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
 

Are you better off cleaning or replacing the IAC? I have a similar problem.

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

Anyone have the GM numbers for the IAC and TPS?

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.

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

UPDATE


Well... the IAC didn't make it in time for the weekend. So I went down to the boat and took the throttle body off. It was a little gummed up. I then pulled the IAC and it was loaded up with carbon. After cleaning it I noticed half of the "pintle" looked like it was rusted/coroded. I couldn't get that part clean. So I cleaned and followed the reseting instructions from the Merc. man.

We let the boat idle for awhile to warm it up. Then motored up the creek and so far so good. As soon as we put load on it the surging came back. Not as severe as it was. It was definatley an improvment. So next weekend I am going to swap the old out with the new and hope it'll cure the problem.

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

UPDATE:

Well, I dropped in a new IAC and the problem still occurs. When the engine is cold its was fine. The surging happens only when the motor is hot. The idle issue wasn't as rough. So the new IAC helped a little.

We had a fairly long run (about 45) minutes @ 2500 rpm. Once I came to a stop and put the engine in nuetral the engine would rev (lope) from 900 to 1100?? It did this a couple times.

So what would be the next step????

-TPS
-ETC
-ECM??? (please no)

I'm stumped, please help??

Thanks!

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

One possibility may be that the TPS does not go down all the way to 0% when the throttle plates close. In that case the ECM cannot identify that you want it to idle and it will surge in the same manner as it often does when you set the throttle in "fast idle" position. If you have a scantool you can check in what position the ECM believes that the throttle plates are when the throttle lever is at idle.

Another possibility on the same theme is that the throttleplates actually dont close completely when you have the throttle lever at idle with the same result.

This would however not explain the surging under load.

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

The "under load surging" only happens a low rpm. 1100 rpm and under. Like I was saying when I hold my hand on the throttle lever at the low speed idle the surging stop. Anything above that the engine runs like a champ.

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

My experience with the MPIs that I have had is that they do surge a little when in " fast idle" (around 1000 rpm). This of course becomes worse if it is not in gear. However, when getting close to idle position of the lever the TPS should signal a voltage of max 0.7 V which the ECM interpretes as 0% opening and it goes into idle mode which should be 600 rpm, basically rock steady. I would check that the throttle plates close completely and that the TPS voltage is not more than 0.7 V when the throttle lever is in idle position.

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

Mats..

Thanks for the help. I don't have a scan tool but I do have the manual so I can test the voltage.

How about just replacing the TPS? Would that cure a issue with a bad TPS? I will also check the throttle plates.

Now when you mean is is common for MPI's to surge. It is the same issue where the surging jumps from 900-1100 and back down pretty quickly??


Thanks again,

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

Regarding changing TPS; Suggest to start with finding out if it delivers faulty values :D . If it does it may be either an adjustment issue or the TPS itself.

Regarding surging; From the two MPIs that I have owned, I have observed that they sometimes have difficulty to maintain a steady rpm at "fast idle", especially when not in gear. In the one I have now it is actually worse when the engine is cold. It may sometimes vary some 100 rpm when in gear. At idle, it is of course key that the throttleplates close completely and that there are no other vacuum leaks such as unconnected vacuumhoses. Otherwise the ECM is not able to maintain 600 rpm within the dynamic range of the IAC and ingition advance that it plays with.

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

I will check the voltage values before I start ordering more parts.

It's just after I replace the IAC motor she started up just fine, idled properly. Then as soon as I got under load @ 800rpm she would slightly jump to 900rpm and then drop back to 800. It's a 23ft boat so you can feel the jump in rpm :mad:. Some cases worse

Then I did some high rpm running, boat running smoothly. I would then throttle back to neutral and that's when the 900-1100rpm irractic surging would happen. I know I am repeating myself I just want to make sure I'm on the right track.

Thanks!