How does exhaust leaks cause high EGTs?

[Detroit80]

I have a 1990 CCLB, Banks Sidewinder with wastegate, that's been basically parked for about a year due to multiple issues...biggest ones being some pretty decent oil leaks, AC failed (which is a HUGE bummer here in Phoenix, Arizona, lol), and had been dealing with high EGT/high coolant temps for a while when pulling trailers. I took the easy way out and picked up a Chevy K2500 6.5 diesel..that now is also having issues of it's own. I was never happy with the lack of power from the 6.5 compared to the 7.3 anyways, so now I'm looking at doing some repairs to the Ford and putting it back on the road.

I understand the basics of what causes high EGTs - too much fuel/not enough air, and timing being out of spec. I was reading over some other threads where it was mentioned that pre-turbo exhaust leaks are also a big deal when it comes to EGTs, and I'm not understanding how an exhaust leak would cause EGTs to go up. I'm not arguing it, just trying to understand it better.

Is it just a matter of exhaust leaks will reduce the ability of the turbo to spool up to higher pressures thus causing a problem in the "not enough air" department?

 

[Laine D]

Pretty much what you said. Even a small exhaust leak will effect the turbo’s efficiency. So you have all that fuel going in and not enough air to keep up with it. I’m sure there’s a much better explanation but that’s mine

 

[Detroit80]

Hmmm...that may well explain at least part of the issues I was having before I parked it, namely boost falling off and high EGTs. I'll definitely add that to the list of things to go over when I start tearing into it.

 

[u2slow]

Both on the right track. Its a circular/compounding issue though.

Your turbo runs on heat; or in other words... the expansion characteristic of hot gas (exhaust) spins the turbine.

Any heat you lose pre-turbo (leaks are lost heat) reduces the response and peak rpm of the turbo, and that reduces the compressor output (less air). The reduced air builds boost slower, and if the IP boost-references (like the AFC on a cummins) then the governor delivers extra fuel for longer.

These leaks develop easier on a V engine with a single turbo because of the extra exhaust connections.

 

[Detroit80]

Which also then explains why additional RPM didn't really do much for me in bringing the EGTs back down then as well (at least I'm thinking it does, anyways, lol)..I had read plenty where people were saying to drop a gear and wind the motor out as that will push more air into the motor as well, but I could go from 2K rpm to hitting the governor and see _maybe_ a 50 degree drop in EGT, but then it would quickly come right back up again.

Then this compounds as you said, where less air means higher EGT, but also drops the turbo efficiency. Less efficiency means less air, and round we go until I get back off the throttle again, as the IDI doesn't have a computer to pull back on fuel automatically.

I'm thinking too that this may help explain the power loss I seemed to feel after bumping timing up. Didn't have a timing adapter, so basically just winged it by moving the pump a dime's width. Seemed to lose a ton of torque down low, truck would seem to fall on it's face for a second or two if I tried to do a 2nd gear start (5 speed manual), and boost went from 10psi to 8psi, with really no effective drop in EGT. Shortly after that, I got the Chevy, and hadn't played with it any more.

 

[The_Josh_Bear]

When you go looking for exhaust leaks, clean up all the fittings from headers to turbo and smear a healthy amount of copper RTV on them. It's only rated for 700* or so but somehow survives great anyway even with EGTs over 1100*. Anyway it seals up those old connections really well, I've run this way for 1.5 or 2 years and the darn stuff doesn't melt off or push out. Crazy if you ask me but 100% worth it!

 

[Detroit80]

Hey, I'm willing to try anything that keeps me from having to pony up to replace the entire exhaust system, lol

 

[u2slow]

as the IDI doesn't have a computer to pull back on fuel automatically.

Does the IDI have low-boost fuelling ability? I've never learned enough about the DB2 pumps to know if they were adapted for boost sensing when used in factory turbocharged applications. The cummins (Bosch) AFC is merely a diaphragm that overrides the governor to some degree - nothing computerized.

 

[Philip1]

Does the IDI have low-boost fuelling ability? I've never learned enough about the DB2 pumps to know if they were adapted for boost sensing when used in factory turbocharged applications. The cummins (Bosch) AFC is merely a diaphragm that overrides the governor to some degree - nothing computerized.

No there is no boost reference on the db2 pumps unless you get an aftermarket top cap. The factory turbo models (from what i understand) have a torque screw which defuels at low rpm to simulate a boost reference but isn't always accurate.

 

[chillman88]

Does the IDI have low-boost fuelling ability? I've never learned enough about the DB2 pumps to know if they were adapted for boost sensing when used in factory turbocharged applications. The cummins (Bosch) AFC is merely a diaphragm that overrides the governor to some degree - nothing computerized.

The only thing I know of is available from Hypermax to limit fueling below boost, and is not stock equipment. It does serve that purpose though, so I figured I'd mention it. "Hypermax smoke puff limiter".