Engines hard to start - only sometime

[Flytrade]

Sometime both 6.2L Mercruiser engines are hard to start. They will turn over but fail to fire. I then turn the ignition on and off a few times to make sure the fuel pump is getting as much fuel to the system as possible. Then I advance the throttle and turn the engine over till it starts. I have always been able to start both engines, but only with trouble. However, at other times they both start as normal. Also, once they do start, they run perfectly.
I have tried replacing the Idle Air Control Valve and Throttle Position Sensor on both engines, but with no luck.
My next thought is to replace both fuel filters.
If you have any other thoughts I would appreciate it.
Also, how hard are the fuel filters to change? Could you tell me where they are located.
Thank you.

 

[Chip S]

Mercury recommends changing the fuel filters every 3 years. Since you haven't replaced them before, I would start with the fuel filters. I replace mine every year, but I tend toward overkill when it comes to maintenance.

 

[sbw1]

Agree with the yearly change out of all fuel filters and water separators if you have them. Some setups have two filters and a separator for each engine. Do them all yearly.

 

[Flytrade]

I ordered two fuel filters, and they should be here next week, so I'll let you know if they fix the problem. Thank you for your responses.

View attachment 104979
Agree with the yearly change out of all fuel filters and water separators if you have them. Some setups have two filters and a separator for each engine. Do them all yearly.

 

[Lazy Daze]

You have 4 filters. Each fuel/water separator canister has two filters as SBW mentioned. At least take the smaller, disc filter out and inspect/clean it.

Next step would be to check fuel pressure. Also inspect ALL rubber fuel lines for interior failure (can usually tell by feeling them).

 

[Flytrade]

I have the filters, and there are two for each engine. They will be installed Wednesday.
I also had my mechanic check the fuel pressure, and he says it's normal.
However, here is another piece of the puzzle I have noticed.
I first press the start switch to the first detente to activate the fuel pump. After 5 seconds or so I engage the starter and listen as the engine turns over.
Now I know this is going to sound crazy, but try not to laugh as you read this.
I can tell the engine is going to start normally when it turns over at the normal speed and sounds "rough".
However, I can tell the engine will not start if it turns over faster than normal and with a "smoother" sound.
The rpm is noticeably faster when the engine will not start.
I inspected the serpentine belts, and shot them with belt dressing, and so far the engines are starting as normal.
So now the question is - how can the engine turn over faster than "normal" during start?
I know this sounds crazy, but this is the way it is.

 

[ttmott]

It sure sounds like the base timing has changed.

 

[Flytrade]

I had the fuel filters changed, but the problem still remains. However, I can always get the engine to start by holding the start switch and advancing the throttle to half or more. It does take a few seconds to get the start, but at least the engine will run. And once started, both engines run perfectly.
However, I noticed that once the engine is running, and I shut it down after just a few seconds run time, that it will then start normally.
This would seem to indicate the engine is not getting sufficient fuel on original start. I then tried turning the ignition on and off a few times to activate the fuel pump several times. But with no improvement on original start.
I also changed the IAC's (Idle Air Control Valve) and this did not help either.
My mechanic is at a loss. Any thoughts would be appreciated.

 

[Lazy Daze]

As far as the engines turning over faster, the first thing that comes to mind is a loose spark plug(s).

 

[Flytrade]

As far as the engines turning over faster, the first thing that comes to mind is a loose spark plug(s).

That is a good thought, but after the engine was started, and ran for just a minute or so, I shut it down and it restarted normally. But I agree with your thinking - why would the engine rpm be different when it will not start?

 

[scoflaw]

I have the filters, and there are two for each engine. They will be installed Wednesday.
I also had my mechanic check the fuel pressure, and he says it's normal.
However, here is another piece of the puzzle I have noticed.
I first press the start switch to the first detente to activate the fuel pump. After 5 seconds or so I engage the starter and listen as the engine turns over.
Now I know this is going to sound crazy, but try not to laugh as you read this.
I can tell the engine is going to start normally when it turns over at the normal speed and sounds "rough".
However, I can tell the engine will not start if it turns over faster than normal and with a "smoother" sound.
The rpm is noticeably faster when the engine will not start.
I inspected the serpentine belts, and shot them with belt dressing, and so far the engines are starting as normal.
So now the question is - how can the engine turn over faster than "normal" during start?
I know this sounds crazy, but this is the way it is.

I've noticed the same when an engine is flooded

 

[Lazy Daze]

I dunno, honestly. I'm just thinking outloud here - that's a tough one to diagnose via a forum.

But, again, thinking outloud... the engine could start easily the second time since it was just running and it's kind of "ready to go"... if that makes sense. Also... please forgive the extremely technical nature of this post

Edit/add... however, if once it's running, you're getting your normal top end speed and RPM, I'd veer away from the plug idea. But it wouldn't be a bad idea to check them just for giggles.

 

[Flytrade]

This is the way I picture the engine in my mind.
When I try to start a cold engine, there is a bowl in the fuel system that is empty. As the engine turns over, and I advance the throttle, this bowl starts to fill with fuel. Once full, the engine starts and runs normally.
If I shut down the engine and restart it in a short period of time, this bowl still has sufficient fuel to start the engine. But, if I wait a longer period of time, the fuel in the bowl goes someplace and we are right back to square one.
However, this still does not explain why the engine would turn over faster when the bowl is empty. It really is amazing that I can tell by the sound/speed of the engine as it turns over if it's in a good or bad start mode.

 

[capecodcruiser]

You have fuel injectors, not "Bowls" which are on carburetors. My theory is that you have leaking fuel injectors. Hence you needing to advance the throttle which gets more air into the engine and it starts. If fuel is leaking into the cylinders, that might be why you hear a different sound on initial start due to extra fuel trying to compress in the cylinder. And injectors can leak sporadically, hence why it happens sometimes and not others. Timing doesn't all of a sudden change, spark plugs don't just get loose and serpentines have nothing to do with starting other than keeping batteries charged. There are stats on how long high pressure fuel side should hold pressure. Say 40psi for 1 hour. Totally just made that up, but you get the idea. I would find those specs and do that diagnoses to start pinning down the issue. Don't just throw parts at it, unless the money tree in the back yard is in full bloom, than go for it!

 

[Flytrade]

You have fuel injectors, not "Bowls" which are on carburetors. My theory is that you have leaking fuel injectors. Hence you needing to advance the throttle which gets more air into the engine and it starts. If fuel is leaking into the cylinders, that might be why you hear a different sound on initial start due to extra fuel trying to compress in the cylinder. And injectors can leak sporadically, hence why it happens sometimes and not others. Timing doesn't all of a sudden change, spark plugs don't just get loose and serpentines have nothing to do with starting other than keeping batteries charged. There are stats on how long high pressure fuel side should hold pressure. Say 40psi for 1 hour. Totally just made that up, but you get the idea. I would find those specs and do that diagnoses to start pinning down the issue. Don't just throw parts at it, unless the money tree in the back yard is in full bloom, than go for it!

Thank you, and I like your analysis. If an injector is leaking, does it need to be tightened, cleaned, replaced, other?
Is there any way for me to analyze the problem by looking at the engine, or are special tools needed?
I had my mechanic look at the problem, but he was of no help.

 

[capecodcruiser]

Fly, nothing you can really do externally. "Leaking" means the internals of the injector are worn and the fuel that stays in the system is leaking into the cylinders. Hence doing the residual pressure test suggestion. If you aren't able to do the work, the hard part will be finding someone competent to do the work. If it were me, I would chase that possibility first and than go from there. Diagnostics is process of elimination, but with some intuitive thinking and understanding on how systems work. I would never look at the transmission for a starting issue for example. You have used your common sense to help in diagnoses which is a huge help.

 

[Lazy Daze]

You can have your injectors rebuilt - typically it's about $20 per injector and at 15 years old, it's not really a waste of time and money to do so. Seeing as you're in Florida, you shouldn't have any problems finding a fuel injection shop. They're more common than you might think.

 

[Flytrade]

Thank you for the info on the injectors. My mobile mechanic was not able to diagnose the problem so I'll have to find someone else. I could take it to Marina Max, but they have such a long wait list. I'll keep you posted how this plays out. But, thanks again for all your help.