!996 5.7l LX EFI Bravo III

[wakina]

1996 Mercruiser 5.7l LX TBI (2BBL) serial # OF756740 with Bravo III drive
This motor starts fine and idles fine to the ear but when powering up it sounds like a Jon Deere Tractor with no power. Took it to a well respected repair shop to have the codes on the ECM read. Absolutely no codes were stored. new fuel filters and water separator filter have been installed, new plugs, the old ones with less than 40 hrs. were fouled very badly but it still runs the same with the new plugs. Any Idea where I should look next?
I should have added that this motor ran just fine the weekend before but when we tried to make the run to our favorite fishing spot the next weekend it fell flat at the first throttle up after idling to the lake.

 

[Don S]

Re: !996 5.7l LX EFI Bravo III

Start with a compression test.
Were the plugs fouled with oil or what?
Codes aren't everything on the TBI systems, you could have problems but it's not something that would set a code, just a sensor out of range. You would need to compare the sensors to typical readings.

 

[wakina]

Re: !996 5.7l LX EFI Bravo III

Compression check ran less than 40 hrs. ago during a survey at time of purchase, I can not remember the numbers but they were all well with in specs and within 5# of each other. since the plugs were out for the compression check I had them install new ones at that time and keep the old ones for my visual inspection which they appeared to be fine. This is not just one spark plug but all 8 plugs with one being worse than the other 6 and one being a little better showing signs of some gray on the electrode they all reek of gas smell and were fouled with carbon deposit/soot badly. Oil is clean and at proper levels and engine does not smoke out the exhaust, nor does it use oil, it does have an un burnt/ partially burnt gas smell from the exhaust when idling.

 

[Don S]

Re: !996 5.7l LX EFI Bravo III

You need to get the engine hooked up to a diagnostic computer. Your problem could be as simple as an engine coolant temp sensor telling the ECM the engine is cold, so it would be running a lot richer than normal. Or a number of other things,
I take it the engine ran fine for a while and then the problem started suddenly, or did it just get worse and worse over time.

 

[Tail_Gunner]

Re: !996 5.7l LX EFI Bravo III

That is a very old motor..Take a neon light and illuminate your injector's there should be no drips and a nice conical spray pattern. Can you run the engine in netural at 3500 rpm for a minute or two to see if it will clear. You could also take a timing tape and watch your advance as you idle up. It really does sound like stuck injector's and a few minutes at 3500 should clear it..If that does work take the time to clean all of your contact point's..maf...etc...knock sensor's...injector leads Your IAC maybe a prime suspect..

 

[wakina]

Re: !996 5.7l LX EFI Bravo III

A diagnostic test was done at the same time the code check was done they found nothing.
The boat set idling for quite awhile in neutral while waiting for a dredging operation to clear the channel we were unaware the channel would be closed off and on that day from time to time. For saftey reasons I left the boat idle in neutral just in case we needed to move for any reason. Could this long idle have caused an injector problem?

 

[wakina]

Re: !996 5.7l LX EFI Bravo III

Yes this problem came on fast from good the weekend before to bad the following weekend. The boat ran great the weekend before and like crap the following weekend.

 

[chads516]

Re: !996 5.7l LX EFI Bravo III

I am having what sounds like the exact same problem. Went out on wed. Ran great , Thursday everything started, I replaced plugs, cap and rotor, water/fuel seperator today and still there. Haven't had chance to have someone come hook up to motor yet. When I first start boat in slip it revs fine in neutral and then once I've been on lake and tried to see if I fixed problem it then even in neutral shakes, misses and runs rough. Please let me know if you figure anything out. Even shoot me a text or call EDIT thanks

 

[wakina]

Re: !996 5.7l LX EFI Bravo III

Do to high winds last week the boat will be taken on a sea trial this week to try and determine what the problem is! Could a long idle have caused an injector problem?

 

[Don S]

Re: !996 5.7l LX EFI Bravo III

They should be able to idle all day with no problems.

Have you actually checked the fuel filter for water? Just changing the filter isn't going to cure the problem if there is a lot of water in the tank, just another filter full of water and the water goes into the engine again.

 

[wakina]

Re: !996 5.7l LX EFI Bravo III

Yes dumped the gas from the old filter into a clear clean glass container only thing visible was gasoline. Left it set for several hours undisturbed just to be double sure there was no water suspended in the gas.

 

[wakina]

Re: !996 5.7l LX EFI Bravo III

Follow up. Repair tech ran compression check and all cylinders were at 185 to 190 except #3 which was 170# put a couple of squirts of oil in the #3 cylinder and it still tested at 170#, replaced cap and rotor and new plugs again. Started on muffs no noticeable mis-fires. Took for water test ran smooth for about 1/4 mile then started to make the same noise only louder this time, noise started out mild and increased to a much louder sound the longer it ran. Put boat back on trailer and put muffs back on and a definite funny engine sound from the exhaust. Cooled engine did another compression check all cylinders tested at 185 to 190 except #3 and #5 both of those were tested at 0 lbs. Repair tech says that I have a possible/probable blown head gasket between the cylinder walls of 3# and #5. Could not explain the reasonably good check before water trial but was suspect of head gasket problem or a valve problem because of the oil test. Any how will have to have the head removed and replace the head gasket and will check the valves also. There are only 855 hrs. recorded in the ECM, 60 of them are my hrs. Survey comp. check showed 180 lbs on #3, and 796 hrs.

 

[chads516]

Re: !996 5.7l LX EFI Bravo III

Follow up. Repair tech ran compression check and all cylinders were at 185 to 190 except #3 which was 170# put a couple of squirts of oil in the #3 cylinder and it still tested at 170#, replaced cap and rotor and new plugs again. Started on muffs no noticeable mis-fires. Took for water test ran smooth for about 1/4 mile then started to make the same noise only louder this time, noise started out mild and increased to a much louder sound the longer it ran. Put boat back on trailer and put muffs back on and a definite funny engine sound from the exhaust. Cooled engine did another compression check all cylinders tested at 185 to 190 except #3 and #5 both of those were tested at 0 lbs. Repair tech says that I have a possible/probable blown head gasket between the cylinder walls of 3# and #5. Could not explain the reasonably good check before water trial but was suspect of head gasket problem or a valve problem because of the oil test. Any how will have to have the head removed and replace the head gasket and will check the valves also. There are only 855 hrs. recorded in the ECM, 60 of them are my hrs. Survey comp. check showed 180 lbs on #3, and 796 hrs.

Mine turned out to be the same problem I tore mine down yesterday and I ran to long with the gasket gone and it actually basically torched a channel on my head between cylinders.... new head on order....