6.5L diesel excessive exhaust pressure

[Jeffd99]

Hello all, thanks for letting me join the site. To get right to it I have a 1999 6.5L diesel suburban 309k on the clock, bone stock. I noticed while walking around the truck recently the pressure coming out of the exhaust pipe at idle was a little more excessive than before. No mods, relocated pmd, no gauges bone stock steady oil pressure of 25psi at idle. Any ideas or concerns I should look out for? Thanks!!!

[Yukon6.2]

I acquired a 2000 Sub that the PO could not get running and keep running.
I returned all the patches and disconnections to stock.
The Sub would run for about 30 sec and die.
Disconnected the Catalytic Converter and the Sub stayed running.
The Cat was plugged solid,removed it and the Sub runs great.

[Jeffd99]

My burb starts right up and idles just fine just blows a forceful stream of air out the exhaust. I know the stock turbo doesn't make that much boost to make it pump put like that. If I punch it while driving on a highway there is a puff of black smoke momentarily.

[DmaxMaverick]

The turbo does nothing at idle. In fact, it's a restriction in the system until it builds boost at higher speed under load. A little puff of black smoke when you romp on it, before the turbo spools, is normal. At normal unloaded idle RPM, the amount of gas exiting the pipe is the same, regardless of anything else. Higher RPM moves more gas, lower RPM moves less gas. The engine is a fixed-displacement pump. A cold engine idles at a higher RPM than warm. Engine speed is governed at a specific RPM, mechanical or electronic. If the engine is running at governed speed, what remains is your perception.

That said, a plugged soot trap (cat) adds load, so the tail pipe gas can be greater to compensate, as Yukon suggested. As counterintuitive as it sounds, more exhaust restriction results in increased exhaust gas, but it doesn't sound like that's your problem.

If it starts and runs great, don't fix what ain't broke.

[Jeffd99]

Thank you for the reply and information. I am starting to wonder if the "tuna can " is causing back pressure???

[DmaxMaverick]

Even if it works perfectly as designed, yes, it does. Every minute of operation increases that, ultimately to a point of failure. If it is lawful in your area, losing it or rendering it unrestrictive (AKA: gut it) is an improvement.

Just so we are understanding, when you say "tuna can", you are referring to the exhaust system. Correct? There are other items that may resemble a "tuna can" that have nothing to do with this discussion, and the cat isn't an item I'd suggest resembled a "tuna can". A coffee can, perhaps. It could also refer to a muffler or resonator, which are less restrictive than the soot trap, and are rarely an issue that isn't obvious.

[Jeffd99]

I was referring to the crankcase depression regulator on the passenger side valve cover. If it not opening to vent pressure back into the turbo wouldn't it be rerouted to the exhaust?? Just brainstorming...

[High Toned SOB]

Jeff if the exhaust is stock then thats your issue. Drill a hole upstream of the converter and muffler, see if it whistles or even put a pressure gauge there.

Is the exhaust necked way down anywhere? My older 6.2’s would have a good “stream” of exhaust coming out the pipe, but it was like single 2” or something, lol

[DmaxMaverick]

I was referring to the crankcase depression regulator on the passenger side valve cover. If it not opening to vent pressure back into the turbo wouldn't it be rerouted to the exhaust?? Just brainstorming...

Was concerned you meant that. The CDR does ABSOLUTELY NOTHING, actively, as suggested by its nomenclature. It's purpose is to LIMIT the amount of crankcase vacuum at higher engine speeds. It is normally open, and gradually closes as intake vacuum increases, such as during higher engine speed or a fouled air filter. Excess crankcase vacuum/suction can increase engine oil vapor/consumption (messy intake) and/or seal failure. The CDR gets way too much credit for just about any reason you can imagine, when in fact it's a passive device that has almost no contribution to problems outside of greater oil consumption when it fails, which is very rare, even when they are filthy-dirty. The CDR is very often replaced unnecessarily, while chasing the wrong suspect.

There is no "back pressure" in regards to the intake or compressor. Flow can be restricted, but any form of positive pressure is measured as boost. Any intake restriction may result in less exhaust pressure, but power (as well as gas flow) would decrease.

[Jeffd99]

Thanks for the info...i was about to get a CDR off Amazon and try it...i will try and tap a hole upstream and down stream off the muffler and see if the readings are the same. The truck is starting to have a hard time getting going from a dead stop and up steep hills as well. Was hoping it was connected with the exhaust issue...fix two issues at once but I'm not that lucky lol.

[Yukon6.2]

just unbolt the exhaust in front of the cat,and take it for a drive. no holes to worry about

[High Toned SOB]

just unbolt the exhaust in front of the cat,and take it for a drive. no holes to worry about

Or this, yukon is def right, and faster. You just seemed to me like a “try to diagnose it with pinpoint science types, or whatever,” lol
Either way will tell you. But again, a “excessive” stream is kinda opposite of a restriction..

[DmaxMaverick]

.... “excessive” stream is kinda opposite of a restriction..

Not necessarily, such as in this case. A restricted soot trap causes the engine to load, which increases governed fuel, which increases exhaust gas. As long as the cat isn't completely blocked and the engine runs, the gas will get out. This causes other issues, such as higher EGTs and ECT's, and a skewed power map, which can often cause erratic auto tranny shifting.

[High Toned SOB]

Not necessarily, such as in this case. A restricted soot trap causes the engine to load, which increases governed fuel, which increases exhaust gas. As long as the cat isn't completely blocked and the engine runs, the gas will get out. This causes other issues, such as higher EGTs and ECT's, and a skewed power map, which can often cause erratic auto tranny shifting.

True, but i figured since he hasnt really complained about any performance or driveability issues, no abnormal noises,it runs just fine. He just “noticed” a higher flow of exhaust than usual.
I know when i put the 351vgt on my truck and closed the housing down, the truck would sound choked just idling. And the exhaust would just kind of “waft” out of the pipe. Granted i have 4”, but still, very little flow. Try and drive it like that, good luck, it would start knocking as soon as you tried to move it. Let alone “punch it on the freeway”. You could hear the difference in the intake as well. A plugged exhaust makes the same intake sound as trying to tev something with the exhaust brake on.

Thats why Im partially wondering why we’re looking for a restriction when he says “excessive”.
Maybe he fixed it? Maybe nothing was wrong.

[arveetek]

When you say excessive amount of air coming out of the exhaust, do you mean that it seems like there is just a huge flow of air coming out the tailpipe? If so, that is quite normal. Compared to a typical gas engine, the diesel engine has no throttle plate or other type of restriction in the intake, so it is sucking in (and therefore expelling) as much as air as possible, even at idle. On a typical 6.5L diesel, you can barely keep your hand over the exhaust because it is pushing so much air out. However, on a gasoline engine (at idle), there is very little air being pushed out.

I suspect that everything is normal if this is what you are referring to.

Casey

[Robyn]

Cat plugged up.

Been down this road a few times on these rigs.

If you have to pass smog test...get a fresh one....If not there are other options.....

The back pressure will cause issues as mentioned.

[Jeffd99]

Thank you all for your information. I wasn't able to get on yesterday to respond. The truck is having some performance issues taking a longer time to accelerate from a dead stop and holding highway speeds of 70. When we mention soot trap are we referring to the muffler or the resonator before the muffler? I think this ultra low sulfur diesel in PA is not good for older diesel burners. The exhaust comes out hard and you can feel it standing two feet away from the pipe...normal rpms..but delayed shifts at times that makes it feel like its towing something. Vetified it not starting out in second...have all gears, but fuel mileage decreased significantly.

[DmaxMaverick]

Soot trap = Catalytic Converter. A resonator is different, and won't plug like a cat, or a muffler. Your Suburban likely has a cat, not a resonator. That isn't to say it hasn't been changed out or "modified" at some time.

From your description, it sounds like the fuel filter, fuel delivery, or plugged cat, in that order.

Anything that causes a power loss for a given throttle input almost always causes issues with auto tranny shifting, specifically delayed shifts.

ULSD is not good for older Diesel engine fuel systems, but it shouldn't degrade performance at any measurable level. It can cause premature wear in a system that isn't compatible or updated.

[Jeffd99]

Would it be wise or do you recommend to cut the soot trap out and tap in a straight pipe in its place? I'm located in PA and diesels don't require emissions in this state. Would that hurt the muffler at all??

[DmaxMaverick]

No damage to anything, including the muffler. Most folks just bust out the internals with a punch and hammer (it's quite brittle when cold), but you can drive a pipe through it if you want.
(Disclaimer: for testing and off-highway use only, of course).

[Jeffd99]

That sounds alot cheaper.....to test i mean....thanks to all that responded with insight and suggestions, it is much appreciated!!!!