Mercruiser 7.4 MPI Idle Speed Very High

[tpenfield]

During the prep of my twin Mercruiser 7.4 LX MPI engines (serial #'s in my sig) the starboard engine started idling very high, like in the 1200-1500 RPM range, even though the throttle was closed. A week ago, when I first started the engines after the winter lay-up it ran fine. Today, not so much.

I figured maybe it was the IAC failing, so I checked all the sensor connections and replaced the IAC, since I had one on-hand. No difference. Occaissional the engine will idle down to about 900, but then it drifts back up to 1200 and even high, all by itself.

Anyway, I have the Diacom software, but forgot to bring my Windoze computer. So, I loaded the software to another computer and ran some tests.

Looks like it is the MAP sensor that is bad . . . Starboard Engine.






I also ran the Port engine for comparison . . . looks OK.


I did not run the port engine all the way up to temperature, but after about 10 minutes, it looked OK.

anyway, although I got an error code for the MAP sensor, the values that it was sending the Diacom software seemed to be similar to the Port engine, which was OK, no codes, etc. Just sort of curious what exactly signals the malfunction to the MEFI ??? (MEFI1 BTW)

Also, I think these MAP sensors are a bit pricey, IIRC, as a Mercruiser part ($100 + ). Wonder if there is an automotive equivalent :noidea:

 

[tpenfield]

I took out the MAP sensor



As best as I can tell, this is an automotive part. that is original to the engine. A '039' type (1 Bar) MAP sensor. Mercruiser 's price (P/N 864856A1) is about $175-190. The same part through the automotive suppliers is about $40.

Seems like a rip-off . . . wondering if I should just get the part through the automotive retailers. :noidea:

 

[alldodge]

I would get the auto part. When my motor was rebuilt the guy installed a new auto map sensor

 

[Scott Danforth]

I would get the GM part from napa or your FLAPS (Friendly Local Auto Parts Store)

remember, GM pays about $9 for that part (and about $3 for a TPS) Mercruiser buys it for the $40 and marks it up by 4.2X

 

[tpenfield]

OK, thanks guys. It looks like the part is a GM 16137039 or an ACDelco 213-185

 

[tpenfield]

I do have a follow-up question . . .

It looks like the GM and ACDelco part numbers are older numbers, as much of what I am finding on the Internet are 'NOS" (New Original Stock) parts.

I have found some cross-reference parts on various forms, but perhaps not of an authoritative source.

Old #'s
GM 16137039

ACDelco 213-185

New #'s
GM 12569240

ACDelco 213-1545

There is also a Standard Motor Products # AS5 or AS5T that seems to be referenced, but I'm not sure what the difference is between the AS5 and AS5T :noidea:

My preference would be to get parts that are in current production, but want to make sure they are the proper equivalents.

Anybody got a link or something to a definitive cross reference source?

 

[Scott Danforth]

Interchange Part Number: 16137039, 213-1545, 213185, 2131545, 12569240, 16254719

 

[tpenfield]

I ordered 2 of the the ACDelco/GM part, figuring to stay as close to OEM as possible. Got 2 parts, in case the MAP sensor on the other engine decides to fail.

 

[alldodge]

The testing lead to the MAP, and hope it fixes the issue

I'm wondering if it will

 

[tpenfield]

The testing lead to the MAP, and hope it fixes the issue

I'm wondering if it will

. . . if doesn't, then there will be a boat for sale in my yard

 

[tpenfield]

BOAT FOR SALE . . . BOAT FOR SALE :facepalm:

I put the new MAP sensor in (ACDelco 12569240) and brought the engine up to temperature. I had to clear the 33 code from before, but no codes returned during the run of the engine.

However, the idle is still high and will not come down. Here is my MEFI-1 data.



It seems to be pretty much like before. AllDodge you were thinking that the new MAP sensor would not fix the idle problem . . . (correct) . . . just wondering what your thoughts are?

I shut the engine down after it had reached running temperature (10 minutes running) and then re-started it. The engine rev'ed to 2K RPM and would only settle back to 1500 RPM. At that point I came in the house opened a beer and started looking through the boat classifieds

FWIW - I keep smelling a slight burning smell from this engine (starboard engine) while it is running . . . sort of like a hot wax or crayon smell. Not sure if that has any significance. I did a hand test on the MAP and the MEFI to see if they were warm/hot after running the engine for the 10 minutes. They were both cold.

Not sure what to do at this point, but I do have enough beer for the evening.

All ideas, thoughts, knowledge and wisdom are welcome and greatly appreciated at this point.

 

[nola mike]

I was wondering that as well. If your IAC is closed (looks like it is from the software? ), could we have just a plain 'ol air leak somewhere? Also, same symptoms with warm/cold engine?

 

[tpenfield]

I was just thinking that too . . . gonna go check the hoses that lead to the intake plenum.

Thanks.

 

[alldodge]

I would put a Tee inline with the fuel pressure regulator and connect to a vacuum gauge and see what your reading is, disconnect the vacuum line going to the fuel pressure regulator and put the vacuum gauge on it. Want to see if the gauge matches what the ECM thinks is there.

If that checks out, check for 5V (Gray wire) on the MAP wiring with the key ON and not running. Then ring the connection from the MAP to the ECM. If all that checks out send the ECM to OBD to see if its OK.

 

[tpenfield]

I would put a Tee inline with the fuel pressure regulator and connect to a vacuum gauge and see what your reading is, disconnect the vacuum line going to the fuel pressure regulator and put the vacuum gauge on it. Want to see if the gauge matches what the ECM thinks is there.

If that checks out, check for 5V (Gray wire) on the MAP wiring with the key ON and not running. Then ring the connection from the MAP to the ECM. If all that checks out send the ECM to OBD to see if its OK.

thanks AD, but not quite following your suggestions:

Fuel rail pressure? or vacuum line to the VST pump? not sure I follow you as far as connecting to the pressurized fuel rail with a vacuum gauge :noidea: Do you mean the suction tube on the regulator?

Not sure I have a vacuum gauge . . . probably would need to get one.

Also not sure about the comment about the connection from the MAP to the ECM?

I have come across a post or 2 where the high idle turned out to be a malfunctioning ECM. (oh that could get expensive :facepalm: )

 

[tpenfield]

I got 5.04 volts on the gray wire at the MAP.

 

[alldodge]

Fuel rail pressure? or vacuum line to the VST pump? not sure I follow you as far as connecting to the pressurized fuel rail with a vacuum gauge :noidea: Do you mean the suction tube on the regulator?

Your regulator connects to the intake manifold, only issue is the hole which feeds the regulator is a pin hole. so if there are any sticking valve it may not pick up the fluctuations, but also don't currently see that as the issue. If it is disconnected and go direct to gauge, the fuel pressure will rise a bit, but also don't see that as an issue right now. If it was Tee'd then there would be no change.


https://www.harborfreight.com/fuel-pump-and-vacuum-tester-62637.html

Also not sure about the comment about the connection from the MAP to the ECM?

Here is your wiring



Good on the 5V

 

[Scotvl]

Have you tried swapping ecm's between the motors to see if the fast idle moves with the ecm? Also you said the throttle was closed but did you take the linkage right off of the throttle body just to see if its keeping it open slightly?

 

[tpenfield]

Have you tried swapping ecm's between the motors to see if the fast idle moves with the ecm? Also you said the throttle was closed but did you take the linkage right off of the throttle body just to see if its keeping it open slightly?

Thanks, I have thought about doing that . . . It would prove or disprove an ECM fault. I have the spark arrestor off for diagnostic purposes, so I can see the throttle plates. They look closed. I can certainly disconnect or adjust the linkage to be sure.

If the ECM swap does not pan out, I may take the plenum off and re-check the hoses, etc.

 

[tpenfield]

AllDodge with the fuel pressure regulator, are you thinking that the suction tube is letting air into the manifold, or just not reducing the fuel pressure when vacuum is applied? I could just clamp off the tube to see if it makes a difference.