OMC Overheating issue...Help!!

[thannah84]

Had the boat out today (1984 Starcraft with 2.5L 400 OMC Stringer) and noticed that after running at 30mph, The temp gauge was a lot higher than normal. I think I have a 160* thermostat but not sure. Usually runs dead in the middle of my temp gauge at any rpm and any load. Today it crept up to the third line and above. I throttled down and crept for a few minutes and it came right back down to normal temp. Throttle up again and after a few miles it crept right back up to the third line and above. Limped her in and when I got back into the no wake zone before the docks, it was right back to normal within a minute or so. After reading some posts on here, I pulled the exhaust elbow off thinking there was a clog in there, because that seems to be the nature of the problem with the symptoms that I have. There was no blockage at all, note, there was some rust, but I am fairly certain that this is semi-normal. I have to stop tomorrow and get some new gaskets or some high temp gasket maker to put it back together. So with all that said, what else could it be and where do I start? It doesn't sound like the thermostat, because if it works at idle and everywhere else, why would it say that it is overheating when high rpms? Also, I came home today and hooked it to the muffs and let it idle for awhile, and ran at normal temp. I increased the rpm's to around 1500 for a few minutes, and it came a little above normal, but not much. Bought the boat two years ago and previous owner stated he had impeller changed recently. So...I guess there may be three things that it could be. 1. Thermostat (I'll probly get a new one tomorrow and change it anyways since they are inexpensive) 2. Impellar 3. week water pump.

So in short, how do I troubleshoot each of these and is there something that I am missing?

Thanks in advance!

 

[mkast]

Re: OMC Overheating issue...Help!!

What temperature thermostat is supposed to be in the engine?
Replace the impeller.

 

[thannah84]

Re: OMC Overheating issue...Help!!

Seems to cycle water alright when the muffs are on, but I know that the water from the hose is pressurizing it through the system as well...Would impellar cause it to only overheat at high rpms?

Also another tid bit of info.....I recently replaced the points style ignition with an electronic ignition and played with the timing to get it to run perfectly...Would this also play a role in engine temp?

 

[studdy05]

Re: OMC Overheating issue...Help!!

As someone else stated above, replace the impeller.

Also, when you slide the upper back into the lower, check the seal on the lower unit...its called an exaust seal...when that seal is weak or cracked, exhaust bubbles at high RPMs make it into your upper units water pump...like drinking coke with a straw with a hole in it, this just will jack you up at high RPMs but allow enough water through to cool you some when the engine isnt working as hard...and it has a grease fitting...grease the fire out of it...then grease it again.

Timing can be an issue for overheating, but it would have to be way the heck off...Ive tried to recreate severe overheating advancing my timing and its difficult to do with a good impeller...

Lastly, you cant test crap with muffs with your type of issue...I have fought it and researched it to no end, you have to dunk the stinking stern drive...and only when it is WOT pushing the boat on the hottest day of the year will you know if your problem went away...testing an OMC on the lake at Christmas wont give you good results.

 

[studdy05]

Re: OMC Overheating issue...Help!!

This time of year, you can actually pull the thermostat and not worry about replacing it until fall...but if you do - OMC's get restricted a little by the thermostat so you are going to run cooler by more water flowing - which is fine...unless and until you realize it is probably your impeller starting to come to pieces which they do, and even running without a t stat wont keep you cool. Impellers dont generally wholesale fail, the chip away a little at a rotten time in sizes of about BB's...some go faster than others...leaving less and less water flow...a good OMC impeller can push a gallon in 25 or 30 seconds at idle in the lake. They can fight fires across the county at WOT.

Wouldnt hurt to make sure your sending unit to the temp guage is working right if you are so inclined to spend the 15 bucks.

A small amount of rust in the exaust and risers are normal, but scaling will cost you a few degrees...use muratic acid from Lowes to clean the scale.

On rare occasions the engine water pump goes, but it usually leaks when it does...and once in a blue moon, a tiny piece of impeller makes it way to far down the system and clogs something in your engine...but very rare...

So here is the list in order of common things first:

Impeller
Temp Sending unit
lower unit Exhaust seal plus grease
thermostat
risers
timing
blown head gasket

 

[thannah84]

Re: OMC Overheating issue...Help!!

Well....Just ordered a repair manual for my model....How hard is it to replace the impellar. Never done it before and from the sounds of it, it's a 3-$400 job at the mechanic....Also, looks like there is 2 different temp sensors on my motor. One behind the fuel pump and one on the thermostat housing...What is what?

 

[mkast]

Re: OMC Overheating issue...Help!!

This time of year, you can actually pull the thermostat and not worry about replacing it until fall...

I think the thermostat was installed in the engine to heat the engine to operating temperature.
I keep reading in maintenance manuals that engines should be at operating temperature before adjustment/s are made.
The engine will operate without a thermostat, just not the way it was designed to operate.

 

[thannah84]

Re: OMC Overheating issue...Help!!

So here is the list in order of common things first:

Impeller
Temp Sending unit
lower unit Exhaust seal plus grease
thermostat
risers
timing
blown head gasket

What is the difference between the temp sending unit and the thermostat. What sends the temp to the gauge? Also, the thermostat is above the water pump in the thermostat housing. Where is the temp sending unit. Looks like it is on the block behind the fuel pump I think.

 

[thannah84]

Re: OMC Overheating issue...Help!!

Okay...Stopped on the way home and got some parts. I got a thermostat, a sending unit and the o-ring and gasket for the thermostat house. I also got the 2 gaskets to put the exhaust elbow and end plate back on that I took off to investigate. I also talked to my mechanic that I have been going to for years and years. I really trust this guy and he would never stear me wrong. He tells me like it is and is always totally honest with me. He told me that I probably need a new water pump (Impeller and housing and the whole thing) He advised me not to try to do it myself. He asked me if I ever did it before. I said no. I told him I bought a repair manual to try to do it. He said that if you never did it before you would really be in a lot of trouble. He said the last guy that came to him that tried to do it himself ended up needing a new impeller again and also a new drive shaft because it was stripped. About $1000 bucks he said. So, In short. I am going to try the thermostat and sending unit and see if that was the problem. That will atleast rule that out. And the manifold and elbow is also ruled out. Could be circulating pump as well, but doubt it.

 

[thannah84]

Re: OMC Overheating issue...Help!!

Well, I installed the new thermostat and new temp sensor tonight. Also put the gasket (that was the right size) on the end cap of mainfold.. The gasket that I got for the exhaust elbow was not the right size, so I used high temp gasket maker. Hopefully that holds. I have used it for race car applications (Headers and valve covers) where the aluminim gaskets blow out and it seems to hold so we will see. I didn't get to fire it up, the gasket maker has to cure for 24 hours so, I will find out tomorrow....The thermostat looked old and rusty and would not close all the way, maybe 1/8" gap. Maybe it wouldn't open either? Don't know. But when I pulled the elbow and end cap off the manifold it was full of water so, it was obviously letting some water through. The temp sensor wasn't in bad condition but replaced it anyway.

 

[WAVENBYE2]

Re: OMC Overheating issue...Help!!

I do know that when I would run my boat with muffs the water is cooler and temp gaige didn't raise above 140, but in the water where the water is 84deg the temp gaige runs higher 150-160 when i run her good, so differant reading where normal for me

 

[superbenk]

Re: OMC Overheating issue...Help!!

I'm concerned about this too. I have a 30 yr old boat and as far as I know the impeller is original. I'm sure it's probably ready to be replaced at this point anyhow. I've read in the forums that at least one person was told by their mechanic that it's not a trivial job, but my Solec manual doesn't make it look that hard. Which is it? Are there any special tools required? What possible issues should I be aware of if I do this myself?

 

[thannah84]

Re: OMC Overheating issue...Help!!

I don't know...Seems that a lot of people on here have done it themselves and been sucessful..Other have not though...I am not mechanic by any means..But then again, I am not totally incapable either...I don't know what to do..I am afraid to mess it up though and create bigger problems..

 

[thannah84]

Re: OMC Overheating issue...Help!!

Well after replacing thermostat, Temp sensor and gaskets for exhaust elbow I fired her up today on the muffs. She is still running hot...So on to the next thing...I pulled the plugs...Cylinders 1 and 3 look perfect. Cylinders 2 and 4 look like they are burning to hot. So I messed with the timing...It is suppose to be 4*BTDC...I had it running at 2*ATDC...This is at idle (500-600 RPMS) My next question is, at what RPM's should I set my timing? Also, no water on plugs or in oil for the head gasket wonderer's...Don't see water anywhere on the block....So I guess next it is impellar kit...Still am not sure about doing it myself...???? I either need a confidence boost or someone to help me that has done it before...Also, another tid bit of info...When running on muffs at normal temp...I look at the exhaust and there is A LOT of water cyling through...I know this is with muffs and pressurized, but still running hot...Put my hand in the exhaust water and it feels about as hot as bath or shower water...???? So any other idea's where to look???? Symptoms I should be looking for????

 

[NW Redneck]

Re: OMC Overheating issue...Help!!

I've got a 1975 stringer and I replaced the impeller without any problems. Actually I had one problem and that was busting off a siezed bolt head, but that's a hazard with wrenching on any older machinery. I was a little apprehensive about doing mine, but it's not that bad of a job. Took me about 3 hours and I pulled the leg right off the boat to do it. One of those 'workmate' style table/vices works great for holding the leg to work on.

Good luck!

 

[thannah84]

Re: OMC Overheating issue...Help!!

Thanks NWR...Yours looks a bit older than mine...I think I have to pull the lower unit from the upper to get to mine..Waiting on my repair manual to do it...

 

[thannah84]

Re: OMC Overheating issue...Help!!

ttt

 

[thannah84]

Re: OMC Overheating issue...Help!!

One more question for Now....Is there a way to test the Water Circulating pump on the engine?

 

[thannah84]

Re: OMC Overheating issue...Help!!

Well...My story for tonight is: I pulled the hose coming from the impeller off the thermostat housing and put the outdrive in a big deep tub and filled with water. The impeller pumped a 5 gallon bucket in about 35 seconds. I think that would be sufficient enough water supply but??? I am going to get a compression tester tomorrow and test the compression on all 4 cylinders to check the head gaskets so...Any thing I should be looking for???

 

[thannah84]

Re: OMC Overheating issue...Help!!

ttt....No one will answer any of my questions....