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E7 400 Mack, Misfire, seems to Move, need help


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Hi, new to Mack and need help

I have a E7 400 Mack with a jaccob Brake set up, here's the story  the truck had a head gasket problem that was building coolant pressure that we were adding coolant hopping to get a few more weeks out if it till we had time to get the truck in the shop, then we noticed a lack of power and noticed a misfire it seemed to be cylinder 2 and 3 we checked this by cracking the fuel lines loose one at a time, when removing the valve cover we found cylinder 3 exhaust yoke broken, from there we, 

Replaced both heads, gaskets , all 6 injectors , and injector lines , and the one broken yoke, piston's and cylinders seemed fine on visual inspection, adjusted valve timing according to the book and have rechecked since 16 on intake and 24 on exhaust, put all back together there is no more coolant pressure problem to still has a misfire , we checked compression and leak down all seem ok, we tried checking witch cylinder the misfire is from by disconnecting each individual fuel pump wire one at a time and it seems that the misfire jumps around, OHM checked all fuel pump wires seems to be ok , swapped the computer with another truck and other truck ran fine with computer not sure where to go next    

Edited by rspatola88
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  • rspatola88 changed the title to E7 400 Mack, Misfire, seems to Move, need help

Leave the unit pump wires hooked up and try cracking the injector lines 1 at a time to try and locate.  My guess is you have a bad eup and it's not supplying enough fuel for the new injectors.

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How many miles are on this engine? Are all the valves adjusted about the same.... as in you didn’t have to crank one to the end of threads to get adjustment correct? Were the lifters or the EUP lifters checked? The ceramic rollers break or the metal rollers score up and cause a miss that way, especially if you had broken valve train components

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So this comment holds some weight being as you had a yoke failure  to begin with you should check the lifters !on the failed yoke cyl! That being said the fact that the miss seems to jump around sort of indicates the computer is possibly trying to compensate for irregularity in engine rpm smoothness not sure on this! But I have been personally involved in chasing this issue several times wasting time To no avail ! My advice is  After you check the lifter! if the engine is only doing this at Idle and is not effecting performance, Leave the dam thing alone and drive it! Its an annoyance thing only!  Not an indication of a failure of any sort! 

Just an opinion 

Edited by fjh
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have you installed a pressure gauge on the fuel inlet hose to the engine, or "T" into the output line before the regulator and run the truck with a load and read the pressure? I have one made up with 20' of hose so it can ride with me in the cab.. 80-100 PSI is good. if the pressure bounces a lot, may even get down to 50 or less theres a good chance the pump is slipping or failing..  If you have the fuel sensor located at the regulator, you ca take it out, install a "T" then put the sensor back in and put the gauge on too, and test it...  if it's good, Then refer to "F's" post. Jojo

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