View Full Version : 496HO problem solved

07-19-2011, 11:33 AM
RE: 1989 Z-33 w 496HO

I have been chasing this problem since last October and I think I have got it fixed. While taking some vistors for a cruise last October, the port engine went into the Guardian mode and I had to idle home. I haven't had large blocks of time since then to fully tackle this problem; but, July 4th that changed and I was able to really get on it. I drained the fuel tank, checked the entire the fuel system, installed new fuel filters, checked the throttle body, exchanged the idle assmbly with the strarboard side engine, exchanged throttle bodies, installed a new cut off switch; and, then I did what I should have done in the beginning. I called a guy with the Smartcraft big computer machine for 496 engines. ( A generous member on this site had allowed me to borrow his Renda machine but that did not have the increased information of the big computer). The guy found the problem in 5 minutes. The machine said that the computer was reading 29 lbs of water pressure from the sensor at the Transmission cooler; and, the pressure at idle should be around 2 to 4 lbs. So, I replaced the sensor for $113 and "bingo" it works fine.

Here's what it looks like and when I bought it, the parts guy at Marine Max said this was a common problem with 496 engines........go figure.

07-19-2011, 11:39 AM
Congrats Richard. Took some time but you found it. Curious, transmission cooler?


07-19-2011, 11:39 AM
Great to hear it is solved, and THANKS for the follow-up. Nothing better than hearing about a clear resolution of an issue/mystery. -Mike

07-19-2011, 12:48 PM
I assume you have a TRS drive with a Borg Warner trans. Did the original setup sense water pressure at the trans cooler or was that input added when you upgraded your engine management controls? I know that my TRS with a Merc Trans does not monitor water pressure at the trans cooler.

07-19-2011, 02:07 PM
I had a TRS with a Warner Velvet Drive trans in my old Formula.
It originally had a 330 Mercruiser carb Big Block Chevy.
Call me old fashoned
But I still like the simple carb & distributor engines.
Usually I can fix them quickly myself .
The old Merc TRS never had any such sensors on the original engine/drive packages.

With the modern FI engines sold today you really need access to some sort of scan tool to properly troubleshoot strange sensor issues like this.
Who would have guessed that this problem was a faulty trans oil cooler pressure sensor issue ?
I would never have guessed this in 100 years.
Without a scan tool you are really sunk at trying to trouble-shoot many issues today with the FI fuel & Ignition electronic computer management systems in use on modern marine & auto engines today.

I'm glad you sorted this strange issue out & are now up & running again.

07-20-2011, 08:08 AM
........sorry ..... it was the power steering cooler.
The orginal engines were 454 with Bravo drives from 1989. Two years ago I upgraded to 496HO with Bravo X drives and of course the nre transom plate too. I found that I had to replace the transom as well. This era Z-boat always seems to have problems with rotten stringers, bulkheads, gussets etc.