Dying# 4 cylinder

[Murph d]

I have a 1986 3.7 Lt 190 hp Merc I've developed a miss at an idle sometimes, I've narrowed it down to the back cyl. The miss will clear up if I rev it up a bit. Also I had a backfire at 3500rpms ,it ran great for about a half hour at 3500 then backfired , backed off to 2800 then ok. Parked for approx. 1.5 hours , then ran great at 3500 for about 20 mins. then back fired again. I have replaced plugs, plug wires,cap, rotor ,and condensor, the problem still persists . Do you think these symptoms are related ? And do you think this is spark or a gas problem?

 

[Murph d]

Re: Dying# 4 cylinder 3.7 Lt

Re: Dying# 4 cylinder 3.7 Lt

I have a 1986 3.7 Lt 190 hp Merc I've developed a miss at an idle sometimes, I've narrowed it down to the back cyl. The miss will clear up if I rev it up a bit. Also I had a backfire at 3500rpms ,it ran great for about a half hour at 3500 then backfired , backed off to 2800 then ok. Parked for approx. 1.5 hours , then ran great at 3500 for about 20 mins. then back fired again. I have replaced plugs, plug wires,cap, rotor ,and condensor, the problem still persists . Do you think these symptoms are related ? And do you think this is spark or a gas problem?

 

[TilliamWe]

Re: Dying# 4 cylinder

Exhaust manifold/elbow failure, in my guess. Be very careful with this issue. You can bend a pushrod fairly easily.

 

[stonyloam]

Re: Dying# 4 cylinder

Been there done that (exactly the same symptoms)! Do a compression test, if #4 is low, you have a bad head gasket. Need to pull the head and replace the gasket, and get a valve job done. Use a OEM Mercruiser gasket. Good luck!

 

[Murph d]

Re: Dying# 4 cylinder

I've done a compression test , it's 125psi , around the same as the other cyls. The motor has fresh water cooling . Do my symptoms sound like a worn exhaust riser ,and are there new manifolds available?

 

[stonyloam]

Re: Dying# 4 cylinder

I've done a compression test , it's 125psi , around the same as the other cyls.

That is a little low. How much difference? If #4 was the lowest (or if 3&4 are low and the same), even by a few pounds, I would still bet on the head gasket. It is logical to assume that If you replaced all of the components (plugs wires etc.) and you still have a miss in #4 the problem is with #4. These engines are noted for head gasket problems affecting # 3 & 4 cylinders. BTW is the backfire though the exhaust or the carb.?

 

[Murph d]

Re: Dying# 4 cylinder

Thanks for the replies , this forum is great. The backfire is through the carb. If it is the head gasket wouldn't there be signs of coolant in the oil.

 

[stonyloam]

Re: Dying# 4 cylinder

Thanks for the replies , this forum is great. The backfire is through the carb. If it is the head gasket wouldn't there be signs of coolant in the oil.

Not necessarily, I had none, but I had a bad head gasket leaking coolant into 3 & 4. My machinist took one look at the head and said "coolant leak". My compressions were 155, 155 135, 35. so a lot worse than yours. Replaced 4 exhaust valve seats (eroded by the coolant), and intake seat in #4. My symptoms were EXACTLY what you describe, even to the backfiring through the carb in the 3000-4000 range. It idled rough (pulling #4 wire had little effect), but smoothed out at higher RPM. After the valve job it runs perfectly, smooth as silk. Pulling the head is not a real big deal, I used the Clymer manual and it was fine, better than the Merc manual for this job. If you do need a head gasket, make sure you use OEM Mercruiser, they are made specifically for the Mercruiser engine.

 

[TilliamWe]

Re: Dying# 4 cylinder

The second reason I am telling you to check the exhaust components first is that properly re-installing the head on those motors is Difficult to say the least. With the open deck design, you have to get it on and retorqued EXACTLY right, according to the Mercruiser manual, or you will ruin a brand new headgasket. Since you would have to remove the exhaust riser and manifold first, do that, and inspect them interally, very closely for signs (rust usually) of water. However, one of your exhaust pieces is probably aluminum, so it won't show rust in the exhaust passages, but might show corrosion. I "think" you can buy manifolds/risers for that engine still, but they are Pricey. Good luck.

 

[stonyloam]

Re: Dying# 4 cylinder

that properly re-installing the head on those motors is Difficult to say the least. With the open deck design, you have to get it on and retorqued EXACTLY right, according to the Mercruiser manual, or you will ruin a brand new headgasket.

I have to disagree a little with the first statement, it isn't that bad. There are alignment pins to position the gasket and head, and the new OEM gaskets have been modified to seal better, not too difficult just PITA. I will completely agree with the second statement. You have to follow the book exactly, including cleaning the bolt holes and bolts, perfect seal on bolts, torquing in stages and in the correct sequence, and retorque after running the first time. Again not difficult, but major PITA, just take your time. TW is right once the exhaust is off the hard part is done.

 

[Murph d]

Re: Dying# 4 cylinder

Thanks alot for the advise I'm going to get to the bottom of this on sat.

 

[Murph d]

Re: Dying# 4 cylinder

Thanks for your informative help guys , it turns out that my manifold riser was quite corroded . Past time to change, I'm told there good for between 3-7 years .The sea water outlet was rusted back far enough to leak into the 4th cyl. causing the missing and backfiring.

Thanks again Murph.

 

[myoldboat2]

Re: Dying# 4 cylinder

how cool is that, within one day you were given the two most likely causes, and one of them was it. what a great forum. nice job, guys.

 

[TilliamWe]

Re: Dying# 4 cylinder

And you caught it before you bent a pushrod in that cylinder? Hooray! Good for you! Good luck in the future, and go boating!