MAF Sensor/Fuel Trim/ Vacuum Leaks

[Marcos Beltran]

I have a 96 gmc sierra with 4.3 V6. At first truck idled normally but bogged down when accelerating, sometimes would shut off. Borrowed a maf sensor from a buddy and is driveable now and made a big difference but if i get on it from a stop it's sluggish at first. If i gradually get into it, it drives good. Now im getting some long term fuel trims that are off big time. On LONGFT1 and LONGFT2 im getting between 16 and 25 which points to a vacuum leak. I made a homemade smoke machine and im not getting any leaks. Ive searched up and down using a flashlight and all. Jacked up the truck and nothing underneath either. I do get smoke coming out of the box where the air filter is but im sure that wont cause the vacuum leak. What else could cause the LONGFT's to be off like that? Fuel pump is new and has the right pressure. Any ideas anyone?

 

[thegawd]

so this is a reoccuring theme... failed brake boosters with vacume leaks. you can test this by holding the brakes and watching the fuel trims. if they rise when holding the brakes that indicates a vacume brake booster leak.

ya...

what happens if you hold the brakes for an extended period of time then release them and stomp on the gas? does it go or does it almost die? if it dosent go you are probably dealing with a failed vacume brake booster.

honestly lately like over the last year there have been many people with these ideling problems.... my burb completely ruptured its brake booster last year.... scary **** coming to a stop sign while also blowing the front passanger line.

Anyways after I put a new brake booster on my truck it immediately had so much more power than it ever did and it was obvious to me that I had gotten used to a gradual decline in performance due to this faulty leaking BB.

I think all of these boosters in everyone's trucks are reaching the end of there life and because it is a brake part we dont realize it until after trying many other things....

Al

 

[thegawd]

I think I'm guna buy one for my sierra incase they become scarce. LMAO it's so rusty it's original for sure.

 

[Marcos Beltran]

so this is a reoccuring theme... failed brake boosters with vacume leaks. you can test this by holding the brakes and watching the fuel trims. if they rise when holding the brakes that indicates a vacume brake booster leak.

Will look into that tomorrow morning. So this is a common/ reoccuring problem huh? I just did the newer style master cylinder upgrade about a week before this problem started cause of a spongy brake pedal. It helped big time, brakes feel way better. If the brake booster was failing/ leaking wouldnt i be able to hear some kind of hissing or something? Really dont hear anything. Could the upgrade might have damaged my brake booster if it was already on its way out?

 

[thegawd]

when you hear hissing it has ruptured. it can leak and not be audible.

swapping MC's might not be a good idea due to mismatched brake parts.... I change brake parts by rpo code only. sometimes people get lucky and sometimes they dont. I also believe that you may have swapped out a faulty MC and replaced it with one that works but it might not be the best option, I mean a working proper MC would be the best option.

the MC piston is matched to the wheel cylinders and brake calipers and there are low drag calipers and conventional calipers that have a different MC design with a stepped piston and a visibly different MC....

sometimes this works out for people but Iv read so many times where it dosent. i almost did this swap but with JB5 brakes on my sierra and JB6 brakes on my Burb, they are more than adequate for my trucks. both systems work flawlessly including the abs. but I exercise the abs every chance I get.

Al

 

[Marcos Beltran]

Just went to the garage and tested the brake booster. When pressing the brakes the LONGFT stays the same. The SHORTFT rises from 0 to 3.5/5.5 when pressing the brakes and holding it then goes back to 0. When releasing it does the same, goes to 3.5/5.5 then goes back to 0. Also when brakes are pushed it goes from 650 rpm to 720 rpm then back to 650 rpm when holding brakes. Then when releasing brakes it does the same thing, goes from 650 to 720ish back to 650 rpm. This cant be normal right?

 

[thegawd]

I dont know... I never knew it was leaking... it ruptured coming up to a stop sign. but it was leaking before because after I replaced the booster my truck was running like I just did a tune up on it. from what I understand is they fuel trims will rise all the way to 100% if you hold the brakes long enough.

Al

 

[Marcos Beltran]

I dont know... I never knew it was leaking... it ruptured coming up to a stop sign. but it was leaking before because after I replaced the booster my truck was running like I just did a tune up on it. from what I understand is they fuel trims will rise all the way to 100% if you hold the brakes long enough.

Al

So pulling off the hose to the brake booster and plugging the end that goes to the engine should be able to pin point if the brake booster is the problem right?

 

[Erik the Awful]

Or just vise-grip the hose shut.

 

[Marcos Beltran]

Or just vise-grip the hose shut.

Aint the brake booster, just checked and made no difference. Any ideas anyone?