engine cuts off when shifting in/out of gear

[mcknight]

Hi everyone,

I have a 2000 Baja 302 with twin Mercruiser MEFI 3 MCM 7.4L MPI Bravo's. One of the engines cuts off when at low rpm/idle when I shift it into or out of gear. This makes maneuvering very tricky!
I've read about shift interrupter switches, but I've also read that these engines don't have them. I'm looking for a little help on how to troubleshoot and/or find/fix the interrupter switch.

The problem only happens once the engine is hot. ie. after making a long run and trying to enter a marina. It only happens on one of the engines. If I set the rpm's higher with the throttle, I can keep the engine running when I shift, but as you can imagine, this makes shifting pretty violent. Other than when shifting in/out of gear, the engine runs fine. It starts without any problems, idles just fine, but as soon as I put it in gear it dies.

I'm not super mechanically-minded, but from the research I've done, these symptoms appear to line up with a shift interrupt issue. Unfortunately, I can't find anything specific about these motors and where the switch might be and if the engine doesn't have one, what else could be causing the problem?

If it helps, the engine serial number is: 0L836417

Any help would be appreciated.

Thanks to you all in advance,
-Michael

 

[alldodge]

You have bravo drives so there is no interrupter switch. The serial number is for the drive

Is it only one of the motors which cut out or both?

What rpm is the motor idling at while in gear?

It might be the IAC needs cleaned but could be other things

 

[mcknight]

Sorry - posted the wrong SN. The engine SN is actually: 0L633801

The problem only happens on one of the engines. Idle rpm when in gear is right around 800. I've replaced the IAC and MAP sensors with no change in behavior.

Thank you

 

[alldodge]

Did you inspect the air passage on the throttle body that the IAC uses?
Can you check the idle rpm with another tach to verify?

Really need a scan tool to see what the IAC is doing, could also check fuel pressure at the rail, should be 43 psi with motor running

 

[mcknight]

We've looked at the RPM's with the scantool... I don't remember the exact numbers but they were about the same as the engine that has no problems. I want to say it was around 800. Maybe a little less in gear vs. idle. I don't have the scantool right now. It was a mercruiser scantool.

We also tested fuel pressure. It was dead-on and we didn't see any drops in pressure between idle and shifting into gear when cold. It only stalls once the engine has been up to temp for a while.

When we changed the IAC, everything appeared to be clean.

We also tried to adjust the throttle idle dead-stop(?) so the rpm's wouldn't drop too far. This was just a test to see if it would make a difference. The boat would idle a little higher, as expected, and for a little bit it seemed like that had worked, but then we started to see the same failures all over again.

When the engine stalls, the exhaust smells a little rich. Not bad, but it's like the ignition is being cut while there's still a little fresh fuel in it. It behaves as though there's an ignition interrupt happening. What I mean is that it doesn't spit-and-sputter... it dies instantly.

Is there something with RPMs in particular we should look for? ie. If I get the scan tool back and can read them again, what should I be looking for?

Thank you

 

[alldodge]

Your OSO thread states in only dies going into gear, not into neutral, which would be more understandable from the engine being loaded.

With everything dead-on and the only issue is when it goes in gear, maybe a vacuum leak, low compression or plugs not firing correctly. If spark is weak its not getting a good burn (plugs, wires, cap or rotor)

 

[mcknight]

Yes, sorry about the confusion around when it happens. It is only when going into gear. Where would I check for a vacuum leak? Do you have any thoughts on why the problem would only happen once the engine has been running for a while? Are any of your suspicions more likely when engine temperature is factored in?

I'm going to try to get it on the water tomorrow, weather permitting. I won't have the scan tool with me, but I'll try to get as good a reading as I can from the gauges. Anything else I should try or watch for while I'm out there?


Thank you

 

[alldodge]

If the motor is weak (low compression) or there is a vacuum leak the IAC will try to open further to keep the motor running. If IAC is almost wide open (60 to 80 counts) it doesn't have much to work with. Normal position of IAC is 40 counts

With fuel pressure dead-on, then the only other thing could be the injectors. When cold the ECM provides more fuel, once warmed up the fuel is reduced.

Need more info, voltage reading

 

[mcknight]

I was able to get to the lake yesterday. At idle in neutral, good engine idles at 600 and the bad one just under 800. When I put the drives in gear, there is no appreciable drop in rpm's at all. Shifting is smooth and effortless. This is after a long no-wake travel, with both engines showing just over 150 degrees.

After a long run -- approx 20 miles -- coming off plane to an idle, the engine rpm's return the the same levels as mentioned above, no perceivable difference at all on the gauges. Engine temps have increased to around 160-165. Idle is still smooth. Once I get to where I have to manouver and shifting in and out of forward/reverse, the bad engine starts to cut out.

I did notice that sometimes the cut-off is immediate, and other times its a few seconds later. In either case, when it cuts off, it's instant, again meaning that it doesn't spit-and-sputter as if it's running out of gas... it dies as if there is an ignition loss.

Voltage (by the gauges) doesn't seem to change throughout my test runs. I tested with no accessories turned on, but in the past I've not been able to tell a difference whether or not the lights, radio, blower, etc were on or not. It just didn't seem to make any difference.

 

[alldodge]

Problem with boat gauges is they can be dead on or off and there is no verification. IF the bad motor is idling at 800 rpm (if accurate) would lean me toward a vacuum leak or IAC is not being controlled

Need a scanner

 

[mcknight]

I finally was able to get time with a scantool (Mercruiser Quicksilver Digital Diagnostic Terminal) and collect some data...

The scantool showed:
RPM's = ~653 (No noticable change when shifting into gear.)

TP Sensor V = 0.71
TP Sensor % = 0
Inj Pulse ms = 3.29
Spark Advance = -5.13
IAC Position = 0
Base IAC Pos = 0
IAC Follower = 0
Fuel Rate GPH= 0.45

Those were the only IAC-related values reported by the scantool.

None of these values really changed just by shifting in/out of gear. Or, if they did, the change was faster than the sample rate of the scanner. We literally would watch the numbers while we shifted into gear and couldn't see a change. If we revved the engine, things would change, but just sitting at idle and shifting in/out of gear didn't cause a data change.

The boat idles fine. If there were a compression problem or injector problem, wouldn't it affect idle as well? With 8 injectors, would a single bad injector cause this kind of problem? On the good engine, I can unplug a spark plug and still idle and shift without stalling. Seems odd that a bad injector would cause something like this. Compression is ok. You've mentioned a vacuum leak a couple of times... if compression is ok, where else would I look for a vacuum problem? Are there other electrical components or sensors I should look at?

Thanks again,
-Michael

 

[alldodge]

The IAC should be higher then 0, this shows its not being used to control idle. Add to that the idle is at 653 this leans me to say there is a vacuum leak.

If the plenum was ever removed the gasket may be the issue. Are the bolts tight? Check seal on MAP sensor