Well, for some reason my truck is sucking through the tail pipe. I stuck a can in the pipe and it would come out an inch, then suck back in.
Why is it doing this, and is this bad?

 

What engine do you have? Is it true dual? Most dual exhaust have an x or h pipe to balance the pressure/tone of the exhaust. If you put your hand over the tailpipe of a truck with true dual you can feel each cylinder pulse as the fire. That would be my guess.

 

O sorry, Its my '87 302 and it has true duals. no x pipe. I know I should feel the pulse, but it is the sucking in that has me wonderin.

 

You should definitely put something in to equalize pressure...

 

Or just stop putting cans in your exhaust pipe.

 

I always thought that when the exhaust sucked in, you had a burnt (not seating right) exhaust valve?????????

 

LOL! Yeah I wonder how he figured out that his exhaust sucked in... What made him try the can technique?

Anywho, I've always been told though, that if you run true duals you're supposed to use something to equalize pressure between the two pipes, be it an x-pipe, h-pipe, or a dual-in/dual-out muffler. No?

 

That is probably what is known as "scavenging" and is perfectly normal.

 

The movement of exhaust pulses through the tubing is a science in itself. Nascar teams typically change the exhaust tubing length and size, and even the shape and position of collectors depending upon the track that day and the rpm range the motor typically operates at. Tuning the exhaust to help pull the air out of the motor really does improve engine performance, it's the reason longtube headers work.

 

I put the can in as a demonstration to myself. I felt the exhaust and it felt wierd, so I put a can in so I could see what it was doing. I think ya'll are just jealous that ya'll wouldn't have ever thought of doing that to give a visual representation of exhaust pulses.

 

i just use the proper tool, a vacuum gauge.

 

On low horsepower engines true dual isn't bad, but needs to be equalize at higher levels.

 

Those cost dinero. I also don't want to have to pull my manifold out to pull my 02 sensor out to put the vacuum gauge on then put it all back to together then pull it all out again.

What would a vacuum guage tell me anyway? (And don't say the amount of vacuum in the exhaust)

 

Huh? My test gauge was about $15 at Advance Auto, and ANY of the vacuum lines attached to the plenum can be teed if you want it in permanently. Heck, it even comes with a butt load of tees of various sizes and some rubber hose stubs so that you CAN connect it with everything else connected, and WITHOUT cutting any hoses or removing any nuts/bolts.

What the heck are you talking about O2 sensors for? On the speed density 5.0's, the O2 sensor isn't in the manifold anyway. It's in a tiny cross-over in the Y-Pipe. Since his doesn't have an ECM anyway, it won't have an O2 sensor. (Not to mention the absence of a crossover or Y-pipe on true duals) I'm not sure when Ford started using separate O2 sensors on the 5.0, but I'm relatively sure it was after Mass Air became the standard system. (Post '94)

A decent vacuum gage attached to the intake manifold will tell you more about what's going in this vintage Ford engine than ANY scanner or code reader unless the ECM has been retro'd to a newer one with fancier diagnostics than EEC-IV, or even EEC-V. In Australia, 80's and early 90's Falcons had an Economy gauge in the dash cluster. Nothing more than a vacuum gauge. Economy is a good thing to know when you head across the outback, and there isn't a fuel stop for the next 600 Km. Less throttle = more vacuum = economy. It'll tell you about vacuum/intake leaks, it'll tell you about vacuum pulses driving the MAP nuts, it'll tell you about plugged up cats/exhaust, it'll tell you about stuck valves or hosed up lifters, and it'll telll you about a restrictive air intake. And to think this is one of the tools that stealerships don't seem to have in their inventory ....

 

Can you purchase guages to put into your vehicle to measure vacuum? What is the range of vacuum it should read? I have a 5 litre in my pick-up. - Sorry to Hi-jack this thread.

I found this link about vacuum guages after I had already asked the question http://autorepair.about.com/cs/troub.../aa112401a.htm