Boost delay

[Jimmy825]

I have had this roll off truck for just a little over a year.  When I had it inspected, the man told me I had a plus because it had the fuel pump replaced because it was a slightly different color and had yellow markings at the injector lines and a few other places. I about fell out when he said it usually runs about 3 grand to do a Mack pump because I have always been a Cummins dude. Ever since I got it, it hardly had any real amount of smoke come out like most other Macks I see that do and it has a delayed turbo boost. I had a slight miss when idled up a few rpms so I bought a set of exchange injectors from Mack put in by another shop. At first it seemed like it did a little better then went back to the same thing the next day. I returned it back to the shop and he suggested I take it to a Diesel injection shop as he said there was a pump problem and not injectors or the work he did. At times I can pull up to a traffic light and when I take off, the truck wants to go like crazy and a large cloud of black smoke behind in which I barely have smoke come out at all and after that, it is real sluggish as usual.

If I mash the pedal to the floor and hold it, it will end up dying unless I let off the pedal a schmid and wait for the boost to kick in. Or I can just ease down on the pedal going down the road just to gain a little speed and it will kick in and again if I floor it, it will continue to die. Now it seems like I am losing power real bad when I get to a hill while I have an empty or half loaded Vac Box which are normal after dumping the majority of the load off.

The truck has about 350,000 on it, it is a 1997 RD688 with an E7 350 mechanical and a Fuller 8LL. Top end is about 75 to 77 and about 82 downhill .... I can hit 80 and top a hill at 55 and before I could top it at 65 on the same hill any other time so now I know I am having more of an issue ... Any answers to this ???

NOTE: rubber strap in picture was used as a test to make sure I was getting full throttle

[fjh]

Your last picture shows on top  what replaced the puff limiter on the older american bosch fuel pumps!

That has a diaphragm inside It may have a hole in it! To test remove the line at the manifold side and suck on the hose if it won't hold a vacuum the diaphragm needs replacing!  

Edited November 28, 2020 by fjh

[h67st]

I assume you changed all your fuel filters? Did you put a fuel pressure gauge on it? It would help to put a gauge in the cab so when it acts up you can see if the fuel pressure is good. I don't know what the pressure should be but one of the Mack guys may be able to chime in on that.

[Jimmy825]

Yep, changing filters doesn't help it at all. Not many good Mack mechanics left around here in the Baltimore/Washington area (that I can find) that know how to work on mechanical Mack engines. Had 3 look at it so far and they can't figure it out. My problem is hard to explain and best when they drive it and still can't figure it out

 

[h67st]

The reason I say put a gauge on it is because when I was a mechanic we found fuel supply lines collapsing on the inside, tank vents not working right, and stuff floating in the tank that would get sucked up against the standpipe. Any of these could cause an intermittent power problem.

[theakerstwo]

And the fuel pressure should be checked under a load.

[mrgumby]

1 check tank for  crap.                     2 check fuel lines.    3 check lift pump     4ckeck filter housing for crack  5 check fuel tank or fuel cap for ventilation      6 if it has a fuel tank shut off valve check that also I’ve seen some with  rubber in them fall apart and cause a problem but that was usually on old ones that people changed  

[OldRedMack]

Pressure check the intake.