4.3L 205 V6 Engine Losing RPMS

[MojoRisin]

Hang with me here as I know little about my engines. I have twin 1986 4.3L 205HP Mercruisers on a 270 Sea Ray Amberjack.

Two seasons ago, the port engine started losing RPMs. Prior to last season, we (i.e. my mechanic) put on a new carb. The old was really shot. That did not fix the issue and at the very end of last season, the engine started to back fire and we pulled it.

Leak down tests were performed on both motors and both were rebuilt this past winter, except for the cam shafts which were in good shape.

This season, when the port engine hits around 3200 RPMs, it drops RPMs fast and if you don't drop down the speed fast, it backfires through the carb.

Here is a list of all things done to date:

- started the season with the carb from starboard engine
- replaced the fuel pump
- replaced the gas line and with an air compressor blew back into the tank and out the overflow
- swapped gas lines between the motors after previous test
- removed and replaced the water separators and fuel filters
- swore loudly
- replaced the distributor, pickup, and rotor
- swapped the electronic ignition boxes
- checked the timing numerous times throughout the process
- replaced the coil and wire
- kicked the cats
- replaced the spark plugs and wires as part of the rebuilds
- removed the shift inhibitor and later put it back after test failed
- replaced the cam shaft which was not done as part of rebuild as they looked great
- drank beer and swore loudly again


After the cam shaft test, which failed, my mechanic took over the helm and when the engine started to drom RPMs, he pumped the heck out of the throttle and a couple of times got the engine to over 4000 RPMs before it dropped. He's now thinking the carb again seeing the the old one from the starboard side is on the port engine now.

Anyone care to throw some more ideas out there.

I appreciate everyone's time!

Joe

 

[SuperNova]

Re: 4.3L 205 V6 Engine Losing RPMS

I would put a vacuum gauge on the intake manifold and a fuel pressure gauge on the fuel line. It honestly sounds to me like the exhaust is blocked partially, did you look to see if your flappers are still in place? If they fell off, which they do, they could be blocking the exhaust. The vacuum gauge should tell you if this is the case. It'll have appr 20 in Hg then drop steadily when you hit the point where the engine is not breathing. The other thing I've seen is the pickup inside the fuel tank gets clogged or damaged in such a way that when you apply pressure it seems fine, but under suction it collapses, and that is the reason for the fuel gauge- you should be able to maintain at least 5 psi at all times, even under full throttle load.
--
Stan

 

[MojoRisin]

Re: 4.3L 205 V6 Engine Losing RPMS

Stan,

Thanks for the input. When the boat was last pulled to have the cam shaft replaced, my mechanic said he checked for all exhaust blockages. I did just tell him your idea of the vacuum check.

In regard to the fuel line, we did switch fuel lines between the 2 engines and replaced the fuel pump.

Thanks again,
Joe

 

[Coors]

Re: 4.3L 205 V6 Engine Losing RPMS

What do you mean, the camshaft test failed?

 

[MojoRisin]

Re: 4.3L 205 V6 Engine Losing RPMS

After replacing the cam shaft, the same problem was still present. Sorry for the bad verbiage. Again, I am mechanically illiterate!

Joe

 

[Bondo]

Re: 4.3L 205 V6 Engine Losing RPMS

my mechanic took over the helm and when the engine started to drom RPMs, he pumped the heck out of the throttle and a couple of times got the engine to over 4000 RPMs before it dropped.

I Don't Know Why,...............

But it sure Sounds to Me,....
That you've Been,+ are continuing to fight a Fuel Delivery Issue...........

And,....... Your Mechanic prefers Living in your Checkbook..............

What's the RPMs run at,....??... Historically Speaking..............

 

[MojoRisin]

Re: 4.3L 205 V6 Engine Losing RPMS

Just wanted to first note that my mechanic is an excellent guy and he has not charged me anything for what has been done, other than the rebuilds which were being done anyway.

He has felt all along that this is a fuel delivery issue and you can see what has all been done to try to rule out the fule delivery side. No matter what we have switched, the port engine still has the issues and the starboard works great. You can also see what has all been replaced. I believe he pumped the throttle to see if extra gas flow would do anything.

The engine used to get 4500+ RPMS and then last year struggled to get 3500 tops on great conditions and now I have this 3200 max issue before she drops.

I was sent here via a suggestion from GLANGLER and I was told there are great posters here that might be able to help.

I hope I answered the questions. Thanks for any and all opinions!

Joe

 

[Gary H NC]

Re: 4.3L 205 V6 Engine Losing RPMS

See if there is anyway your mechanic can run that engine on a portable fuel tank.Even a 6 gallon plastic tank should last long enough for a test.
I'm with Bondo on the fuel starvation..

 

[Bondo]

Re: 4.3L 205 V6 Engine Losing RPMS

Don has a Most Excellent write up on this Issue,......

But I haven't saved it to favorites Yet,...... Maybe He'll see this,+ Post it again......

The only other Thought that comes to mind is an Exhaust Restriction,.............
Are your Flappers where they Belong,..??..??.....

 

[MojoRisin]

Re: 4.3L 205 V6 Engine Losing RPMS

I requestioned this to my mechanic and he stated the flappers are where they belong and the exhaust system was checked when the boat was pulled to do the cam shafts.

As per the separate gas can, wouldn't the test we did of switching the gas lines from port to starboard and vice versa basically check that out? We were hoping that would be it but the starboard engine continued to run fine using the fuel line from the port side.

Thanks again,
Joe

 

[MojoRisin]

Re: 4.3L 205 V6 Engine Losing RPMS

Any ideas on things we have not tried? I think my mechanic and I are past frustration on this one.

Thanks,
Joe

 

[Don S]

Re: 4.3L 205 V6 Engine Losing RPMS

Here is a list of things that can cause low WOT rpm.

I think most of the above have been ruled out as possibilities, but maybe you should give This Test a try.
An easier method might be to hook up an outboard tank to that engine eliminating your boats fuel system and giving it a try.

 

[MojoRisin]

Re: 4.3L 205 V6 Engine Losing RPMS

Great information Don. I will print to discuss with my mechanic.

When fishing with someone else today, the idea of a cracked intake manifold came up. Is there merit to this thought?

Thanks again,
Joe

 

[fendersfender]

Re: 4.3L 205 V6 Engine Losing RPMS

ant thing is possible...vacuum test??

 

[MojoRisin]

Re: 4.3L 205 V6 Engine Losing RPMS

I don't believe a vacuum test was done but the whole exhaust system was inspected manually. I will mention this to my mechanic.

I found out yesterday when talking to him that he had swapped the intake manifolds during the rebuild process.

Don, in regard to the gas test you noted, we ran the starboard gas line to the port engine and vice versa hoping the port engine would work and the starboard would fail. Of course, that did not happen. Wouldn't that suffice for your gas test?

Thanks,
Joe

 

[Don S]

Re: 4.3L 205 V6 Engine Losing RPMS

Don, in regard to the gas test you noted, we ran the starboard gas line to the port engine and vice versa hoping the port engine would work and the starboard would fail. Of course, that did not happen. Wouldn't that suffice for your gas test?

Yea, that eliminated the boats fuel system.

I would sure do a vacuum test on the engine also. Might also consider re-adjusting the valves, if you are reusing the old lifters, they may need backed off a little from book. Or, just back them all off 1/2 turn and run it and see if the problem changes.

 

[SuperNova]

Re: 4.3L 205 V6 Engine Losing RPMS

How about clogged flame arrestors? And I still say run that vacuum test. A bad coil could do it as well.
--
Stan

 

[MojoRisin]

Re: 4.3L 205 V6 Engine Losing RPMS

Thanks again for the replies. I just got off the phone with my mechanic and we'll be doing a vacuum test tomorrow. He said the flame arrestors are very clean.

As for the coil, we already replaced it with the wire.

I'll let everyone know how tomorrow goes.

 

[MojoRisin]

Re: 4.3L 205 V6 Engine Losing RPMS

Here is the latest in the order it was all done, with a test at each level. I had 2 mechanics onboard yesterday and mine had a well respected mechanic friend come up from Milwaukee as a second set of eyes and hears as well as brainstorming.

- Hooked up a vacuum gauge to each engine to compare and the all tests showed no apparent issues with the exhaust flows
- Replaced the ignition switch
- Hot wired the engine to remove thoughts of external electrical issues
- Disconnected the tach

Both mechanics feel that the engine is running out of gas at higher RPMs causing the backfire. The 2nd one listened very closely and felt he heard the carb pinging. They both now want to go back and re-eliminate the fuel flow/delivery portion. The next test on Thursday will be to run a gas line from the starboard fuel pump to the port carb and vice versa. If that fails, the 2 carbs will be swapped. Note that the carb on the starboard engine (a ?new? rebuild as of spring of last season) now was on the port engine before the rebuild and the whole intake manifold assemblies were switched with the rebuilds.

I now have 2 very perplexed mechanics but they are trying like mad and using lots of technical resources for ideas.

The saga continues!