496 ho surging/rough idle
 

I had a fuel injector get stuck open in my 03 496 ho so I decided to replace them all with Delphi 25348180 injertors because I was told they are the ones that come in it from mercrusier. So after I replaced them and fired it up and at idle rpms the motor surges up and down with a air sucking sound. I have no faults so any idea what I should try or is that just a normal thing.

A small amount of surging is normal (in my experience) as the IAC cycles. This is more noticeable audibly than actually seeing a rpm fluctuation on the gauge.

My 2001 system did not have a muffler (small piece of foam material) in the throttle body at the IAC air intake opening and it does make some audible noise, again varying with the cycling of the IAC. I have purchased one of these mufflers but have not tried it yet.

Now, if the performance is different than it was prior to changing the injectors, then you may have an issue. Is that all you changed?

Yes that was all I've changed. I was getting a code saying that injertor 6 was open now that I replaced them the code went away. I haven't had a chance to run the boat in the water yet I only hooked it up to the hose.

Well, a bad injector will certainly cause all sorts of idling issues. Perhaps you've just gotten it back to the way it should be.

Again, I am saying that a very small amount of "hunting" on the idle seems to be normal in my experience. I am not implying that missing or roughness or surging over several hundred rpm is correct.

Good luck.

Are you sure you didn't cut a seal putting them in? Sounds a bit like the IAC is trying to compensate for a lean issue.

IAC could be going bad. Also the cylinder that you had the injector stuck open, you could have fouled the plug so bad, it doesn't clean up and could be bad. If the Injector was stuck open it was dumping fuel on that cylinder. It most likely was dumping it for a while before it completely got stuck open. I would pull the plug and check it out. IAC is also a possibility, they don't seem to be lasting as long as they once did. Good luck

They used 2 different style of Delphi injectors for the 496 depending on engine number serial runs ---- they used a pintle 3 hole and a pintle 2 hole and yes it matters which one you use for idle quality and the rest of of the throttle range for running quality.

With that said have no idea what yours takes without an engine serial number and have no idea how the person that sold you these 100 % gave you the correct ones without it either.

Also if you bought these injectors off the net its a 50 / 50 chance that they are cheap knock offs junk injectors that will never flow correctly. Very common today with fuel injectors advertised on the net and or ebay.

Possible your running issue is not injector related but a scan would help pin point the area for the problem. Possible IAC problem along with a real dirty intake tract system.

Do you have DTS if so NO IAC and NO IAC muffler have no idea info from your end is missing exactly what have and engine serial number.

My motor is the 0m025400 and I bought the injertors from ausinjertors.com

I have a Mercruiser scan tool to get the readings but I can't start the boat out the water pump is out of it I have to replace it and the new one is not yet here.

ok from the scan tool what is IAC PWM % at dead idle fully warmed engine - what is it when key on only non running engine but warmed up ??? and what is from a cold engine with key on and at start up ?

Whats your tps voltage ?

Whats your MAP readings

whats your fuel pressure(s) readings

I hoping the new water pump is here tonday so I can get it put back in and start it up and put on the scanner later tonight. I'll try to get the hole print out and send it to you so you can take a look at it and see if you see anything. I'll do one with the motor warm and started and one with it warm and not running.

Well it ended up being the IAC Once I changed that out it ran great

What IAC did you get? Merc or auto version?

Mercrusier

Thanks. Just replaced mine with an auto one and we'll see how it hold up.....

That's what was in it that I just replaced it was put in back in 2013

I wish I could figure out who makes them for Merc anyways some of the auto cross overs do not do as well plus have different PWM % compared to what Merc uses. Some auto cross overs even after new install will have a slight rough idle.

The harder the IAC works the faster it fails and of course on some IAC failures it has knocked out the PCM. I probably see 1 per year on an average.

Merc IAC do not even come painted anymore they use to. Just saying.

Ran mine for the first time today after replacing IAC with a NAPA one and it was obvious the idle was too high, around 900rpm per tach. Even after it warmed up it around 850-875 again per tach. Had my scanner with me but never hocked it up. May scan it tomorrow to see the exact rpm. I saw your other post BUP on running the test so I don't have premature failure. ay do that tomorrow too. I know theres also a thread on here somewhere that talks about the high idle but haven't found that one yet. It may be buried in the same thread as where you talked about testing it..... In case I don't find it, what do you do if it idle is high? I know it's from the IAC so do I mess with the idle itself or swap out the IAC till I get one that has the proper idle setting?

i'm having similar issue as you vindicator101 on my port side motor. Did you ever resolve the high idle?

Hi yes I did it was the Idle air control valve.

Nope never did get it sorted out. Just ran it like it was. Ended up dealing with some other problems that were more pressing. I may revisit it when the season starts. If you've got twins you can move parts around, which is helpful.

496 idle/load
 

I had a 496 ho that was having the suction sound fluctuating but ran really good. Noticed last weekend when i took it out the suction sound stopped but the motor started and idled smooth but when its under load and you romp on the throttle it sounds like a miss where compared to the opposite motor it runs and sounds good. I dont feel anything rough in the driveline but I plan on checking that too.