moisture inside distributor cap

airshot

Rear Admiral
Joined
Jul 22, 2008
Messages
4,219
My search did not show anything usefull so if this topic has been used please let me know. I have been helping my neighbor trouble shoot his hard starting and failure to run problem with his 350 4 bbl engine. He has a constant problem with moisture inside his distributor cap. We have removed it dried it out, used wd-40 and other drying agents but the next time we go out to his boat it fails to start or runs terrible due to moisture inside the cap. We dry out the cap and it runs great, but a day or two later and the same problem. Is there a trick or a special cap or ???? do we do to correct this issue? Thanks for any recommendations..
 

Fun Times

Moderator
Staff member
Joined
May 16, 2009
Messages
8,776
What engine year & distributor series (#) do you have now?
Is the cap OEM or aftermarket? Does the cap have a vent? (small 90 degree brass elbow type for example)
Is there a gasket of any sorts under the cap to the top of distributor housing?
Any other part of the engine seem to get a heavy amount of moisture on it? Heavy rains lately in the area?
Some distributors tend to have a screen type hole on the bottom to help with moisture problems...Is it clean, etc..
 

wellcraft-classic210

Master Chief Petty Officer
Joined
Sep 22, 2010
Messages
839
I have seen leaking intake gaskets adding moisture to the crank case -- It would works its way it the cap but that often shows signs elsewhere also -- such as on the dip stick milky film or eventually rust. Not sure if that applies but just in case.
 

kenny nunez

Captain
Joined
Jun 20, 2017
Messages
3,056
I also had a customer with a similar problem. The block had a pin hole crack in the rear under the manifold. The moisture was going right up the distributor shaft.
 

bobterisch

Seaman Apprentice
Joined
May 16, 2008
Messages
38
Although not as bad as you I have the same problems with a 350 MAG MPI. Make sure the vent screen in the bottom of the distributor is clean. I have also started using the NAPA Echlin caps with aluminum terminals. It seems to last longer than the copper ones. The crab style cap is also know as the "crap cap"do a google search for "350 crap cap". There is a lot of info on them. Improvements to NAPA Echlin GM 6 & 8 Cyl. Caps
The Problem
For a long time GM has admitted a design
problem with the ignition systems that use
the RR207 and RR253 distributor caps.
These GM distributor body design flaws
have led to premature corrosion of the cap
& rotor and ignition system failure due to
improper ventilation.
Remember, this is not a NAPA Echlin product
problem but rather an original equipment
design flaw. In fact, GM has released a volunt
ary consumer return notice in addition to
numerous TSBs relating to
this specific problem.
The Solution
NAPA Echlin has made the following
improvements to compensat
e for the faulty OE
design. Both the RR207 and RR253 will now
be manufactured with aluminum terminals.
Due to the OE distribut
or flaws and corrosive
environment, in these in
stances, aluminum will
actually provide a long
er service life than the
brass. Additionally, both numbers will now use
a special formulation manufactured from a
polyester base plus a unique blend of mica
particles and glass fibers. This improvement
results in superior performance due to the
higher dielectric strength of the cap.
Product Differentiation Between NAPA Echlin and Mileage Plus
Although both the NAPA Echlin and
Mileage Plus will use aluminum
inserts, the NAPA Echlin
caps (RR207, RR253) contain 35% mica-glass content in the plastic as compared to 30% for
Mileage Plus (RR207SB, RR253SB). Th
is results in a 31% improvem
ent in dielectric strength
for NAPA Echlin! Higher dielec
tric strength means less voltage
leakage, greater part durability
in adverse conditions and extended service life. See comparison chart.
 

Fun Times

Moderator
Staff member
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Messages
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Fun Times

Moderator
Staff member
Joined
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Messages
8,776
That number just fits within the serial number range of the now considered informative bulletin posted above...Had time not expired, a new distributor may had been granted to help....Glad you found another alternative to help control some of the moisture issues though.

Hope airshot is doing okay...He hasn't been back online since posting this topic which typically he's a pretty regular contributor here.:encouragement:
 

isaacs

Petty Officer 2nd Class
Joined
Oct 15, 2013
Messages
153
My 2002 Bayliner, 5L Mercruiser has had this problem since day one; I usually have to replace the cap every two years. It develops little green "boogers" at the contact points. It is possible to clean them off and wipe out all the moisture if I'm in a hurry and don't have (or feel like buying) a replacement cap. There is a gasket and a small vent in the side with an elbow that points down toward the bilge.

One cause that I read about is that if any water gets onto the cushions above the engine it will drip through a crack between cushions right onto the distributor cap. Bayliner put a thingy there to divert the water, not sure if it helps too much.

Another thing I noticed is the cap will fail sooner if the oil is overfilled. Apparently it travels up the shaft into the cap--not sure if this is related to moisture but it seems to cause the same problems.

Having a good water tight cover and keeping standing bilge water to a minimum seems to help.
 
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