Mrercury 700 dropping boost
 

I am having issues with a 700 mercury dropping boost . Boost will go up to 7 to 8 lbs and then drop to 4 . The boost control bypass has been replace and checked it is good.. If we work the throttle sometimes it will come out of it. Any Ideas??
thanks

My AT had that issue and we traced it back to the motors "talking" across the electronics when we had the batteries on "both". When we had them set individually it never happened. Not a true answer on why but it fixed the issue.

mine has a battery isolater deal first boat i had with one . Its running triple 700 Its only running 2 alternators .I have been told the isolater takes care of the rest.

...and just another reason to not run your boat with battery switches on both!! It makes your boat faster! :-)

I'll add that to my list of reasons when the battery switches thread comes back again. LOL!

Any chance the belt is slipping at higher throttle settings???

the first thing i would do is replace the belt and check the tensiner.

I wouldn't think it would be intermittent then but I will check it out .thanks

I was recently looking over a 700sci engine. I noticed the belt tensioner was stamped "dayco" . I remember the 600sc and 800sc merc's, had GM tensioners off an LT4 vette. Even had the GM part # stamped on them still. 50 dollar tensioner at GM dealer, vs 400 bucks from Mercury. Same goes for the belts. Basic 10 rib belt, 30 bucks from parts store, or 120 bucks from merc.

Anyhow, any noticeable belt dust anywhere?

no and it is like a light switch when when it looses it

Have you had an IAC fail on this same engine in the recent past? The boost controller and IAC are on the same circuit/fuse and it is not uncommon for a failed IAC to cause damage to the PCM, which could be impacting the boost controller.

Just a thought,
GF?

Yeah, it would've been helpful to know the EXACT reason but since it seemed to work I wasn't re-fixing it. LOL

Even if you have a isolator you can't run batteries on both...it will still overvoltage the system causing ECM problems.

Assuming your feeling and seeing the diff in that one engine. Rpm loss coincides with boost gauge?

yes it is map sensor is good so is iac

Have you tried swapping the PCM with another engine?

GF?

yep no change

That is a Bad Idea running batteries on Both , seen that melt an Altenator on the Port Engine smoke the wiring harness . Then burned the whole back off the boat!
The transients that the combined Charging systems setup, when combined, will wreak havoc on the ECMs. New Belt , New Tensioner then Id swap the ECMs and Boost controllers between Engines and see if the problem follows it

did ecm swap new boost had it no the computer to check map sensor all good .

The PCM uses inputs from the MAP sensor, crank sensor (RPM) and the Throttle Position Sensor to calculate the signal it sends to the boost controller. Barring any mechanical issues with the engine you might want to try a new TPS. The TPS is a potentiometer and it might have a bad spot that is causing the output to fluctuate, which might explain why it works sometimes. BTW, the TPS is AC Delco part #213-911

GF?

did compesstion check on all cylinders there fine tps cheap enough will try . would that throw a code . I got a race vision smartcraft display nothing

I also have SmartCraft and my failed TPS did not throw a code. I had an idle issue and we found that the TPS voltage on that engine was fluctuating at idle but was steady everywhere else.

Good luck,
GF?

thanks