Mercruiser 350 MPI - Hard to Start & Overheats - Please Help!

[Wilkness]

Howdy, all. Long time reader; first-time poster. I hate to ask what is probably two unrelated questions in a single post, but here goes:

I got my hands on a steal of a deal on a 2003 Monterey 268 SC with a Mercruiser 350 MPI and Bravo III out-drive about a year ago. Ran fine for the first six months, then overheated on the water hose one day in the yard. Decided it could probably use a good tune up and seasonal prep from a pro, so I took it down to the local marina repair shop. The guy called me back and told me he'd like to replace the impeller, thermostat, and manifolds and risers for good measure--all to the tune of about $5k. I figured, "Hey, I'm a mechanically inclined kind of guy, and YouTube is my faithful guide in all things," so I decided I'd do it myself. Needed to learn my boat anyway, and that sounded like a great start.

Long story short, I whipped out the manual and started learning all I could. After about a month of reading this forum and watching videos, I ultimately bought a bunch of parts and replaced the following: Starter, manifolds, spacers, risers, spark plugs, wires, coil wire, distributor and rotor, impeller, thermostat, and fuel filter. I used all new gaskets for everything like all the pros suggested, and gave her an oil change and new filter there, as well. I removed and back-flushed all of the raw water cooling system hoses that I could access from the front and sides of the engine (not a ton of room to work with or even see in the back area) and got some corrosion flakes, but not enough to scare me. I even rewired a bilge pump under the cuddy cabin that was acting up.

When I had her all back together, I stood back in awe of my latent mechanical prowess and gave a little nod to all the guys who posted videos and forum answers. Thanks to all.

I turned the dial to wake the batteries back up, and that pesky bilge fired right up and cleared the cutty cabin hull space. I was well on my way to posting my own helpful answers one day; I just knew it. I put the key in, ran the blower for a minute, and read the gauges--five by five, all looking great.

Then I turned the engine over...starter doing its job great, but no fire. Scratched my head for a while, gave it a few connection checks, and tried again. No fire. Hit the forums. Troubleshooting. Troubleshooting. Downloads. Reading...Finally found a guy who was talking about faulty IAC modules, and he suggested giving it some extra throttle with the prop bypassed, so I tried that. It fired up. Maybe need to replace that IAC unit. I put it on the list, but now its running, so I pull her back to idle. Runs good, sounds great. Water coming out of the stern like you'd expect, a little coming through the side exhaust; I'm not sure how much to really expect, so I assume that might be normal until the thermostat opens up. We'll see.

About five minutes go by... heat rising. I see it climbing up to just over 150 F, so I'm expecting that brand new 160 F thermostat to kick in and knock it back down any second now. Except it doesn't. Gauge hits 175 F, and I freak out and shut her down. Time to do some more reading. I'm all over the web, but no spot on advice.

I flush the hoses again, pull that thermostat, clean up what I can, and put it all back together. Put her on the hose and try to crank again. No fire. Try the increased throttle. Doesn't work this time. Scratch head. Check connections. Pull the coil wire and reinstall. Hmm... I remember a video where a guy was talking about engine breathing, so I pull the air intake and find that little muffler filter on top. It's filthy. I know they're like three bucks, but I just clean it up with Dial Ultra and rinse the heck out of it and dry it completely and put it back in. Back to the console, give it some throttle.

It cranks again! We're in business. Overheating issue happens again! I shut her down at 175 F.

Points I haven't brought up yet: I saw a few posts about distributor caps saying to make sure that some little black wire is put back in place; no good details, though. I have some little black wire that is loose, but I have no idea where it should go and I didn't see it come off in any of the repairs. It sits on the left side of the engine as I'm facing the belts. It's long and is secured on the floor of the hull with about two and half feet of wire. Also, I've seen videos where a sensor wire with a plastic housing end is fitted into the back of the distributor cap area, but I can't find a wire set that would fit there. I don't remember removing one in the replacement, either. Does it have to have something plugged into it, or is that just for running diagnostics? Finally, the left riser (and the water coming out of the left exhaust elbow gets hotter than the right riser and the water coming from that exhaust. Maybe a blockage there?

I'm really stumped here, guys. New pretty much everything having to do with ignition and temperature control, yet both problems are persisting through all of the repairs.

What could be causing my ignition issues, but would seem to cause no problem once I've got it cranked?

What is the most likely reason that I'm still having overheating issues?

Anything helps. Thanks so much for all the reading.

J

 

[tpenfield]

MPI engines run great, until they don't

Post your engine serial number. Closed cooling (coolant) or open (raw water) cooling?

Check to make sure the shifter is in neutral. If you get the engine running again, see if the temperature stabilizes a little bit above 175.

After a few posts, you can post some pictures, which may help. You will have to reduce the 'size' of them down to about 800 x 600 in order for them to upload to your posts.

:welcome: to the posting side

 

[Wilkness]

Thanks, Ted. I should have known all of that info would come in handy... Dadgum rookies, right? :joyous:

Engine Serial #: 0M166001

Open Raw Water Cooling Only

If by "shifter is in neutral" you mean that the throttle control is in the locked vertical position when cranking, then yes, it was until I started giving it gas to assist with the crank, which is how it ultimately started. If by "shifter" you mean I need to check something located elsewhere, then I don't know where that is and would need to learn some more, I guess.

The temperature does not stabilize around 175. I let it go a little further and the alarm sounded, so I shut her down.

Thanks again, so much!

 

[TurtleTamer]

Try putting in your old thermostat or another new one? Or try the pot of hot water test on the existing one.

How did the impeller look? Was it missing any of its fins? They've been known to get lodged in bad places and cause hot spots or overheating.

Also, double and triple check your hose routing/plumbing. With that being said, is it possible you got the wrong riser gaskets? For instance, you may have accidentally got block-off type for closed cooling and now you're trapping water in the manifolds.

I appreciate your entertaining writing style and attention to the details. I'll be interested in seeing how this turns out.

 

[tpenfield]

If the temperature rises uncontrollably to the point of the alarm (which I believe is around 200 F), it is a fair bet that the engine is not getting enough water in total. Therefore, take a look at the sea water pump on out through the outdrive.

Is the impeller new (sea water pump)?

Oil & P/S coolers clear?

Bravoitis at the transom assembly?

Debris caught in the outdrive inlet hose?

I have on a couple occasions found marine life (barnacle, shell) trapped in the water hose at the bell housing. Spent much of the boating season chasing the cooling issue, only to find the real problem when I took the outdrive off at the end of the season.

 

[Wilkness]

Try putting in your old thermostat or another new one? Or try the pot of hot water test on the existing one.

How did the impeller look? Was it missing any of its fins? They've been known to get lodged in bad places and cause hot spots or overheating.

Also, double and triple check your hose routing/plumbing. With that being said, is it possible you got the wrong riser gaskets? For instance, you may have accidentally got block-off type for closed cooling and now you're trapping water in the manifolds.

I appreciate your entertaining writing style and attention to the details. I'll be interested in seeing how this turns out.

TurtleTamer,

I'll pull the thermostat in a day or so when another new gasket comes in. I'll do the hot water pot test, but since the thermostat is brand new, I'm sure hoping that isn't the issue. I guess it never hurts to pull and flush the hoses and cooling components, but, man, what a lot of frustration in the meantime.

I've now pulled all of the cooling water system hoses in the engine compartment and flushed a water hose through each. Dang, that was a job, but I found no blockages in the hoses or the cooling components. I have not yet done as Ted suggested, though, and checked any of the hoses to and from the stern drive. I'll need to find a video and that'll be next, but I do have good warm water outflow at the stern, so who knows?

The impeller looked like I replaced it for nothing, actually. No missing tines, and I didn't find any pieces from prior ones lodged in any of the hoses or cool fuel thingamawhatzit.

As for riser gaskets... Well, I used the ones that came in the kits for the manifolds and the risers. There were two types: one that had all four holes wide open (these were the only kind that came with the manifolds, so I used them there); and one that had those little restrictor-port holes offset for each top and bottom riser. I did use those for the contact point between the 3" spacers and the risers because I saw a guy do that in a video. That left me with two of the four-full-openings ones sitting in a "someday" box in the garage. Do you think someday is now and I should remove and replace those restrictor-port hole gaskets?

Thanks for the help!

 

[Wilkness]

If the temperature rises uncontrollably to the point of the alarm (which I believe is around 200 F), it is a fair bet that the engine is not getting enough water in total. Therefore, take a look at the sea water pump on out through the outdrive.

Is the impeller new (sea water pump)?

Oil & P/S coolers clear?

Bravoitis at the transom assembly?

Debris caught in the outdrive inlet hose?

I have on a couple occasions found marine life (barnacle, shell) trapped in the water hose at the bell housing. Spent much of the boating season chasing the cooling issue, only to find the real problem when I took the outdrive off at the end of the season.

Ted,

Impeller is new, but the housing is not; it's the brass type. I was hoping to not have to replace it, but there is a little bit of scoring in there. I don't think it's much, and it doesn't look as bad as some of those the guys in the videos are replacing. Obviously, I was hoping it was still moving enough water, but maybe that's my problem. How do I know if I have enough scoring to cause a lack of water? Are there signs that can be checked externally based on water flow at the outputs?

Oil and P/S coolers are clear.

Googling Bravo-itis was entertaining. That looks like a last resort kind of job considering it looks like I'll have to learn how to take the whole out-drive off to do it properly.

I'll take a look at the out-drive inlet hose this evening if the Wife will let me out of the house again. This has become my new obsession, I'm afraid.

 

[TurtleTamer]

I'll let the experts answer your riser gasket question. Sounds to me like you have the correct ones (the offset vent holes for letting air out) but I'm not experienced enough to tell you one way or the other. Did you pay attention to what kind came out of it?

 

[devildogae]

post pics of the wires your talking about. Also these engines run hotter on muffs than on the water. Might want to take it down to the dock and see how she runs there. The hoses from the outdrive into the motor would be the next place I look at. Sounds like your covered most the other areas. Find out if that thermostat is working.

 

[Wilkness]

So, the sensor port on the back of the distributor is called a hall effect sensor or a cam shaft sensor, and I'm having a hard time getting a straight answer as to whether there should be something plugged into that on a 2003 Mercruiser 350 MPI. I've seen it answered both ways with equal confidence and apparent authority. I can't find a loose wire fitting that would go in there just hanging around in my engine compartment somewhere.

Anybody actually know if something should be plugged into there, and if so, from where would that wire originate so I might track it down? This might have something to do with my starting issues.

Thanks, all.

 

[Wilkness]

UPDATE!!!

New boating idiot alert! My problems were as follows:

Cranking Problems:
1. Hadn't cranked the boat in about six months, so the systems were cold and sluggish. After the fifth or sixth crank, she now fires right up;
2. Batteries were a whopping eight years old and were barely kicking over the 11-volt mark. A new battery had that thing cranking up like a champ after about 1.5 seconds of key switch.

Over-heating Issue:
1. The raw water pump housing did have too much scoring to effectively push water through the system. I bought one of those el cheapo Gen 7 rebuild kits and now it runs right in the suggested operating range, even off the hose.

Related Follow-ups:
1. No Bravo-itis occurring. It turns out that the previous owner was great about flushing the system after any brackish water outings;
2. IAC module was working fine;
3. The new thermostat was working properly, as I learned after employing the hot water stove-top test as suggested above;
4. That little black wire is just a mystery. I looked back over some of the photos I had taken of the engine compartment prior to beginning any of the above work and could just barely make out that it had been off the engine just sitting there loose by the fuel pump all along. Not sure where it goes, but it must not be anything absolutely vital...for now...;
5. The hall effect sensor (rear distributor cap area) does NOT have anything that needs to plug into it on the 2003 Mercruiser 350 MPI.

Thanks to all for the help. It's cranking and running beautifully as of today!

Best,

Wilk