98 Mercruiser 5.7l V8 Thunderbolt V 2bbl carb problem! l

[ksmauck]

I bought a 1998 21' Reinell Coho that has a 98 Mercruiser 5.7 Liter V8 250 HP motor with the Thunderbolt V ignition and 2bbl carburetor. The thing ran beautifully the first several times I used the boat. It cruised with no odd noises and top speed on calm water was 48 mph according to the GPS. Plenty of power, idled perfect.. started easy.

One morning when it was a little cooler outside I decided to do the motor a favor and gave it an extra pump on the throttle. Needless to say bad idea..it wouldn't start and would diesel after turning off the key. Eventually it started but I had clearly flooded the motor. Could smell fuel and it would aspirate fuel through the carb. After getting it started it ran fine... Went out on the water and it ran fine. Next cold start.. same thing.. hard starting. I figured i had fouled a plug so I pulled the #1 plug and took it to Napa and spent $87 on NGK iridium plugs matching the plug I had removed. (should have checked the manual). When I put the new plugs in I had a friend (mechanic) who was helping me. (yes I can change plugs but change 4 or 5 of those bastards and you'll let your friend help if he's willing with the remaining plugs). Pulled the rest of the old plugs and they were all some autolite brand.. apparently he only had 1 NGK. My friend when putting one in said oh I think I owe you a plug.. I may have cracked the porcelain. He then told me that it was good to go and he hadn't.. I should have verified but I didn't.

After changing the plugs it started easy and ran smooth. Took it out on the water and it ran fine... I do think that it wouldn't reach the RPM it should have on the top end but I blamed it on the water and the tides.

Today I took the boat out.. started harder than the last time but still not bad. Idled fine.. maybe a little rougher than normal. Cruised about 15 miles at about 3500 RPM and was hearing this sound. It sounded like a rattle but to me it sounded like a higher pitched rattle like maybe a spark issue.. like it wasn't timed right or something... Went back lifted motor cover while we were cruising along trying to find where came from.. hoping it was a vibration.. pushed on a few things etc.. walked back up took the helm and could feel the motor struggling.. I started to slow slightly and it fell on its face and started running horribly. Came down to an idle and clearly not running on all cylinders..

Limped home and pulled the plugs and sure enough... porcelain was cracked on the #4 hole. Spark plug electrode strap looked like it had broken off or melted. Figured I had found my problem so I went and bought all new plugs that are recommended for this particular engine.. (not iridium). Decided while I was at it I would change the cap and rotor and the coil (i guess i felt like I had an extra $80). Figured it would run like a champ... put it all back together.. started it up and it ran like crap still. Put a timing light on it and it looks like it's horribly advanced at idle..

My buddy got it to where it ran smooth (it would die if you even came close to 10 deg btdc). Started easy.. ran smooth.. sounded good.. I figured hmmm maybe the cylinder had been loaded up with fuel and it just cleaned itself out..

We took it out on the water and you could hear that awful noise at high rpm again. He adjusted the distributor under load and underway at about 4,000 rpm (which is about all I could reach) and got rid of that annoying sound... came back down to idle and boom... ran rough and almost died... gave it some more gas to try to get back up and it would barely run.. like it was not firing on all cylinders again..

Any help is greatly appreciated! I'm confused as all hell.. I did an OHM check on the plug wires and they all seem to be higher than normal.. As an example the #4 wire was about 14.1k ohms..

Right now I'm thinking everything from fuel issue (carb, vent) to ignition module, knock module, timing chain jumped a tooth, stuck valve.. junk motor.. have no idea.. It's all just confusing to me right now!

 

[achris]

You need to set the ignition system into 'Base Mode' to set the timing. Ground the white/purple wire coming from the distributor and set the timing like that. But before you do that, for piece of mind, do a compression test on all cylinders, and check/change fuel filters.. Just make sure you haven't done any more serious damage. Once you have the timing set correctly, then start working on the fuel system....

Chris...............

 

[ksmauck]

Ok so I did some more testing... Started with the compression test... Hole #1 and Hole #3 were 145 PSI.. Hole #5 80 PSI and Hole #7 was ZERO PSI..
Didn't bother checking the others holes other than #8 didn't have any compression either and #4 felt very weak as well. So pretty much crappy news!!!

I explored why no compression and determined it is the valves... Air coming back through the intake...(not the exhaust and not the crankcase). Used a boroscope and crosshatching looks good on the cylinder walls from what I can tell.. Didn't see any damage to the top of the pistons but couldn't get a perfect view either.. but the valves and valve seats look like total crap. It looks, from best as I can tell, as though there is detonation damage to the cylinder head valve seats and valves. And of course the intake valves are hanging open.

Did a little more exploring and I can see that this is not the stock OEM engine. It looks like it has a crate motor in it and I confirmed with the guy I bought it from that this is not the original motor (a little fact he completely left out when selling me the boat). The stamp on the motor says 10243880 Hecho in Mexico. From what I can determine this is a 95-99 350 V8 4 bolt ZZ4 roller cam, one piece rear main seal motor.

Can anyone tell me anymore about this motor and if it is/was an acceptable replacement for the OEM motor? Clearly I am going to need, at a minimum new cylinder heads. Can anyone tell me what cylinder head I should use for this motor?

Can anyone give me some suggestions on why I would have such horrible detonation problems? Could it be the wrong iridium spark plugs that were in it? Could it be horrible ignition timing? Could it be a bad carb or wrong carb for this motor (stock 2bbl carb is still on it)? Last thing I want to do is put new heads on it and have the same problem a 100 hours down the water...

Thanks!

 

[achris]

What plugs were in it? If it was like that when you bought it, then the PO MAY have advanced the timing just to get it to run well enough to sell... As your engine does NOT have a knock sensor unit, any knock (timing/bad fuel) would just lead to the sort of damage you've seen, and no retarding of the spark...

Not sure I can help you much on your other questions... But when you do get it back together, just set the timing in base mode and you have the right module for the engine.... Can you find a number on it? (you should find a number like 807264T3 on it)

HTH,

Chris.......

 

[ksmauck]

He had a combination of Autolite iridium plugs (dont' know the number had already thrown them out) and NGK IR plug ITR4A 15.
The # on the ignition module is: 861253-1 5.7L Alpha
The other module that sits on top of that is: BCTU 16184069 and then under the barcode it has the number (which is smaller) 1375582

I think you are definitely on to something as far as advanced timing. I do know when we hooked the timing light up while it was running it was definitely way advanced. Too even keep it running at 600-800 RPM it was advanced way behind the timing mark I would predict 30 btdc.

 

[achris]

Those are the right numbers for your engine based on year. But if the timing was that far advanced, then the knock module would not have been able to pull the timing back enough... (Yes that engine does have a knock module. For some reason I was looking at 97 diagram :facepalm

Do you have an engine serial number? It's much easier to track the right engine with a serial number than a year. Not all boats have the same year engine as the hull is.

Correct spark plugs are: AC - MR43LTS or NGK - BPR6EFS... NOT iridium! Iridiums are for the later MPIs....

Chris.........

 

[ksmauck]

Actually I do have the number off the engine.. It is 10243880 That's the stamp on the block and I've found that it's not the original motor. The original was suppose to be a mercruiser GM (all black) this one is a volvo GM (red) and it has a volvo tag on the motor as well. It's a 1995-1999 GM 5.7 Small Block ZZ4 4 bolt main. The ignition module has the knock module but there is not a knock sensor on this particular motor. The PO just plugged that pur/wht wire so it's not hooked up.

What I found: I did a compression test and found 2 dead cylinders in #7 and #8 and #5 was only 80 lbs. Leak down indicated top end as it was leaking past the valves through the intake. Pulled the cylinder heads and found that the intake valves in those cylinders was cupping. Looks like they got too hot and weren't sealing. Exhaust valves looked ok. Cylinders look good with good cross hatching and the pistons look fine.

Appears as if it was definitely a pre-ignition problem.. I'm hoping it was just not timed correctly! I'm getting the heads re-done and will reinstall with correct plugs, new plug wires, new rotor and cap, and sending unit and coil. Also rebuilding the carburetor. Hoping it fixes the problem... if you have any ideas I'd love to hear them. Last thing I want to do is put it back together and still have a problem that's not discovered until it's too late. Also replacing the risers...it is a closed cooling system.

Is it okay that this thing has a knock module but no knock sensor?? There is nowhere on the block for one

 

[achris]

If there's not sensors, disconnect the module, or fit sensors. There will be a blind hole in the port side of the block, somewhere near the water drain and the oil pressure sensors. Into that hole screws an adaptor, and the sensor screws into that....

Chris......