91 Mercury 135 hp Blackmax

[labguy]

Hey guys,

Took the boat out for the first time since buying it and it ran awesome. The only thing is wanted to see if you guys could tell me what the optimal operating temperature was. I was assuming it was around 150 since that was in the middle of the temp gauge, but it ran around 170. It would drop to around 150, then go back to 170 whether idling, or wot. If 170 is a little high, do you guys think its the impeller?

Thanks,

Don

 

[j_martin]

Re: 91 Mercury 135 hp Blackmax

It's hard to tell without a pressure gauge or some history. It will vary up and down some, and depending on ambient temp, 170 probably isn't too high.

I consider impeller change to be an annual maintenance item.

hope it helps
John

 

[Texasmark]

Re: 91 Mercury 135 hp Blackmax

The thermostat for that engine is probably in line with most Mercs of the vintage and that is an opening temp (pellet stamped temp) of 143F. Once the stat opens, engine block temp will rise and 10-15 degrees over that is reasonable. Stat is only operational for lower speeds. As rpm's increase to 1500 to 2500 range, there is a pressure relief by pass that bypasses the stat allows the engine to remain cool. The size of this port (cross sectional area) vs the stat port is like 10:1. So temps could possibly drop as a result of ambient water temps at engine speeds exceeding 2500 rpm's.

All I know for now.

Want more ask.

Mark

 

[labguy]

Re: 91 Mercury 135 hp Blackmax

Thanks for the info. So, as the water temp rises during the summer months, the operating temp will rise also? Just makes me nervous to see the temp gauge that high. Just so that I understand you correctly, the cooling system is regulated by a thermostat under 1500 rpms, but as they surpass 1500, the thermostat is automatically bypassed? I ask this because the temp was varying wildly between 170-150 at all speeds including idle. Didn't seem to be any rhyme or reason why it was at a particular temp.

 

[sschefer]

Re: 91 Mercury 135 hp Blackmax

Change the impeller and the check the poppet valve.

My 135 runs at 145 at idle and 130 at speed. 130 at speed is just about right in winter and 140 is about right in summer.

The thermostats (your's has two, one on each head) are only players up to about 1800-2000 RPM when the water pressure is high enough to open the poppet. If your impeller is worn or your poppet is stuck you will not flow enough water. These impellers are high pressure/low volume.

A water pressure gauge, (as mentioned before) is a good way to find out what's going on. IMHO your engine is running too hot and that can be deadly for these engines.

There is a chance that it's the advance module that's causing the problem. You can eliminate that by removing it and setting your WOT timing to 21 degrees. Most of us that have these engines know about the deadly advance modules and have removed them.

 

[labguy]

Re: 91 Mercury 135 hp Blackmax

Thanks for the info. One more thing I forgot to mention. I noticed the stream of water coming from the engine was pretty weak. I noticed some motors yesterday as I was waiting to pull out of the water shooting out a strong stream of water and then mine was just a weak little stream. Thanks again for the help guys.

 

[j_martin]

Re: 91 Mercury 135 hp Blackmax

Sometimes that's a weak impeller, sometimes it's just a spider nest in the tell tale hose.

If it's an unknown, I'd service the water pump and go from there. I still think you need a pressure gauge. They're about 25 bucks. Simple buordon tube gauge and a plastic line.

hope it helps
John

 

[Dukedog]

Re: 91 Mercury 135 hp Blackmax

My way of thinkin' on a "new to you" motor" without any known background is to replace tha complete waterpump, t-stats and poppet. You have no idea how old or new these parts are. Cheap insurance. This way there is no is this good or that good. Not a good thing when it comes to coolin'. Like J says get a pressure guage. Temp guages give ya an idea but are known to be very inconsistant and inaccurate untill you know tha true value that its reading no mater what it says. ex. your guage reading 170 might be really 130 actual temp or might be 190 actual temp. as long as you don't get a warning buzzer your safe.....................

 

[labguy]

Re: 91 Mercury 135 hp Blackmax

Thanks for the advice. Im going to replace waterpump impeller, tstats, and poppet and go from there.

 

[labguy]

Re: 91 Mercury 135 hp Blackmax

Alright guys, I tested the tstats and they popped open around 143 or so degrees and replaced the impeller and still running a little warm. I put my hand under the "pee stream" and it was hot but not scalding. Is there a way to test the poppet valve to see if its defective? Also, do you think that my temperature gauge could possible be off? Should I invest in a digital gauge? Thanks for your input.

 

[j_martin]

Re: 91 Mercury 135 hp Blackmax

Is there any way to test the poppet valve?

You use the pressure gauge.

I'm outa here.

 

[Dukedog]

Re: 91 Mercury 135 hp Blackmax

Temp guages give ya an idea but are known to be very inconsistant and inaccurate until you know tha true value that its reading no mater what it says. ex. your guage reading 170 might be really 130 actual temp or might be 190 actual temp. as long as you don't get a warning buzzer your safe.....................

99% of temp guages on motors today read metal temp. Not actual water temp.

Like he says get a pressure guage..............

How hot is 140 degrees? Can ya hold your hand in 140 water for very long? Put it in tha water and run it after you get a pressure guage. If tha pressure doesn't jump around and act "stupid" and is consistant at different rpms, run it. If you get an alarm (buzzer) then ya got a problem. If not it doesn't mater what tha "temp" guage says. Like said it is just something you have to get use to in tha way it reads. Just take notice of where tha needle is on tha guage at different rpm. As long as tha guage goes to tha same place ever time at tha different rpms then all is well.......................

 

[labguy]

Re: 91 Mercury 135 hp Blackmax

You use the pressure gauge.

I'm outa here.

Okay, okay, okay, lolol. Will get one this weekend. Do I tee off the pee hose?

 

[j_martin]

Re: 91 Mercury 135 hp Blackmax

Okay, okay, okay, lolol. Will get one this weekend. Do I tee off the pee hose?

NO

Sorry, got carried away.

There's a pipe plug in the top of the block behind the flywheel. You remove that and put a hose adapter where it was.

The pee hose has only dump water from the T-stats in it.

hope it helps
John

 

[Dukedog]

Re: 91 Mercury 135 hp Blackmax

John, he should already have a 1/4" hose going from there (no plug) down to tha bib at tha poppit. So he can just install a T in that hose at tha top......

 

[labguy]

Re: 91 Mercury 135 hp Blackmax

Alright, found where the hose is coming from the top of the block behind the flywheel. So I just tee of that hose or do I need to buy another connector?

 

[sschefer]

Re: 91 Mercury 135 hp Blackmax

I would never argue that a pressure gauge isn't a great compliment to any engine diagnostic system, that's a given.

These engines are aluminum and aluminum spreads and dissapates heat rapidly. On a V-6, Merc puts a temp sender for a gauge on one head and a temp switch for the alarm on the other. They haven't deviated from that configuration for many years. They don't sell their engines with water pressure guages but they do warranty their engines. That's a given.

We've all been scared to death of burning up our engines. That's a given.

Considering that a poppet valve is designed to increase water flow through the engine under periods of demand. A stuck open poppet valve would cause a low water pressure reading and a low temp reading and a stuck closed poppet valve would cause a high water pressure reading and a high temp reading at speeds above approximately 1800 RPM's.

A bad impeller by itself would cause a low pressure reading and a high engine temp even if the poppet valve was stuck open or closed

My vote... run them both but use good quality gauges. Doing so allows you to pinpoint the problem and can save you a lot of grief and money.

 

[Dukedog]

Re: 91 Mercury 135 hp Blackmax

Yes thats it.
sschefer, They didn't come with temp guages either. They only came with a warning buzzer. Everything else was optional.................

 

[j_martin]

Re: 91 Mercury 135 hp Blackmax

John, he should already have a 1/4" hose going from there (no plug) down to tha bib at tha poppit. So he can just install a T in that hose at tha top......

I guess later ones had a pressure hose that went down to the poppit, modifying it's operation. You can T into that. the T-stat hosing is useless for this purpose, however.

Duke, most of them had the temp gauge sender installed at factory.

hope it helps
John

 

[Dukedog]

Re: 91 Mercury 135 hp Blackmax

Yeah John. That hose and bib were added when tha water cooled pos reg came out. It was supposed ta help eliminate air pockets in tha divider plate that was causing tha reg ta overheat. Didn't work out to well but they left tha hose from then on.
None of tha new motors I bought in tha "old" days came with a temp sensor for a guage. Even tha first production efi ('86 or '87 I think) I bought only had two wires on tha sensor for tha ecu. I remember that one very well. They wanted an unbelievable price for tha one with tha extra wire for tha guage. I put a real "water" temp guage in just ta keep from payin a stupid amount a money.................