really a safety concern.

Indeed I can slightly adjust the idle screw, for sure that will help when the engine is hot with the correct low rpms and avoid stalling. This is not ideal but for safety now a proper workaround. I believe the carb must be overhauled at this point eventually and the TKS should run properly, I get a new temp switch today. That should help to keep the spark plugs clean at least.

I will also start disconnecting alarm circuit sensors, check wiring etc. This is seperate from the TKS circuit anyhow, see where the alarm is coming from. Logical advice here as well.

I am on the boat later today to follow up on all the advice, got my drawings and all, so I report back.

Many thanks,
Regards,
Martijn

 

[alldodge]

The problem is here, when I shut down the engine, the solenoid is immediately starting to open again since the temp switch is not proving current while the engine is hot

This should not happen. The temp switch should stay closed and pass current when motor is OFF. The current only stops flowing when the motor cools down.