8.1 Surging at low RPM's

JMD

Member
Jul 31, 2017
33
Chicago
Boat Info
2006 Sea Ray 340
Engines
8.1 V-Drives
Hi all,

2006 8.1 Horizons

Engine starts ok, maybe slightly slower than other motor, but certainly starts ok. It then idles around 650RPM. Cruises fine, develops power fine.

However, if left in neutral and set to 1000 RPM, the motor surges in the 800-1200 RPM range.

I've attached a video. The first part is at idle, you will then notice when the RPM start varying after the throttle is increased.

IAC swapped with good engine this morning. Same result.

 
On list to replace this week.

Cool fuel filters
Plugs / wires

Anything else?
 
Got me a 502 mpi that will do that occasionally, comes and goes.
Let me know if you figure it out. I can say that it's not your plugs, wires or fuel filter
 
I have 02 8.1's i have the same thing. While running at slightly above idle aprox 5 mph my port motor sometimes surges. I have replaced plugs and wires, still does it. Seems to do it less often after a hard run. following looking for an answer.
 
Inside the stainless air intake and inside the round intake is a small square area. There should be a square "slightly larger than a sugar cube" foam cotton like muffler inserted. Over time this little $3 part gets ingested into the intake and should be replaced. I replaced mine last year due to similar issues.
 
Thanks all. While looking at the IAC muffler, I believe it may have been inserted into the wrong place. There are 2 nearly square insertion points on the top of the throttle. I moved it to the location near the 12 o'clock position. The engine ran better, but now has quite the whistle. I'm waiting on a new IAC muffler and will report back.

Many thanks.
 
Old thread - but I noticed the same issue.

Since the op never reported back a solution I am looking for any input.

I am wondering if outlaw is refering to replacing the IAC?

Thanks
 
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[QUOTE="JMD, post: 958470, member: The engine ran better, but now has quite the whistle. I'm waiting on a new IAC muffler and will report back. [/QUOTE]
8.1s whistle at certain RPMs. This is completely normal. Not to worry.
 
[QUOTE="JMD, post: 958470, member: The engine ran better, but now has quite the whistle. I'm waiting on a new IAC muffler and will report back.
8.1s whistle at certain RPMs. This is completely normal. Not to worry.[/QUOTE]

Thanks - Sorry I should have been more clear on my post - I was referring to the surging/higher idle the original poster noted. I started my new to me boat this weekend after the ride home last week and noticed the elevated idle on the port motor. Something like 1020 rpm or so. The STBD is at 650 rpm. Sounds like a slight surging. (I am not hearing any whistling noise).

Any thoughts on where to start?

As an FYI - During inspection I had a compression test (all good) and had my surveyor install new plugs I purchased since they were all out anyway. (560 hrs.)
 
Well I ordered two new IAC valves and mufflers. I will change them next week and report back if this solves the surging.
 
Yes. Don't forget the IAC mufflers while they are hidden under the flame arrestor they do get dirty. It's a quick, easy and inexpensive maintenance item (can't say that about much on a boat).

These are my old and new:
IMG_20220514_123052883.jpg

-Kevin
 
Well I ordered two new IAC valves and mufflers. I will change them next week and report back if this solves the surging.

And you do not need to order a Mercury IAC for an insane amount. You can get a Standard AC423 and a merc gasket for probably less than half the price. I would recommend staying away from the dirt cheap chinese knockoffs and go with a known name brand. While the cheap ones may work they will probably also fail at the worst times.

-Kevin
 
And you do not need to order a Mercury IAC for an insane amount. You can get a Standard AC423 and a merc gasket for probably less than half the price. I would recommend staying away from the dirt cheap chinese knockoffs and go with a known name brand. While the cheap ones may work they will probably also fail at the worst times.

-Kevin
Yes - Thanks Kevin - I searched here and saw several posts about them. I followed nmn999 and saw his results with one and bought that.
Hoping to get them Monday and see if that settles the rpm.
 
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Can't the OP test without them installed and see if it resolves the problem?
I am not sure what you mean but I am thinking by OP I assume you mean "original poster"? If so he hasn't been here since May of 2021. Since my issue was exactly the same, I just re-opened the post. Many of the original responders are no longer active.
 
Can't the OP test without them installed and see if it resolves the problem?
I am not sure what you mean but I am thinking by OP I assume you mean "original poster"? If so he hasn't been here since May of 2021. Since my issue was exactly the same, I just re-opened the post. Many of the original responders are no longer active.
Actually you can by covering the opening with something flat and clean although the idle will be low since or very low it is uncontrolled so open the throttle very slightly before you start it. This is best done with two people and don't use something that can get sucked into the low side.
 
I am not sure what you mean but I am thinking by OP I assume you mean "original poster"? If so he hasn't been here since May of 2021. Since my issue was exactly the same, I just re-opened the post. Many of the original responders are no longer active.
Yes, you in this case. Before you buy/wait for the sugar cube foam thing you should be able to test it without it. I say that based on what I have read online where some do not replace it, not from my own experience.
 
I went to the boat this am with my new IAC valves ready to install.
I started it up just to capture the actual high idle numbers before changing them out. Much to my surprise, no high idle and no real surging. WTF. Well since I have them and all my tools I decided to change them anyway. I started with the port motor as it was running in the 1000 rpm range. When I removed the flame arrester I was shocked how dirty the cube filter was and how hard it got. My new ones are not due till Wednesday so I just removed the dirty ones for now. I replaced this one (IAC) and it started and idled perfect.

The starboard motor ran well before but I decided to change this one too. This one gave me a little grief starting the first time. I had to give some throttle to get it started at first. Once running it sounded fine and I restarted several times, but the second time it sounded like it was about to stall then ran fine. Does this happen often?
 
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