Problems with 5.7L TBI Mercruiser

[Eleonar1]

I have a 1998 5.7L TBI Mercruiser Alpha 1 gen 2, closed cooling, that's having problems. I recently installed a new long block along with manifolds & risers as the original motor blew with about 870hrs on it. The issue I am having is it will only idle for about 15-20 seconds, after this the IAC opens too much, there is a loud hissing noise, and it stalls the motor completely.

Engine Serial # is 0l082496

I have already:
Changed the IAC (with a quicksilver one)
Cleaned TBI passages where IAC sits with carb cleaner, installed new base gaskets
Changed the ECT, MAP & TPS
Changed the fuel pressure regulator & tiny filter under it in the cool fuel II system (original regulator was showing 34PSI, thought this might be too high as book spec is 30PSI)
Tried Unhooking oil pressure sensor (thinking maybe it was telling the fuel pump to shutoff)
changed both injectors, they have a nice even spray, no large fuel droplets or anything at idle
checked for vacuum leaks with a vacuum gauge - none found, pulls a good 20In +/- at idle UNTIL the IAC opens up essentially creating a large vacuum leak and stalling the motor
Changed fuel/water separator
Removed the Anti Siphon Valve (for test purpose only - replaced with a new one) to make sure it wasn't restricting fuel flow
checked base timing (8* BTDC)
Motor when warm runs around 140deg

The only other thing I've found is sometimes it will stay at idle longer if I unplug the ECT completely, which I believe tricks the motor into thinking its cold and injecting more fuel into the TB. But even with this is does eventually stall.

I am at a loss as to what else to try, besides having idle problems the motor runs pretty good, after I get up around 1500RPM or so everything is smooth and I can get up on plane and to WOT without any issues. All the problems seem to be low speed related and have something to do with the IAC opening too much.

I have plugged a RINDA scan tool into this motor and it had thrown a few different codes for IAC/ECT but we had just changed those parts, cleared the codes and they seem to have gone away (j just ordered a Codemate so I can read them myself).

Any thoughts/advice is much appreciated!

Thanks!

 

[alldodge]

Appears to be MEFI-1 ECM

Before the new long block, was there any issues with the motor running?

When setting timing in base timing mode, was the idle rpm increased to 1200 ?

How old is the gas?

Does fuel pressure stay around 30 psi the whole time?

Check that all grounds have been re-connected
Measure the voltage at the MAP and ECT

 

[Eleonar1]

Alldodge,

timing was set in base mode at 1200rpm

gas is fresh (maybe 2 weeks old) I had burned a few tanks of it with the old engine and it was fine, fuel filter had no water or debris in it when I changed it.

fuel pressure (with old regulator) was right at 34psi. I need to check it with the new regulator but it’s doing the exact same thing as before - will update when I check again.

i will recheck all grounds but but I’m 99% sure I put them all back on the two ground studs on the rear of the motor when I dropped the new long block in.

I will measure the MAP voltage / ECT when I’m back at the house and post what I find.

 

[Eleonar1]

AllDodge,

Took the boat out yesterday to mess with it, ended up figuring out that it is leaning itself out at idle, if i unplug the MAP sensor it will idle fine but just runs way too rich. I'm wondering the something in the ECM is messing up?

Do you happen to know if the ECM has its own dedicated grounding circut? I'm wondering if I might've missed one and the ECM doesn't know what to tell the fuel injectors to do? The only ground wire I found that isn't hooked to anything is what looks like 18AWG wire with a small ring terminal on the end that is sort of near the electrical box, the ring terminal is way to small to fit on the ground studs on the back of the motor.. just wondering if this is part of my problem?

Would it hurt anything to run a ground from the ECM casing to engine block just for good measure?

I have tested the TPS with a volt meter and it checked out fine (.51V at idle, and steadily increases as the throttle is pushed forward), I have also checked the MAP sensor circuit at the harness and it seems to be putting the correct voltage out - i had a spare MAP sensor and tried plugging it in with similar results (once plugged back in the motor runs too lean and stalls). IACV is also brand new (quicksilver part). ECT is new as well, checked the harness and it has 5V power going to it between the two wires.

Thank you for the help

 

[alldodge]

If the wire found is black there would be no issue with grounding it somewhere even if it needs an extension to do so. May not change anything but then again it might

I would like to see what a scan tool will show up. Have you connected a vacuum gauge to the intake?

 

[Eleonar1]

I have hooked a vacuum gauge up, has about 20in of vacuum at idle, so I don’t think I have a vacuum leak anywhere

Unfortunately I don’t have access to the scan tool for a bit here until my buddy can bring one up.

at this point it’s very clear that as soon as the motor enters “open loop” the ECM is not giving it enough fuel and it leans out and dies. It’s just a matter of figuring out what is telling it to do so.

also I would like to mention that prior to installing the new long block, the old engine would idle just fine, so I’m wondering what might’ve changed between the old and new blocks.

 

[alldodge]

Agree, sure wonder what happened, Old block idled fine, new one has the issue

Did the injectors come from Merc or ebay? If ebay I would reinstall the old ones

A scanner would tell use what the IAC position is when its cold and where it goes to once warmed up

 

[Eleonar1]

The old block idled fine, however it did have a stumble/hesitation from about 700-1100 rpm (in gear) which I always attributed to the fact that it had a blown head gasket for who knows how long. But again, it would idle in neutral or in gear all day long without any issue.. only thing I found a little odd was I always had to give it a little throttle to start it, but could almost immediately bring it down to idle/neutral and it wouldn't skip a beat.

The injectors came from ebay but they were OEM injectors that a company had remanufactured/flow tested prior to shipping to be, the spray pattern looks equal on both of them, no huge droplets or anything, nice cone shaped spray.

I'll try getting ahold of the scanner next week - unfortunately all I purchased was the code mate reader which is almost useless ( i think if i did it again I'd just buy a RINDA scan tool!)

 

[alldodge]

There are many sellers on ebay which say they rebuild injectors, when the fact is once a injector does not pass flow test it cannot be rebuilt. They are just cleaning them up and resold

If you have your old ones put them back in

 

[Eleonar1]

Put old injectors back in, running a bit better, not stalling at idle nearly as frequently as it was. My biggest issue now is that the RPM’s will surge (I.e. I can be going along at 3000rpm and it’ll bring itself up about 1-200rpm and then come back down to 3000 without me touching the throttle at all) does this throughout the RPM range doesn’t matter if your at 1k or 4K RPM. This wouldn’t bother me much but needless to say it makes docking exciting when the boat starts to randomly go faster when you want to be going slow.

Any thoughts on this?

thank you!

 

[alldodge]

I'm back to the injectors, my guess is they need to be replaced but may be able to be cleaned. I would suggest either BAM or Boats Unlimited. Both do good work and can be trusted with results. The injectors are Mallory part number 9-33106

Unless you know of another I would contact boat and see what they say

https://www.mercruiserparts.com/ultrasonic-fuel-injector-cleaning

https://www.boatunlimited.com/

 

[Eleonar1]

Alldodge,

Is the theory behind the injectors needing to be cleaned that they might be letting in a big droplet or fuel occasionally causing the sudden surge in RPM?

Thanks,

 

[alldodge]

My theory is the on/off time varies due to response time. The ECM wants lets say 20ms and it changes up and down between 20 and 25ms

The other reason to clean is to ensure they are working as required so they can be ruled in or out of what is causing the trouble. Find out what is working correctly

The TPS and MAP as you know are also in the mix

 

[Eleonar1]

Ok I will look into the injectors

ive tested the TPS (Probed wire and watched it steadily climb from .51v to a little over 4v at WOT. So I’m thinking that works fine. I did measure the MAP voltage at the harness which was fine as well (had a spare MAP as well and tried that just to see but no difference)

 

[Eleonar1]

Alldodge,

do you happen to know of merc and Volvo used the same TBI systems in the 90’s? My father has a older TBI Volvo 350 with junk heads sitting in his yard. I didn’t know if I could try the throttle
body off it on my motor, as my thought would be having 2 sets of injectors be junk would be highly unlikely. Before that motor broke it functioned just fine.

thanks

 

[alldodge]

Unable to say for sure if they are exactly the same, but both Merc and VP used the GM TB

 

[Eleonar1]

Alldodge,

I had a chance to swap the Volvo TBI unit and ECM to my motor. The ECM is the same part number and the Volvo it came off was also a 5.7GM block. I also looked online and the Volvo’s call for the same 30psi of fuel pressure for their TBI unit as Mercs do. I have not been able to water test but I ran it on a hose for about a half hour. Started up and idled perfectly at 650rpm in neutral... hoping I might have found the issue!

will updated after I have a chance to take it out when this rain stops...

 

[Eleonar1]

Alldodge,

I was able to go for a quick ride today before more thunderstorms rolled in, boat ran flawlessly up to around 3k rpm but after that it had trouble climbing to 3500+ couldn't get it over 4K RPM at any point. Not sure what might cause this. Again it ran great, idled perfect, no stumbling, hesitation, anything of the sort until I tried to go fast. Was able to get up on plane but it just wouldn't rev out completely.

any thoughts?

Thanks!

 

[Eleonar1]

Disregard, found a small fuel leak coming from the line going into the TBI, took apart and reconnected/tightened everything. I'm guessing as the RPM went higher is was leaning out just a bit and running out of fuel because it was dumping it onto the intake manifold

 

[alldodge]

Disregard, found a small fuel leak coming from the line going into the TBI, took apart and reconnected/tightened everything. I'm guessing as the RPM went higher is was leaning out just a bit and running out of fuel because it was dumping it onto the intake manifold

Could be, sure hope so

Otherwise I would check how much advance your getting