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8.1 rpm's limited to 3500 under load

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bob17412

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2002 Crusader 8.1 standard on my 39 silverton MY. Had a number of problems with port engine in past subject of posts in 2011. Only about 250 hours on engines as for past several years it only goes out a couple times a year. When first purchased boat WOT was about 4400rpm with cruising speed at 3600 rpm and about 24mph. Last year and this year boat runs fine except it maxes out at full throttle on both engines at only 3500 rpm and 20 mph while running. In neutral engines will rev to about 4400. Really puzzled as two separate fuel tanks. Starboard engine is original equipment except for impeller while port has had new ECM, fuel pumps, fuel filters, etc as a result of stalling problems. Last attempted cruise yesterday, was running at 3400 and 19 mph but was able after about half hour run to coax to 3500 rpm and 20. 7 mph. Mechanic at shop says probably just algae build up on bottom but sounds fishy to me. I run in Lake Erie and put 125 gallons fresh fuel in yesterday. All fuel from local marina and no ethanol (at least thats what is advertised). Help and thanks in advance for any suggestions.
 
Probable reasons:

1. Fouled bottom.

2. Fouled bottom.

3. One engine (only) not making enough power for some reason: timing off, carb problems, a bad ignition wire (or two) etc.

To prove out # 3 above, perform the Fastjeff Dyno Test. To wit:

Set BOTH engines at some convenient cruising rpm, say 3,200. Open the Port engine throttle wide open for 5 seconds and note the rpms it achieves; also, note how quickly/ smoothly it accelerated. Return that engine to 3,200, then repeat the test on the starboard engine.

Compare the results: If one is much more powerful than the other, and gets up there quicker/ smoother, dive into it to find out why. (I'll bet one of them is dogging it.)

Finally, it's not obvious but BOTH engines have to be working hard for you to attain the rpms/ cruise you're used to. With one not performing fully, this is what you get.

Jeff
 
could very well be a dirty bottom.....only other thing that comes to mind, based on the original description is stale fuel.....even ethanol free fuel has limited "tank life". Stabilizer helps tremendously but only lasts so long...
 
Thanks for the reply. Will take out this weekend and run for extended period to see if can work off some of the "fouled bottom". Does have me scratching my head as would think that increased drag from fouled bottom would slow top end speed but would still achieve maximum rpm's of 4400 just with less speed over ground. Also getting a scanner to see if it throws any codes. Just as additional information, port tach is fed from ecm and has been inop for several years so can't do fastjeff dyno test . Take it out so few times I just would synch engines based on the sound. Another post suggests stale fuel as issue - any harm in running it out as engines seem to run fine just at low max rpm's.
 
Growth on the bottom increases the effective surface area and the total load seen by the engines....extra load = reduced RPM ....

On the tach, you may want to try cycling the selector switch on it (unless you know the ECU output is bad)....sometimes they will crud up and render the tach inop. Another option would be to disconnect the tach input leads and use a pair of jumpers to 'switch' the tachs' inputs...
 
makomark said:
Growth on the bottom increases the effective surface area and the total load seen by the engines....extra load = reduced RPM ....

On the tach, you may want to try cycling the selector switch on it (unless you know the ECU output is bad)....sometimes they will crud up and render the tach inop. Another option would be to disconnect the tach input leads and use a pair of jumpers to 'switch' the tachs' inputs...
Click to expand...


Thanks, not sure about your comment on using jumpers to switch tach inputs. Isn't the sending unit for the tach directly off of the ecm and thru the J-1 harness ultimately to the tach.
 
There's no sending unit for the tach....the pulsed signal is generated by the ECU as there isn't a single ignition coil....

here's another description.....remove tach wires from tachs. Jumper wire from tach wire in port harness to starboard tach's input and jumper tach wire in starboard harness to port tach. if tachs behave as before, then the port tach is bad (ECU tach driver is ok); if starboard tach is inop, then issue is in port ECU's tach driver.
 
makomark said:
There's no sending unit for the tach....the pulsed signal is generated by the ECU as there isn't a single ignition coil....

here's another description.....remove tach wires from tachs. Jumper wire from tach wire in port harness to starboard tach's input and jumper tach wire in starboard harness to port tach. if tachs behave as before, then the port tach is bad (ECU tach driver is ok); if starboard tach is inop, then issue is in port ECU's tach driver.
Click to expand...


Thanks. It is the driver not the tach. Any fix short of a new ecu which is about a 1000 item?
 
1) First check the wiring very carefully for continuity. We recently fixed a troublesome problem when a mechanic pulled the plugs from the ECU and then made sure each socket had continuity with the one on the other end. Turned out one of the small wires at the ECM plug had come loose. Since all else works, my first reaction is that you more likely have a connection problem than an ECM problem.
2) If all else fails, and you feel especially handy, keep in mind that lots of things use tachs. You can find inexpensive pulse generators for sale on the net. If you did some research to insure you understand the nature of the pulses needed by your tach, you might be able to find something that you could put on your engine to generate the needed pulses.

Good luck.
 
You can always send the ECU out to one of the repair places.....should be much more economical than a new one....do a www search on MEFI repair.
 
Anyone you recommend. Definitely the ECU and not the harness as problem transfered to starboard engine when I moved the ECU to the starboard engine. Hoping the RPM issue was a result of a dirty pancake filter in port engine but won't know until can take it out for a run.
 
I just developed a change in max RPM after I bumped the bottom. I used a scuba mask to see the bottom side of mine, no algae but, I got freakin barnacles on my rudders and prop shafts. Also, I have a ding in my port side prop that will need fixing.
I have the same boat, it is 25,600lbs, it doesn't take much to effect performance. I was idling when I hit something, it stalled my starboard engine but it was the port side that had the prop ding.
 
this season the boat sat at the dock from mid april to early july when I was able to take her out for the first time. starboard motor only ran up to 3400 rpm. a little history...In previous years had major issue of fouling the pancake filters so I added new racor 2 micron filters at the outlet of each tank. boat ran great for the next few years and pancakes never fouled. engines are 1996 crusader 502 xli's with 410 hours in a silverton 41C. After the first run this year and seeing the results of the starboard motor I replaced the racors the following week and then left for a trip for a week on the boat. she ran great at 3400rpm. on the way back I thought I would open it up and see if I get 4200 to 4400 rpm. didnt happen. maxed out at 3500rpm. so now all filters have been replaced, cap and rotor also replaced prior to my trip. I'm thinking fouled bottom. In all of this I never looked at the port motor to see if it is acting the same. will do that this weekend. Any thoughts?
 
I just found the pancake filters on my boat looking through the parts list of my latest small issue. The Filters sound like a great idea on the outlet of each tank!

I am in the Middle of the Chesapeake Bay and I know I have barnacles. I found a calm clear water cove and jumped in the water with a diving mask on and was shocked. I scrapped off my rudders and trim tabs with a long handled Ice scraper but think I still have more work to do on the shafts and supports of the Cutlass bearings.

Where are you located? I have had several people tell me barnacles are bad this year. I am thinking you have a fouled bottom or barnacles. Be careful not to scratch your self with the barnacles, it can be a very bad infection. One guy told me that is where the flesh eating bacteria come from (I don't believe that).

Sorry for such a late response.
 
bob17412 said:
2002 Crusader 8.1 standard on my 39 silverton MY. Had a number of problems with port engine in past subject of posts in 2011. Only about 250 hours on engines as for past several years it only goes out a couple times a year. When first purchased boat WOT was about 4400rpm with cruising speed at 3600 rpm and about 24mph. Last year and this year boat runs fine except it maxes out at full throttle on both engines at only 3500 rpm and 20 mph while running. In neutral engines will rev to about 4400. Really puzzled as two separate fuel tanks. Starboard engine is original equipment except for impeller while port has had new ECM, fuel pumps, fuel filters, etc as a result of stalling problems. Last attempted cruise yesterday, was running at 3400 and 19 mph but was able after about half hour run to coax to 3500 rpm and 20. 7 mph. Mechanic at shop says probably just algae build up on bottom but sounds fishy to me. I run in Lake Erie and put 125 gallons fresh fuel in yesterday. All fuel from local marina and no ethanol (at least thats what is advertised). Help and thanks in advance for any suggestions.
Click to expand...


Just an update on above problem - still a head scratcher. In 2016, replaced pancake filters and fcc filters. Had mechanics check out engine and they indicated was showing any problems with diagnostic reader as fuel pressure, etc was normal. Boat more of a floating condo, so only take out a couple times a year, while taking out for 3 hour run was able to achieve 22mph SOG but still only 3600 rpm. After replacing plugs in both engines, on a test run achieved 3700 rpm and 23.7 mph SOG. I took out this year for first time over fourth of july weekend of 2017 and boat was fully loaded with 13 people was only able to achieve 15.6mph SOG and 3400 rpm. My thought was to have injectors cleaned and check the throttle sensor. I know meaningless but it will achieve 4600 rpm in neutral on starboard. Still can't check the port as tach still inop because of bad ECM tach sender. HELP!!!!!!!!
 
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