Back Pressure?? Does this make sense to someone?

[alex1906]

So, my truck took a crap last weekend and so I took it to the shop. It had a ton of symptoms. Oil pressure was up and down (fluctuated between 20-60lbs). It started leaking oil back by the oil pan a couple of weeks ago. This past weekend it was running EXTREMELY rough and would vibrate and shake pretty violently. So, I take it into the shop and they tell me that the oil pressure sending unit is bad, the lower end main seal is leaking and the exhaust system needs to be replaced. This is a 1995 GMC 1500 pickup with 221k miles and it BARELY has any kind of exhaust system. So, the shop says that there is a leak in the exhaust before the Ypipe and that the exhaust isn't even going through the catalytic converter. He's saying that the engine is running off the exhaust manifold and is running horribly because there is no back pressure. Does this make sense to someone and if so, then can you please explain it to me?

This is a dilemma for me - I'm keeping this truck - it has no rust and the body is in awesome shape. It just has miles. Do I have the engine rebuilt/replaced now or do I make these repairs and see if I can get more mileage.

I guess this is a 2 part post. Any ideas?

[ChevLoRay]

Well, an internal combustion engine is an air pump....in theory. Getting more air out of it requires more air to enter it (less restrictions in the intake, improved air flow into the cylinder) and less back pressure in the exhaust system. I have heard the argument that there has to be "some" back pressure, but I have never seen anyone quantify that with a specific amount. Someone on this website will have some data....I'm guessing.

If having back pressure was something that had to exist to have an engine that would run, doesn't it seem likely that Don Garlits would have run some kind of muffler on his AA/FD dragsters?

Scavenging effects of an exhaust system are more beneficial, in my opinion, although I can't quantify that, either. I had an EMPI scavenger on my then-new '71 VW. I also put a Cyclone scavenger on my '60 VW with a 1600. The X-pipes that can be used on dual exhaust systems balance out the exhaust pulses to scavenge the exhaust gases from the cylinders and to also quieten the tone of the exhaust.

Back Pressure? I don't think we need any stinkin' back pressure. Bada Bing!!

[piecesparts]

#1 fix the exhaust and route it through the full system, the O2 sensor is confused. Change out the oil pressure unit and try putting a mechanical pressure gauge on to actually test the oil pressure, so that you can know what your problem is. Try a tune up on the engine and check all of the engine's sensors for any trouble. As for the leak on the rear seal, the engine is aged, live with it or change the seal out. My engine has 246,000 miles on it and is leaking some on the rear seal, but that is life.

[alex1906]

I think you're right about the O2 sensor being confused. I think I'm going to just let the main seal go. I'm planning on either rebuilding or replacing the engine when the time comes, so the money spent on the seal will be saved for reworking the engine.