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2003 305 Runs Great Cold...Runs Poor Warm

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agifford

New member
2003 Mercruiser (Chevy 305) runs great when it is cold. After 10-15 mins of driving starts missing, coughing, sputtering, backfiring... If you let it cool for a while, it runs fine again. Has new cap, rotor, plugs and wires. I tried pulling one wire at a time to see if I could determine if one cylinder was not functioning properly, but it runs so smooth it does not even notice a missing cylinder. I let it idle for an hour, but that does not get it warm enough to cause a problem.

Buddy suggested I replace the hall effect sensor, trying to find one now. Are there any other ideas on this one for either diagnosis or the actual problem?

Thanks...
 
Re: 305 Runs Great Cold...Runs Poor Warm

Pull the spark plugs to see what color they are. Should be tan to medium brown. If white, it's too lean. Change fuel filters?
 
Re: 305 Runs Great Cold...Runs Poor Warm

I just replaced plugs... A couple old plugs were sooty black instead of brown, I noticed some corrosion on a couple wires so I replaced the wires too. I will replace the fuel filter, but it seems odd that a fuel filter would be the cause of a thermal problem. When the engine is cold everything runs perfectly at both low and high RPMs. It takes 10-15 mins of high RPM driving before missing, coughing, backfiring problem begins. If it were just cutting out I would suspect vapor lock, but it is also backfiring which sounds like something in the timing goes wonky when it heats up.

I will let you know what happens after I replace the fuel filter.

Thanks for your reply
 
Re: 305 Runs Great Cold...Runs Poor Warm

temp gauge does what when this is happening ?

When did this start ? Was there a event ?
 
Re: 305 Runs Great Cold...Runs Poor Warm

Sounds like the choke is malfunctioning................
 
Re: 305 Runs Great Cold...Runs Poor Warm

A couple old plugs were sooty black instead of brown
Click to expand...

That statement leads me to believe it is EFI.

It takes 10-15 mins of high RPM driving before missing, coughing, backfiring problem begins.
Click to expand...

Could it be there is a reduction in fuel at that point?
 
Re: 305 Runs Great Cold...Runs Poor Warm

Boat is a 2003 Bayliner with a 5.0L Mercruiser power plant (GM 305). It is MPI, no carb, although it does have a big single barrel butterfly valve where a carb would be (sans a fuel jet). I have never seen an EFI set up like this before. I have always seen individual lines running to each cylinder. I guess that is just my inexperience with this kind of fuel injection.

I replaced the fuel filter (was going to do that anyway) and low and behold...It did nothing to fix the problem...I was hopeful, even if I did not think it would work.

I have ordered a new hall effect sensor, should be a few days. Meanwhile all I can do is watch the beautiful passing weather and stare wistfully at the serene early morning glassy water which mocks me.

Does anyone have ideas of how I can test this thing to be sure this latest part is actually going to fix the problem? I guess this is a lesson in patience. Note to self: Never ask God to teach me patience.

I saw the coil suggestion, and I recognize that is also a possibility. How common are thermal coil problems? I also saw the question on fuel, and while that may potentially explain cutting out, it would not explain backfiring.

I ran a timing light on this to see where things were. Stupid crank is only marked with a TDC line and block only has the one notch. No additional marks to adjust timing. I read somewhere that you cannot adjust timing on this engine as it is all computer controlled using the hall effect sensor to signal where the crank is. I also saw something talking about a purple and white wire which I have not yet located (you need to be a contortionist to work on a boat in wet dock -- Had to dig a screw out of the bilge with a large magnet. fun fun fun). Timing is currently after TDC (although I have not found the purple and white wire), and I believe it should be 8 degrees before. I have not figured out if this is normal or not. I do know that when the boat starts running like crap, the timing position is still nice and consistent (albeit advanced at normal operating speeds). If you drop it back to idle when it is running crappy and convince it not to die, the timing still appears good.

How much fluctuation in the position of the timing mark is considered normal and acceptable?

Any more ideas on things to test, or how to test them? I am open to suggestions and anxiously awaiting a part. Is it here yet? How about now? Darn....

Oh, I almost forgot....Temperature is rock solid, so the motor itself is not overheating, even if some other part is thermally freaking out.

Thanks again for all your helpful ideas!
 
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Re: 305 Runs Great Cold...Runs Poor Warm

Coils break down under heat; try a different one. Back firing can occur when there is a lean condition. A failing tachometer can screw up ignition timing; disconnect it. I don't think the hall sensor will cure the problem. The carb looking thing is a throttle body injection unit.
 
Re: 305 Runs Great Cold...Runs Poor Warm

you stated MPI as the fuel supplying componet ..if so each cylinder is feed from each injector.

with your description of the carb looking componet I would say you have TBI .. with this system the cylinders in the middle get more fuel than the ends..If this needs rebuilding the middle cylinders get too much fuel ...

this is usually a diaphram that ruptures dumping excess fuel into the intake ..
 
Re: 305 Runs Great Cold...Runs Poor Warm

OK....Back again trying to get this thing working... Problem was NOT hall effect sensor. In fact the hall effect sensor is not even plugged in to anything. I tried soaking the ignition module in cold spray after warming up the motor to expose the problem. No Joy. It appears the problem is not the ignition module. Could still be the coil (although I tried to cool that as well and it made no difference).

A mechanic suggested he had a similar problem with the same motor due to a bad fuel pump, so I am looking at that now. All ideas are still VERY appreciated.

Motor was rebuilt a little more than a year ago and ran perfectly last season.
 
Re: 305 Runs Great Cold...Runs Poor Warm

Just curious but I've never stucxk my head in the engine comaprtment of a boat with fuel injection. If it was a car the first thing I would look at is the O2 sensor, does a FI boat have one?
 
Re: 305 Runs Great Cold...Runs Poor Warm

Not real familiar with a boat EFI setup but if it were a car I think by the problem you are describing I would pull the distributor and check the pickup coil on it or replace the pickup module. It sounds like once you reach your operating temp the module is flaking our on you. I have had many dirt track cars do this same condtion. But like I said that were cars and not boats, I might be looking at it all wrong. Just giving my input to your problem.
 
Re: 305 Runs Great Cold...Runs Poor Warm

if MPI find somebody with a scan tool and read the trouble codes from the ECM and go from there .
did the mech. or you check the fuel pressure on the fuel rail or pressure to each injector? scan tool should tell if an injector is not firing etc.

hate to say it but sometimes the ECM's go bad and when the boat warms up it will do the some of the symptoms your describing.
 
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