Rough idle at startup, fine once warm

[VTmurtha]

Hey everyone,

Long time reader, first time post:

I bought a 1998 Stingray (192RX) with a 3.0L Mercruiser this summer. It's been running well enough all year, but recently has had a pretty rough startup; it starts up easily but I need to keep the RPM's up around 2000 for about 4 minutes for the engine to warm up before I can back down on the throttle. After that, it runs fine, idles at 700.

I have noticed some shaking at low RPM's before its good and warm like it's missing a cylinder (if it doesn't stall). This hasn't been a day-ruiner yet, more of a nuisence but I'd like to know what's wrong (it always runs great once it warms up.)

Some notes:
1) I've noticed temp gauge stays very low even during WOT (130 degs). I've read it should be around 165 but mine never gets that high unless I'm on muffs with the hose only partially open. (Gauge does work, I've seen it climb to 170). Could this somehow be a bad thermostat that is ALWAYS open and doesn't allow the heat to climb quickly at startup?

2) I have NOT had the cylinders compression checked. I'm guessing it could be leaky gasket on a cylinder that "seals" itself once she's warm. (But, it runs fine once hot hence my nonchalance about this.)

I winterized the boat this morning and she's sealed up for winter, but I'd like to have a plan of attack come late March.

Any ideas? I'm dreading the "check the head gasket" response because she still runs (for now...).

Thanks for the help!

 

[Jeepster04]

Re: Rough idle at startup, fine once warm

Started fighting a similar issue with our engines at the end of this season. Engines ran great all summer then towards the end they were hard to start and sometimes flooded out. Once they were started they ran rough for awhile till warm.

For now Im thinking were having ethanol issues since the gas was over a year old even though we keep stabil in the gas. The fuel/water separators finally filled up with water and now were having issues. Will find out for sure come spring when we open the boat back up but its something for you to consider.

Also, check the temp with an IR gun before assuming those gauges are right. Our gauges show 130 also but our engines are running 140-160F.

 

[Fireman431]

Re: Rough idle at startup, fine once warm

You have a bad head gasket (just kidding).

Sounds like your choke may not be operating properly and/or it needs a tune-up. Checking/replacing your thermostat is an easy fix.

 

[Don S]

Re: Rough idle at startup, fine once warm

Sounds a lot like a need for a full tuneup including a carb idle speed and mixture adjustment and look at choke, timing, total advance timing are in order if it hasn't been done in the last year.

 

[VTmurtha]

Re: Rough idle at startup, fine once warm

Not surprised I'll need to adjust idle speed or mixture... I rebuilt the carb earlier in the season but set the mixture to "stock" setting.

I'm fairly new to carbs and adjusting timing... I may be able to learn enough via forums this offseason to adjust all this stuff - but time is money.

How much would a "tune up" at a shop cost roughly to check choke + carb mixture + compression + thermostat + timing/advance? I'll still look into this stuff for my own sake too...

Thanks!

 

[ziggy]

Re: Rough idle at startup, fine once warm

i'm with dons. amazing huh. that's the place to start at. when was the last time ya did that (a minor tune up)? checking compression ain't so hard. here's the low down. http://www.boatfix.com/merc/Bullet/97/97_25.pdf from the diy, adults only sticky of course...
might as well get onto checking it. it's a base for your engine running proper. i've owned my boat 3 years, i have 3 years of compression test results in my folder. i know where i stand..
i'm convinced boats like tunes ups real regular. once a year at the least. they get abused with normal operation.. go to the diy or adults only sticky at the top of the i/o forum. read wot throttle issues, fuel sytems issues, whatever might be relevant to your issue. there's a lot of good stuff there. read your service manual too, many have trouble shooting guides to lead you in the right dirction.
take any wire connections apart and clean their connections with emery cloth or the likes. starting with the battery and go on from there. wire connections seem to cause issues in boats for lack of contact over time..

I'll still look into this stuff for my own sake too...

might as well do that. your service manlual is likely available from the diy stiky, #9, pick you poison. in them manuals you'll find out exactly how to do whatever deed yer wanting to accomplish. what you don't understand, come here and ask. someone will put ya in the right direction.
no idea what all this is worth at a shop. more than i got.......

 

[D_J_Winn]

Re: Rough idle at startup, fine once warm

Before the head gasket, complete a tune up first, if you don't remember when the last time you/someone replaced spark plugs, distributor cap, wires, gas filter, oil/filter thermostat etc. (clean your sensors inside of thermostat). Its time! good luck.

 

[intrepidvoyager]

Re: Rough idle at startup, fine once warm

Agreed ....tune up time ..... but when engine starts ok but won't idle until it is warmed up but then idles fine ....well that's classic sign of choke issue or a vacuum leak ...so look for one ...check your carb bolts are tight , etc.

 

[Fisnfool]

Re: Rough idle at startup, fine once warm

And check for a stuck thermostat too.

If it ran fine in warmer weather It may be just a choke adj. issue.

Like DON said if you can't remember last tune up, and you are not as old and forgetful as me, Oh look there goes a deer through the yard. Now what the ell was I doing? Oh Yeah. It's time for a tune up.

 

[fat fanny]

Re: Rough idle at startup, fine once warm

intrepidvoyager I think that your signature should be the mission statement for the stickies section, that in my opinion has to be the truest statement I have read on this forum since I joined PERIOD!!!!!!. But to the subject I also believe VT is experiencing a choke issue along with the lack of a good basic tune up remember check the results of everything you do for a change.

 

[Don S]

Re: Rough idle at startup, fine once warm

I rebuilt the carb earlier in the season but set the mixture to "stock" setting.

Those are not "stock" settings shown in the manuals. They are "This will get it running" settings. From there you have to adjust the carb mixture to your engine. It will NEVER be the same from one engne to the next.
Many manuals call the book values "Preliminary" adjustments.

 

[intrepidvoyager]

Re: Rough idle at startup, fine once warm

Thanks FatFanny for those few kind words in my shell like ear ...but what the hell is the stickies section !!!!

 

[Fisnfool]

Re: Rough idle at startup, fine once warm

A sticky is any post you see with a blue stick pin icon. That way it stays on the forum topic list and does not fade off. It is usually applied by the Moderator to help save computer space taken by constant repeating of FAQ'S

See any thing here with a pin as an example.
http://forums.iboats.com/forumdisplay.php?f=21

 

[fat fanny]

Re: Rough idle at startup, fine once warm

Go to the top of this forum and you will see posts that are not highlighted (stickies) they contain a large amount of valuable info service manuals you can download,common problems,troubleshooting tips and guides, All of which should be read by newbies before they ask questions on the forum to help aid in the education process of doing exactly what your signatures states. Since I joined this forum I know I will always be able to own a boat.

 

[Don S]

Re: Rough idle at startup, fine once warm

Thanks FatFanny for those few kind words in my shell like ear ...but what the hell is the stickies section !!!!

The first 3 threads in this forum are stickies. 2 of the 3 stickies are shown circled in red in the picture below.

 

[pine island fred]

Re: Rough idle at startup, fine once warm

A thought from left field if I may. If the engine is carburated, chances are that you have a heat raiser valve. Takes exhaust gas and routes it around the intake manifold collector area at the base of the carb. In cooler weather the fuel/ air mix will condense and form droplets of gas in the manifold which eventually break away. Look for some kind of spring/ counter weight assy in this area and make sure it moves. Is a pretty easy assy. to see on an inline engine. The fact that you have increased velocity along with the vaporization of the fuel naturally chills the area until you reach operating temperature.
Not to be confused with carb. ice which has nothing to do with water in the gas. Carb ice is the formation of thick frost in the throat of the carb which slowly chokes off the air pulled thru the carb. Can occure when the temperature is well above freezing with high humidity. regards FRED

 

[Don S]

Re: Rough idle at startup, fine once warm

Look for some kind of spring/ counter weight assy in this area and make sure it moves. Is a pretty easy assy. to see on an inline engine.

That is something on the automotive side. Boats have NEVER used heat risers.

 

[Fisnfool]

Re: Rough idle at startup, fine once warm

Carb Ice. Not that is a neat explanation of something I have hear of but never heard the name.

I love this site. You can learn something every day.

And since I live in a humid area, that is great stuff to know.

THANKS

 

[Showtime22]

Re: Rough idle at startup, fine once warm

Do all the usual tune up and -

I'd do a new thermostat for sure. If the previous owner was a hack like me before IBOATs - He may have put in a car thermostat which opens near 140 instead of near 180.

 

[bruceb58]

Re: Rough idle at startup, fine once warm

Do all the usual tune up and -

I'd do a new thermostat for sure. If the previous owner was a hack like me before IBOATs - He may have put in a car thermostat which opens near 140 instead of near 180.

A hack would put in the 180 thermostat. One who knows what they are doing would put in the temp that is called out in the manual. For my engine , that would be closer to the 140.

The gauger may say 130 but the real temperature may be way different. To really know what temp the engine is would be to use an IR thermometer and measure it that way.