omc 5.0 cobra revs up and down

[weiser boat]

My 1996 four winns with 5.0 cobra has been in the shop for 2 weeks but the mechanic can't locate the problem. I was pullling a skier and it was running great. The skier fell so I turned around to pick him up without turning the engine off. When I tried to accelerate to pull him up again it would rev up and back off every few seconds and continued doing that as long as I pushed the throttle. If I try to go any faster than an idle it revs up and backs off. It doesn't die or backfire. It starts and idles perfectly. My initial thought was fuel or carb. The mechanic focused on electrical issues stating it has electronic fuel injection and has done the following:

1. immediately discovered the altenator belt was broke in half, replaced it but same issue
2. noticed the voltage would drop when push the throttle
2. thought the "computer" needed to be reset because I pull the battery in the winter which shuts it down. To reset it he would let it run for a couple of hours,still same issue
3. sent the altenator to be rebuilt. It needed new bearings. Still same issue
4. has run all electrical tests and all results are fine
5. changed fuel filter
6. checked presure out of fuel pumps, that's fine
7. did note the fuel tank had some "junk" in it.

I checked with two other dealers and they tell me this guy is the best in our area and has a reputable business. Has anyone had this issue and if so what was the problem?

Thanks in advance for your help

 

[Sparkinator]

Your prop hub and engine coupler checked OK?

 

[weiser boat]

thanks for your rely. The mechanic doesn't believe the hub or coupler would cause the type of surge we are getting. He did tell me a couple of other points:
1. He has substituted the gas by using a different tank and that didn't help
2. he is getting a 11-10-11 code which indicates all electrical systems are OK

 

[john cole]

is there a fuel pressure regulator in the system some where? maybe even a RPM sensor that isn't reading right anymore. you can check the individual sensors with an ohm meter, for most sensors there are charts out that has the specs for them.

 

[vinnie1234]

With respect to the alternator, you boat should run smoothly as long as the voltage level is high enough. Even if your alternator dies totally, it should run on battery power until the voltage drops to below 10v. Below 10v it shouldn't even idle well. If that was causing the issue, then it would take a bit of time to run the battery down after you throttle up - and it would run like a bag of hammers afterward and not re-start without a boost. Just the same, to eliminate charging issues as the problem, simply make sure you have lots of battery and try it out. First though you should test the battery (ie: check electrolyte level and specific gravity) - then give it full charge and load test. Once you have verified the battery is good and fully charged - put it in and go for a rip.

I expect with a full battery it will still run the same way.

Have you checked that the spark arrestor is clean? An air blockage could be culprit.

Fuel pressure (as mentioned earlier) could be at fault. To eliminate this as a problem, put a pressure tester on the fuel rail and compare to spec. Even though you may have pressure it may not be enough, only a tester will tell you what the amount of fuel pressure is. This is better than just checking that you have "some" pressure.

I have seen more than my fair share of "weak" fuel pumps. They work, but are unable to provide enough volume for the engine. Fuel injection controls fuel to the engine in a 2 step process. The first step is regulating the amount of pressure in the fuel rail. This requires a strong pump and working pressure regulator. The second step is injector pulse width. We will get there in a minute.

If the pressure regulator is the problem you can eliminate that by temporarily blocking the fuel return line. This will prevent the regulator from blowing off pressure and cause an increase in pressure at the injectors. If you don't have a pressure tester, this can be a good way of troubleshooting the regulator, which is a more common problem.

The fact that your engine is just stalling at idle, makes me think that the computer is not commanding the injectors properly, or there is a blockage in the injector pintle (if it is a throttle body EFI). If it is a throttle body injection system, check to see that there is a nice cone shaped spray coming from the injector. If it is multi port injected, then you may even take the injector rail off and inspect the spray pattern of each injector.

With respect to my comment about commanding the injectors properly: (you may know this already, but...) as fuel demand increases, the computer increases the amount of time the injector is open for. It uses inputs from various sensors to figure this out. I would investigate the throttle position sensor and the MAP (manifold absolute pressure) sensor specifically. The computer uses the TPS sensor almost like an accelerator pump coming off idle. When the TPS sensor goes bad, it can also "limp" by on information from the MAP sensor. If both of these are not working, then you get what you've got.

These sensors will have a reference voltage line (typically 5v) and then a ground and signal output. Check the reference voltage first to see that it is correct, then check that the ground is good (continuity) then check to see that the signal works as described in the manual. If you don't have a manual go get one. Make that step 1 in this list. lol

Anyway, with what you have described, a bad engine ground or wiring related to the sensors in the intake manifold makes the most sense. The other stuff is just basic stuff that you should eliminate first, because it is a lot faster to eliminate that stuff than it is to find a wiring problem.

If that 5.0l is a chevy with the ignition module on the distributor you can check that too. they are notorious for going through ignition modules.

Good luck

 

[vinnie1234]

Oh yes - I fogot to mention - you should check around for air leaks at the intake. If (for example) the vacuum line came off of the MAP sensor, that could cause you some grief. But any air leak at all is not good to have. DOn't just listen, follw all the hoses to make sure that they are tight, and where visible bend them and inspect for cracks too.

 

[weiser boat]

Thanks for taking time to respond. All great ideas. The fuel pressure has checked out OK. I will mention the TPS and MAP sensors to the mechanic. Today he is checking for air leaks at the intake. I really think I have the right mechanic he just needs a break to find the solution.

 

[vinnie1234]

OH! It is still in the shop? I didn't understand that. Dude, if you have a good mechanic let him do his thing. If he cant figure it out then take it to someone else. I can tell you (as a mechanic myself) that a good mechanic has a process to solve a problem. When customers tell me what is wrong I listen carefully to the problems and symptoms related to the complaint and any diagnostic "suggestions" go in one ear and out the other. Not because I don't think they know what they are talking about, just that it is super important to do things in a certain way. ALWAYS. Good habits make for good workmanship. Even when other technicians tell me that they have checked something and "that isn't it" I still check it. I still ignore everything they say but the symptoms of the problem.

There is a "right way" to troubleshoot. If your guy can't do it by himself then he can't do it, and our suggestions can't help him if that is the case. I wouldn't go in there telling him what to look for. Just my $.02 worth.