Greetings guys, I'm trying to help my friend whos in a pickle. Hes got a 96 Blazer 4.3 vortec. He told me 2 days ago it was running rough and then yesterday that it was stalling on him while he was driving and running pretty rough. His brakes were also acting up. So I checked it out and what happens is when you apply the brakes they go down to where they should and then slowly go further down and as this is happening the engine begins to stumble and if its in gear it can stall out completely. When you quickly press the brake pedal to its normal position and let off quickly the engine revs up a little bit.

So its acting like the master cylinder is damaged internally but then it causes this engine stumble which boggles my mind. Are these vortec engines really sensitive to vacumm leaks? Could the booster be leaking and causing the stumble?

 

I'm betting you have a combination of problems. One thing at a time though.

When you remove the check valve from the front of the brake booster, there should be a whoosh sound as the outside air is drawn into the booster which should be held under vacuum by the engine. If this does not happen, then I suspect you have a failed booster check valve (cheap). It is also possible that there is a tear in the bladder inside the booster that may open up more when the brakes are applied, requiring more vacuum.

It also sounds like you have either a brake line leak or an internal leak in the master cylinder. I would first start by inspecting your brake system (lines, calipers, wheel cylinders where applicable) for any leaks. If leaks are found, repair and retest.

Any MAF sensor equiped vehicle is sensitive to unmetered air leaks. The PCM relies heavily on the mass air flow reading in determining the proper fuel injector pulse width. A failing or faulty booster would be a large vacuum leak.

 

I just gave a call to my buddy and he took it to a shop this morning and basically verified what we are both thinking. The guy told him the master went bad and must have damaged the booster in the process and with the vac line clamped it started idling better and what not.

Its too bad he gave up so quickly because now hes going to have a decent bill to have the guy replace that stuff because we could have gotten it done for cost of parts alone but I learned something I guess in the process at least. Thanks for your help