5.7 Engine surge when warm

I have a 1995 Rinker 265 with a Mercruiser 5.7L and Bravo 2 outdrive. At the end of last year it had been running poorly (backfires & hard to start), we had it looked at and they replaced the alternator, spark plugs, cap and rotor, coil, and the fuel filter. Since then we have devolved some additional problems.

It will start and run excellent for a long while. 30 min or more depending, but at some point it will loose power and the RPMs will surge. Going from 3500 to 3000 to 3800 or more and will continue to do that until the engine cools down. I can run at lower RPMs (3000 or less) with no problem, but it will surge or not run any higher then that till it cools.

One odd part is it seems that using any high draw electric such as the trim tabs, or motor trim will set the surging problem off. But not until the engine is warmed up. For example if I try and trim the motor up, the RPMs will drop by 500 or more, and the engine will start to surge.

My guess is there is more then one thing going on here, but I am clueless as to where to start. Our lake has terrible service and it is no easy task to take the boat out of the water and bring it somewhere. Any ideas and help would be greatly appreciated.

Thanks in advance!

Adam

Re: 5.7 Engine surge when warm

hey Adam-
you said that it seems as if problem starts off some times when you use a high draw electrical deviece, I am going to guess at this one. Are your batteries clean around the terminals and across the top, have you done a load test on them, are your batteries being charged less than 13 to 14.5 volts dc, are your grounds clean and tight at the block.
If you have one or more of these issues it could effect the spark module I am thinking and thats the reason it breaks up under heavy load when its warm as the contact of the cables expands then retracts
good luck
rob

Re: 5.7 Engine surge when warm

Thanks for the ideas Rob. I have checked the terminals and grounds. Everything I could see looked fine. The batteries are both easy to get at and check, but the other grounds are a little harder to see. But it is worth checking again I suppose. Thanks again~

Adam

Re: 5.7 Engine surge when warm

You need to check more than just the batteries. From the battery, 12V is routed through the engine connector up to the ignition switch, back through the safety lanyard switch, back through the connector again and then to the ignition. Therefore, you have a few more places to look for the drop.

Re: 5.7 Engine surge when warm

Adam-
just had a couple of more thoughts. first check the grounds and the wire harness at the distributor and module, I am not sure if these modules are like others I have had, were if they get warm and they are on their way out they get intermintent? (spark, no spark) as well last weekend I had no spark and it ended up being the pickup inside the distribuator! that was fun to find that fault!
good luck
rob

Re: 5.7 Engine surge when warm

Thanks again y'all... I will be checking everything over again when we go back this weekend. Although I feel like I have been over every wire in that boat.. lol But it does make me think, we didn't have this problem till after we had the work done and since part of what they did was replace the rotor and dist cap I wonder if something isn't loose behind there. Hummm....

Anyway, I really appreciate the suggestions. Take care~

Adam

Re: 5.7 Engine surge when warm

Adam;
I have an "88" bayliner with a 5.0 OMC and my boat had similar symptoms;
1) The engine would not hit full rpms (4600)
2) The engine would surge from 3200-4000rpms
3) If I hit the trim button it would make the motor bog down momentarly.
4) It would bobble through the hub and back the carb on full throttle dumps.

What it ended up being was my electronic ignition was out of position,timing was incorrect and my timing advance mechanism was sticky. We cleaned and lubricated the springloaded timing advance mechanism,set the trigger plate to the proper position and retimed the motor. THE RESULT; rock steady 4800rpms (prop change needed!!),clean throttle response,no surge and no drop in power during any trimming of the lower unit. I would check your timing advance mechanism to see if it moves freely.

Re: 5.7 Engine surge when warm

Probably will not have exactly the same issues since there is no mechanical advance so if he is not getting total advance, will have the same problem. Should definitely check total advance.

Re: 5.7 Engine surge when warm

Thanks Peanutbay1, I will probably have to dig out the service manual to figure it out but I appreciate the advice. The more options and ideas I have for this weekend the better, so thanks again. Take care~

Re: 5.7 Engine surge when warm

Well it has taken me a while to get a chance to work on this again. But I did try a few things this weekend. First was to clean and double check all the battery cables. Everything was solid, and have now been cleaned.

We also did some tests by disconnecting one battery at a time and running it. Sadly nothing had any effect. The problem persisted regardless of if it was running on battery #1, #2, or with the selector set to both.

Other then that all I was able to do was feel around and make sure whatever wires and harnesses I could reach were solid. Any thoughts on what I can test or check next would be greatly appreciated.

Thanks ~
Adam

Re: 5.7 Engine surge when warm

I am having the same problem. It is only when it gets hot, and after about 30 min on the water. If I use my trim it almost kills the motor, and trying to load it on the trailer is just wants to die. I let it cool off and everything is fine.

It is a mystery. I am going over the entire ignition system this week. Is there a chance the carb is not working right?