surging (hunting) idle when cold....

It happened on the girl friends suv, exactly what your boat did and she called me telling me the idle was going up and down and it finally evened out but give it gas would die but start right up with idle up and down then settle down, told her to let it warm up to temp and see what happened and still wouldn't take gas so she shut it off and sat there for 15 or 20 minutes till I could get there, started it up then and it worked fine. I at first thought water in gas or water froze in fuel line but when I hooked up scan tool it set 2 codes, 1 was IAC other was fuel actuactor, Hasn't done it again. A buddy and me went down to his boat a while after that just to check it and he started his engine up and did same thing, checked intake and they were iced up on outside, same deal let it warm up and everything was fine. Just a fluke with the right weather conditions. I would think the odds of your 2 engines with separate fuel tanks to develop a water in fuel condition at exactly the same time would be pretty high!
 
May I suggest replacing the spin on water separators with the spin on Racor filters with the drainable bowl on the bottom. Make sure on an inboard boat you get the Racors with the cast aluminum bowl. The Racors with the clear bowls are only for outboard boats.
 
So I did some looking into this last night (wife is out of town and I have time on my hands!). Carburetor and throttle body icing is a real thing - I don't think it can happend in a multi-port injected engine. Usually happens on a cold start before the engines have fully warmed up. The venturi effect in a carburetor or throttle body creates low air pressure and significantly cools the air going into the engine. When it is cold, but still enough humidity in the air, it is possible for the ice to form in the carburetor or throttle body. It is LESS likely to happen in very cold conditions as once temperatures drop into the teens, there is not enough humidity in the air to freeze. Pretty sure this is what was going on. Think about small airplanes, they have a heat circuit to heat the carburetor, this is why (my father had his pilots license and a small airplane - he was always yapping about carb heat) - in an airplane, there is not as much heat transfer from the engine to the carburetor due to the cold conditions and airflow around the engine. From my reading the conditions have to be right, but it can happen. In Cliff's case, once the engines fully warmed up it eliminated the freezing in the throttle bodies - a marine engine takes a good bit longer than a car to fully warm up due to the water cooled exhaust. Plus a car engine thermostat is typically 225deg vs 170 in a boat.

So believe it or not -- I'm sticking to this story.
 
thanks for all the replies....I plan to stay on the boat this WE so I should have time to get deeper into this issue....

cliff
 
I never put fuel in the boat without a treatment. I just don't trust that there is no ethanol in the fuel, and even if there isn't, having an open system, the moisture in the air get in the tank.
Put a good treatment in the tanks and double up on it, and run some through.
 
I ordered a case of STP water remover/fuel anti-freeze...I plan to use it this coming WE to see if it helps....

cliff
 
So did you run the boat this weekend? Same problem?

didn't get the chance to get into the issue.....we got to the boat around 3:00 after a 3 year olds birthday party on Saturday and friends joined us on the boat shortly after......then the drinking and eating started and before I knew it was 10:00PM so that day was gone....Sunday was cold and rainy and just miserable so I didn't feel like messing with things.....

cliff
 
as a follow-up this past WE was warm for this time of year (high 60's) but was humid due to rain storms all WE.....on Sunday I started the engines and both started fine and idled fine....they both warmed up to normal operating temp of 170* without stalling.....

both engines would accelerate as expected with increased throttle....but when I brought the throttles back to idle both engines would either surge a couple times and stall or would just stall immediately....

both engines would start back up easily and idle fine but the surging and stalling when off throttle continued...

the problem seems to get better once the engines have run longer say around 25 - 30 mins but does not completely go away.....usually after they run this long when I rev the engines and return the throttles to idle they will surge a couple times but settle down to normal idle of 650 rpms without stalling....

I did not have a chance to check the fuel/water filters for signs of water in the fuel but these symptoms don't really indicate I have water in the fuel...the engines start and run fine with no hesitation or mis-fires....

since the weather was warm I am leaning away from the theory the problem is icing of the throttle body as well...

if I don't find any signs of water in the fuel next WE I guess it is time to call in a marine mechanic that I trust....good marine mechanics can be hard to find but fortunately I found one local to where I keep my boat slipped....

cliff
 
Being a hands on guy, I would think you should own a scan tool, so you can see what the IAC and TPS are doing
 
Being a hands on guy, I would think you should own a scan tool, so you can see what the IAC and TPS are doing

I'd also like to have one of those. What is your recommendation without costing a small fortune? Is the Rinda one good enough for a diy guy?
 
Being a hands on guy, I would think you should own a scan tool, so you can see what the IAC and TPS are doing

yeah, i just don't want to spend the big bucks for a scan tool.....don't think I would use it enough to justify the cost....it would be nice to have though....

cliff
 
my somewhat 'educated guess' is I have a problem with either the IAC valves or the TPS sensors since both of these can cause similar symptoms at idle....just seems odd that both engines have the same problem around the same time....

cliff
 
I'd also like to have one of those. What is your recommendation without costing a small fortune? Is the Rinda one good enough for a diy guy?
I own a rinda, not familiar with others.
First thing to do is hook it up to your perfect running engine and record all the values 1 screen at a time, so you'll have a baseline for when something does go wrong. Many adapters are available for it as well.
 
my somewhat 'educated guess' is I have a problem with either the IAC valves or the TPS sensors since both of these can cause similar symptoms at idle....just seems odd that both engines have the same problem around the same time....

cliff

Yeah, I agree it seems like one of those, just can't get past why on both engines at the same time.
 
You said it was humid, how about moisture in the distributer caps or on plug wires.
 
You said it was humid, how about moisture in the distributer caps or on plug wires.

didn't check for that but I will next WE.....

thanks.....

cliff
 
I have had this happen with my 99 340 dancer with 7.4s in the summer once in awhile through the years if I haven't run it in awhile, Doesn't do it very often so I really don't worry to much about it. Boat has low 700 hrs on it and still works. One thing to look at, and I'm sure these boats have it, is a mass air flow sensor, I think that would also do that.
 
I have had this happen with my 99 340 dancer with 7.4s in the summer once in awhile through the years if I haven't run it in awhile, Doesn't do it very often so I really don't worry to much about it. Boat has low 700 hrs on it and still works. One thing to look at, and I'm sure these boats have it, is a mass air flow sensor, I think that would also do that.

thanks but my engines do not have a MAFS.....they use a MAP sensor in conjunction with a TPS instead.....

cliff
 

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