97 Mercruiser 5.7 slight misfire

[p1mlb03]

Hello all, I read these forums all the time, matter of fact I have an order from I Boat enroute now. I now have an issue with a slight misfire on my 97 mercruiser 5.7lx (carb) alpha drive and I?m hoping you guys can provide some feedback to help me resolve this problem. So here is my story? Winterized boat in December, antifreeze, fogging oil, and stabil in a full tank of fresh gas. Boat was running fine at the time of winterization. I decided to replace manifolds and risers this past weekend. While I had the engine cover removed from the boat I decided to replace the dist cap, rotor button and plugs . While replacing the dist cap I noticed the screw that holds the ignition module and a ground wire was rusted, of coarse it broke when I tried to remove. Drilled it out and replaced it with a SS nut and bolt. I would not think that replacing this bolt would be an issue but wanted to provide full information. After new manifolds and risers were installed along with plugs (AC MR43T) (gapped 0.35 per engine cover, but aftermarket man says 0.45 ??) I started the boat on the muffs. I now have a slight misfire or so it seems. There is no rhythm to the miss, seems to run smooth for a short period and stutters and repeats randomly. Today I replaced the plug wires with no change in status, replaced the new cap and button with my known good onboard spare, no change. Replaced the water / fuel filter and poured gas from old filter into a clean container, looks & smells fine, no sign of water. The flame arrester was cleaned spotless as well. I was thinking that it is possible that I have a bad plug, cracked porcelain or something along those lines. Was going to pick up a new plug and swap it out one plug at a time in an attempt to rule out a failed plug. Was also going to get a gas can with fresh gas and run a piece of fuel line from the can to the fuel pump to rule out fuel. This boat is well maintained and is in great shape, never had any problems. Any ideas / thoughts on troubleshooting this issue?

Thanks, Mike

 

[bruceb58]

Re: 97 Mercruiser 5.7 slight misfire

What plugs were in there? Reason I ask is that in 1997, you may have Vortec heads which require the AC MR43TS plug. 1997 was a transition year when they started putting those heads on.

If you fog in the winter, I would start the engine in the spring with the old plugs so that you burn all the fogging oil away and then change the plugs. Less likely to foul your brand new plugs.

 

[p1mlb03]

Re: 97 Mercruiser 5.7 slight misfire

What plugs were in there? Reason I ask is that in 1997, you may have Vortec heads which require the AC MR43TS plug. 1997 was a transition year when they started putting those heads on.

If you fog in the winter, I would start the engine in the spring with the old plugs so that you burn all the fogging oil away and then change the plugs. Less likely to foul your brand new plugs.

I pulled out MR43 plugs. I agree with the starting on the old plugs, but wit the manifolds off it was just to easy to change them. Is there an easy way to determine if i have the Vortec heads? S/N maybe, or something physical about the heads?
Thanks , Mike

 

[v8power]

Re: 97 Mercruiser 5.7 slight misfire

Vortec heads have the bolts that go throught the center of the valve covers. Pre vortec heads have the bolts around the perimeter of the valve covers.

 

[bruceb58]

Re: 97 Mercruiser 5.7 slight misfire

You can tell from the serial number as well as counting the intake manifold bolts...Vortec has 8 and non-Vortec has 12.

As far as the plug number, the one for the vortec is the identical number but with an "S" on the end so easy to miss.

 

[bruceb58]

Re: 97 Mercruiser 5.7 slight misfire

Vortec heads have the bolts that go throught the center of the valve covers. Pre vortec heads have the bolts around the perimeter of the valve covers.

That's not correct, They started putting the bolts through the valve covers in the mid 80s...way before Vortec came along.

 

[p1mlb03]

Re: 97 Mercruiser 5.7 slight misfire

You can tell from the serial number as well as counting the intake manifold bolts...Vortec has 8 and non-Vortec has 12.

As far as the plug number, the one for the vortec is the identical number but with an "S" on the end so easy to miss.

Ok thanks, I will check the intake in the AM. I meant to say I pulled out MR43T plugs. It has always run great with the 43T, if I do have the Vortec heads would the S plug make that much diff? Would you gap them at 0.35like the arrester cover states or 0.45 that the man states? I gapped at 0.35, could this be an issue as well? Any ideas on troubleshooting this problem other then the plugs?
Thanks, mike

 

[John_S]

Re: 97 Mercruiser 5.7 slight misfire

Check, but don't be surprised that you have the standard heads. I don't know of any merc factory 4bl carb/vortec. Outside of a 2brl (L model), they were EFI or MPI. If you find vortec heads, look over the intake. It probably is not stock.

 

[p1mlb03]

Re: 97 Mercruiser 5.7 slight misfire

Check, but don't be surprised that you have the standard heads. I don't know of any merc factory 4bl carb/vortec. Outside of a 2brl (L model), they were EFI or MPI. If you find vortec heads, look over the intake. It probably is not stock.

I'm thinking they are standard heads, this engine is factory stock, no mods at all. Mine is the 5.7 2bl model... Anyone else have any suggestions / ideas for troubleshooting?

Thanks, Mike

 

[bruceb58]

Re: 97 Mercruiser 5.7 slight misfire

I would revisit your ground wire and make double sure that it has a good connection. Try wiggling it while the engine is running to see if it makes it miss.

How are your spark plug wires? Have they been changed? Maybe they are routed slightly different now that you changed your plugs and you are getting some arcing. Run your engine at night to see if you can see any arcing. A bad plug wire will give off a light show!

 

[p1mlb03]

Re: 97 Mercruiser 5.7 slight misfire

I would revisit your ground wire and make double sure that it has a good connection. Try wiggling it while the engine is running to see if it makes it miss.

How are your spark plug wires? Have they been changed? Maybe they are routed slightly different now that you changed your plugs and you are getting some arcing. Run your engine at night to see if you can see any arcing. A bad plug wire will give off a light show!

I will recheck the ground wire in the AM. I changed the plug wires today, brand new, routed them the same as factory. It won't hurt to run it in the dark to see if I have any arcing.
Thanks,Mike

 

[John_S]

Re: 97 Mercruiser 5.7 slight misfire

Mine is the 5.7 2bl model...

Hmm, an LX model should have a 4brl, any chance you have mistaked throttle body fuel injection for a 2brl carb? If you have serial numbers, that should help.

As far as the miss, I assume this happening at idling or just above, given you are running on muffs. Have you done the timing procedure? See if it misses in base mode. Use the inductive timing light to see if you can find an offending cylinder. Also, if you do have 2brl carb, look down the throat with flashlight while idling. You should not see any dribling or build-up of gas on top of butterflies.

My '96.5 5.7L 2brl had 0.035 gap on the cover, but 0.045 is what they should be gapped to. Doubtful that is causing your miss.

 

[p1mlb03]

Re: 97 Mercruiser 5.7 slight misfire

Hmm, an LX model should have a 4brl, any chance you have mistaked throttle body fuel injection for a 2brl carb? If you have serial numbers, that should help.

As far as the miss, I assume this happening at idling or just above, given you are running on muffs. Have you done the timing procedure? See if it misses in base mode. Use the inductive timing light to see if you can find an offending cylinder. Also, if you do have 2brl carb, look down the throat with flashlight while idling. You should not see any dribling or build-up of gas on top of butterflies.

My '96.5 5.7L 2brl had 0.035 gap on the cover, but 0.045 is what they should be gapped to. Doubtful that is causing your miss.

Thanks John, I am positive it is a 2bl carb, I have to replace the accel pump in the carb every 3 or 4 years, thanks to Eth gas I'm sure. I will regap the plugs as I swap them out one at a time to rule out a failed plug. I have never changed the timing from factory, but I will check it. Could the ignition module be the issue? Or do they either work or not?
Thanks again, Mike

 

[John_S]

Re: 97 Mercruiser 5.7 slight misfire

A 2brl carb on a LX still doesn't compute. Might want to get serial number off plate on engine and not the flame arrester cover. It sounds more like an L vs LX model.

If you have never checked the timing it is way over due. Review the procedures to put in base mode in the service manual.

No, I wouldn't think the module is causing a miss. If it was related, I suspect a loose ground like Bruce suggested.

BTW, a 5.7L non-vortec 2brl would have only a ICM. An LX, Mag, or vortec would have an additional piggy-back knock-module.

Also, do that carb check. It is common for the float needle to stick or not seal and too much gas dribbles in during idle. It would cause misses, most likely on the cyl 7 and 8, for being over rich.

 

[John_S]

Re: 97 Mercruiser 5.7 slight misfire

I highly doubt it is anything fuel related and an intermitant miss is almost always ignition related.

Except, this is very common in the 2brl Mercarbs. There is even two different float needle valves to address this issue. Most rebuild kits include both. Besides that, it is a very quick check. And then you can get back to tracing out the ignition.

 

[p1mlb03]

Re: 97 Mercruiser 5.7 slight misfire

Thanks guys,
Took some pics of the engine and the ground screw in question. I just assumed this was an LX, it is def a 5.7 2bl so I guess that makes it an L. There is no rust in the dist, only on the screw that was holding the module in place on the back side of the dist. I replaced the screw with SS and cleaned the connection good and sprayed some CRC 6 56 on it. I also made a small rubber flap and attached it to the arrester cover so that any water that falls between the engine cover and the fiberglass no longer drips on the module. I am sure I have the correct cap and rotor. I swapped in my spare, which is the cap and rotor from the factory with 90 hrs on them. I will def check the timing. And if that checks out I will pull the carb.
Mike

 

[John_S]

Re: 97 Mercruiser 5.7 slight misfire

Yep, a 5.7L 2brl non-vortec, rated 210HP @ prop.

For the connection as long as the ring terminal surface and ICM metal case tab are clean, and screw is tight, you should have a good ground connection there.

If you don't see that dribbling down the barrel, don't be in hurry to pull that carb. As part of the timing procedure you need to also set the idle mixture and idle speed.

Even if you see dribling, don't jump on the carb. Need to check the fuel pressure. I had a mech pump putting out too much pressure that causes the same symptoms.

 

[p1mlb03]

Re: 97 Mercruiser 5.7 slight misfire

Work schedule is holding up progress on this issue, but this AM I had a couple hours to work on it. I checked compression in all 8 cyl as I was replacing the plugs and they were all good, in the mid to upper 140's. New plugs did not solve the issue, but it appeared that cyl 1 was not firing properly, as the plug was very clean , almost new looking compared to the other 7, they were much darker. I have not been able to check the timing as of yet, my timing light has the old school pick up between the plug and wire and I don't trust it. Going to buy or borrow a better light. I did use my TL to verifiy that I was getting spark on #1 although it may be weak... IDK. May need to buy a diag device such as the smartach that Bubba mentioned. Any idea why #1 may not be firing properly? I've tried two new plugs, new wires, multiple caps and rotors.

 

[p1mlb03]

Re: 97 Mercruiser 5.7 slight misfire

Hate to say it but a really clean plug says water intrusion.

I hate to say that too... Head gasket? Riser gasket failure? Although the manifolds risers and gasket are brand new. How would I prove water intrusion?

 

[p1mlb03]

Re: 97 Mercruiser 5.7 slight misfire

A compression leak down test should give you an idea if its a head gasket. However, if you just put new gaskets in the manifolds/risers I would sure take a look at them as suspect. It doesn't take much on the mating surfaces (or poor machine work) to cause problems.

Thanks for the input, I installed Osco Manifolds and risers which should be good quality, but it is easy enough to pull the risers to check the gaskets for blow by. I cleaned ALL paint from the mating surface and taped off the machined surfaces prior to painting. The manifolds where torqued to 20lbs as well as the risers. Should the risers be torqued tighter?