[DMCForum] Re: Finding vacuum leaks
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[DMCForum] Re: Finding vacuum leaks



Vacuum leaks are indeed idle (low RPM) problem. And Martin G is
correct -- fuel mixture gets adjusted rich to compensate, which then
haunts you at higher RPM's (where leaks go away). That's why vacuum
leaks are MPG killers.

Find them methodically anywhere air can enter engine OTHER than
metering device (air sensor plate). On most engines this is limited to
single piece intake manifold and a few vacuum hoses. PRV is different.
Breathes internally to a degree I've never seen before. Means you're
going to check things not normally sources of vacuum leaks, like valve
covers and oil pans. Note also upper air assembly alone has as many
mating surfaces as rest of block.

Start by isolating engine from rest of car. Can have a vacuum leak at
far end of a hose, like power brake booster or A/C mode switch. No
DeLo vacuum operated accessories are needed at idle, so simply
disconnect hoses and plug nipples. You'll hear instantly if leak goes
away (which then requires one by one analysis to find bad accessory or
connecting hose).

To find leaks on engine itself, I like to use aerosol products with
pin point spray straw. Private label carburetor cleaner is only 78
cents at Wally World, so it's become my solvent of choice. Is
flammable, however, so use judiciously. Ensure ignition isn't arcing.
Electric motor cleaner isn't flammable but is about 3X more expensive.
Push button and straw can be transferred to can of whatever if it
doesn't come with. You can also make a flex extension for straw from
tiny size vacuum hose to reach backside of engine, under intake
manifold, etc.

If solvent hits area that is vacuum tight, is no effect on idle.

If solvent hits minor leak, RPM's increase momentarily as solvent
seals it. Gets sucked into engine of course, so effect is temporary.

If solvent hits major leak, RPM's drag as it is sucked into engine as
fast as you can spray.

Started diagnosis on my PRV at the top: Fuel distributor wasn't
totally tight against upper air assembly. Simply snugging down removed
that leak.

Working down, found leaks where U pipes mate to upper air assembly,
where cold start tube mates to intake, and in most fuel injector
seals. Intake manifold itself leaked around #5 cylinder. Had biggest
leak of all in passenger valve cover caused by slightly uneven match
of timing chain cover to head. Am currently mulling over prospect of
removing crankshaft pully to reach leak in timing chain cover seal
(hey Rich, can I come visit your impact wrench...)

Is longer prospect than simply setting off a smoke bomb next to car to
see where it goes. Take it one step at a time and you'll eventually
find them all. Don't forget to carry diagnosis all the way down to oil
pan -- PRV can have vacuum leak there too.

Bill Robertson
#5939

>--- In DMCForum@xxxxxxxxxxxxxxx, "jamesrguk" <James_rg@xxxx> wrote:
> Hi list,
> 
> My car idles like a dog and I want to fully check for vacuum leaks, 
> I get a good reading of the vacuum gauge when connected to the 
> intake manifold.
> 
> However when I pull the rubber plug out (the one that sits above the 
> mixture adjustment screw on the metering head) and put my finger on 
> it I can't feel/hear any suction? the car runs, all be it badly, and 
> the idle is terrible.
> 
> What is the best thing to spray around the engine bay to detect 
> vacuum leaks? 
> 
> Rgds
> 
> James



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