Liquid Addiction

I have a 33 ZX with twin 496HO's and recently pulled the motors to replace the transom assemblies. The work is finished and the boat runs great with the exception of the starboard tach reading about 100 - 150 RPMs less than the Port. Also when taking off, the starboard motor tach lags the port motor until up on plane and leveled off. When cruising at 3500 the tachs are very close but still not right on. I have put a computer on the motor and the reading on the computer matches the tach. To add further complication, I recently had the starboard motor cut off completely when cruising...found that issue to be a bad ground connection in the wiring harness to the crank position sensor. When I fixed the ground pin and test ran the boat the tachs were perfectly in sink with each other. The next time I took the boat out the tachs were off again. I did put in new plugs and have a brand new set of MSD wires to put on. (I heard the wires can affect the tachs if they are worn or laying up against the exhaust) The motors feel like they have plenty of power and the exhaust sounds fine. It is just in the tach that I see the difference in RPM.

Has anyone experienced this with their 496's?

Thanks

Sydwayz

iTrader: (1)

Yep, EXACT same scenario.

Turned out to be a bad Crankshaft Position Sensor.

My experience:
http://www.offshoreonly.com/forums/g...iac+cause+this


Another local buddy is experiencing exact same thing. He is troubleshooting this week on his 32' Sunsation with twin 496HOs.

They are a blitch to get out. And they will likely crack when they do. Be very careful not to have it crack so far as to break off when you are pulling them out.
I replaced both.


I also bought Camshaft Position Sensors, but have not installed them as problem went away with Crank sensor R&R.
There are OLD and NEW versions of the crank sensors that are VERY important to know the difference as there is an interference problem if you mix up.
Member "BUP" has put some very informative posts up on this topic as well.
I have one newer style (white) cam sensor, and 3 older style (black) ones, all brand new if by chance you need to check cam sensors.
However, I'll bet 2 rounds your issue is Crankshaft Position Sensor.

Liquid Addiction

Thanks for the reply.
I actually replaced the crank & cam POS on both motors when I had the motors out...not to say they could not still be bad or maybe the bad ground wire caused the crank POS to go bad...

Liquid Addiction

The port motor runs great with no issues.
I did save the old crank POS that were not actually bad when I replaced them. I was just trying to do preventative maintenance while I had them out.

Sydwayz

iTrader: (1)

I think you could be right on the Crank sensor being soured by the bad ground. If it had + signal to it and no ground, it may have created it's own ground to the block messing up your tach signals.

Since you replaced the crank sensors recently; they should be easy to pull. I'd swap them and see if problem follows.


If that doesn't do it...

How low can you be running the RPMs and see the difference?

You could back the boat down the ramp, bump up throttle to 1000-1500 rpm, and shut down each cylinder one by one with the Rinda scanner.
When you get to one that does NOT affect the RPM of the motor, you have found a cylinder not firing. You could have wire or coil problems at that cylinder.

You could do that on the hose, but I don't recommend that unless you have a really strong water pressure water supply.

Liquid Addiction

Pretty much can see the issue all the way from idle to cruise speeds and more so when trying to get on plane.

donzi matt

iTrader: (1)

I have the exact same issue on one of my motors, the port motor lags a bit getting on plane, after adjusting throttles at ~3200 they are pretty closely matched, at 4000 they are off again, and at WOT it is about 100-200 behind.

I have also noticed the tach slightly glitchy on this motor, although to the best of my knowledge the motor sounds fine. My tachs are Livorsi Vantage View tachs so I can display digitally the RPM the tach is reading from the ECM, and the tach and digital are spot on. I could also be imagining it, but the port motor appears to turn over a little longer than the starboard when starting but I have never had a stalling or erratic idle issue. As much as I hate firing parts at a situation like this, I might just put a CKP sensor in it and see how it responds.

Sydwayz

iTrader: (1)

Everyone is certain that Thottle Position Sensors are reading same at Full Open, Full Closed, and a few portions in between, right?
(Check with Scanner, engine off; ignition on.)

Remember, cables stretch and also become a little less flexible as they age. Given there is a linkage at both the engine & the throttles; cables and bushings/linkages could be attributing to butterflies not being synced well between motors.

Liquid Addiction

Not sure about throttle position, but mine also cranks a bit longer to start on the motor with the RPM issue.
I did notice when I fixed the ground wire on the Crank POS the motor seemed to start right up and then like I said the next time I took the boat out it cranked a little longer and the RPM issue was back.
I found the issue last Tuesday and fixed it...then ran the boat on Saturday.

Liquid Addiction

RPM issue is still present.
I decided to swap out the new crank POS with the old one I replaced thinking maybe the new was bad or going bad. There was no change. The RPM variance is still there.
There has to be another ground wire somewhere not making good contact...I'll keep looking...