deboatmon

I have a problem that has been with me for almost two years now. My 1996 502EFI's are built a little with Crane cams, roller lifters and rockers and J&E Pistons and a little head work with valves, springs and a few more minor items. I use Stainless Marine exhaust with long, through transom risers. Great running and sounding combination somewhat similar to a 500EFI.

My problem happens on one motor only. Usually right at 3500 RPM cruising the starboard motor tach will flicker a few times and then drop approximately 200 RPM to 3300. I can easily tell that the motor is actually changing by the sound and also speed decreases 2-3 of MPH and as it flickers it's easy to feel the boat surge with the variation in RPM. When missing like this max RPM is somewhere around 4000 RPM whereas normally it sees about 5000-5100. Sometimes I can go full throttle and all of a sudden the engine will "catch up" . But usually I'm stuck with this problem. So, it can come and go. Last Saturday I ran 50 miles without a miss. First time in two years I've been able to go that distance minus the miss. On the return trip it started missing after just a few minutes and stayed that way the remainder of the trip.

I've had my local guys put their Merc computer hookup to it several times and they are unable to isolate the problem. It is extremely frustrating as they are good at general engine problems.

Does anyone have any suggestions at all where to start or maybe which parts to swap between the motors to test? Is it possible that a bad tach could cause this. I don't know exactly how the tach works within the actual ignition system but am aware that it is directly wired in.

I've posted this problem before ( over a year ago ) but obviously have had no results. Any ideas will be greatly appreciated.

stevesxm

the injectors on these motors, ( like all, i suppose) are prone to several ways to fail... the simply stop, they get dirty, they get intermittent... if it were mine, i would pull the injectors, hook them up to my scanner and pulse them with a good injector cleaner...

this will tell you if they spray well... if they seem to be hesitant, if they are working at all....

it sounds to me like you drop a cylinder everyonce in a while... beyond all the simple and straight forward stuff you would do anyway... caps rotors wires look at the plugs etc... i would have a look at the injectors. thats what it sounds like to me.

if you DO pulse the injectors, remember to pulse them BACKWARDS as well... thats an easy way to clean the screens.

mats

My 502 MPI behaved in a similar way and it was driving me nuts. It went away when I finally changed the plug wires. My guess is that the wires need to be in perfect condition for the voltage to be sufficient for the spark to be able to jump the gap in the distributor at the max ignition advance

Scarab28Germany

Hi,

I had a same Problem a few weeks ago. The sound change, lose some power. and the speedo flickers.
It was a lose Sparkwire.
When a Sparkwire is not Ok The Highvoltage could jump anywhere to mass (Enginblock, Exhaust, or something like this) The result ist that you lose the power of this zylinder about 50 HP (with org. Motor) you can hear this as a "popping" noise at the exhaust Tips. The Sparkfire to mass could have 40000 Volt and more that make your speedo flickering.
I would check the sparksystem first , itīs the easyest thing of all. When the wires are not soft and fexible change them its just a few Dollar in the US. If you want make a test, change them from one to the other engin and make a testdrive. When the problem is now at the other engin you know it absolutly. A injektor failer could make the same problem but you will not see the flickering speedo because the signal is comming from the ignition not from the injektion.

Hans

(please excuse my english)

SSComp

Sorry I can't help...but I'm doing the exact same thing. Who did your ECU reprogram and what cam did you use.

deboatmon

Sorry, but I don't have the Crane part numbers here with me. That info is filed away somewhere at home. They aren't too aggressive. Very similar to 500EFI profile. I did get new valves when motors were built. Had all new valvetrain including Crane rockers. It's been great for 400 hours. I replaced springs and lifters @ 250 hrs.

Longblockbenny

Iwill guess you have a bad VST valve. In the back of your efi plenum you will find a small tank where the fuel lines go to, this is the VAPOR SEPARATOR TANK "VST" on top of the vst there is a plastic tube conected to it. Inside there is the air release valve that' if you never have change it , it will keep the air mix with the gasoline and you will end up with air on the injectors and you will loose power and your mecanics will never find a code for your problem. This is easy to diagnose: conect a fuel pressure gauge to your gasoline manifold fitting "front of engine under the plenum" and run the boat and you will see your pressure drop "most mecanics will change the fuel pumps at the momet " hope this help