MerC 5.7 TBI Won't Idle

J

j-d

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This is a 1996 pre-Vortec flat cam serial 0F800414. We ran two patrols, Friday and Yesterday, for a Blue Angels air show, 70 NM over those two days. Ran faultlessly Friday. Yesterday engine got to where it would not idle. Linkage is closing the throttle plates. Idle Speed Over Ground (SOG) is 5 KT at 1100 RPM. Neutral Idle at that point was 1500 RPM. At end of yesterday Idle was 2200 RPM. This is too fast to shift, especially with Alpha Drive and today we have another 40 NM. I have a spare used Idle Air Compensation (IAC) Valve that I'll try to change at the marina.
Any other suggestions?
 
A marine scan device could reveal whether the IAC or TPS are out of parameter specs. Even a clogged IAC "muffler" (foam filter) that I believe Merc uses could cause abnormal idle. You could check the MAP sensor , engine coolant temp sensor (CTS) and base timing as well. If a sensor is faulty the computer may stay in cold-engine high-idle default mode. And if the distributor base has moved, that will affect idle rpm and other running as well.

You said the linkage is closing the throttle plate, but just to make sure, do you still have very high idle with the throttle cable disconnected from the engine? Is the throttle body clean so the plate can truly close properly?

Are you sure the "spare used IAC" was not removed because it was faulty ?

If you change out the IAC, be sure to give it a little time when 1st running again as the engine control module will need to learn the new part's response to it's commands. For example, when I replaced a bad IAC in my previous Grand Cherokee with TBI I drove it for about 15-20 minutes before the computer and IAC got sufficiently acquainted for everything to run really normally again.
 
Isn't there a warning buzzer that sounds every minute if the IAC goes bad? Seems mine always did.
It will be interesting to hear the cure.
H
 
What is your no load idle? 2200 RPM? What is the in gear idle RPM?

I just don't have the experience with fuel injection. Isn't there an idle adjustment screw?
 
The boat has an undiagnosed stall at idle about one slowdown in hundreds. I bought an IAC hoping it might clear the problem. Still a stall once in awhile, very rarely. MerC master mechanic with scanner could not find anything.
The IAC I put back is the one I replaced and did not change anything. I think it has about 1000 LESS hours on it than the one I put back. I was very fortunate to guess what tools I needed and packed them up along with some throttle body spray cleaner. One connector and two small Torx screws. Removed one, sprayed cleaner, installed new and ran bilge blower till exhaust didn't smell like cleaner. Started right up, idled right, ran simply GREAT all day. Very fortunately. We were 25 miles from home and had a mission to run that went about 20 miles, then all 25 back. Faultlessly.
Michael, I'd like to test TPS sometime, for now thankful to be running and likely won't mess with it unless symptoms return.
Sandy, There is not a filter. I think it relies on air through one part of the throttle body diverted to another part. I've heard of that learning and seen it on a Ford but didn't happen this time on the GM Rochester TBI system. The TBI seems to be automotive but with a MerC version of the ECM. There is no MAF or MAP sensor.
Harlan, I think you're talking about a SmartCraft MerC engine and ours is not. You should have seen River City Marina yesterday after the air show! Tide running out hard, river all chopped up with part of the 1000 spectator boats running through. Needed Sea Legs to walk the Fuel Dock!
Charlie, Idle speed SOG when things are right is around 2.5KT. Yesterday at 1500RPM neutral idle the "idle" RPM was about 1200 and SOG 5.6KT. 2200 would be close to 9KT, but I couldn't check it yesterday in that running current.
I've had a long three days. 135 miles on the three days of air shows. Thanks for all your suggestions. Next problem post before long... Water dripping out of TOP of Lower Shift Cable!
 
The only problem I ever had with FI was the TPS.(on car) There was a dead spot just above idle. Sounds like you fixed whatever it was. Maybe just disassembling and reassembling did the trick.
 
The one I took out looked pretty crusty on the shaft that brings the valve plunger in and out. My mechanic cleans IAC's but what I've read says not to. Clean the Passages but Replace the IAC.
 
Now if you had a CARB, The diagnosis of stalling during slow down would be dampner pot letting butterfly close too fast.

All these fancy new fangled Do-Dads just getcha' more stuff that can break. Do what Thuddie does, put in a Briggs and Stratton.
 
Agreed on the Dashpot. I can see where the IAC could be closing just a little too far at idle, cutting off ALL the air to the TBI. Or maybe the seating area for the plunger gooped up and causing the correct closing position going into a restriction. I'd have to study up on how the IAC works. It has several terminals so I'd guess that some position the plunger on command from ECM and others tell the ECM where the plunger is.
 
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