3.0L rough idle issues

[havoc_squad]

Brief summary:

1993 Mercruiser GM 3.0L engine.

Boat ran great last year, winterized and I believe the fuel was stabilized. No carb issues whatsoever.

New spark plugs were installed before de-winterization.

De-winterization attempted, fuel pump failed on me. No start, primed intake with gas, ran fine while it had gas to ingest.

Replaced fuel pump, fuel filters changed, runs however idles badly on muffs. The longer it runs at idle, the more it seems to lose strength and basically stumbles nearly to a stop.

As soon as you let the throttle make up the slight difference, the boat runs fine at 1100 to 1700 rpms. I think it would run fine on the water in higher RPM's. However I don't think it would even let me get in gear with how rough that idle is.

I am going to run a half a can of Seafoam through the fuel if possible.

Any tips on gauging if this is clogged idle port(s) or something else?

I'm tempted to want to see if the idle mixture adjustment needs to be changed, or the idle speed adjustment. However, I don't want to incidentally create a high lean mixture issue or do something really stupid.

I'm just asking so I can make sure before I go and pay for a $450 carb rebuild on this thing. It's bad enough that I boneheadedly spilled a 1/4 of a gallon of gas over my boat carpet only 9 months old at the back while trying to run a test tank. It delaminated up and I've now got to pull up the damaged area to splice in a new piece, man am I glad I kept my leftover carpet.

Meh, its always something that needs fixing, or that needs breaking, like last weekend.

 

[Bondo]

Re: 3.0L rough idle issues

I'm just asking so I can make sure before I go and pay for a $450 carb rebuild on this thing.

Ayuh,... Welcome Aboard,..... While that sounds like an ungodly Price, I think the carb Needs to be Rebuilt...

 

[NHGuy]

Re: 3.0L rough idle issues

Time to redo the test tank run, just don't spill again.
If it runs well on a test tank you might have a clogged fuel vent, antisiphon valve, supply line or fuel pickup. The pickup clogs can be really frustrating if the blockage falls off the tube when the motor isn't running. A leaking fuel line before the pump will allow air into the fuel.
If it runs badly on the test tank you have eliminated the fuel supply as the culprit.

 

[havoc_squad]

Re: 3.0L rough idle issues

Near 100% certainty it is not fuel fuel issue to carb or bad gas. Other than the new gas I added, all the old gas was 89 ethanol free with stablizer and no older than 9 months old. It's either clogged idle in carb or the idle settings are way out of whack.

Probably clogged, I guess the stabilizer did a poor job this year.

I'll probably see it clears up this weekend some running that Seafoam through it before taking it in for a rebuild. Doubtful it will, but one can hope.

If this was a two stroke engine, I would default to a rebuild without hesitation.

 

[fat fanny]

Re: 3.0L rough idle issues

Paying that kind of $ for a re-build is insane if you changed the pump and filters you can re-build the carb just get the numbers off the carb get a kit and some carb dip and as long as you have access to compressed air it's no harder than building a model car stay organized and follow the directions easy breezy plus everyone here is always willing to help out if you get stuck.

 

[havoc_squad]

Re: 3.0L rough idle issues

I didn't quote the exact price of that rebuild, but I was going off what I "roughly" remember from about 3 years ago for a carb rebuild.

I believe it was $400 including tax on the rebuild on a different boat, same setup 140 1980's IL4 mercruiser engine.


Sorry guys, but there are some tasks such as carb rebuilding or bellows jobs that I just don't have time to do and don't have the space to store expensive specialty tools.

 

[mark1905]

Re: 3.0L rough idle issues

You've already messed with your idle air/fuel mixture screw and verified that it's still in adjustment, right?

 

[havoc_squad]

Re: 3.0L rough idle issues

You've already messed with your idle air/fuel mixture screw and verified that it's still in adjustment, right?

No, I never touched the idle speed or idle mixture ever on this boat. I was thinking about it. First I was going to mess with it while on the muffs to get it "good enough", then when its stable put it in the water and see how it behaves.

I didn't want to do a bunch of compensating that would end up in futility, hence this is why I asked now in this forum.

 

[oldjeep]

Re: 3.0L rough idle issues

New spark plugs were installed before de-winterization.

Verified correct plugs, correct gap and plug wires all the way on and in the right position?

 

[fat fanny]

Re: 3.0L rough idle issues

s is a phillips and flat blade screwdriver and compressed air. And for that price you can buy a little one from harbor frieght and still be under that cost alonng with the carb kit and dip.

 

[Scott Danforth]

Re: 3.0L rough idle issues

a carb kit will be under $100.

how do you know you dont have a fuel quality issue?

 

[havoc_squad]

Re: 3.0L rough idle issues

a carb kit will be under $100.

how do you know you dont have a fuel quality issue?

Mainly because as soon as the electric choke opens from a cold start, it starts idling rough. Which if my memory serves correct signals a lean condition on the idle jet flow.

Second, I don't ever let gas sit unused over 9 months in the tank. It gets used and replaced, then stablized at the end of the season. I use my boat, not stare at it in storage for two years.

Third, fuel was 100% gas in it, no ethanol and when I pulled the water seperater filter, no water showed up when I poured its gas in a canister.

Most likely on Monday I'll be getting the carb rebuilt while I'm at work so I can focus my time and energy to carpet glue removal on the floor at the damaged area and installing the new replacement carpet piece that should keep the seams down to just one that I would like to make sure is barely visible.

Yes, I might save $200 in rebuilding it myself, but if it takes me 8 hours of total time to get it right and drags on for a week and a half, I'm shooting myself in the foot.

 

[havoc_squad]

Re: 3.0L rough idle issues

I got an ambush surprise today.

Today it idles perfectly wonderful now.

After I ran it yesterday on muffs with Seafoam dumped into the gas tank for a good while in investigating the rough idle issue.

Got home from work today, pulled the flame arrester off the carb, hooked up the muffs, started, let run for about 15 seconds at 1500 rpms, and lowered the throttle back to idle. Ran bloody perfect at idle for all the time it ran.

The choke operated properly, and no issues were noticed other than my tachometer is starting to fail. No surprise there, as the voltage gauge stayed constant and it was slightly off always last year.

I'm not saying Seafoam is the cure all, fix all for carb/fuel injected issues, but I'm not saying it didn't help my situation either.

Lake test tommorow to verify no hidden issues with carb, then hopefully I can focus on the carpet repair fix without dumping more money to fix crud.

 

[mark1905]

Re: 3.0L rough idle issues

No, I never touched the idle speed or idle mixture ever on this boat. I was thinking about it. First I was going to mess with it while on the muffs to get it "good enough", then when its stable put it in the water and see how it behaves.

I didn't want to do a bunch of compensating that would end up in futility, hence this is why I asked now in this forum.

It's a simple check and you're not going to mess anything up. While running, turn it either direction until it runs best. Simple enough.

 

[fat fanny]

Re: 3.0L rough idle issues

Keep an eye on the electrical gremlins they can bring up all sorts of issues I know when my temp sending unit and ignition switch was givin me pains of blowing fuses my idle fluctuated some and affected the engine idle irradicaly. Had to pull the whole wiring harness out and go thru it just to find one bad connection on a bullet connector and re- wrapp the whole thing again with kevlarand seal all the connections with heat shrink. Not fun! Glad to hear it's runnin better

 

[havoc_squad]

Re: 3.0L rough idle issues

Keep an eye on the electrical gremlins they can bring up all sorts of issues I know when my temp sending unit and ignition switch was givin me pains of blowing fuses my idle fluctuated some and affected the engine idle irradicaly. Had to pull the whole wiring harness out and go thru it just to find one bad connection on a bullet connector and re- wrapp the whole thing again with kevlarand seal all the connections with heat shrink. Not fun! Glad to hear it's runnin better

Just to reinforce what I meant to say previously, the tachometer is the only electrical sending device that is flaking around (sometimes accurate, sometimes not). The voltage gauge showed a stable level of slightly over 13 volts at idle.

I tapped the plastic cover of the tachometer with my index finger and the gauge went nuts. Hence I think its just about on its deathbed.

I'll post the results of the lake test if I get a chance to getting around it this weekend.

 

[mpdive]

Re: 3.0L rough idle issues

When you said new plugs before winterization, did you fog the engine? If so, were the new plugs in at the time? I ahve had to replace new plugs that were fouled from fogging. Just askin.........

 

[havoc_squad]

Re: 3.0L rough idle issues

When you said new plugs before winterization, did you fog the engine? If so, were the new plugs in at the time? I ahve had to replace new plugs that were fouled from fogging. Just askin.........

Unless situation requires, I never replace plugs before winterization.

It's always easier to just change them out late February or early March at my leisure since it is winterized.

Easy is better, is it not?

 

[mpdive]

Re: 3.0L rough idle issues

Unless situation requires, I never replace plugs before winterization.

It's always easier to just change them out late February or early March at my leisure since it is winterized.

Easy is better, is it not?

Yes. Just wanted to make sure you didn't have a fouled plug

 

[havoc_squad]

Re: 3.0L rough idle issues

Well, crud I had a strong feeling this would not be fixed.

Did a lake test in the morning and ran the boat on the muffs this afternoon.

The idle issue pops up whenever the engine gets out of cold start and warms up enough.

I'm mainly ruling out electrical because after a long run at throttle, it would idle well when in gear for about 1 minute (residual gas remaining) or so, then the idle would start getting rough.


Back at home with the muffs, I changed the idle mixture some, no difference and I thought I reset it back to previous state while running on the muffs. However, when I went to restart the engine after I turned it off, I think it slightly backfired through the carb, releasing a little smoke through the flame arrester.


With that subtly scary moment, I'm done testing and debugging it today, Monday it goes for a carb rebuild.