Stumper - 350 MPI Running Cool at idle

[ELUSIVE]

Howdy all -
have done tons of searching but not finding what I'm looking for and this problem is stumping my dealer too.

2000 Tige 20i with the Mercruiser 350 MAG MPI.

I'll be cruising along at 22-34 mph and my temp will be fine. Slightly below center (170 ish) on the gauge. As soon as I pull the throttle back into idle, the temp gauge drops all the way down within seconds. When the temp is down the systems throws more fuel in and it runs rough and I've got no power and it often stalls.

I have learned that if I reduce my speed slowly that my temp stays up. I have also found that if my temp does drop then I can rev the engine up to about 2500 rpm (in neutral) and leave it there for 20-30 seconds and then slowly reduce it back to idle and the temp will stay up.

My dealer has replaced the thermostat (twice), done tune ups, cap/rotor, spark plugs and nothing has seemed to fix this issue. They are stumped because usually water flow issues result in high temps, not low.

Any ideas?
Thanks.

[boatwakes]

Do you have a heater or hot water system plumbed into the engine? Improper plumbing can do it.

[mjsti]

Did they check your IAC?

[ELUSIVE]

I do not have a heater, but I do have a hot water system.
Any suggestions on what could be going on? I'll try playing with the hot cold settings.

I've rarely used this hot water system in the 12 years i've owned the boat.

I believe they did check the IAC.

Thanks so much for the responses!

[NICKYPOO]

No water at the temp sensor at full stop? Maybe something clogging up that particular water jacket?

[boatwakes]

Remove the hoses that feed the hot water system and plug those ports (there will be two hoses). One hose is usually in the intake manifold and the other will either be on the circulating water pump or plumbed into the raw water inlet. If, with the hot water system bypassed, the condition still occurs, check the thermostat housing for a crack or obvious hole where the intake water could be bypassing the thermostat.

As for your ECT overfueling condition, that particular engine has an air temp sensor that likes to foul and cause problems too. It's located directly behind the air cleaner assembly on the intake manifold, slightly starboard side of engine, vertical orientation with a two wire connector. Pull it out with a 7/8 deep socket, clean it with brakleen or carb cleaner and reinstall.

[ELUSIVE]

I plugged the hot and cold water hoses bypassing the shower, and also cleaned the air temp sensor that you mentioned with carb cleaner and i'm still experiencing the same problem.

At this point I'm thinking that there must be something wrong with the thermostat housing. I'll take that off tomorrow and do some experimenting.

Any other possibilities?
Where is the IAC located on this motor?

Thanks again for all of the suggestions.

[boatwakes]

The IAC is at the back of the throttle body and has a waterproof connector on it, horizontal orientation.

[ELUSIVE]

Everything seems to be in order...but the problem persists. Any other ideas?

At first I thought that perhaps the water temp was really going down so quickly (problem with thermostat housing). I'm beginning to think that the there may be something wrong with the temperature sensor and the water temp is in fact fine.

As soon as the temp gauge drops you can clearly begin to tell that the engine isn't running well because it's throwing more fuel in?

New temp sensor?
Faulty wiring?
Needs a computer upgrade?

Thanks.

[bigdaddy_houseboater]

So, have you taken the thermostat out and replaced it? If its stuck in the open position, it will run very cold at idle. If you dont trust the sending unit, take a radar temp gun with you and check it at idle and at cruising speed.

[ELUSIVE]

I've replaced the thermostat twice and have seen exactly the same problem.

If I bring the throttle back slowly from cruising speed to idle I can keep the temperature gauge up. If it bring it back rapidly the temperature drops rapidly.

[boatwakes]

There should be two temp sensors in the thermostat housing, one for gauge temp and one for engine coolant temp (computer input), make sure that you physically see two and they have two different style connectors. The only way to know for sure the water temp is to hook up a mechanical gauge in place of the electric gauge sending unit and watch it while underway, or use the thermo gun technique. Either way, if the water is truly running cold, there is a problem in the thermostat or t-stat housing. Water comes into the t-stat housing and fills both the block (through the circulating pump) and the exhaust system with pressure from the raw water pump until the thermostat opens and exchanges heated water out while the circulating pump moves new, cooler water in. If the t-stat sticks open because of a sudden pressure differential (reduced raw water pressure/circulating pump pressure because of dropped rpm) or piece of debris gets lodged in it, the same scenario will happen. I'm seriously debating sending you a known good unit for comparison because this is really odd. Keep us posted.

[boatwakes]

Did you plug the heated water pickup on the engine, not just the hose, but where it's plumbed into the motor?

[lohe]

Also I had some odd temp reading so I just replaced both sensors in the t-stat to find out it was a gauge issue with mine so I have two sensors you can test with if you want. One is for the gauge and the other is for the ECM. if you want to just pay the shipping then ship them back when you are done you can pm me your info.

[ELUSIVE]

Yes, there are two different style connectors on the thermostat housing.
The one on the starboard side of the motor for the gauge (if I disconnect it, the gauge drops instantly) and the one on the port side of the motor for the engine coolant temp. They have different connectors.

I don't have a mechanical gauge or a thermo gun to test the water temps, so I really have no way of determining if the water temp is actually correct.

For the heated water pickup for the shower, I cut the red tube about 2 inches outside of the motor and plugged it. I did this opposed to plugging the 3-4 foot tube at the end.

At this point I'm debating buying a new thermostat housing. It's a gamble though because it's about a $300 part and it would suck to not have it fix the problem.

Should I buy new sensors for the thermostat housing first?