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.

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!

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.

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.

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.

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,

Regarding changing TPS; Suggest to start with finding out if it delivers faulty values . 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.

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 . 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!