Mercruiser 7.4 LX MPI EFI problem

[Hoplite808]

Hello all, I have a 1996, made December 1995, Sea Ray 270 Sundancer fitted with a single engine that is a 7.4 LX MPI EFI with the Bravo 3 outdrive. (Engine serial OF764XXX)

The issue I am having right now is a bit of a rough idle and when put into gear it will not rev past 1300 RPM’s. If I open up the throttle all the way the engine really starts running bad and backfiring and shaking the whole boat. There is also a significant smell of unburned fuel coming from the exhaust.

I checked all the spark plugs and found no evidence of water or oil intrusion in any of the cylinders and compression on all 8 cylinders was within acceptable perimeters. The plugs did show evidence of having a very rich mixture however. The cap rotor and wires were new in August 2021 and still appear to be in good order.

I am thinking that it is perhaps the idle air controller and/or the throttle position sensor have gone bad. I do not have access to a computer that could plug into this motor to help diagnose the issue and I currently am unable to locate a mechanic to hire in my area that is available to fix the motor either.

For those more knowledgeable on this motor what are your thoughts on what could be causing my issue?

 

[Wanderer 290]

If an inductive timing light is available check the timing to start and then put the inductive pick up on each wire to check for consistent spark.

 

[Hoplite808]

Update: The IAC and TPS I ordered came in yesterday and I was able to install them. It took a minute for the computer to organize itself after putting in the new parts but I think that fixed it. The idle smoothed out quite a bit and I’m not smelling the unburned fuel in the exhaust. The engine will also rev up now. I plan to take it out today for a test run to verify but it looks good.

 

[Hoplite808]

Update on the update. Took it out today after it ran great last night and it ran horribly today. Couldn’t get it to throttle past 1900 rpm’s under load. The unburned fuel smell came back again. The engine is not throwing any codes and I can’t find any other potential faults. I have no idea what the issue is now.

 

[Fun Times]

Hi, typically speaking, normally when you have/smell a higher amount of fuel coming out of the exhaust system you would want to include performing a fuel pressure check to see if it's within your normal psi values which on this engine should be right around 36 psi +/- 4ish psi with the vacuum line R&R'd...
To see where to connect the fuel psi gauge, it would connect to item number 13/14, Genuine Mercury & Mercruiser parts. VAPOR SEPARATOR TANK & FUEL RAIL, SN & Up (See Note)
Or seen here in the first photo, https://www.offshoreonly.com/forums...-fuel-ii-low-fuel-pressure-3.html#post4748208.

With the fuel pressure mentioned and the description mentioned of your engine running issues there may be an issue inside your fuel VST tank mounted towards the rear of the engine such as either a fuel float level problem or a fuel diaphragm problem which on this system could put out too much fuel flow volume but not show up while performing a fuel pressure check of the fuel pressure regulator which is item number 15.

If you open the VST tank, just be as careful as you can as the "service" parts are becoming hard to find and very costly.

The top cap screws are hard to remove too and the O-ring kit is high $... An impact screwdriver is usually needed to get the screws loose as there are usually held with a thread lock type sealant if never opened since new.

Next time you're at the boat try first removing the vacuum type rubber tubbing and see if fuel is coming out while the engine is running...Also try lightly tapping all around the VST tank top cover area to see if with help unstick the fuel float and/or the needle and seat which could be putting out raw fuel into the cylinders/out the exhaust system...Genuine Mercury & Mercruiser parts. VAPOR SEPARATOR TANK, SN & Below (See Note)

Try not to run the boat to long with this issue as it could washout the piston rings, etc..

Hope this starts as a helping point, good luck.

Genuine Mercury & Mercruiser parts. 454 EFI (GEN. V) GM 454 V-8 1994-1996 - 0F115700 THRU 0F802349

 

[Hoplite808]

Thank you Boat Tech, just got back from going through it again with a mechanic friend who could help me today and we checked everything you listed and it checked out okay. Decided to check compression again since I found the plug in cylinder 4 wet with gas when I pulled it and when I spark checked it it was working. Compression test found 80 psi. Looks like I’m going to have to overhaul the motor. As the Tiger King is quoted saying “I may never financially recover from this.”

If it weren’t for bad luck I’d have no luck at all.

 

[Chris-380]

Do a leak down test, might be a sticking valve not closing all the way. Did you have the engine completed warmed up, good and hot before the compression test? What were the other 7 comp numbers?

Thank you Boat Tech, just got back from going through it again with a mechanic friend who could help me today and we checked everything you listed and it checked out okay. Decided to check compression again since I found the plug in cylinder 4 wet with gas when I pulled it and when I spark checked it it was working. Compression test found 80 psi. Looks like I’m going to have to overhaul the motor. As the Tiger King is quoted saying “I may never financially recover from this.”

If it weren’t for bad luck I’d have no luck at all.

 

[Hoplite808]

Another update. Began pulling everything apart to take the heads off to get them rebuilt to solve any stuck or leaking valve problems. Found the exhaust valve on the #5 cylinder completely seized and both rocker arms off the valves for that cylinder with their push rods both bent. Hopefully there is no damage to the piston and I can just repair the heads and all will be well again.

 

[Hoplite808]

Another update for those who care.

Got the cylinder heads back from the machine shop and mounted along with the intake manifold which I had to repair due to erosion from the water jacket. Currently trying to get the exhaust manifolds mounted which is no easy task by my lonesome. Hopefully I’ll have her up and running like a champ again soon.

 

[Fill Ess]

Another update for those who care.

Got the cylinder heads back from the machine shop and mounted along with the intake manifold which I had to repair due to erosion from the water jacket. Currently trying to get the exhaust manifolds mounted which is no easy task by my lonesome. Hopefully I’ll have her up and running like a champ again soon.

Updates always appreciated!

 

[Hoplite808]

Got everything put back together yesterday and was getting ready to install the last part before test firing the engine to see if I turned the wrenches the right way or not. But of course a singular sensor on the bottom of the intake plenum decided to break apart. Now gotta wait another week for the part to come in. At least all the heavy lifting in a confined space is done.

 

[Hoplite808]

Part came in today and got it installed. Engine started and ran reasonably well. I think I just need to dial in the timing and it'll be ready to go. Unfortunately the manual I have doesn't detail the procedure well. I'm certain that there is something that I should unplug or run a jumper to in order to stop the automatic advance system while setting the timing with my light but I haven't figured it out yet.

 

[techmitch]

Part came in today and got it installed. Engine started and ran reasonably well. I think I just need to dial in the timing and it'll be ready to go. Unfortunately the manual I have doesn't detail the procedure well. I'm certain that there is something that I should unplug or run a jumper to in order to stop the automatic advance system while setting the timing with my light but I haven't figured it out yet.

You pics in the previous post show rusted parts & bolts and watery chocolate milk oil.

 

[Hoplite808]

You pics in the previous post show rusted parts & bolts and watery chocolate milk oil.

Both heads were removed, rebuilt, and replaced. New valves, valve guides, and seats. Also replaced the push rods. Intake manifold was also repaired and refurbished. Both exhaust manifolds were replaced entirely and while at it I changed the flapper valves too. Plugs, wires, cap and rotor also replaced. No damage was observed to any of the pistons or cylinder walls. The "watery chocolate milk oil" you see in the photo was just from water that dripped on it while I was taking it apart. No signs of water intrusion in the oil pan oil. Also rusty bolts were either wire-wheeled and repainted or replaced and painted.

 

[Hoplite808]

Well, I thought I had the engine back together and running good now but of course if it weren’t for bad luck I’d have no luck at all. The Bravo 3 outdrive gimbal bearing has now failed during my sea trial and it began to struggle above 2500 RPM's again. Hopefully I can find someone to help me get it hauled out and repaired without completely breaking the bank.

 

[Hoplite808]

Decided to double check my work with a leak down test since it was still giving me issues and found that I somehow screwed up the head gasket on one side and have zero compression in the #7 cylinder now. Fun times, I get to strip the engine all the way down again.

 

[scoflaw]

How did you come to that conclusion? Don't sound right

 

[techmitch]

Decided to double check my work with a leak down test since it was still giving me issues and found that I somehow screwed up the head gasket on one side and have zero compression in the #7 cylinder now. Fun times, I get to strip the engine all the way down again.

You found low compression with a leak down test?

 

[Hoplite808]

You found low compression with a leak down test?

I was getting zero pressure in the #7 cylinder when I was testing all the cylinders. It had 150 according to my notes before the rebuild. I know the cylinder walls and the piston looked free of damage when it was apart so its not likely the piston or rings that failed. I loosened the rocker arms to make sure both valves were still closed and the cylinder still would hold zero pressure. Unless the piston failed after assembly which I would expect more noise and symptoms if it had. The only thing I can think of is I somehow screwed up the head gasket and the pressure is leaking out the gasket. Either way I'll only know for sure once I tear the whole thing down again. Also either way this boat is becoming exhausting both physically and financially with just trying to get it to run. I really don't want to be doing this myself but I still can't find a single mechanic willing to work on it for me. All the ones I trust here either are too busy or now say they refuse to work on gasoline engines and will only work on diesel.

 

[scoflaw]

Sounds more like the valve is seized in the guide. Who rebuilt the heads?