Correct operating temp for 7.4 MPI?

I finally got a chance to take the 99? Crownline 248 BR I bought out for a long run this weekend and really test it out. The 7.4 MPI ran like a top?.I?m very happy with the motor and Bravo III drive it?s got.

The one thing I noticed was that the water temp gauge would seem to fluctuate and between 170-180 depending on if you were idling, or running fast. I did a fresh oil change on the motor and water pump impeller before I took the boat out, so I know everything should be in good shape. It may be a problem with the gauge because I noticed that when I would turn the blower on, or switch some other accessory on for a second?the needle on the temp gauge would jump up just a hair?..turn the blower off, and the needle on the gauge would drop back down.

What?s the normal operating temp this thing should run? At wide open throttle?it seems to stay right at 175 on the gauge, but then at idle everyone now and then it would creep up to around 180, and then it would go back down to 170. Sometimes pulling a wakeboarder it would creep up to around 180 or so also. Is that normal? Should I replace the thermostat? If so?what temp?

Thanks!

Re: Correct operating temp for 7.4 MPI?

my 7.4L carbed runs 170 - 180 at the guage . A good way to tell if your cooling system is cycling enough water is lay your hands on the top of exhaust risers. You should be able to keep them there w/o burning for a few seconds. If not, theres a good chance you'll need a new impeller for the raw water pump. FYI, the impeller supposedly needs replacing every 100 hrs of use.

Re: Correct operating temp for 7.4 MPI?

It's got a new impeller in it. And yes, the exhaust risers are pretty cool....I can hold my hand on them for a while before it gets too hot.

Re: Correct operating temp for 7.4 MPI?

IQRaceworks said:

What’s the normal operating temp this thing should run? At wide open throttle…it seems to stay right at 175 on the gauge, but then at idle everyone now and then it would creep up to around 180, and then it would go back down to 170. Sometimes pulling a wakeboarder it would creep up to around 180 or so also. Is that normal? Should I replace the thermostat? If so…what temp?

Thanks!

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Howdy,


It would help if you reveal a little more info on what you have and if you have closed cooling or not.

According to Service manual #23, you should change your sea water pump impeller:

Whenever insufficient Sea Water Flow is Suspected (If operating Temperature Exceeds Normal Range)

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Your engine uses a 160 degree t-stat. normal operating temp is 160-180.

If you have ever had an impeller failure there's a possibility of impeller fragments clogging your oil cooler.

You have to remove the cooler to ensure you get all the fragments out.

Here's a link to an online service manual. You can check to see if the serial numbers covered include yours.

Boatinfo - Mercruiser Service Manual nr. 23 - 1998-2001 - Engines GM V8 454 & 502 CID (Gen VI and L-29)


Regards,


Rick

Re: Correct operating temp for 7.4 MPI?

Assuming you do have a rwc unit, then yours is mine just a year or two older depending on when it was manufactured.

When I first obtained my boat (from original owner) I saw the gauge a shade over 175 when warmed up and running at cruising speeds.
I had my mobile mechanic do a raw water pump replacement along with the water circ pump since a bearing in there was starting to go.

When we then did the water test the gauge would still show the same position slightly over 175 but on his scan tool that was plugged in the actual temperature was 169-170 degrees. He commented that the tool is accurate and as long as I don't see the gauge go much above where I see it now then all is good. Maybe at some point I may want to invest in trying to figure out why the gauge is not accurate but not worth it right now.

I also use the hand-on-top-of-the-elbow(riser)-should-not-be-too-hot method as a double-check.

Mark

Re: Correct operating temp for 7.4 MPI?

MarkSee said:

Maybe at some point I may want to invest in trying to figure out why the gauge is not accurate but not worth it right now.

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Because the gauges are dirt cheap - bottom dollar bits meant to make the most profit. If the gauge was sold for $5, the manufacturer would still make profit. The air motors for many gauges come from one or two manufacturers. Most gauge manufactures simply make the bezels and face plates with a purchased air motor and cup.

The good news is you have a correlation between the gauge and real world (scan tool).

Re: Correct operating temp for 7.4 MPI?

Thanks for all of the info guys. The boat is a Crownline 248 bowrider with a Bravo III drive. I have already replaced the raw water pump impeller, but the old one that was in there actually looked great. I did it just for peace of mind on a used boat.

As far as checking the actual water temp with a scan tool....where is the best place to shoot the motor to get a good temp? I've got one I can use.

I'm guessing that the 14yr old cheapo temp gauge is probably not all that accurate, so that's probably the problem...but I would like to double check just to make sure.

I did not flush the lines when I did the impeller install, but since the old impeller looked really good...I figured that there weren't any chunks of old impeller in the system. But I guess I could always do that just to make sure.

Also, what about putting in a new thermostat? I guess the first order of business is to shoot the motor with the temp gun and see how accurate (or how far off) the temp gauge really is.

Re: Correct operating temp for 7.4 MPI?

IQRaceworks said:

The boat is a Crownline 248 bowrider with a Bravo III drive. I have already replaced the raw water pump impeller, but the old one that was in there actually looked great. I did it just for peace of mind on a used boat.

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The boat manufacturer doesn't help much. (they didn't build the engine or drive and could have used a 1999 OR 1998 or in some rare cases 1997 model year engine and drive) We really need Mercury serial numbers since that's how Mercury identifies the engine and drive. Service manuals specify which serial numbers they cover.

As far as checking the actual water temp with a scan tool....where is the best place to shoot the motor to get a good temp? I've got one I can use.

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Mercury puts the temp sender in the t-stat housing. The temp at the housing is where it should be "shot"....... I would verify it with an infrared "gun".

I'm guessing that the 14yr old cheapo temp gauge is probably not all that accurate, so that's probably the problem...but I would like to double check just to make sure.

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They're frequently not accurate. That's why you would verify with a different sender/gage/infrared/etc

I did not flush the lines when I did the impeller install, but since the old impeller looked really good...I figured that there weren't any chunks of old impeller in the system. But I guess I could always do that just to make sure.

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If you bought the boat new and you haven't had an impeller failure, I would say no problem. If you DID NOT buy the boat new and are just relying on what someone else told you then I would pull the oil cooler and look inside (raw water inlet)

The guy I bought mine from told me it had a new impeller and he never had temp problems.
My oil cooler was SO CRAMMED with impeller fragments that I cannot see how it even ran at all without over-heating! They were in there from a previous failure (probably a couple of impeller replacements-"back")

Also, what about putting in a new thermostat? I guess the first order of business is to shoot the motor with the temp gun and see how accurate (or how far off) the temp gauge really is

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After you check your entire raw water circuit, and you know you have no blockage. (and assuming your gage is correct) you could take your old t-stat out and check it. It should *start* opening at 160. They don't fail that often (in non-salt-water engines)

Is this a raw [salt] water cooled engine?
And if it is, how old are the manifolds and risers?

The key to keeping the raw water "circuit" working correctly is free flowing water with no restrictions.

Restrictions will occur from impeller failures (fragments) , or other debris clogging the raw water lines, oil cooler, PS/cooler, cool-fuel cooler etc AND rust in the t-stat housing, exhaust manifolds, risers, circulating pump, block and heads.

Any one or all of the above can restrict raw water flow and cause overheats.

Rust or other blockage in the block, heads, & circulating pump doesn't affect the over all raw water flow from the raw water pump through the system and out the risers at all unless the manifolds and risers are clogged too!


Oh, and btw, in my example of the clogged oil cooler, it DID flow *some* raw water with the fragments stuffed in the oil cooler inlet. It just didn't flow ENOUGH for a full throttle run. It seemed adequate for low power operations......

Re: Correct operating temp for 7.4 MPI?

Well, I got the boat out on the water this weekend and got everything all warmed up. The gauge was saying 175-180 again.....and when I got the area near temp sender at the thermostat with my temp gun it was reading 155-160.

So it looks like everything is fine, and the motor is running nice and cool. I guess the sender and/or the gauge is just a little off. At least I know everything is good.