1997 454 Merc. MAG MPI - Idle/Surging issue
 

Hello..
I am having an interesting issue that I cannot diagnos for the life of me. I am having some trouble with my 97 454 Merc. MAG MPI (500 hrs). When the engine is cold she starts up just fine idles at 1100rpm and drops down to the appropriate idle. Just like a fuel-injected car. All season I have been having this problem when the engine warms up the idle surges. For example, I am cruising up the creek and the rpms will be set at 800-900 and then all the sudden the engine will jump up to 1000-1100 and then back down. Sometimes I have to hold my hand on the throttle to keep it steady. Then I was waiting for the gas dock at idle the engine would rev up to 900-1000rpm and then drop back to the ideal rpm. Sometimes the idle would stay at 900rpm. I then put the boat in neutral and rev the engine and it would back down. This issue only seems to happen when the engine is warm.

Some have suggested:
-Replace the ECT (because the ECM might not know the exact engine temp?)
-Replace the TPS (just a guess)
-Clean the distributor (Merc. manual suggestion)
-Bad ECM??

Should I try and replace the ECT sensor?? Any suggestions would be greatly appriciated. I am stumped and my marina mechanics don't seem interested in helping me.


Thanks!

Rob

EDIT: I did replace the MAP sensor. When the marina hooked up the laptop to the ecm they saw some issuse with the map sensor. They said that was the only abnormal reading.

Re: 1997 454 Merc. MAG MPI - Idle/Surging issue
 

You most likely have a dirty IAC motor and/or dirty throttle body. You need to remove and either clean or replace the IAC motor and clean the throttle body where the blades close. Also make sure the seat where the IAC pintle contacts is clean.

Re: 1997 454 Merc. MAG MPI - Idle/Surging issue
 

Thanks insptech! I was just reading up on replacing the IAC. I did take the flame arrestor off and the butterflies looked dirty. I am going to order a IAC and TB gasket right now.

What would you recommend I use to clean the throttle body?

btw: I notice your in G-Burg. Are you the shop off Gude Dr?

Thanks again.

Re: 1997 454 Merc. MAG MPI - Idle/Surging issue
 

I just did mine the other day with Swipes and it worked great, however any fuel induction cleaner will work. For the throttle body, spray it on a rag and wipe so it doesn't end up on paint or rubber seals. I tried the Swipes just for grins since they are much less caustic, turns out they did the trick. I am not affiliated with any shop, but I can steer you in the right direction if you need "professional help" (on the boat) :D

Re: 1997 454 Merc. MAG MPI - Idle/Surging issue
 

Thanks for the help! I am going to replace IAC and then clean the throttle body as the next step. It seems this surging/loping idle is common on the MPI's and the IAC is the right fix. While I'm at it I'm going to clean the ETC sensor.

:D

Rob

Re: 1997 454 Merc. MAG MPI - Idle/Surging issue
 

On my own, I clean it once, then replace if it sticks again. On other's boats I usually tell them to change because I'm not going in there twice :D Most IAC motors are fairly cheap-even from Merc. You will find that if you pull the throttle body, clean it, and the IAC at the beginning of the season when you change the plugs, you will never have a problem.

Re: 1997 454 Merc. MAG MPI - Idle/Surging issue
 

Sometimes it is on the part.

Re: 1997 454 Merc. MAG MPI - Idle/Surging issue
 

I ordered the IAC and throttle body gasket for a total of $75. Seems like a simple repair.