question about brakes and AC [solved]
 

So, this is me trying to prevent some insanity. Need your help.

Problem: poor gas mileage, best hypothesis is brakes are dragging.

I drive a 91 22re with about 70K miles. I get about 15 mpg.

at some point I had the front wheel off the truck and I noticed that the front tire did not spin too freely. I thought maybe the previous owner re-did the bearings and tightened them down too tight.

But I had taken the front caliper off, the wheel spun better.

I didn't have time to rebuild the caliper, bought a refurb, but it still sticks (I think).

Today I had my A/C on and it felt like the brakes weren't so sticky. And I thought to myself that The A/C has some vacuum hoses, as does the brakes, and so I wonder whether there may be some vacuum issues between these two systems.

That maybe when the AC is off, some sort of vacuum issue is preventing the master brake cylinder/brake booster to not completely release the brakes.

This just seem crazy to me, but I'm ignorant of vacuum lines generally and could use some of y'all's expertise.

I suppose I could test this by lifting the front tires off the ground, turn the engine on, and see if having the AC on makes the wheels spin better. (yeah, this just sounds nuts).

Cheers.


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update to solve my fuel issue...
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I'm up to 18 mpg and am going through my posts to update.


Here is what I did:
1) though I wasn't getting any CEL warnings, I checked all computer diagnostic connects (02, tps, anything that conceivably would be related)
2) all codes checked out, I took off my intake plenum to clean it out and to remove the injectors.
3) sent the injectors to RC engineering to have them tested, cleaned and calibrated
4) cleaned the heck out of my plenum. There was about an 1/8 of black gunk through the plenum, including and especially the pipe that carries the exhaust from the EGR valve. that part was completely clogged.
5) cleaned the and tested the egr valve

when I put it all together, I had to shut down the idle screw, which was basically all the way open.


It's only been a week that I've been driving it, but the fuel consumption is about 18 mpg in seattle, minimal highway driving.

Disc brakes naturally have a little drag. If you can spin the front tires and they continue to rotate under their own momentum, it's likely just normal drag. If they stop immediately, you have a problem.

And have you ever had better fuel economy with that truck? 15mpg is not uncommon for the 22R-E depending on maintenance (or lack thereof).

I've gotten about 18mpg, which happened after swapping out the front right caliper. But typically it bounces between 14 or 16mpg.

perhaps a better question is: how is it that replacing the caliper, putting on the tire, it spun more freely. Now, not so much.

I would keep coming to the bearings being too tight were it not for my witnessing how much better the hub spun while the caliper was off the truck (and how it spun well once it all was put together).

exactly. This is what I have. Which is why I orginially thought it was teh brakes, I swapped out the front caliper, still has a problem.

That got me thinking flush the brakes lines. I did that, though maybe not well enough?

Even so, is there something about how master cylinders work (and perhaps the brake booster) that might impede the release of those calipers?

cheers

Not a brake expert but as far as I know the master cylinder simply stops applying pressure when the pedal is released. The calipers then stop squeezing the rotors but don't 'back off', unlike drum brakes where springs relieve pressure from the shoes. The booster simply utilizes manifold vacuum to increase line pressure. I don't believe the master cylinder or the booster are your problem, leaving the calipers in question. Where did you get the replacement caliper(s)?

Caliper that I replaced performed like the new one. That is why I'm thinking it's not the caliper, which I got at a local auto store, good reputation and past experience with them.

The bit about the booster is what I was thinking too. Maybe I just need to do a really good flush.

Still, somewhere ideas that if the flush or caliper fix doesn't stop the prob something is wrong with the master cylinder.

Thanks for your help.

Okay, first of all the a/c has nothing to do with the brake system. They are in no way interconnected; also there are no vacumm hoses in either except for the booster. However, a/c does put more strain on the engine at lower rpms. Mpg suffers because of that.
Youre hetting 14-15 mpgs? Thats pretty low; i get closer to 20 with oversized tires. Have you tried adjusting the brake pedal? Also, there is a push rod between the booster & master that if left too tight actually causes the master to be engaged partially even when you let off the brake.

Right...I knew the question was a little odd to begin with but since there the booster does work on the vacuum system, my thought was that if the AC uses that vacuum, maybe it makes the booster work 'less well'.

I have tried adjusting the pedal. Looks okay..

That I did not know. I will check. The master cylinder looks pretty new, and so I wonder if the PO didn't set it right.

Thanks all.

If it was an adjustment issue it would be grabbing on both sides. Unless I'm mistaken his issue is just one side.

Good call. passenger side is more draggish than the drivers, but both drag some.


EDIT 6:42 PST, I just adjusted the push rod. No big deal. Just loosened the lock nut, the rest spun. I checked the brake lights and took a trust drive. I'm not sure if it helped or not, but I took a vid and will try to post it. Seems significantly better...




I'm thinking I'd like to adjust the pushrod, but most of the instructions I've seen say to do it when the MC is off.

Under the dash, I see a threaded bar that looks like it goes into another threaded bar. That threaded bar goes into the MC.

------||---------------------<>===========|===M/C

Key:

-----. . . . . . : Threaded Bar
||. . . . . . . . : Pedal lock nut
<> . . . . . . . : Odd Groove nut
====== . . .: Larger thread bar







Is it possible to adjust that push rod without any special tools? And can it be done without remove the MC?

It looks like I just need to loosen those two nuts and spin the bars such that the "larger threaded bar" goes away from the M/C.

But now I'm thinking that there is a nut on the MC side of the firewall that needs loosening for this to work.

Thanks for your help and for listening to me think out loud..

Cheers!

That may work, but what i was referring to is the pushrod inside between the booster & master. You dont have to completely remove the master; just the four nuts that hold it to the booster and there should be enough slack in the brake lines to move it out of the way. You can get a small key in between and adjust it that way.

Ah, sweet. thank you for this. I have not read this anywhere in yotatech in all my searches including "booster", "pushrod", "push rod", "adjust".


I can't get my vid's to load up from my phone, but the passenger wheel spins much better, and both my wife and I feel the truck rolls better. I'm picking up speed on hills much more quickly than before, and all it took was maybe 1/8 of an inch to thread the pedal push rod under the dash.

I still have clearance for the brake pedal. When I get some time and warmer weather I'll see if I can make a proper push rod adjustment of the booster to the MC.

Cheers!

Might look at replacing the rubber lines between the frame and the caliper. They'll break down on the inside and cause the pressure to not release all the way