350 Mag MPI will not run at 800 rpm.

[c1steve]

2006 Merc. 350 mag MPI, Horizon model. Idles great, runs fine at 1100 rpm and above, but when setting the throttle in the 700-900 range, the rpm's drop down to 625 after a few seconds. I thoroughly adjusted the control cables and control, which had no effect.

Would a small vacuum leak cause this? I replaced the IAC valve, there was no change. My Rinda Tech software detects no faults. The engine came to me used, with 1200 hours on it. This issue has been there since I installed the engine.

Engine used to run at 1,000 rpm, but after I installed a prop with less pitch that number went up to 1,100.

 

[achris]

That is a funny rev range. It's the 'coss-over' between idle control, which uses the IAC and TPS to determine fuel/air mixtures, and the higher engine speeds which uses MAP and revs to determine fuel/air mixtures. I have the same on my engine at 1400rpm out of the water. Without the exhaust back pressure, it WILL NOT hold anything between 1100 and 1500 rpm.

The only thing I could suggest is to have a look at the Diacom ECU data dump, and see what the defined TPS 'idle' range is, then adjust the TPS so it's closer to the top end of that range (at throttle closed). That will (hopefully) push the ECU into MAP/revs control a bit earlier. But make sure you're still getting the right numbers at WOT...

Chris..............

 

[muc]

Is this a inboard engine?
What is your serial number?

 

[jbcurt00]

Bt Doctur

Finally found it

Edit: posted w no error msg

Edit2: edit posted w no error

 

[c1steve]

Is this a inboard engine?
What is your serial number?

This is a sterndrive engine. s#OM675521.
Chris, I will check the TPS numbers this weekend.

 

[QBhoy]

Hi

mine is the same almost. It will sit at idle rpm of 650 on smart craft all day. The lowest it will sit with out being unhappy above that is 850 rpm. It’s just what they do. The IAC is doing it’s job likely

 

[QBhoy]

I religiously watch the smart craft all the time. It’s totally normal. I’ve never seen my rpm at 700-800 rpm at a constant. It’s just what they do. I’ve also never had the inclination for it to have to do it either. As I mentioned above. It’s either idle tick over speed of 650 or 850 and above. It’s just how they are. Nothing wrong at all. My 2 friends with the same engine also do the same.

 

[muc]

You have an open recall on this engine. service bulletin 2006-02 has to do with the alternator wiring.

Service bulletin 2003-06 has some miss fire info.

When you replaced the IAC, did you check the hoses going to it to make sure a muffler didn't get stuck in there?

Might want to get the part number and calibration number off of the ECM and compare the cal number to what shows up on your scan tool. You could call MerCruiser with that info to see if they have a newer cal for your engine that might address this issue.

I have run across this issue before and it was always the distributor or a recal for the ECM. But we sold very few Horizon models and I don't remember having this issue with them.

 

[c1steve]

Thanks for all the help. Right now it is either 650 or 1050, when in gear. If I could get 850, I would be totally happy.
I did not check inside the hoses to the IAC, but will later this week.

I will also get the part number and calibration number off of the ECM, and run my Rinda software again. Trolling for salmon, but should be 2.5 to 2.7 knots. Right now I am either at 2 knots or 4.5 knots.

TPS: 0.12% reading, .58 vdc, at idle. I should have re-checked this thread earlier. For a while there were no more responses, so I sort of gave up. Thanks for all the help.

-Steve