cysflorida
Recruit
- Joined
- Oct 22, 2015
- Messages
- 2
I have a 2008 mercury 90 efi 4s. recently the engine would stumble rough idle and stall or catch itself and come back up to running fine. 100 hour service completed as I originally thought this was a water in the fuel problem flushed fuel tank added new fuel new seperator etc. in the middle of trying to diagnose this the whole engine went into failure and fuel pouring out of the IAFM. thanks to top notch thread posts i figured this out to be the vacuum hose split in the FSM. so today I installed that and started the engine on ear muffs. fired and ran for about 20 minutes at various RPMs and ran it both in gear and neutral high throttle. same thing happened almost (now it doesnt seem to stall everytime like it was doing but just stumble). the engine stumbled for about 90 seconds but caught itself and came back up to what ever RPM it was running at. I put a fuel gauge on the rail and get all kinds of crazy but consistent readings. it seems to really like to be at 20 psi.regardless of RPMs. but after about 10 minutes it went into "stumble"mode for about 90 seconds. pressure dropped to around 12 psi once it caught back up the pressure went to 30 psi steady thru all RPMs with no variable movement. after about 4 or 5 of these "cycles" of failing and running good I jumped it up to 5000 RPM the pressure then went to 35 PSI for a while and when i dropped it back to idle the pressure came down to 30 and stayed there thru the next two failures. I tried increasing the RPMs during the failure period that does not change things. I also tried squeezing the primer ball rapidly during a few of these times but only seemed to correct the problem once. the primer ball does feel like it is going soft on me prior to one of these short episodes but not every time. also i detected a high pitch whine randomly coming from one of the pumps in the FSM but again some times it whined while running good and once it made that racket during one of the brief stumbles.
I have not timed the episodes to see if there is a distinct pattern i.e. good run 10 min bad run 1 min over and over but it sure feels like this is close to being a pattern.
the inline fuel filter from the tank appears to stay full all of the time and the inline filter going back down the port side of the engine aft seems to stay 1/2 full all of the time. I have no idea why this thing does not seem to get to the 40 - 60 psi fuel range stated in a service bulletin i read and i do not understand why the fuel pressure does not change with a change in the RPM like i thought it was supposed to. nor do i understand why it runs perfect at 20 psi then stumbles and runs perfect at the same RPM but at 30 psi then stumbles. it appears that the fuel pressure has no impact on RPM or smooth running/failures
when i did have the FSM apart fixing the vacuum line the tank itself is spotless and the screen at the bottom of the pump was in mint condition. I also believe the previous owner ran a ton of ethanol fuel thru this thing and it did not have a water seperator on it when i bought it.
I have not timed the episodes to see if there is a distinct pattern i.e. good run 10 min bad run 1 min over and over but it sure feels like this is close to being a pattern.
the inline fuel filter from the tank appears to stay full all of the time and the inline filter going back down the port side of the engine aft seems to stay 1/2 full all of the time. I have no idea why this thing does not seem to get to the 40 - 60 psi fuel range stated in a service bulletin i read and i do not understand why the fuel pressure does not change with a change in the RPM like i thought it was supposed to. nor do i understand why it runs perfect at 20 psi then stumbles and runs perfect at the same RPM but at 30 psi then stumbles. it appears that the fuel pressure has no impact on RPM or smooth running/failures
when i did have the FSM apart fixing the vacuum line the tank itself is spotless and the screen at the bottom of the pump was in mint condition. I also believe the previous owner ran a ton of ethanol fuel thru this thing and it did not have a water seperator on it when i bought it.
