Engine starts to miss / rough after 5-10 minutes with gas odor after 15

JohnEGraham

JohnEGraham

Active Member
Aug 5, 2008
1,013
Long Island, NY.
Boat Info
250 Sundancer 1998 5.7.
Engines
350 Merc / Bravo 3
1998 TBI Delco EFI 5.7 Mercruiser. The engine starts right up and runs fine, Revving up is no issue even when the engine is hot
. But after I take the boat out the engine starts to run Rough / Missing after about 5 -10 min and continues to get worse and then a fuel odor is observed after about 15 min.
I Filled the Gas tank with Premium fuel and replaced the Fuel / Water filter, The Throttle body was cleaned There are only 2 Large injectors and the Spray pattern looks good. Installed a New TPS and IAC, New Distributor Due to another issue of getting the Engine started set with advanced timing light. I assume fuel pump? But in tight spot willl need replacement by smaller mechanic..
Thanks

Thanks in advanced
John G
 
Oh, I did change the Wires last week and the Plugs a couple of months ago. I'll Check for fouling
 
Got this reply on another forum.. $500plus for 2 new injectors: will have to do more research

“I don't think the spray pattern is correctly visible to the naked eye. Almost guarantee one of those tbi injectors is done and you're experiencing fuel wash dumping into the manifold. It is not hard to rebuild those throttle bodies and you can start with these guys here: https://m.motormanfuelinjection.com/...s-17104288.htm
 
They call it a Dump-type injection system with only 1 injector for each side/. Not sure how a spray pattern at the injector makes a difference if it just flows down to each cylinder.
 
JohnEGraham said:
They call it a Dump-type injection system with only 1 injector for each side/. Not sure how a spray pattern at the injector makes a difference if it just flows down to each cylinder.
Click to expand...
It still relies on the ultrasonic transducer to more or less thoroughly atomize the fuel before entering the intake manifold. If one (or both) is bad, the fuel will run down into the manifold as a liquid, rather than a proper vapor. Not good for economy or even distribution between cylinders.
 
Noid lights and spark tester
 
If you spray a little starting fluid into the intake manifold will it fire
 
Scott215 said:
If you spray a little starting fluid into the intake manifold will it fire
Click to expand...
The engine starts and runs fine except under load, it states studdering running rough with a fuel odor. so stating replacing the two 25 Year old injectors
 
JohnEGraham said:
The engine starts and runs fine except under load, it states studdering running rough with a fuel odor. so stating replacing the two 25 Year old injectors
Click to expand...
Injectors are either good or bad. If anything they tend to become restricted with varnish as they age which reduces fuel causing a lean condition and not a rich gas smell condition. They could possibly stick in a open condition when they get hot - but I've never seen that.
First it was stated the engine/boat starts and runs fine until after 10 minutes when it becomes rough running and a gasoline smell. Now it seems it runs fine except under load???

Anyhow, ignition coils can exhibit this exact condition when they age and start to fail. As a part of saturation of the coil they become hot, this is normal, but they can also become erratic in producing the high energy discharge if there is an open winding. This can and does happen.
 
ttmott said:
Injectors are either good or bad. If anything they tend to become restricted with varnish as they age which reduces fuel causing a lean condition and not a rich gas smell condition. They could possibly stick in a open condition when they get hot - but I've never seen that.
First it was stated the engine/boat starts and runs fine until after 10 minutes when it becomes rough running and a gasoline smell. Now it seems it runs fine except under load???

Anyhow, ignition coils can exhibit this exact condition when they age and start to fail. As a part of saturation of the coil they become hot, this is normal, but they can also become erratic in producing the high energy discharge if there is an open winding. This can and does hap
Click to expand...
 
Call High mean by “under load” Is driving the boat not just sitting in the slip. Than it runs up and down fine…
I have a timing light to be able to check the spray pattern.
It needs atomization properly, air and fuel, if it isn’t than raw fuel can runs into the cylinder… that’s what I’ve been reading…
 
John,

Honestly this has been a tough year to diagnose things over the internet. We had a similar thread that took off to a couple of hundred posts that started with the engine not running right under load.

While in a lot of cases it is simple....fuel or electric....I have just learned that a properly done compression test at the start of the diagnostic process rules out the mechanical question. The great part about a compression test is you kill two birds with one stone since you can assure yourself that the mechanical is sound and you replace the spark plugs which can also tell you part of the story.

I hope it is just the injectors or pickup/ignition coil but I have seen enough issues this year to just start at a place which builds confidence that the engine is sound.
 
I did do a compression test before I replaced my manifolds and Risers and it all checked out fine at 165lbs plus or minus 5lbs after 4 compression strokes.
I did this early in the year due to water in the cylinder, and none in the oil. The water was due to the exhaust shutter falling out of place which I replaced.
 
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