i have written a few threads on my bad fuel mileage. After all of the work that was done to the truck it still is not working out very well. The truck has 5" exhaust from the turbo back and the cat is still in the system and there is a cold air intake.

I have had a couple of people tell me the smoke (black espically the tip) and bad mileage could be because of no back pressure.

Any other thoughts.

The next thing i have to try is send in an oil sample. If fuel is getting into the oil what does that mean? I can only imagine...

 

thoes must have been gas engine people? on a diesel, or any turboed engine for that matter, the less back pressure from the exhaust system, the happier engine will be, i cant remember what truck you have but a cat is still very restrictive, what milage do you have, what are you doing with it, do you have any gauges, what do they read????

 

ALL engines will work better with less back pressure. PERIOD.

However, if the exhaust gas velocity is too low on a normally aspirated engine, the scavenging of fumes from the cylinders won't work as well and you lose power. A lot of people get confused and think that back pressure is what makes the power, but it's not. It's an unfortunate side effect that you cannot avoid.


If back pressure is what was really needed, you could take an 8" exhaust and stuff it full of socks until it registered the same amount of back pressure as a 3" exhaust, and I would bet it would perform worse than a proper 3" exhaust.

Velocity is key.

Why does having a too-large exhaust hurt velocity?

Think of it this way - put a garden hose into a drain pipe. When the water flows through the garden hose it has a good velocity, but then it dumps into a huge pipe and it loses velocity - not because of decreased back pressure, but because it has a much greater volume to fill. Exhaust gasses are hot, which compounds this slowing effect because as the gasses cool off, they contract. When they contract, it takes even more gasses to fill the pipe.

It's not because the pipe lacks back pressure, it's because it's not containing the gasses enough and they're losing velocity.



Ahem.

Anyway....

What year truck are we talking about? What have you done to diagnose the problem?

5" exhaust isn't going to be a benefit to you unless you're running aftermarket turbo(s), but it shouldn't hurt your fuel economy - turbo charged vehicles have positive manifold pressure which pushes the exhaust out of the engine. Scavenging isn't important.

Have you tested for a leaky/stuck injector? I'll be honest I'm a recent diesel convert and I'm not all that knowledgeable on the specifics of each component, but if someone tells me they're getting poor economy and too much fuel (smoke), it sounds like a fueling issue. Either too much fuel, or not enough air.

 

The truck is a 06 w/129000 miles. So far 2 places have done all kinds of diagnostics and found nothing wrong and there are no codes. They have all check fuel pressures and everything is coming back normal. One place did remove the injectors and had them tested. One injector came back with a high return but it still did not solve the problem. The last thing I was told to do was take a oil sample and send it in. What are they check for?

Oh I also have an issue with the TIPM but that is posted in the Gen 3.

Thanks Aaron

 

Perhaps there's something hinky going on with your fuel timing. Sounds like maybe a computer or sensor issue.

I think your best bet might be to take it to an actual Cummins dealer/service shop and have them dig into it.

 

Well I just had the TIPM replaced and I had A LOT of electrical issues going on with that thing going bad. Could that have something to do with it. Because now I have noticed a couple of things with the truck that I didn't have before.

 

So how is the timing corrected or checked? I can't afford sticking more money into this thing.
Thanks

 

Way beyond my level of understanding.

Here is a good start though.
http://www.cumminsforum.com/forum/3r...explained.html

 

I upgraded my air intake to a full cold AFE system. I added a bully dog. I upgraded to a full 5" from the turbo back. My fuel mileage went from 19-21mpg at new down to 14-15.5 now after 85k miles.

 

my computer is all wacked up. it was saying 19 then 20 on the highway at 70mph. hand calculated 16mpg. That sucks. Truck all stock. I was really expecting 19mpg. I'm blessed with another gas guzzler.