Sorry to hear you bought a new ECM to throw parts at this issue.....Honestly I'd venture to say you may as well keep the ECM as they are becoming harder by the year to obtain as this system is starting to date itself out... Should you ever sell the boat then maybe sell the ECM online/eBay or offer it at a higher cost to the new owner for a just in case situation.
Typically 3 things could put this system into "SLOW" mode which could be either High Engine Temp, Low Engine Oil psi and "should" your engine model have these, there maybe two little sensors found near the top of the exhaust manifold risers/elbows...If you do have them, it would look something like item number 8 for example only,
http://www.volvopentastore.com/Exhau...view_id.769069
Here's a test to try as well,
http://www.justanswer.com/boat/58m57-volvo-penta-5-7-gl-c-vortec-fuel-injected-think.html
If you do have them and either of the two exhaust elbow sensors become compromised from physical damage or read to high of temp, they could put you into slow mode.. Anything over 160 degrees could do it or if the sensors where over tightened including overtightening the wiring nut should it be of that design could damage the sensor.
Here's some good info regarding the systems that go into "Engine Protection Mode" vs a "Rev Limiter" which one system turns off one injector bank/side while the other turns off both banks.
http://www.justanswer.com/boat/2e4rc...reduction.html ...Versus
http://www.boatinfo.no/lib/volvo/man...y_efi.html#/24
Just recalled that the older engines call this feature SLOW Mode while the later engines changed the name to Engine Protection Mode.
^ With that, When you bough the new ECM, was it the same exact design or did you have to upgrade to a different design + MEFI number which may have had a different wiring harness kit too?
Since you don't have access to a marine scan tool, here's an ingenious way to check for Data Trouble Codes on the MEFI fuel injected system/ECM like you have,
https://www.youtube.com/watch?v=fSEwwa6D2jM
Due you happen to know your full engine model such as 5.7GSIPBYDCE for example? If needed, the info below may help you find the model type.
DECODING THE VOLVO PENTA MODEL NUMBERING SYSTEM 
Typical Volvo Penta Engine ID Plate
All Volvo engines come from the factory with a metal tag riveted on the engine or outdrive which has the serial number and the engine identification.
From the late eighties and up, the serial number should be ten digits in length. If you can provide us with this number, most of the time we can tell you exactly what the engine or outdrive is along with the year of production.
Routine engine maintenance, including the timely replacement of gas and oil filters on your boat is the key to a long marine engine service life. One challenge that people often face is how to correctly identify what they have for an engine.
Volvo Penta has over the years had several different methods of identification. Engines built before 1993 are painted red in color. Until the late eighties, the engines had a prefix which told you what type of application it was: inboard (MB or BB) or inboard out board (AQ), followed by a displacement or horsepower rating number, depending on the year. A letter on the end indicated the production version. So if you have an engine that says AQ131B on it, we know:
- it is an inboard/out board (AQ131B)
- it has approximately 131 shaft hp (aq131B)
- it was produced in 1986 or 87 (aq131B)
The later models in this series (early nineties) changed to a system based on displacement. Engine models had a three-digit number followed by a letter. For example, the 43 in 430 signifies that the displacement is 4.3 liters and the third digit is for model control. A 434 and a 432 are both 4.3 litter engines but the different third digits indicates a difference between the two engines. Volvo manufactured this family of engines until 1993.
Incidentally, from 1993 to 1998 or so Volvo supplied both engines and sterndrives to OMC. The drives were branded OMC Cobras but were really manufactured by Volvo. These engine models had a two-digit number and followed by a string of letters to identify the year and model. For example a
4.3GLPNCA is 4.3 litters, and the:
G identifies that it is a General Motors block,
L tells you that it is limited output. L= limited, S= Superior, X= exceptional i = stands for fuel injection (the lack of and “i” means that it is carbureted)
P tells you that it has power steering,
NC identifies that it is a 1996 model year and
A is the service code for version control.
The fourth and fifth (or fifth and sixth) letters always identify the year of the particular engine in question. The year codes for different years is as follows:
MD = 1993-1994
HU = 1994-1995
NC = 1996-1997
BY = 1997-1998
WT = 1998-1999
EF = 1999-2000
