Hmmm … I’d still be quite confident that acting with someone at the loud pedal, if necessary on a lift, you should be able to identify either acoustically where the hissing comes from or optically where the tube collapses. I’m aware that I’m privileged with a UK car and both intake and booster on the same engine side. Don’t know where the tubes are located on LHD cars.
Good luck
Jochen
75 XJ6L 4.2 auto (UK spec)
Further to the brake booster, I did some tests today.
I have normal brake when I start the car and let it warm up before driving off. Driving at low speed down a flat street I get two normal braking before the pedal goes hard.
Driving off in low gear and staying in low gear, I get as much boost as need as long as I keep the RPM’s up. After I slow down, I get one or two stops and then a hard pedal.
Low rpm’s going on a flat street moving slowly, may one brake before going hard.
I’ll bring home the vacuum gage from the garage and run some tests this week. What should I test?
Lou
Lou,
do you really need to test the obvious? Just search for the leak or maybe the place where the vaccuum hose collapses. Have a helper operate the gas pedal and follow the vacuum tube … well, that’s at least what I would do …
Good luck
Jochen
75 XJ6L 4.2 auto (UK spec)
**
Disconnect vacuum hose from the one-way valve, Lou and connect hose to the vacuum gauge…
In idle, you should read some 18" Hg vacuum, and, unloaded with constant throttle in the same region…up to a point. Manifold vacuum is a function of load and rpms - it will drop while reving and with increased load.
The point here is simply to verify that the manifold vacuum is reaching the booster at all times. Ideally you should have a second vacuum gauge - reading manifold vacuum from a different source…
Testing procedure is tricky; you have to repeatedly disconnect and reconnect the gauge at the hose - to dump the vacuum built up in the gauge between measurements. Otherwise, with the hose clogging, the gauge will retain each reading…
A leak is unlikely; the engine draws a lot of air at any rpms/load - and
However, Jochen’s point is well taken; either there is a leak - or the hose collapses under high vacuum. Odds are that changing the booster hose will resolve the issue…
Initially, booster gets vacuum (air is sucked out), then, if the hose collapses, the one-way valve retains booster vacuum - enough for a couple of brake applications (filling the booster with air). But with a clogged hose, air is not sucked from the booster - so there is no booster assist and the pedal goes hard…
Varying manifold vacuum due to pedal variations (vacuum drops when pedal is applied) may intermittently unclog the hose - rebuilding booster vacuum. And the cycle is repeated, but cannot really be controlled - vacuum is not a plain rpm issue…
A hose leak is unlikely; the engine draws a lot of air at any rpms/loads - and symptoms would be different.
However, if the gauge reads manifold vacuum at all times; there is some internal faults in the booster - or the one-way valve itself is causing the clogging. which should be tested…
Frank
xj6 85 Sov Europe (UK/NZ)
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has anyone suggested squirting a bit of carb cleaner on each of the hose connections to see if there is a change in RPM? this would also include around the intake bolted to the head. I’ve identified many a leak using this method.
Mark:
That sounds like a very good idea. If it ever stops raining in Pennsylvania, I will try it.
Lou