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Baja 302 cuts off when shifting gears

Old 06-21-2020, 08:23 PM
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Originally Posted by mcknight View Post
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.

Originally Posted by mcknight View Post
Hi SB, sorry for the confusion... it only stalls when going into gear, not when putting into neutral.
Dont take this the way you should. Your fn killing me. Lolololololololol l

IAC , IAC passage dirtiness, minimum air (throttle blade setting or dirtiness) or base timing biggest offenders.

Last edited by SB; 06-21-2020 at 08:27 PM.
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Old 06-23-2020, 05:04 PM
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Yes, I see that now. Sorry!

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 maneuver 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.





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Old 08-09-2020, 12:25 PM
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Bang on one of those gauges, it could simply be sticking. You can't use new dash gauges as an RPM indicator when trouble shooting let alone any that old.
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Old 09-25-2020, 02:42 AM
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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.

When we changed the IAC, the passages were clean.

Thanks again,
-Michael
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Old 09-25-2020, 12:28 PM
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The IAC values should move when you shift into gear.

BTW, I moved this thread to the tech Q & A for more exposure
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Old 06-26-2022, 10:16 PM
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Hello,

I just wrote an article about troubleshooting marine engines that start but die when put into gear. It has 9 simple tests you can use to find the issue with your engine! Please let me know if it helps you out!

All the best and happy boating!

Boatkeep
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Old 06-27-2022, 12:41 AM
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Assuming the IAC values are correct, I'm thinking vacuum leak.
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Old 06-27-2022, 07:41 PM
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Originally Posted by BoatKeep View Post
Hello,

I just wrote an article about troubleshooting marine engines that start but die when put into gear. It has 9 simple tests you can use to find the issue with your engine! Please let me know if it helps you out!

All the best and happy boating!

Boatkeep

Thanks for the article link. From what I can tell, the items listed in the article would affect the general running of the engine, not just when shifting. My problem is that the engine otherwise starts, idles, etc. just fine. It only acts up when shifting into gear, and only after the engine is at temperature. When I first launch it, I don't have any trouble at all. After running for 20-30 mins, the problem shows up.
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Old 06-27-2022, 07:43 PM
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Originally Posted by zz28zz View Post
Assuming the IAC values are correct, I'm thinking vacuum leak.
Thanks for the tip. Where would I look for a vacuum leak? I don't see a bunch of vacuum hoses running everywhere like I used to on old car engines.

Can you help me understand the diagnostic logic you're following? I'm so new at this and have no idea how engines without a shift-interrupter work, and in what way vacuum plays into it.
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Old 06-27-2022, 09:36 PM
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When setting up idle speed on a fuel injected eng, the IAC is manually closed ( or air passage blocked to simulate a fully closed IAC) and the throttle blades are adjusted to obtain base idle speed. Base idle is set high enough so that eng won't die and is lower than the idle speed the ECM will ever call for. When the IAC is brought back on-line, it opens a little to get the RPMs up to what the ECM wants. When a load is applied, like going into gear, the IAC will open a little more to keep the RPMs where the ECM wants them.

Now introduce a vacuum leak. ECM see eng RPMs higher than it wants so it closes the IAC valve to cut off some air entering the intake. If RPMs remain higher than the ECM wants, it will continue closing the IAC valve until it hits zero. At that point, there nothing more the ECM will do to lower idle RPM.

Depending on where the vac leak is, it can effect one or 2 cyls more than the others which can cause drive-ability issues during low RPM operation. When eng is cold the ECM enriches the mixture which tends to hide small vacuum leaks. As eng warms-up and leans out, the effect of the vacuum leak becomes more apparent.

My experience is mainly with cars but the theory is the same. I'm not very familiar with your set-up so it hard to point you to a suspect area to look at. Intake manifold gaskets are an obvious place to start along with throttle body gaskets and misc vacuum hoses..






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