The brake pedal on my QX4 is stiff during the first application on a very cold morning after parking it overnight. After I hit the brake a couple of times, its fine. From what I've read, it looks to be the relief valve in the brake booster.

Is there a write up showing how to change it out? Is it an easy fix (one that can be done when its freezing outside and I can swap it out in the parking lot)?

on my 2001 qx4...the relief valve is under the hood on a vaccuum line coming right off the brake booster to the intake manifold. might be the same on yours

How do you check the relief valve? It was -25F today here and my brakes were basically inop/unassisted till the engine warmed up (which took 20 mins ).

I tried looking for relief valves for the QX4s with adaptive cruise control and I couldn't find any online. Any suggestions where I can find them? Or are they interchangeable between vehicles that have the adaptive and non adaptive versions of the cruise control?

There’s a big black circular metal thing on the firewall of the drivers side. This is the brake booster. There’s a hose coming off the top of this headed to the top of the intake manifold. In the path of this hose is a small one way check valve. Pull the hoses off it and take it out and gently blow air through both sides. It should only go one direction. I it goes both it’s bad. Make sure you install it in the same direction you removed it

https://www.infinitipartsdeal.com/parts ... 5w500.html

I went outside and checked the car. Because I have ICC/ASCD, I don't believe my car has a check valve in it. There's just a solid hose going from the connection on the booster to the bolt that attaches to the solenoid. The link attached above also seems corroborate this

Minus 25 is pretty nippy! Have you changed out your brake fluid since you got the Q? You might have some moisture in your brake fluid - it will freeze in the fluid at 25 below. That might be why it took 20 minutes to get brakes.

I looked at the diagram you linked - there is another diagram for vehicles with production dates after 7/2001. It does not show a check valve. If you look closely, the attachment point for the vacuum line from the manifold is on the driver side of the brake booster, which should match yours. In the diagram you linked to the attachment point is on the passenger side. I think they may have put the check valve inside the brake booster.

A new booster is kind of pricey!

THE FSM for 2002 pathfinders definitely shows a check valve in the hose that I mentioned earlier. So I’m not sure

Rockwood wrote: ↑

Tue Jan 22, 2019 1:02 am

Minus 25 is pretty nippy! Have you changed out your brake fluid since you got the Q? You might have some moisture in your brake fluid - it will freeze in the fluid at 25 below. That might be why it took 20 minutes to get brakes.

I looked at the diagram you linked - there is another diagram for vehicles with production dates after 7/2001. It does not show a check valve. If you look closely, the attachment point for the vacuum line from the manifold is on the driver side of the brake booster, which should match yours. In the diagram you linked to the attachment point is on the passenger side. I think they may have put the check valve inside the brake booster.

A new booster is kind of pricey!

Yeah, -25 wasn't pleasant. The t-case was acting funny. It was engaging 4H even in auto for the first few minutes of my drive. The nav system was stuck on the initializing/loading page and the HVAC split screen would not auto hide, the overdrive would not engage for the first 5-8 miles of my 50 mile commute . The turn signal stalk felt like it was filled with grease

But I digress. Interesting thought on the brake fluid. I've not changed the brake fluid since I got the Q and per the PO's detailed maintenance records, the brake fluid hasn't been changed/flushed in the last 80K miles. I did have the rear wheel cylinders replaced when I first bought the Q. But the shop did not flush the system. They simply refilled whatever fluid was lost. Does the rear cylinders need to be bled as well? I've never owned a vehicle with drum brakes before.

I looked at the 7/2001 onwards diagram and you're right. I can't find a check valve and the vacuum line is on the driver's side - which is what I have on my Q. Given the cost of the new boosters ($600+ for just the part), I'm debating if I should find a used one off car-part.com. Or should I do a brake flush first instead at a local shop?

heavy hitter wrote: ↑

Tue Jan 22, 2019 6:13 am

THE FSM for 2002 pathfinders definitely shows a check valve in the hose that I mentioned earlier. So I’m not sure

I re-read the FSM and it doesn't show any differences in the boosters between vehicles equipped with ASCD vs. regular cruise control. That being said, I can't seem to find the check valve.

Do you happen to know of any write ups/how-tos on how to replace the brake booster?

have you tried doing the brake booster diagnostic in the fsm yet to see if the brake booster is even bad?

can you take a photo of your brake booster assembly?

I ran the diagnostics called out on page BR-20 of the FSM. The one I can't seem to figure out is #2 under airtight check -"Depress brake pedal while engine is running, and stop engine
with pedal depressed. The pedal stroke should not change
after holding pedal down for 30 seconds"

How do I prevent the pedal stroke from changing once the engine is off??

The Q is currently at the shop getting its brake fluid flushed. I can try to get a picture of the booster assembly tomorrow...

What its saying is to depress to pedal to the full extent that it will depress. turn off the motor. The pedal should stay in that exact position. if at some point in 30 seconds it depresses further....its because there is a problem with the brake booster not remaining airtight

That test checked out as well. Makes me convinced with high degree of certainty that my booster is ok. The brake fluid flush didn't do anything except give me a slightly improved pedal feel.
I decided to run the engine for a minute after it was warm and take off the hose at the 90 degree elbow in the manifold. Didn't hear any vacuum/hissing. Put it back and ran the engine for a minute again (not touching the brake pedal). This time I pulled the hose off the brake booster and guess what - hissss.

I took the hose completely off the Q and was able to feel my way around the hose. Turns out, they integrated the check valve and buried it inside the hose about 40% of the distance from the booster side . I ordered the a new hose for about $30 from the Nissan dealership (part number 47471-5W600) and should have it in the next day or two.

I'm hoping that fixes the issue.

Did you verify whether you could blow air in both directions through the hose with the check valve on it?

Well I had no idea they had hidden the check valve inside the hose. I thought they had made it a part of the booster.

Looks like you are going tp see some really cold weather this week. 20 below zero is very possible. So you will get to check and see if you have really fixed it. Sounds like you are on the right path tho.

heavy hitter wrote: ↑

Thu Jan 24, 2019 11:35 am

Did you verify whether you could blow air in both directions through the hose with the check valve on it?

Given how slow the leak was, blowing air did not verify the problem. I took the hose in at work and had air blown in at 60psi, 10psi and 5 psi. All verified that the check valve operated as normal. I couldn't replicate an extremely slow leak (i.e. overnight leak) in cold temperatures.

Rockwood wrote: ↑

Sat Jan 26, 2019 4:29 pm

Well I had no idea they had hidden the check valve inside the hose. I thought they had made it a part of the booster.

Looks like you are going tp see some really cold weather this week. 20 below zero is very possible. So you will get to check and see if you have really fixed it. Sounds like you are on the right path tho.

I thought they had integrated it into the booster as well. The prospect of having to spend $500 or more on a 17 year old vehicle didn't seem particularly appealing to me. So I got the hose off and felt around to find the check valve! Never seen a set up like this but I think they use a mandrel to push the check valve in while the hose is hot post molding.

I ended up installing the new booster hose on Friday. Starts up at 23F, 17F and 10F all have resulted in normal brake function from the first actuation!
We have -20 to -25F coming up on Wednesday which I believe will be the ultimate test

Great!!

Did you ever hear it hissing under the dash....or was the pedal the only problem?

I never heard the hissing under the dash. I was looking up similar symptoms on other forums. On a bimmer forum, they said that the hissing was more linked to issues within the booster(?).
The stiff pedal was the only problem I was having. If the problem stays fixed with all the cold weather expected later this week, I would be happy to have my Q running tip top for about $30.

brickbox wrote: ↑

Mon Jan 28, 2019 6:38 am

...
Given how slow the leak was, blowing air did not verify the problem. I took the hose in at work and had air blown in at 60psi, 10psi and 5 psi. All verified that the check valve operated as normal. I couldn't replicate an extremely slow leak (i.e. overnight leak) in cold temperatures.

Pity I didn't catch this thread any earlier. Someone in the sedan forums had a thread trying to locate that check valve, and it seems that for the newer models they ditched the individually replaceable check valve.
https://forums.nicoclub.com/brake-boost ... l#p6783945

Anyway, onto the test that you were doing here. Normally the check valve allows air movement away from the booster, towards the engine. If you were to blow air on a bench test, then you should observe this:
- Air can be blown away from the booster, but not towards it.
- There shouldn't be much of a restriction blowing it (especially beyond 10 psi) away from the booster. So the slow leak was either a sign of a blockage or a failed check (depending on direction).

It seems your original check valve was stuck closed/obstructed. Either way, please let us know how the replacement fares on in unfair weather.

***

Addendum that I've used for booster diagnostics - https://www.google.com/amp/s/m.roadkill ... oster/amp/

I expect to hear a couple of stories about blown freeze plugs, maybe a cracked block, blown radiators. This is getting close to "Alaska cold" weather in a few places.

Rockwood wrote: ↑

Mon Jan 28, 2019 2:47 pm

I expect to hear a couple of stories about blown freeze plugs, maybe a cracked block, blown radiators. This is getting close to "Alaska cold" weather in a few places.

It was down to 0F with windchill today morning and the brakes worked fine. I'll report back tomorrow and day after as well on how things go. I'm hoping I have no horror stories to tell!

EdBwoy wrote: ↑

Mon Jan 28, 2019 10:06 am

brickbox wrote: ↑

Mon Jan 28, 2019 6:38 am

...
Given how slow the leak was, blowing air did not verify the problem. I took the hose in at work and had air blown in at 60psi, 10psi and 5 psi. All verified that the check valve operated as normal. I couldn't replicate an extremely slow leak (i.e. overnight leak) in cold temperatures.

Pity I didn't catch this thread any earlier. Someone in the sedan forums had a thread trying to locate that check valve, and it seems that for the newer models they ditched the individually replaceable check valve.
https://forums.nicoclub.com/brake-boost ... l#p6783945

Anyway, onto the test that you were doing here. Normally the check valve allows air movement away from the booster, towards the engine. If you were to blow air on a bench test, then you should observe this:
- Air can be blown away from the booster, but not towards it.
- There shouldn't be much of a restriction blowing it (especially beyond 10 psi) away from the booster. So the slow leak was either a sign of a blockage or a failed check (depending on direction).

It seems your original check valve was stuck closed/obstructed. Either way, please let us know how the replacement fares on in unfair weather.

***

Addendum that I've used for booster diagnostics - https://www.google.com/amp/s/m.roadkill ... oster/amp/

I was able to verify that at 10psi and 5psi, the check valve was behaving as it should i.e. unidirectional flow. But so far, the brakes have been behaving well and I think my problem is solved. Although I'll report back how it fares over the next two days.

On a completely side note, the ATTESA on the Q was phenomenal while driving through 4-6" snow while everyone else was sliding and rolling into ditches. ATTESA + aggressive snow tires = FTW!

Brakes seem to be working good. It was -16F today morning which felt like -40F with wind chills. The car did NOT like the extreme cold weather at all, but it ran like a champ after the first few mins

Is it normal though in these extremely frigid temps for the transmission to behave oddly? For the first 5 miles, I only had 3 gears. After which I got 4th gear and then after another 3-5 miles, overdrive kicked in.

I don’t know that it’s normal....but it’s possible. When was your last transmission fluid change?

I did multiple drain and fills (about 8 gallons) with the Valvoline Transmax stuff over this past summer. Trans runs fine otherwise except when its bitterly cold like this. Outside of that, it does have occasional hard 1-2 shifts when it shifts the very first time and the weather is 25F or colder.

The hard 1-2 shift is a known issue with this vehicle.