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Old 04-04-2002, 03:00 AM
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Question amateur vacuum leak hunting

I need some advice / pointers, please.

I am in the process of tracing an erratic idle on a 102 engine (I read that this is referred to as 'stumbling'). Anyway, from the archives I learnt that a vacuum leak can be one of the culprits.

Before I throw money at replacement parts, I want to narrow down the possible sources of a vacuum leak.

How would a vacuum leak be traced?
- listening is out of the question I assume
- spraying a liquid (water) on any possible hose / gasket should temporarily seal the leak but what happens if the liquid is sucked in to whatever causes the vacuum ? Maybe selectively spraying something like carb cleaner or penetrating oil will minimise any damage ?

I have replaced the rotor and the distributor cap and my fuel consumption dropped remarkably, but the 'stumble' is still very evident. It's most noticable at idle. I can feel the car 'shake' at each 'miss' (not violently - just a mild shake of the engine) and the burble at the exhaust is interrupted as the plug misses.

If there is no vacuum leak, the plug leads and plugs are next.
I'll also try some injector cleaner with the next tank of gas.

any ideas will definitely be appreciated.
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Old 04-04-2002, 03:54 AM
Registered User
Join Date: Dec 2001
Location: Yorkshire England
Posts: 70
I would always start with the ignition system. If you haven't recently changed the plug and leads, I would suggest do it now.

BTW, does your car have a catalyst?


E200 Wagon 1994 108k Miles
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Old 04-04-2002, 05:23 AM
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hi, Barry

nop, not cat. unleaded fuel only became available here during mid 1990's.

i am having difficulty relmoving the plug wires from 3 of the 4 plugs. I'm afraid I might do more harm than good by yanking on the lead with all my might. what could cause this ?

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Old 04-04-2002, 06:46 AM
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I can't remember where I bought it, but I have a stethoscope looking thing with a long piece of hose about 1/8" ID. I simply put on the stethoscope and probe around connections and places where there might be a vacuum leak and listen for the leak.

This is a fancy and more effective version of the piece of hose that I watched my Dad hold to his ear and probe around with looking for vacuum leaks.

Good luck,
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Old 04-04-2002, 11:39 AM
Registered User
Join Date: Dec 2001
Location: Yorkshire England
Posts: 70

The plug leads are "only" a push fit on the plugs.
What can happen though is a bit of corrosion between the metal end of the plug lead connector and the plug can cause a bit of "tightness"
Also, maybe there is a trace of oil or similar on the plug ceramic, causing a suction effect with the rubber end piece on the plug lead.
Either way, they should respond to a firm (!) straight tug.
The bad news is: You can break either the plug ceramic, or maybe pull the lead out of the plug connector, so you may need alternative transport to get you to the parts shop!
Alternatively, buy the parts first! The plug leads from M-B are expensive as each part is separate, the lead and the end pieces.
But the leads are really high quality. I don't know if other guys know of another make that's any good. I always have used M-B leads.
Check out web site. They're in the UK, but will deliver overseas.

Good luck


E200 1994 108k miles.
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Old 04-04-2002, 04:59 PM
Posts: n/a
want a stethoscope cheap? Attn: all members - I have bought from these people and found their prices hard to beat. Remember the prices shown are in CANADIAN DOLLARS - so multiply by about .65 to get US Dollars. The stethoscope is $2.99 CDN. Their url is Hope it is useful to some members.
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Old 04-04-2002, 05:34 PM
Senior Member
Join Date: Jul 2001
Location: San Antone
Posts: 408
There is a special tool to grip the spark plug wire leads/caps to pull them off the plugs and (usually) not damage the wire. The tool looks like a pair of pliers with rubber coated jaws shaped to grip the plug wire lead/cap connected to the spark plug. Take the pliers and grip the plug wire lead/cap firmly, then twist and pull on the tool at the same time to remove from the plug (don't pull on the plug wire itself). When installing the plug wire lead/cap onto the plug use a bit of dielectric grease on the plug wire leads/caps to prevent rust/corrosion and the plug wires should be real easy to remove next time.

Vacuum leaks can be found using a tool called the Mighty Vac. The Mighty Vac connects to your vacuum system and then you create your own vacuum to check for leaks in the various vacuum system's sub-systems. Once you find the sub-system where the leak is, then you check the various components (lines, connections, valves, etc.) until you find the leak. Very useful tool since you can check for leaks in areas where a line or part is buried in a panel or other hard to reach place that other tools may not reach (like a stethoscope). You can also check your engine's condition, bleed brakes/master cylinders, remove fluid from hard to reach places, etc., etc.

Good Luck!
America: Land of the Free!

1977 300D: 300,000+ miles

American Honda: Factory Trained Technician & Honor Grad.
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Old 04-13-2002, 02:40 PM
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Thumbs up found the culprit

Hi, All

After extensively wiggling the plug leads on #1, #2 and #4, the plugs leads came free. Boy, were they stuck ! It was the grip around the metal tip and not vacuum or anything else that caused the reluctance to unplug .... The new plugs also grip the plug leads with a vengeance ....

I also found the culprit responsible for the erratic idle.

It turned out to be the spark plugs on #1 and #2 cylinder. Both had the nose porcelain cracked, probably causing spark to be lost to earth. It also looked like the plugs had worn past their useful life (I was under the impression they were changed at 120 000 miles at the dealer - I did it now at 136 250 ).

Anyway, the engine now idles smooth as silk.

Thanks for all the replies. It sure did help a lot in preparing for the job ahead.

I'll keep the vacuum tracing tips filed ....

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