165 HP Mercury Inboard - Gm engine

Engine ran well when put in storage.

Now, only fires on 3 cylinders instead of 4. Have checked wires, spark plugs, & distributor. Vales are not stuck. Appears to be gettin gas. Compression is 150 lbs in all 4 cylinders.

Still - one cylinder won't fire.

What have I failed to check??

Ed

 

I dont recall a 165 4 cyl GM engine.
How about a photo

 

I believe the 165HP, 4 cyl. BUILT by Mercruiser, is not an Auto transplant, but Is basically Half of the Ford 429/460. It utilizes the Ford Big Block Head.
You might do a compression check. Compression on One cylinder might be too low to fire ?

 

Thanks Jango .

After posting my inquiry, I learned that the engine was not a GM engine but a mercruiser engine.

I really appreciate the correct information.

Compression is 150 lbs in all 4 cylinders.

Ed

 

This was Mercruiser's one and only attempt to build an engine from Ford parts, but they quickly discovered buying GM Vortec products was cheaper.

With equal compression and 3 cylinders firing, the problem can only be electrical. You checked wires, plugs and distributor, but how?
A visual inspection is not sufficient. The distributor cap may have microscopic cracks resulting in the entry of moisture reducing isolation resistance, the #4 cable may have too high a resistance if it has a carbon core etc.

Try another plug with less contact gap in #4 cylinder. If there is no change temporarily replace the cable with a piece of isolated wire, the stripped ends pushed in the distributor cap and wrapped around the plug's top thread. If still no luck, you have eliminated two possibilities, so the problem must be in the distributor wiper or cap.

 

I agree with CDK it would have been a bigger success if they used Chev parts!!

Not much else to add other then pull the plug leads off the distributor and then rotate it 90 degrees, put the leads back where they were and re check the timing.
That will eliminate or confirm a problem in the distributor

 

Thanks

Thanks to CDK and Powerabout for their advice. Every piece of info helps.

I am getting closer to the solution.

I must try all these new leads.

Ed

 

You need to isolate which cylinder is not firing before you can troubleshoot.

 

You've never seen an individual cylinder miss when there's an intake leak? This person's issue may very well be electrical, but ruling out everything else is bad practice.

If there's a leak right at the intake port for a given cylinder, it can be leaking bad enough to lean out the mixture for that cylinder just enough to cause it to miss, yet not be enough of a leak to lean out the rest of the cylinders to the extent that they're just as affected.

 

Yes I have seen that, or better: heard it because it makes a loud hissing sound when the engine idles, so you can easily diagnose it.
It happens when the intake manifold is not firmly attached or part of the gasket is missing.

But is such a case all cylinders fire as soon as the throttle opens and the vacuum decreases.
The poster wrote "engine ran well when put in storage".

 

I'd replace the plugs one more time, I had a engine that was put up for storage. When it was pulled out, new plugs were installed. The oil that was put in the cylinder for storage caused a very high compression that broke the seal in the plugs in 6 of the 8 cylinders. I had rolled it over with the starter with the plugs out to clean out the cylinders but it still took me a few days to replace the plugs, hey they were new! Lesson learned just cause it's new does not mean it's good.

 

Funny stuff.

First, you said that if the compression was equal and only 3 of the cylinders were firing, the problem had to be electrical.

Now you say you have seen a vacuum leak cause a miss.

Which one is it?

And by the way, a vacuum leak doesn't always cause a whistling noise making it "easy to diagnose". And where does your throttle comment come into play? I see nothing in the original post about engine rpm or throttle positon when the miss occurs.

You're talking out of both sides of your mouth....again.

And you're declaring something as fact, when it isn't.....again.

Lastly, thanks for the explanation of what a vacuum leak is and how it can occur. Before you explained it to me.....I had no idea.

 

PS
If you knew how the manifold to cylinder hed joint looked like on a 470 then you would know its just about impossible to have a vacuum leak

 

No need for hostility broke_not.
I merely explained why, given the way the question was phrased, the cause of the problem must be electrical.

You are the one who entered vacuum in the equation and asked me whether I ever saw that.

There are even more exotic reasons possible why one cylinder doesn't operate, but electrical seems the logical one.

Btw, I do have some doubts about the statement that all four have 150 lb compression. I've never seen that in an old engine, have you?

 

Work methodicly! Get it running then using an insulated tool, pull off each plug lead in turn until the non firing cylinder is identified by no change, then check for a lead spark to the block, if ok change the plug, check for a spark at the plug first and any shorts, if still not firing it can only be the plug is fouling, poor compression/non seating valve or fuel, presumably this is fresh? Check it's not running on choke. A bit of running may clear it and settle it down.

If no decent spark, change that lead, if still no spark change the cap then the rotor arm and so on! Shouldn't be that difficult but need to know what ignition system it has, seen some with twin or individual coils, one that was the cause.

Any idea of the age, model No. I might have the manual, there was a four cylinder Renault engine used in the eighties, can't remember the HP though.