Idle speed, Mercruiser 2.8 165hp D-tronic

[Aye Cap'n]

I've got a bit of an issue.....anyone any suggestions? After a lovely afternoons cruising at circa 1200 rpm I throttled back to idle when coming alongside. The engine revs immediately fell back from the recommended 750rpm down to 3-400 rpm which sounded, lets say, a tad rough. The engine will now not hold idle speed - unless I actively 'massage' the throttle. An easy solution would be to make a throttle lever stop and a screw to act as a mechanical idle adjustment and insert this at the throttle lever housing on the engine. However, a mechanic has told me that is not actually resolving the original issue. Getting a (Mercruiser) mechanic down to look at the engine is proving very difficult if not impossible. I'm thinking it may be a faulty ECM or a faulty throttle sender thingy - I'm not sure. A new ECM could be expensive, especially if its not the actual problem. Any ideas/thoughts out there would be very gratefully received.

 

[alldodge]

I would start with the basics.
Air cleaner
Fuel filter (is it clean and full, if not full check antisiphon valve)
Exhaust system (manifold, turbo, pipe)

I would lean toward the fuel system since this just happened and was doing fine. If a piece of crud got into one of the injectors it would reduce it enough for something similar.

You getting any smoke?

 

[Aye Cap'n]

I would start with the basics.
Air cleaner
Fuel filter (is it clean and full, if not full check antisiphon valve)
Exhaust system (manifold, turbo, pipe)

I would lean toward the fuel system since this just happened and was doing fine. If a piece of crud got into one of the injectors it would reduce it enough for something similar.

You getting any smoke?

Hi thanks for that. She 'will' happily hold 750 rpm if I manually advance the throttle lever above the normal position at the engine. Would that still happen if there is a fuel/air constriction?
If it wasn't a 'tronic' engine I would simply adjust an throttle idle adjuster screw - if you see what I mean? ie it doesn't have an adjustment screw as I imagine the revs are controlled electronically

 

[alldodge]

The ECM adjust to keep idle within spec, but can only adjust so far. If one or more injectors are not getting the correct amount of fuel then there may not be enough room for the ECM to adjust.

There should be a code thrown and the MIL light would be illuminated. You didn't mention the light so guessing its not lit. Would be best to scan the motor for codes

Can do an injector test. Need safety glasses, rag and gloves. Get it idling as low as possible then one at a time loosen the injector line at the injector. Note how mush slightly loosing the nut if it changes the idle and by how much

 

[Aye Cap'n]

The ECM adjust to keep idle within spec, but can only adjust so far. If one or more injectors are not getting the correct amount of fuel then there may not be enough room for the ECM to adjust.

There should be a code thrown and the MIL light would be illuminated. You didn't mention the light so guessing its not lit. Would be best to scan the motor for codes

Can do an injector test. Need safety glasses, rag and gloves. Get it idling as low as possible then one at a time loosen the injector line at the injector. Note how mush slightly loosing the nut if it changes the idle and by how much

This is good. I am beginning to see the logic here.
For reference, No codes are evident, no MIL light illuminated. Honestly not sure about smoke - would need to test again as this happened over eight weeks ago - and I'm still trying to acquire a mechanic who is prepared to travel to the boat......
There is a centrifugal Racor filter and a disposable secondary filter before the engine but I suppose it is still worth checking. I know that earlier in the season I had to empty the Racor filter of crud.
Thanks Guys, will report back my findings

 

[Scott Danforth]

check the connection at the electrical throttle. check the reference voltage. doesnt take much to throw the signal voltage off.

you may have to plug in

 

[alldodge]

With the Racor having a lot of crud, the first thing I would do is replace the secondary filter

https://www.mercruiserparts.com/bam/subassembly/31205/2673/170

 

[Aye Cap'n]

check the connection at the electrical throttle. check the reference voltage. doesnt take much to throw the signal voltage off.

you may have to plug in

Yep good one

 

[Aye Cap'n]

With the Racor having a lot of crud, the first thing I would do is replace the secondary filter

https://www.mercruiserparts.com/bam/subassembly/31205/2673/170

Thanks All Dodge

Soon as I can get replacement filters I will test. Hoping this weekend, failing that the following one. All sounds a bit cheaper so far than tackling an ECM problem. Fingers crossed.

 

[Aye Cap'n]

Well, I've changed the filters and no improvement.
Adjusting the throttle cable to 'hold' the engine throttle arm above idle position, achieves 750rpm, but it proves inconsistent and won't accurately hold position when constantly playing the throttle at the helm.
I'm getting the lend of a diagnostic device on Monday for the day, although hav'nt a clue how to use it. Hoping this might suggest something.

 

[alldodge]

Have you tried this yet?

Can do an injector test. Need safety glasses, rag and gloves. Get it idling as low as possible then one at a time loosen the injector line at the injector. Note how mush slightly loosing the nut if it changes the idle and by how much

 

[Aye Cap'n]

Have you tried this yet?

I had a guy who services the engine undo the feed lines into the injectors. I wasn't present. He seemed to think that they were ok - but then he added that it would be difficult to assess what he was looking for as he had nothing to benchmark against.

 

[alldodge]

Was listening/looking for the same change in rpm with the loosening of each injector. If they were all the same change then there should be no issue

 

[Aye Cap'n]

Ahh I see.
Ok will perhaps try this again on Monday. Thanks Alldodge

 

[Aye Cap'n]

Ok. So I have managed to get hold of a diagnostics machine. We've got the following results:-

Fuel Temp 163 f.
Intake Air Temp 59 f.
Coolant Temp 123 f.
Low idle Set Point 600rpm ***** ?
RPM at injector 715rpm (reflected by throttle position note below)
Fuel Shutoff Solenoid 100%
ECM Ref Volt 2.49
TP Sensor 4.96 volts
Throttle Position at 2% (throttle lever held manually to achieve 715rpm)
Barometric Pressure 30 HG
MAP Supply volts 4.96 VDC
Manifold Pressure 30 HG

Anyone care to have a prognosis based on the above results?

 

[alldodge]

Two readings give me concern
Fuel temp 163* (where is this reading coming from)
ECM ref V 2.49 (I would figure it would be 5V)

Any codes?

 

[Aye Cap'n]

Two readings give me concern
Fuel temp 163* (where is this reading coming from)
ECM ref V 2.49 (I would figure it would be 5V)

Any codes?

The only fault code that appeared was P0180. Which is supposed to be Fuel temp sender

I personally dont understand 'low idle set point 600rpm' - cause that's the actual problem, this engine is supposed to idle at 750rpm?

 

[alldodge]

The P0180 gives me some good thoughts. If ECM thinks the fuel is that hot, then it may change the idle. Don't know because manual 22 does not list any trouble codes

Guessing again: The low idle set point would guess this is the lowest it will allow the motor to go if there is a problem.

The ECM should be putting out 5V on pin 57 Pink/Yellow

 

[Aye Cap'n]

Would that suggest a possible faulty Throttle position sensor?

 

[alldodge]

Would that suggest a possible faulty Throttle position sensor?

I wouldn't think so. I used the TPS test point only because it was easy to show. The supplied voltage should be 5V on pin 57, if its correct then the TPS should be correct. Have been unable to find where the ECM reference point is located

Is it real cold where your at?