5.7 merc overheat alarm once then runs ok-gctid821519

[Big_Ben]

Ok so I have a strange one. First off I have a 2004 5.7 merc, carb, full closed cooling with San Juan kit.

And I'll get this out of the way - its a long story (over 9 years now so not new) the gauge on the dash reads 20-25 F hotter than the engine. Gauge always reads 175F and I run a 140F t stat so the engines true running temp is 155 F or so but dash says 175 F.

So I has always run perfect at 175f (on the gauge) till the other day. Now every time I start it, it will once climb up to 200F on the gauge and alarm goes off and then it quickly cools down to 175 and stays. I can be a low idle at the dock, hi idle at the dock or going 5 knots down the channel out from my slip. I don't have to pull back on the throttle or change anything just watch it heat up, alarm and then cool off and run ok. The other day I had 750 lbs of men on board, we started off down the channel, alarm went off, then cooled down, I then worked it hard on plane for the next 20 minutes out to our fishing spot and it ran a perfect 175F. It alarmed once when I restarted the main engine to come back in again after it warmed up.

So the gauge sensor and alarm sensor are different which tells me this is probably a mechanical thing not electrical. They are both 2" to the right of the t stat housing in a fitting that has a heater hose for the hot water heater and cabin heater take off.

Using my temp gun at low idle at the dock

T stat - 155F perfect

2" left of t stat - 155 perfect

2" right of t stat - 180-185 f when overheating then cools to 155-160 (remember my gauge reads 20 higher)

exhaust manifolds - 160 F - perfect

heat exchanger inlet - 160 F perfect

heat exchanger outlet - 90 F - perfect.

Every temp is right on except the R/H side. I thought maybe I had a lazy t stat the way it climbs and then drops and runs good.

Done so far - change t stat to a new 140F, change dash gauge (just because), change alarm sensor, pull raw water pump (perfect but put a new impeller in anyways), back flush p/s cooler, scrub and feel leg water intake with my hand in the water, bled and bled the system trying to find any air -none. No difference. With the heater take off out and t stat out I ran my fingers between the 2 and had wires shoving in and out looking for blockages - none.

So do I have a "hot" bank on the RH side ? Why does it climb then drop and run good every time I start ? I only run for 20-30 at a time to get out to my fishing grounds so I haven't done lots of testing with it other start, alarm, cool off and work good for the next 20 or so. Even at low idle at the dock it will alarm and then just come down and settle at proper temp. At no point can I find the t stat housing climbing in temp - just beside it where the heater/alarm/gauge sensor take off is the only spot I can get a hot reading

Anyone else ever have this ?

I attached 2 pics - one showing the heat take off with the sensors beside the t stat. The second shows my take off - coolant in the bottom and out the top. Alarm sensor on the left and gauge on the right

Thanks,

Dave


Attached files

[Pcpete]

I had a similar problem lots of years ago and it seems to me that it was a bubble or void in the sensor "tower". I think I ended up replumbing the heater takeoff/return but I don't remember how. Sort 'bout that part.

[Big_Ben]

Thanks Pcpete, I'm hoping its something like that. I bought some different fittings but they are to tall and hit my heat exchanger so I will regroup and try something else. I will let you know if/when I get it figured !

[Big_Ben]

UPDATE - Well in case anyone has a similar problem....here is what I found. So pulled the fresh water pump and inspected - looked perfect. Something told me it was in the bypass loop which on my boat goes to the cabin heater then hot water tank then back to the water pump. This is the same circuit the gauge and alarm sender are plumbed in. I felt the only thing left was that it has low flow so I started pulling all the lines and flushing both ways and then flushing the heater cores (cabin heater and hot water). Well I crawl in to look closer and see something strange in the inlet of the hot water tank for the coolant - a piece of plastic folded over. It was the thin black "coating" on the metal pipe (I assume for corrosion protection ?) had come half off and was blocking off 80% plus of the inlet - thus causing low flow. So when the t-stat is closed the coolant has no place to go other than the bypass loop. This ensures flow to allow even warm up and allow the t-stat to properly work. When mine was plugged the coolant basically had no flow so was just getting hot spots and not opening the t-stat until it was too late as the coolant was not flowing or getting the hot coolant to the t-stat. You just never know what you will find !

[builderdude]

Never would have thought a blocked up bypass would cause this problem, but do remember another member stating the same. Good info

[builderdude]

Never saw this post. The " bypass" you refer to is the cabin heater/water heater loop? (Supplied at the intake manifold upstream of the t stat and returns to the engines circ pump) Shes running normal now?

[Big_Ben]

Never saw this post. The " bypass" you refer to is the cabin heater/water heater loop? (Supplied at the intake manifold upstream of the t stat and returns to the engines circ pump) Shes running normal now?

Yes that's the loop. I have done hrs of reading on the internet after I found this and yes whether it be Ford Mustang Forums or Hot Rod etc there is an on-going debate about this - some say that would never cause overheating and don't even have that loop connected on their motor and other say when their temp rises, falls, rises, falls etc and that circuit was the problem - again automotive has lots of variables.

I do have a San Juan closed cooling kit on my boat. It makes sense for mine - So the water pump puts water into the block. The water should flow through the block and up into the heads then out the t-stat, to the manifolds, heat exchanger and then back to the inlet of the water pump. If the t- stat is closed where is the water circulation ? No where. And therefore you get hot spots, no flow, and an overheating engine. But have the bypass loop and you get some flow and the t-stat can open when it should as it has warm coolant going by. The 2 - 1/8' holes some of us drill in the t-stat do not allow enough flow this is more for air than anything.

I have the merc service manual and all factory closed cooled models show the bypass loop plumbed. Even found a warning in my merc service manual that says restricting the bypass loop can cause engine damage and overheating.

And yes now it starts, warms up and holds and maintains it engine temp perfectly