960 vacuum leak

Paul-C · Apr 30th, 2007, 14:20

Having changed the inlet maifold gasket and then experienced high and low revs at idle (alternating between 1000 & 2000 revs)I cleaned the throttle body, air control valve, AMM sensor and this changed the idle speed to around normal but it is very erratic, also I have noticed no vacuum on the brake servo so I presume I have a vacuum leak.
Can anybody point me in the right direction as to what hoses I should change? as the car has 134k on the clock so it might be prudent to change hoses rather than trying to identify the one leaking.
At the moment I have stripped the manifold down to re-check the inlet gasket, I did intend removing the manifold from the car but I cannot get to the bottom bolt
Thanks in anticipation
Paul-C
PS where is the vacuum created and whats in the vacuum circuit?
Thanks again

Dan R · Apr 30th, 2007, 16:50

I think you in fact do have a vacuum leak.

Vacuum is created inside the intake manifold, it is controlled by the throttle valve. Vacuum is highest when the throttle is closed, opening drops the pressure some. On worn out cylinders or bad valves, vacuum pressure drops quite a bit, due to blow-by being forced into the intake stream.

No vacuum at the brake booster may be a clue, did you try to remove the hose from the diaphram or where the hose connects to the manifold? Maybe something was damage at the diaphram. Also the connection point down low on the manifold, where all of the little hoses join, that part is plastic and maybe it was cracked, it also can be removed and there is an o-ring that can also go bad, but the o-ring would cause just a small leak, a crack plastic fitting could be the culprit.

That elusive bolt at the bottom only has to be loosened, don't remove it. I have found with my big mitts for hands, the only possible access for me is from the top, it might be accessible for me if the vehicle was on a lift, but lying under, there is no way my arms can contort to reach it. I remove the plastic cover over the spark plugs and just kneel on the engine and blindly reach down.

Did you use a torque wrench on reassembly? Those bolts strip very easily, maybe the leak is still at the head.

You could also, seal off all of the vacuum hoses where they fit at the manifold and re introduce them one at a time. Does the engine idle smoothly?

DanR 94 964 329,000 miles (95,000 on the new engine)

andykem · Apr 30th, 2007, 17:10

Where the vacuum pipe connects to the servo, there is a plastic elbow (there is on my 960 and has been on every Volvo I've ever had) that contains a check-valve to stop the vacuum you built up in the servo leaking back the way it came. There should be enough vacuum in a servo for a couple or three assisted brake applications as I recall, after stopping the engine.

To make sure, you can/ought to use a new check-valve with a new rubber grommet that goes into the servo body for the new check valve to sit in. But, just to see, if you have the sort of srappy nearby that lets you take bits off, get one off a old 'un and see if it works then. Best to get a new one anyway then so the seal/grommet and valve are both known good.

HTH
Andy

Paul-C · Apr 30th, 2007, 20:57

Thanks Dan R and andykem for your response, the situation is that I have just loosened the manifold bottom nut and will get the manifold out tomorrow, after re-shaping my right arm (`o' for a car ramp!)
The brake servo rubber hose was disconnected at the servo end so I could check the workings of the non-return valve, which `blowing' through each end seems to be working fine.
When I have removed the manifold, I will be able to re-check the gasket security and also torque the studs up. I could not torque the studs with the manifold in situ it was impossible to get to the studs other than with a short ring spanner.
What will be the effect on the car if I remove all the small vacuum hoses blank them (with tape) then refit them individually in order to chech each one out? Where do these small hoses go to?
I have just ordered a brake servo hose from the dealers as, being rubber, the internal part of the hose could be perished, acting like a `check valve!
Thanks again for your assistance
Paul-C

Dan R · Apr 30th, 2007, 21:04

There are two things that I would not be surprised to learn. When the designers/engineers at Volvo planned the 900 series, they laid out that bolt on the manifold and the vacuum motor for the vent setting on the climate control. From there they designed and built the whole car around them. I suppose every make has it's nearly impossible to reach parts. That might be a good topic for discussion.

Hope it works out for you.

DanR 94 964 329,000 miles (95,000 on the new engine)

Apr 30th, 2007, 14:20	#1
Paul-C Member Last Online: May 8th, 2024 07:15 Join Date: Jan 2006 Location: Riccall Nr. York	960 vacuum leak Having changed the inlet maifold gasket and then experienced high and low revs at idle (alternating between 1000 & 2000 revs)I cleaned the throttle body, air control valve, AMM sensor and this changed the idle speed to around normal but it is very erratic, also I have noticed no vacuum on the brake servo so I presume I have a vacuum leak. Can anybody point me in the right direction as to what hoses I should change? as the car has 134k on the clock so it might be prudent to change hoses rather than trying to identify the one leaking. At the moment I have stripped the manifold down to re-check the inlet gasket, I did intend removing the manifold from the car but I cannot get to the bottom bolt Thanks in anticipation Paul-C PS where is the vacuum created and whats in the vacuum circuit? Thanks again

Apr 30th, 2007, 17:10	#3
andykem Member Last Online: Mar 10th, 2020 12:16 Join Date: Jan 1970 Location: cheltenham	Servo check valve Where the vacuum pipe connects to the servo, there is a plastic elbow (there is on my 960 and has been on every Volvo I've ever had) that contains a check-valve to stop the vacuum you built up in the servo leaking back the way it came. There should be enough vacuum in a servo for a couple or three assisted brake applications as I recall, after stopping the engine. To make sure, you can/ought to use a new check-valve with a new rubber grommet that goes into the servo body for the new check valve to sit in. But, just to see, if you have the sort of srappy nearby that lets you take bits off, get one off a old 'un and see if it works then. Best to get a new one anyway then so the seal/grommet and valve are both known good. HTH Andy

Apr 30th, 2007, 20:57	#4
Paul-C Member Last Online: May 8th, 2024 07:15 Join Date: Jan 2006 Location: Riccall Nr. York	latest from the lost vacuum saga Thanks Dan R and andykem for your response, the situation is that I have just loosened the manifold bottom nut and will get the manifold out tomorrow, after re-shaping my right arm (`o' for a car ramp!) The brake servo rubber hose was disconnected at the servo end so I could check the workings of the non-return valve, which `blowing' through each end seems to be working fine. When I have removed the manifold, I will be able to re-check the gasket security and also torque the studs up. I could not torque the studs with the manifold in situ it was impossible to get to the studs other than with a short ring spanner. What will be the effect on the car if I remove all the small vacuum hoses blank them (with tape) then refit them individually in order to chech each one out? Where do these small hoses go to? I have just ordered a brake servo hose from the dealers as, being rubber, the internal part of the hose could be perished, acting like a `check valve! Thanks again for your assistance Paul-C

Apr 30th, 2007, 16:50	#2
Dan R Junior Member Last Online: Jul 22nd, 2010 00:21 Join Date: Nov 2003 Location: medinah, illinois	I think you in fact do have a vacuum leak. Vacuum is created inside the intake manifold, it is controlled by the throttle valve. Vacuum is highest when the throttle is closed, opening drops the pressure some. On worn out cylinders or bad valves, vacuum pressure drops quite a bit, due to blow-by being forced into the intake stream. No vacuum at the brake booster may be a clue, did you try to remove the hose from the diaphram or where the hose connects to the manifold? Maybe something was damage at the diaphram. Also the connection point down low on the manifold, where all of the little hoses join, that part is plastic and maybe it was cracked, it also can be removed and there is an o-ring that can also go bad, but the o-ring would cause just a small leak, a crack plastic fitting could be the culprit. That elusive bolt at the bottom only has to be loosened, don't remove it. I have found with my big mitts for hands, the only possible access for me is from the top, it might be accessible for me if the vehicle was on a lift, but lying under, there is no way my arms can contort to reach it. I remove the plastic cover over the spark plugs and just kneel on the engine and blindly reach down. Did you use a torque wrench on reassembly? Those bolts strip very easily, maybe the leak is still at the head. You could also, seal off all of the vacuum hoses where they fit at the manifold and re introduce them one at a time. Does the engine idle smoothly? DanR 94 964 329,000 miles (95,000 on the new engine)

Apr 30th, 2007, 21:04	#5
Dan R Junior Member Last Online: Jul 22nd, 2010 00:21 Join Date: Nov 2003 Location: medinah, illinois	There are two things that I would not be surprised to learn. When the designers/engineers at Volvo planned the 900 series, they laid out that bolt on the manifold and the vacuum motor for the vent setting on the climate control. From there they designed and built the whole car around them. I suppose every make has it's nearly impossible to reach parts. That might be a good topic for discussion. Hope it works out for you. DanR 94 964 329,000 miles (95,000 on the new engine)

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