Overheating Issue

Boat: 1992 Supra Comp w/ PCM 351 Pro Boss converted to electronic distributor ignition (530 hrs)

My boat is having an overheating issue. When I first take it out, it runs absolutely fine for a couple hours. After a while of driving around, it will start to stutter and hesitate while in forward and then soon after it will just die altogether. When this happened, I lifted up the doghouse engine box and found that the engine as a whole was putting out some serious heat. I touched the part of the engine where the fresh water is pumped into (hose from top of water pump into engine) and it was hot. I can start the boat and run it in neutral just fine and I can actually feel that part of the engine slowly cooling down. After sitting in neutral for a while I can then reengage it into forward and it will work for a while, and then the cycle repeats.

This happened to me last weekend, and I needed to replace the impeller anyways for the season so i went ahead and did that. Yes, I made sure to reinstall the water pump correctly and not upside down. With the new impeller, same problems.

Could this be a faulty thermostat? http://www.skidim.com/prodinfo.asp?number=R026002

At the beginning of the day my temp gauge will sit at the bottom of the gauge (120) and then will sit around 150-180 after the boat is warm and I am driving around. The gauge will stay in this range even when I have this problem and i'm sure it much warmer than that. I'm guessing that this means that either my sending unit or gauge is bad.

Has anybody had issues like this or suggestions? Any input will be greatly appreciated.

When you replaced the impeller, were any of the vains missing? Have you ever had missing vains from an impeller? If so, check around the thermostat and make sure there's nothing blocking the flow of the water, that's where they'll usually get stuck. Also check to make sure there's not any trash or weeds, or other blockage from your water pickup all the way to the impeller housing (and anything else between there). I'm not familiar enough with the older supras to know all the components, but there might be a transmission cooler along the way. Pull all the tubes out and make sure there isn't any blockage.

I wouldn't just assume the guages are bad. 150-180 is pretty normal operating range for the 351 (I think they have a 160* t-stat in them). These motors will always feel hot. They're sitting in a tiny little doghouse with very little airflow.

None of the vains were missing from the old impeller. Thanks, I'll pull all the hoses and check for blockage.

First things first -- we need to find out if you're actually over heating. The gauges rarely go bad and the fact that you're this close to "normal" operation on it makes me think that it's probably OK. A sender is a cheap way to double-check or you could get one of those infrared/laser thermometers and double-check.

I suspect that you are not overheating at all and that your hesitation/hard starting issue is something else.

Are you using last seasons gas? Have you checked/changed your fuel filter?

I plan on picking up an infared thermometer from harbor freight sometime this week.

Just to clarify, I'm not having hard-starting issues. I can start the boat fine and run it without engaging the transmission all day long. It's just after continuous use in forward that this issue turns up.

I'm not using last seasons gas and I have checked the fuel filter. There was no debris or junk in the filter and the boat runs fine in neutral so I didn't think the fuel filter could be the culprit.

Does it die as soon as you put it in gear, or will it idle in gear as well and then die as you give it more throttle? Can you rev the motor while it's in neutral without it dying or will it sputter off?

Here's some more things to check from another forum I'm on. This is related to an MC with the pcm 351.

Quote:

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1. Vacuum leaks around carb base. If air is leaking underneath the carb, you are going to be lean no matter what you do. Test this by spraying some carb cleaner around the vaccuum lines.
2. Point gap check minimum (prefer to go with dwell as it is substantially more accurate) Replace if necessary....bad points surface will look pitted.
3. Condition of distributor cap & rotor. If any crud on contacts...scrape it off. Replace if necessary.
4. Verify timing.
5. Verify correct spark plugs, condition and correct gap. Replace if necessary.
6. The balance between mixture screws and idle speed screw is easily thrown out of whack. If the idle mixture screws are turned in too far, not enough fuel can be provided for adequate idle speed and the idle speed screw needs to be turned out far enough to open the throttle blades to the point where fuel is provided by the main operating circuit. If the idle screws are out too far, it is hard to get decent idle quality as it will be rich no mater what you do. Screwing them in all the way, then backing out 1 1/2 turns is where to start. Small adjustments from this point.

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Some other things to verify as well...

Pull the flame arrestor off and make sure the carb is dumping fuel when you put it into gear.
Inspect your fuel line - Ethanol eats away at these and they will eventually collapse and cause blockage. Has your fuel line ever been replaced?
Is your ignition system in verified good working condition? How old is the coil? Plugs? Wires? You said you have electric ignition, which setup do you have?

It will die as long as its in gear, regardless of what level of throttle i'm at. Yes, I can rev it up when I lock out the transmission.

The electronic ignition/distributor is brand new. I bought the kit from skidim (http://www.skidim.com/prodinfo.asp?number=RK107025A) and put it on last month. I had a marine mechanic install it and verify the timing.

I thought about the spark plugs, so I replaced those this weekend as well (OEM from skidim), still having the same problem but they did need to be replaced.

The fuel line could need replacing, I could only inspect the outside of the fuel line and there was no debris from degrading fuel lines inside the fuel filter (tank to filter line). Is there any good particular way to inspect it?

I'll adjust the idle and mixture screws as they definitely have the potetial to be out of whack and see if that helps. I know the carb is dumping fuel when in neutral, however I haven't checked after I put it in gear.

I just assumed something was overheating since the boat will work fine from cold to 2-3 hours and then it'll act up, then i'll just let it sit for a while and it will start working again and then a cycle begins.

It doesn't sound like overheating just because you're in the normal operating range of engine temps, and you can run it so long before the problem persists. To me that sounds like fuel delivery issues since eit runs so well at all times besides under any sort of load. It makes sense that it will run for a while since any debris/buildup in the fuel line would settle, then get jarred around and shaken loose when in use, finally creating enough blockage to restrict fuel flow enough to stall your boat. If you don't know when the fuel line was replaced, you might as well just do that anyway. A 20+ year old boat needs it anyway. Try that and see where it gets you.

So an update to my problem...as much information I have on it....

I have replaced all of the fuel lines (they definitely needed replacing).

After that, I was still having problems, I hooked up an external 6 gallon tank while on the water straight to the fuel pump (to see if I had a fuel pump problem or if gas wasn't getting out of the tank). This external tank had a priming bulb and I realized that the fuel pump was not supplying enough pressure. I took the fuel pump off and took it apart and it was actually in pretty rough shape. So i'd figured I would go ahead and replace it.

This weekend I had the same problem (however I did fix some fuel/throttle drop off issues while driving around). I drove around for about an hour or so. Then the boat started hesitating when I put it in forward and would sputter and die as soon as I tried to give it more throttle. I lifted the dog house and realized that the fuel pump was very warm, I thought I may be having a vapor lock problem. I planned for this and I brought a squirt water bottle to cool off the fuel pump, I kinda limped it along between wakeboard pulls by using this method. Sometimes it would actually let me accelerate and sometimes it would die on me.

Eventually I turned the engine off and sat around for about 20 minutes with the doghouse up to vent hot air out of there, It ran fine for a couple more hours.

The gas I was using was not ethanol-free, however I do add the prescribed amount of sta-bil to any gas that I put in that has ethanol in it.

I think the issue is that the doghouse is getting too hot. Either vapor lock or some kind of electrical problem where the additional heat is increasing resistance somewhere.

One time I just dropped the anchor and ran the boat in neutral for a while and the boat eventually cut off, this leads me to think that the general doghouse area is overheating (with no ventilation) and is causing engine shutoff by either fuel cutoff or some electrical demon.

Thoughts? Any and all are appreciated. I'd like to diagnose before I throw any more parts at it.

I'm just going to try and help spit-ball ideas here but how about an eletrical fuel pump WITH a return line? That should be able to keep the fuel circulating and will prevent it from getting hot. Is your current fuel pump electric or is it mechanical?

My current one is mechanical. I just got this guy from skidim . Maybe getting an electrical one would've been a good idea...

What electronic ignition did you out in? Most need a minimum voltage to trigger. Heat increases resistance. You maybe having a heat related coil issue. There cheap. Get one that is resistance spec'ed for the ignition.

I used this kit from skidim. If I remember right that would mean this ignition coil.

Looks like the coil comes with it. Make surethe installer removed the ballist resresistor. Additionally whars your thoughts on the hot discharge hose from your pump? Something sounds wrong here. That hose shoukd be cold. Water does not circulate here. It goes from the pump to the tstat housing an either goes into the block or out the exhaust manifold with no restriction.

My boat came with an electric fuel pump and it has always amazed me that it's worked. It's pretty small and doesn't seem like it would provide enough fuel, but I've never had an issue.

It looks exactly like this one -- http://www.ebay.com/itm/AIRTEX-E8337...a48eeb&vxp=mtr

It would be something cheap to try if you suspect that the problem is with your mechanical pump. I'd check your coil next time you have a problem, though. Maybe CornRickey is on to something.

Vapor Lock maybe? Just my 2 cents.

87 Supra Sunsport, 351Ford. Took out forfirst time this season. When running at idle, heat would jump to 200-205 but when running under throttle, temp would drop to normal.
Thermostat or Impellar or should I check both, Thanks