98 Bravada Perished Vacuum Tubing
 

Hey Team,

A bit of background. My daughter has a 1998 Bravada. A few days ago, on the way to work, it started missing badly and is basically undrivable. It also threw a P0300 code (multiple misfires).
I have changed the rotor, cap, plugs and HT leads with no effect. I'm also going to change the O2 sensors when I get back from traveling for work next week.

When I was checking things out I have discovered a tube that has deteriorated badly and has broken. Here are a couple of pictures:

The first is of the perished tube itself. It is running left to right and is to the right and below the white connector. It goes into the the left fender.
https://www.gmforum.com/attachment.p...ine=1391052945

The second shows the other end of the tube. I'm not sure what it goes into (PCV?) but whatever it is also has a breather from the valve cover.
https://www.gmforum.com/attachment.p...ine=1391052945

Can anyone tell me what this tube is/does, and could it be the reason for the engine issue?

Thanks
Paul

Yes, it is the very issue for your missing/P0300. Any vacuum leaks on an engine can cause issues.

That long section of tube you marked perished, goes to your vacuum chamber. It's just a chamber that helps create a vacuum reserve for the system.

The smaller section that goes down to a smaller tube and disappears, that goes to your vent controls. No vacuum= no control of floor/vents or defrost. Thankfully, the engineers were smart enough to design the system to that if it should lose vacuum, it will automatically revert to defrost mode.

And yes, you need to replace/fix those hoses for the engine to run properly.

the best way to replace that is to go to the parts store and get tube that is specifically designed for vacuum tube and some radiator clamps and replace as much as you can because if that one is going chances are the rest of them are too and the tube is cheap and will cause issues like mike said

Thanks very much team, the fast responses are appreciated.

A bit of an update, I replaced the three pieces of tubing and there was very little (if any) change in the way the engine runs.
We got a new battery as the old one was no good and I thought it might have been part of the problem, it wasn't.
The fuel pressure is about 57 psi KOEO which drops to about 55 psi when the engine is running, from what I've read this is normal.
I've also replaced the O2 sensors

So, any other ideas? When I hold the revs up a bit (1500-2000) it seems to run ok but it won't idle very well and when I rev it you can hear it missing. I've even heard the odd backfire.
Thanks for any help.

Justin's suggestion was you should replace all the vacuum tubing, not just the 3 you found to be deteriorated. If 3 are bad, others are likely bad as well. You can test this by spraying carb cleaner (engine running at idle) around the varous hoses and connectors. A change in RPM's means you found a leak as the carb cleaner is sucked into the intake system.

Good point. I'll get that done as soon as I can. Travel for work is really getting in the way of this !

As a test, you can cap off the port on the engine side to see if the engine runs any better. Last I knew, you only have those 3 small vac lines. Blazers and S10's have 1 more because of the vacuum actuated front differential.

Another question, what's the engine idling at when at normal operating temperature? Sometimes a vacuum leak will cause a high idle.

In case anyone is interested or has a similar problem. We had a shop look at it and the injector cluster/spider was causing problems and had to be replaced
But, there was also an issue with the distributor as well. There is a plastic part that the shaft runs through that had "slopped out" causing the rotor to move around. This caused weak/bad spark.
Anyway, an expensive fix but the Bravada is running way better than it ever has so all good in the end.

Thanks for updating us with your findings, glad you were able to get it fixed.