how to do compression test excalibur 330. damage coil?

[laurie88]

since doing compression test by mechanic my boat has started to missfire at 1700rpm and engine starts shuddering , popping

he did remove only plugs and we cranked engine in each cylinder

he did NOT
REMOVE ANY LEADS FROM COIL or distributor
Disable fuel pump


Has this damaged my coil, (or anything else) as we have replaced everything else and engine still misses but only under load at 1700rpm . we have no engine warn light, boat starts first time, idles ok

pls help

[scuff97]

I would not think there is damage to the coil. One of the spark plugs or spark plug wires was damaged when removed

[91xl]

misfire

like any misfire you need to isolate which cylinder then find the cause. is it fuel, spark, or compression, why did you do a compression test? if there was no issue before i would look at the plug and wire of affected cylinder.

[laurie88]

misfire

tx guys

i did compression test because i had leak from exhaust man into cylinder and had water in oil and on 2 plugs..we replaced manifold and changed oil

i changed wires and plugs.....could it still be these items...incorrect gap or something else

[91xl]

misfire

new means never ever worked, i would start by finding which cylinder it is. it's also possible that oil may have partially plugged the injector. u would need a pressure gauge and a tool to toggle the injecters then compare the pressure drop between them. need to start buy re checking the plugs and wires if the part store droped a plug in stocking it it could be damaged.

[DanielC]

Modern ignition systems can generate very high voltages, if the current does not have somewhere to go. If you do not provide a place for the current to go, It will try to find a way to ground, and when it does it can leave a "path" or a track through insulation, and this may provide a short circuit past the spark plug, and cause the misfire. Long story short, you need to either ground thew coil output, or disconnect the primary side of the coil, (or coils) when doing a compression test.

Since the spark plugs wires were replaced, make sure the firing order is correct.
Another possibility is the distributer is not indexed correctly, and the rotor may be closer to a cylinder not firing, making that spark plug fire. It may be that your distributor cap was damaged by cranking the engine with nowhere for the spark voltage to go.

The voltage needed to fire a spark plug on a normally operating engine is highest when accelerating, off idle, with a load. Misfires happening then, usually point to a weak ignition system.

If after checking the obvious, and you still cannot find the problem, I would find a shop with an ignition oscilloscope. This is a piece of test equipment that can show the actual voltages needed to fire each spark plug, and it allows you to quickly isolate the problem down to the affected cylinder, or cylinders. A good technician can also tell from the waveform displayed what the problem is, usually.

[laurie88]

tx

pls clarify the distributor indexing thing

i changed distr cap and disst rotor., the white plastic rotor when fixed moves one way and then back ever so slightly, presume this normal...........but have not touched the fixing bolt for the distribtor assy.

[AirTool]

tx

pls clarify the distributor indexing thing

......but have not touched the fixing bolt for the distributor assy.

then your indexing should be right if it was right before. Generally, out of index range (+/- 2 degrees) will turn on the check engine light.

[laurie88]

guys

its been 6 wks and the boat is lying there unused we are going crazy

get to 1700rpm and pop , pop misfire

changed most of the sensors incl oil, ect, map, tps, all wires, plugs, pumps, filters, disrt cap, rotor, fuel, fuel press good, no engine warn light, starts first time only 400 hrs its practically a new boat

all this started after the fitting of new exhaust man and doing compression test without removing coil lead by mechanic


the only thing we havent changed is coil, i put multi meter on this today and 0,04 volts, book says 1volt...........could this be the prob

anything else we should check

txs guys

[91xl]

misfire

if the mechanic didnt disconnect any thing the engine would of started during the compression check. i have never seen a coil fail from it arcing a few times. i dont work on boats but I am a ase certified auto tech for 14yrs now. really need some specialized tools to check actual kv on each plug wire, and also injector flow, i have seen lots of plugged injectors on vehicles lately, some times they clear up with an induction service and others need replaced hopefully you will figure it out before spring

[DanielC]

Laurie88,
It is very difficult to diagnose even common problems over the Internet. Honestly, I probably could tell you more about your engine, if it was a GT-40 Ford engine like the one in my boat, but you have a Excalibur GM motor.
I have downloaded the Gt-40 service manual for my boat, from this site. I believe you should look and see if you can find the manual for your GM engine in the downloads section. If not there, try looking on the PCM website. You need to do some of your own research.
By randomly changing parts, you have a very poor chance of finding the part or parts that is actually the root cause of your problem.
The GT-40 manual has a very good diagnostic section. I would think the GM service manual also has a diagnostic section.

This is how I check the ignition system on my engine, a GT-40. The tester in the picture is available at Auto zone. It will not tell you the actual KV available, but you can get a good idea but how large of a gap the tester is set at.
http://www.planetnautique.com/vb3/sh...=Ignition+test