E6 350 Popping from Exhaust

[Sy McCarty]

Hey everyone! I have an 86 Mack 550 Superliner dump with an E6 350 4 valve. One day it started popping from the exhaust that almost sounds like the Jake popping when you roll to a stop without taking it out of gear. We noticed some white smoke as well. We’ve gapped the valves and yolks 5ish times and can’t get it to come out of it. We’ve changed fuel filters as well but still hasn’t helped. Book calls for .024-.028 gap on exhaust valves and we have gone all the way to .035 and we are still getting the popping like the exhaust valve is open. What’s next? We’ve unplugged the Jake solenoids to rule that out too.

[Joey Mack]

When you had the valve covers off, did you remove the rocker shafts and use a small straight edge to check valve stem height? Do you know which cylinder is acting up? Did you pull the engine brake celenoids to inspect the sealing rings, are all the push rods properly seated in the lifters? Ok... Ill stop here. Good luck.  Oopps, 1 more thing... Are the valve bridges set evenly to .010" ?  Ok thats it ..

[Joseph Cummings]

13 hours ago, Sy McCarty said:

Hey everyone! I have an 86 Mack 550 Superliner dump with an E6 350 4 valve. One day it started popping from the exhaust that almost sounds like the Jake popping when you roll to a stop without taking it out of gear. We noticed some white smoke as well. We’ve gapped the valves and yolks 5ish times and can’t get it to come out of it. We’ve changed fuel filters as well but still hasn’t helped. Book calls for .024-.028 gap on exhaust valves and we have gone all the way to .035 and we are still getting the popping like the exhaust valve is open. What’s next? We’ve unplugged the Jake solenoids to rule that out too.

Only when you back off the throttle? and as soon as it hits idle, it goes away? Dropped exhaust valve seat. With the truck idling crack the injection line at the pump one at a time. When you crack the one and it goes pop pop pop from the exhaust, that is the cylinder with the dropped seat.

When there is zero fuel delivery the seat comes loose and makes that popping noise, as soon as there is even enough fuel even to idle, the seat expands and seals in the head. It's a pretty common problem, especially with heads that have been redone by some third party, They don't fit them tight enough. It happened to me so many times that I only trusted heads I did myself, or the heads Mack rebuilt at their Hagerstown plant

[terry]

Once had a 237 engine when you backed out of it sounded like bongo drums out the exhaust, exhaust seat was loose, you could tell be the carbon around the seat, ran a long time like that. But it could be one of those things that could let go at anytime.    terry

[mechohaulic]

agree with all stated above. have seen  loose valve seats create the problems indicated. the fun days of building your own 

[fjh]

On 4/13/2025 at 7:32 AM, terry said:

Once had a 237 engine when you backed out of it sounded like bongo drums out the exhaust, exhaust seat was loose, you could tell be the carbon around the seat, ran a long time like that. But it could be one of those things that could let go at anytime.    terry

Loose valve seats were fairly common on the two valve heads v8 and 6 they usually didn’t have smoke issues thou! Until we get some more info  most things have been discuss ! 4 valve engines rarely dropped seats v8 was quite common! If a seat has partly dropped a seat the symptoms would fit the crime check your turbo exhaust wheel for nicks or chunks out of it!