Changing IAC valve on 7.4 Mag Horizon motor

Larry

Larry

Well-Known Member
Oct 19, 2006
1,284
Oakland, NJ
Boat Info
2004 42 Sundancer
Hydraulic platform
Cockpit air
KVH M3
Engines
450hp Cummins
Has anybody changed the IAC on one of these motors? I can not see how to do this without pulling 1 exhaust elbow! You have to pull the throttle body off and turn it over to get to it, and I can't pull the studs off that hold the flame arrester and throttle body to the plenum without pulling the elbow. Has anybody done this before on this motor???
 
Nope, I have the 7.4 MPI engines. When I bought the boat they had one with the 7.4 MPI and one with the Horizon engines. Both were the same price, found no advantage to having the Horizons, so I went with the MPIs.

The service manual shows that you have to remove the throttle body to remove the IAC. These IAC motors rarely fail, as opposed to the ones on the newer engines that fail regularly. If you're sure that the IAC is bad you might find it easier to remove the plenum first and then the throttle body.

Removing the exhaust elbow isn't really a big deal. What you want to do is to wet the fiberglass exhaust collector so that you can slide the exhaust tube backward after removing the hose clamps. Then you can pull the elbow.

You need to pull the elbows every couple of years to inspect them anyway. Might as well make this year the year.

Best regards,
Frank
 
Thanks's Frank, I guess this leads to another question has anybody removed the MPI plenum on these motors? I guess all I would need is a gasket set. But I will probably just remove the elbow... I actually bought all new elbows early this year (manifolds and risers are fresh water coled) and they are sitting in my garage waiting for the motivation to change them, well I think I just got motivated.

FYI: The sympton I have is that the engine RPM's hunt up and down if I bring the idle up to about 1000 - 1400 RPM, It starts going up and down by as much as 400 RPM's constantly. Now I just developed an additional problem that when I put it into gear it has stalled 2 or 3 times this summer (while docking, not fun!). I should buy a scan tool to confirm but, I think based on the symton the IAC was the rite place to start, I just didn't realize what a PIA it was!!!
 
I think you are right. The plenum isn't hard to remove. I looked at the shop manual and the high performance one isn't terrible to remove. The elbow is easier, except for the part where you have to slide the exhaust tube back.

To get those tubes out of the way, first remove the clamps. Then slide a hose removing tool under and work it around to break the seal. A tiny screw driver would probably work OK, but don't gouge the fiberglass exhaust collector. Then wet the fiberglass and try to spin the rubber hose until it breaks free. Then wiggle, turn, and slide it back until you have just enough room to remove the elbow. Install by wetting, wiggling, and sliding forward. It's a pain in the butt.

Best regards,
Frank
 
Thanks again Frank, your idea of wetting the fiberglass tubes is a good one, I have removed these atleast six or seven times (changing water pumps, replacing blower motors, repairing trim tab pump, and so on...) and none of them were allot of fun... We have great boats when we don't have to work on them...
 
I'll agree with that. I'm not all that big, which helps.
 
Does replacing the IAC correct that problem???
 
above idle? no. there is no fix. even talking to merc. You can give them the number off of your computer just to see if it needs to be updated but the last time I ran in to that they told me some of them will hunt like that in neutral and even in gear around 900- 1400. then will smooth out. And some motors will do it more than others.
 
flflatrate said:
above idle? no. there is no fix. even talking to merc. You can give them the number off of your computer just to see if it needs to be updated but the last time I ran in to that they told me some of them will hunt like that in neutral and even in gear around 900- 1400. then will smooth out. And some motors will do it more than others.
Click to expand...

Well I guess that kind of sucks.... The hunting I have had for years and didn't really bother me, I am more concerned that the engine now has stalled a few times at idle shortly after going into gear, makes docking not so much fun.... I have changed cap and rotor this year.
I am planing on going to the boat tommarow PM to change the IAC unless someone has somthing else I should be checking.
 
I have the same problem 1997 400 sedan bridge port engine only .
NEW IAC & TSP NEW WIRING SWITCHED ECM IT MADE NO CHANGE.
IM LOST

Lizcat72
 
Last edited:
I have had the same issue with my port engine since it was new to me in 2003.
Changing the plugs seemed to reduce the hunting, but not for long. I do a lot of slow speed running. It does smooth out at around 1100 rpm.
 
Well to update everybody, I changed the IAC motor and that did nothing for the stalling or the hunting... It is very infrequent that it stalls but this is still not acceptable. This weekend I changed out all the spark plugs on a recomendation from a local Merc mechanic, replacing the NGK platinum plugs with new ones ($10+ each...). This seemed to fix the hunting issue, and the motor did not stall, yet... I think it sounds like the idle is slightly lower then the other engine but according to the sinc it isn't, I just don't know how accurate the sinc is at idle speed.
I think it is time to spring for a scan tool...
 
The sync gauge is accurate at all speeds. Never seen a problem with it. Oakland isn't far from me if you want to borrow my scan tool.
 
I changed my port side saturday am., took about an hour. Disconnect your throttle arm pin by removing the small nut, spacer and washers. Remove the flame arrester, and with a small vice grip, just inside the outer threads of each stud, remove the four studs. Tap on the throttle body w/ a wide, heavy blade (scraper) to dislodge it from the old gasket. Remove the old IAC, clean all parts, and reinstall w/ new gasket. Works like a dream for me. My engines have about 1100 hrs each, and I by the way the old IAC looks. it may be the 1st time out !

Good luck !

Ed.
 
Ed,

Yours is allot easier then mine, remember mine are V drive transmisions so my motors are backwards which puts the flame arestor next to the exhaust elbow, mine was a pain in the a$$...
 
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