Loss of engine rpm at WOT or trim activation

David Runk

Recruit
Joined
Jun 19, 2003
Messages
3
The problem started quite suddenly. I took the boat (1988 24' Crestliner Saber with OMC 5.0L 2 barrel model 502APRGDP) out a couple of times for several hours with no problem. Third trip, however, I noticed that at WOT the engine rpms woild fall and then rise again to normal (a surge). The boat, otherwise, runs fines at all but the top end of the throttle. I initially thought maybe bad gas. I treated the fuel, replaced the water seperator and cleaned the in carb filter, and put 35 gallons of fuel in the 52 gal. tank. The dwell is correct and spark plugs look good and are gapped correctly. One strange thing I've noticed is that when running at or near full throttle the engine will also lose power if I engage the trim motor, either up or down. My next thought is a faulty coil. I'm running out of ideas and my troubleshooting skills just about exhausted. Any ideas? Also, are marine coils and automotive coils interchangable?<br />Thanks
 

Trent

Captain
Joined
Nov 17, 2001
Messages
3,333
Re: Loss of engine rpm at WOT or trim activation

Iv seen the trim/engine reduce power several times. Every time it was a bad ground. Check and clean ground wires.
 

a.d.mobile

Petty Officer 1st Class
Joined
Jul 15, 2002
Messages
326
Re: Loss of engine rpm at WOT or trim activation

There is a bulliton from omc on this. there is too much restence to the coil in other wise to long of restence wire feeding the coil I will look for the bulliton and try to post on monday .A.D.
 

sea wolf

Lieutenant Junior Grade
Joined
Apr 3, 2002
Messages
1,219
Re: Loss of engine rpm at WOT or trim activation

i had the same problem last yr. on my 3.0 omc. it was the alternator. had it rebuilt, problem solved. watch your ammeter gauge when trimming lower unit. how much does it draw down?
 

David Runk

Recruit
Joined
Jun 19, 2003
Messages
3
Re: Loss of engine rpm at WOT or trim activation

Thanks for advice so far. Yesterday I checked and cleaned the grounds, they looked good. I also replaced the coil. Didn't have a chance to test, it's been raining here on the upper Chesapeake. I will check the resistance between the coil and alternator to see if it is within spec (I believe between 1.5 and 2.5 ohms acccording to Clymer manual). As for the alterator, I don't have an ammeter, just a volt meter on-dash. It reads about 14V normal and drops to just over 12V with trim engaged. Thanks again. I'll keep working on it.
 

sea wolf

Lieutenant Junior Grade
Joined
Apr 3, 2002
Messages
1,219
Re: Loss of engine rpm at WOT or trim activation

sorry, i meant voltmeter.
 

P.V.

Chief Petty Officer
Joined
Mar 14, 2002
Messages
452
Re: Loss of engine rpm at WOT or trim activation

As noted above, check out the OMC servive bulletin pertaining to your exact problem. Also, there was newer brkr points and condenser, spark plugs and an high output coil.If needed!
 

JasonB

Lieutenant
Joined
Feb 10, 2003
Messages
1,455
Re: Loss of engine rpm at WOT or trim activation

Had a similar prob on my mercruiser. Engine would cut out if I trimmed up . Fixed it by replacing the keyswitch for a seemingly unrelated problem.
 

akriverrat

Chief Petty Officer
Joined
Oct 31, 2002
Messages
588
Re: Loss of engine rpm at WOT or trim activation

is trim switch in throttle handle?
 

a.d.mobile

Petty Officer 1st Class
Joined
Jul 15, 2002
Messages
326
Re: Loss of engine rpm at WOT or trim activation

I FORE GOT THE BULLITON!! THIS IS THE PROBLEM will get it for tomarrow you have to much resitence to the coil.
 

a.d.mobile

Petty Officer 1st Class
Joined
Jul 15, 2002
Messages
326
Re: Loss of engine rpm at WOT or trim activation

still looking been a while
 

ndemge

Commander
Joined
Jul 15, 2002
Messages
2,644
Re: Loss of engine rpm at WOT or trim activation

My '82 OMC 5.0L has the EXACT same problems you described..... <br /><br />hope to find the solution here with that bulliten
 

a.d.mobile

Petty Officer 1st Class
Joined
Jul 15, 2002
Messages
326
Re: Loss of engine rpm at WOT or trim activation

Guys i have the bulliton can you post a fax nunber? i'm posting from work and i do not have a scanner here. and thur is my last day for 2 weeks vaca...
 

a.d.mobile

Petty Officer 1st Class
Joined
Jul 15, 2002
Messages
326
Re: Loss of engine rpm at WOT or trim activation

well here you go i will type it:<br />Bulliton 4091 jan 93<br />subject: high speed misfire with trim activation<br /><br />Models: all breaker point ignition 4.3 and ARY/GDP 5.0 litre<br />Problem: a misfire or loss of rpm can be noticed when the trim button is depressed. this condition can be caused by worn ignition components and combustion chamber deposits.<br />Solution: 1. Test resistor wire as outlined in service manual. if the resistance value is approximately 2 ohms, unrap the harness, and trace the red/purple resistor wire from the coil. cut 21 in off the ciol end of that wire and solder a new ring terminal to the end. the resistance value of the wire should now be 1.25 ohms + or - 0.1 reattach resistor wire to the coil and retape the harness.<br />2. replace the spark plugs with champion RV15YC4 or equivalent, gapped to 0.30<br />3. remove, clean and install the distributor breaker plate. install new points p/n 987894 and condenser p/n 987895<br />4. if steps 1 thru 3 fail to correct the problem replace ignition coil with p/n 987673. hope it helps someone.......
 

David Runk

Recruit
Joined
Jun 19, 2003
Messages
3
Re: Loss of engine rpm at WOT or trim activation

the trim switch is in the throttle handle, akriverat. I've replaced the coil and it seems to have solve the problem of reduced rpms at WOT. However, the drop in rpms is still noticable with trim activation. I will try a.d.mobile's advice next.
 
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