1974 Chryler outboard 135hp overheat alarm

[WinnerCougar74]

I replaced the impeller in my 1974 Chrysler 135hp outboard at the beginning of the season last year. It ran with no problems for 3 months. Now if I idle around for more than 10 minutes while trolling the overheat alarm comes on. If I drive faster it goes way. Where should I start? How hot should the water be coming out of the telltail? I could not hold my hand in front of it. There seems to be a faint black sout around the tell tail opening.

 

[matt167]

impeller service required

 

[Nordin]

Check the water jackets and flush it. If salt water use there can be a lot of deposits in the jackets.
3 and 4 cylinder Chrysler/Force engines up to late 1980.es was equipped with a idle relief valve.
The purpose of it was to open up and by pass the cooling water if the pressure become to high when idle.
I know that Frank A wrote about in a post in this forum for a long time ago and the symptom if this valve is stuck is that the block get hotter at idle and the alarm can go off when idling and then stops when open up the throttle and run faster.
Unfortunately Frank A is not with us any more. He was the Chrysler/Force guru.
I have not work with this so much cause I live where we have most fresh water engines.

 

[Nordin]

Sorry some wrong information from me again.
The idle relief valve opens up when pressure rise to a predetermined value and allow the cooling water to by pass the thermostate and circulate through the water jackets.
If valve stuck shut it maybe will cause overheat at higher RPM and if stuck open maybe overheat at idle if water jackets has lot of deposit.

 

[jerryjerry05]

Change the impeller.
Check the thermostat?

You mean soot? Normal.

I bought a heat gun on eBay 16$
One of the best tools I have ever bought.
Check the temp and make sure it's really getting hot??

 

[WinnerCougar74]

Thank you everyone. Very helpful. Primarily freshwater and I always do a flush with muffs after a saltwater run. I forgot to mention that when I checked the thermostat, someone had already taken it out. Based on what everyone has said it seems like I have a clog somewhere although it seems weird that it only happens at extended idle. What is the best way to flush a Chrysler outboard, maybe backflush at the thermostat opening or backflush at the bypass valve opening? Also, there is always flow at the tell tail. How hot is the water supposed to be coming out of the tell tail? And just to double check what jerryjerry05 said, it's normal to see faint black soot at the tell tail right? Thanks again everyone!

 

[ExplorGM]

That is a great looking boat! This is a little off topic, but what kind of top speed do you get with that 135?

 

[WinnerCougar74]

I have 2 props for it. the 13x19 that came with it will hit 38mph (gps) at 4500rpm but that it, it won't rev beyond 4500 rpms. With the 13x16 I found on craigslist it will hit 32mph at 5000rpm. Overall I like the 13x16. Much better hole shot and since it's and old motor I like the lidea of it not working as hard. I do have to be careful though because it will rev beyond 5500 now. Not exactly sure but it seems like the 13x16 uses more gas. The perfect prop is probably 13x17 but props for these motors are hard to find and pricey. I only paid $20 for the one I found on craigslist.

 

[ExplorGM]

I'll have to hook up a tach... Thanks for the info!