Changed exhaust manifolds, now engine won't start

RubberDuckeeToo2

RubberDuckeeToo2

Active Member
Oct 21, 2006
472
Connecticut
Boat Info
'98 370 Sundancer
'97 SeaDoo Speedster 14'
Engines
7.4 Mercuisers
Ok twin 7.4l MPIs (late 90's Gen VI?), changed the exhaust manifolds on both engines which went somewhat smoothly.

Starboard engine runs like top. Port engine turns over, but will not start. The engine is getting fuel, so that's not the problem. There is no spark.

Here's what I have done so far:
1. Switched coils - both are functional. This is the advantage of having twin engines.
2. Switched computers, relays, etc. with the other engine with the same result. Starboard starts, port does not.
3. Replaced distributor cap and rotor, and spark plugs.
4. Followed and checked the engine wiring harness from the schematic for continuity. No popped circuit breakers or fuses. All connectors fit securely and make good contact.

i've read the Mercruiser manual from cover to cover on all parts dealing with ignition. I think it comes down to the distributor and the electronic ignition module in it. It showed no signs of failure prior to doing the work and I'm lost as to how it could fail when I did nothing to mess with it.

It looks like replacing the distributor is more than just a title involved because setting the timing isn't simple matter of using a timing light as in the old days. But it appears to be the last link in the chain. I don't have access to the computer diagnostic tool to do any further analysis.

ideas? Did I miss anything?

Gene
 
Check the 10 pin round connector to make sure all the pins are making good contact.

10 pin.jpg

Did you have to unplug any sensors when replacing the manifold that may have been damaged or not reconnected?
 
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On our 454 are fuses the push kind 50 AMP I think.On the 3.8 V sixs were real fuses. The fuse went in a far away port and a local mechanic knew what to do. Asked for $40 to fix it I gavehime $60 as I felt it was worh it. Look for fuses. Mine are on the starboard sideofthe engine attashed to the riser.
 
Switch the coil wires.

That looks like the only thing you didn't check out.
 
Gene, you can't just change the ignition module? If you have to pull the distributor I don't remember it being a big deal when I had mine out since the timing is all computor controlled. I think I just marked the position of the housing and the rotor and put it back where it was. I'll keep thinking about it and let you know if anything comes to mind. Good luck, Mark
 
I would be triple checking all engine connections and sensors for something you may have dislodged during the manifold change.


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OK, was delayed due to visitors from out of state (we're not in Kansas anymore). So here's the update from today.

Switched the coil wire. Works on the starboard engine that runs, not on the port engine.

Switched the electronic module in the distributor. Works on the starboard engine that runs, not on the port engine.

Called a local mechanic who says I've covered pretty much all the grounds, so now it is time to hook up the computer to see what the error codes say.

Gene
 
Gene, try putting the port motor on the starboard side and see what happens! Sounds like you tried everything else.
 
Had a marine mechanic out to the boat today with his computer to read the error codes. We found that the engine is getting spark, but errors were coming from the intake air temperature sensor and the manifold air pressure sensor. We pulled both of them and found them fouled with oil. I do not fog my engines because they are MPI which shouldn't be fogged. I made that mistake when I first got the boat and was corrected by several mechanics not to do it again. I didn't have any problems as a result as the fogging oil was rapidly consumed when I next ran the engines.

So now I have been instructed to perform a compression test to see if there is a stuck intake valve and other damage which is spitting oil up the intake. And I have to take the intake cover off to clear any oil left in it and see if I can determine which the oil came from, if not from a blown valve.

Any comments or suggestions? Here's hoping I don't need a new engine.

Gene
 
RubberDuckeeToo2 said:
Had a marine mechanic out to the boat today with his computer to read the error codes. We found that the engine is getting spark, but errors were coming from the intake air temperature sensor and the manifold air pressure sensor. We pulled both of them and found them fouled with oil. I do not fog my engines because they are MPI which shouldn't be fogged. I made that mistake when I first got the boat and was corrected by several mechanics not to do it again. I didn't have any problems as a result as the fogging oil was rapidly consumed when I next ran the engines.

So now I have been instructed to perform a compression test to see if there is a stuck intake valve and other damage which is spitting oil up the intake. And I have to take the intake cover off to clear any oil left in it and see if I can determine which the oil came from, if not from a blown valve.

Any comments or suggestions? Here's hoping I don't need a new engine.

Gene
Click to expand...
Hey Gene....
I just replaced my 98's with a pair of 99's. Port runs great, Stbd was running great, and is now rough, but that's a full story for another time. Initially, we did find oil in the MAP, IAC, and especially in the throttle position sensor. These came out of a crash boat, so we are beginning to think it might have been laid over, or perhaps upside down. Before changing the TPS with one of my old ones, it would not start or stay running if it did. They are all pretty easy to change without pulling the intake cover. If you start going there, be real careful with the torx bolts. One year they came off just fine, and last time, they were easily stripping out. One of the reasons I found new engines.
 
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One more thing. 18436572
It's easy to get them backwards because the engines face backwards. My 100.00 per hour mechanic replaced my distributor cap while I was out of town, and mistakenly swapped 7/8 and 5/6. Ran like crap until I finally checked his work. Ruined a brand new set of plugs.
 
370Dancer said:
Hey Gene....
I just replaced my 98's with a pair of 99's. Port runs great, Stbd was running great, and is now rough, but that's a full story for another time. Initially, we did find oil in the MAP, IAC, and especially in the throttle position sensor. These came out of a crash boat, so we are beginning to think it might have been laid over, or perhaps upside down. Before changing the TPS with one of my old ones, it would not start or stay running if it did. They are all pretty easy to change without pulling the intake cover. If you start going there, be real careful with the torx bolts. One year they came off just fine, and last time, they were easily stripping out. One of the reasons I found new engines.
Click to expand...

Hey 370Dancer,

You do realize this is a 4 year old thread? I think Gene figured it out by now...
 
So here’s the rub. While replacing the exhaust manifolds, I somehow disconnected the tach leads and reconnected them in reverse. So the spark wasn’t getting full charge (or something like that).

The oil was a red-herring. Not sure what caused it, but it never re-occurred.

Gene
 
RubberDuckeeToo2 said:
So here’s the rub. While replacing the exhaust manifolds, I somehow disconnected the tach leads and reconnected them in reverse. So the spark wasn’t getting full charge (or something like that).

The oil was a red-herring. Not sure what caused it, but it never re-occurred.

Gene
Click to expand...
duhhh. That's what searches will get you. I was so caught up in the symptoms, I didn't notice the vintage date. Glad yours was easy.
 
Well, if Gene would have followed up on this thread back in '13....... (Just kidding, I'm glad you got it fixed AND are still around to update the thread!)
 
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