Bought a dump truck about 8 months ago. I did test drive it and it seemed to have plenty of power then. The truck was in a bad enough state of repair I did not want to drive it from Tulsa to Spfd mo so I trailered it home. Have been working on this truck for 8 months. Did a test drive a couple of days ago and it has low power. Really noticable in trans hi range. Wont come up on turbo. Starts out good and pulls hard on low end just when turbo starts to come up it falls flat and really feels like its not getting fuel. Have driven trucks with the same issue and was a blown afc bellows and it feels just like that. Ok so I set out today to put a new bellow in the pump. Did the usuall gymnastics and contortions to get all the bolts of and the pump on my bench. When I pulled the cover for the Afc bellow I got a large bit of fluid from the back or pump side of the bellows. The bellows itself looked great and I saw no defects. It was kind of thick and black and really looked like engine oil. Inspecting the pump it just cannot be engine oil and I have to conclude that its fuel that has been in there some time and is dirty. I was pretty perplexed about having fluid in the afc spring chamber cavity. Seems to me that fuel or fluid should not be there. I dug out my old pt pump manual and read about the old and new style plungers. I have the new style plunger that is supposed to vent this fuel back into the pump before it gets to the afc spring cavity. I am sumizing that my barrel and shaft are now worn enough that its leaking fuel to the spring cavity and has no where to go. This basically creates a hydraulic lock and the boost pressure cannot move the plunger as it is bound by hydraulic lock in the spring cavity. The book talks about venting that cavity to the fuel return line back to the tank. My set up does not have that line and has a plug in the pump housing. My question is should there be a drain vent line in the hole im pointing at in the picture? Also pictured is my plunger and you can see its the new style? Has any of you come across this and what have you done to fix it?
Sorry but no help here.. but that plunger is pretty scuffed up.. I don't think I would worry too much about old & new or venting as u said it "did" run great.. Replace the bellows & the plunger if u can & slap'r back together & see what happens.. & don't forget>> when diagnosing a low power problem> TAKE OUT THE AIR FILTERS..
Yes the plunger does show some wear. Thats probably why I now have fuel in the spring chamber side of the afc bellows. I just keep thinking that fuel should not be there and with no vent its hydro locking the plunger movement. The book talks about the old and new style plunger. The old style had a seal to stop leakage. The new style like mine has the pin hole in what used to be the seal groove and has a hollow shaft to vent the fuel back to the pump body. I think the only thing that makes that work is a good fit between plunger and the barrel. If I replace the plunger shouldnt I replace the barrel also? I am going to put it back on the truck tommorow and run it with that plug removed so the spring chamber side is vented. The book lists the vent being plugged as a possible cause of low power. Mine has no vent set up wgatsoever! Also book says the spring chamber side vent is run to the fuel return line. I have heard before that if the fuel return is restricted it could cause low power. I never could understand how. Now i see it if the afc is vented to fuel return then a restricted return line would cause fuel pressure to build in the vented side of the afc bellow!
Back when the Internal Aneroid systems began a lot of the Road School mechanic/drivers tried the restrict the return line trick to gain power, found a good deal of home spun small hole drilled inserts in fuel line fittings the idiots would buy and try only to see pump damage or fuel in the oil issues from pushing past weak o-rings later.
Ok looking at some other trucks I have they do have the vent line from the port I mention to the fuel return line. The first pic is an 88 pete. Heck even my old 79 cabover has that vent line in the last pic. Thinking im on the right track. Gona be hard to find a fitting. The plug is something less than 1/8" pipe plug. Going seaching for parts now.
Could it be 1/16 pipe thread? Not real common but does exist! Just one example I found with Google search: https://www.amazon.com/Parker-2MSC1N-B-Compression-Fitting-Adapter/dp/B000FN0IJK/ref=sr_1_9?hvadid=198216462861&hvdev=c&hvlocphy=9005174&hvnetw=g&hvpos=1t1&hvqmt=b&hvrand=5393794866967097898&hvtargid=kwd-324887258785&keywords=1/16+npt+to+1/8+npt&qid=1553695094&s=gateway&sr=8-9 Also seem to recall adapters in grease fitting kits we had at the shop, 1/16 male to 1/8 female pipe. Or is it a straight thread with oring seal?
It looks like straight pipe but hard to tell that small lol. At cummins now trying to hunt up parts to put together a vent setup.
On some small propane carburetors I have found 1/4 x 28 taper thread holes. That is not pipe size it is actual thread size, as in a 1/4 x 28 screw will thread in there. Adapters are available from Alemite to go to standard pipe thread.
What fittings where on the bottom of those older air shift knobs on the Roadranger transmission? ? I believe they were smaller than 1/8 pipe. Anyway you seem to be on the right track Good luck !
I do belive they are 1/16". I do have one and it does fit that plug hole.
Would check the CPL & Pump number with Cummins as to if that PT unit has a Internal feed to return, not that hard to check then would have to think what else may have goo in it.
Better check the return fuel hose before you go any further. If the hose has deteriorated and or swollen inside it will put pressure against the Aneroid which will in-turn cut the fuel back to the injectors. What year is this thing? The Aneroid is there to cut fuel back to control excessive smoke from turbo lag.
I did check that and the return is plenty open to flow. The way this pump is set up it cannot put fuel pressure on the back side of the bellow as it does not have a vent line to the fuel return system. It vents the afc to the front of the pump cover. I figured that oit when i had the afc completly assembled and removed the plug on top of the pump. Blew air into that port and it exshausts out the front cover where tje tach drive would go if it had a tack drive. Put 15lbs of static air pressure to tje air port on afc and still have same problem. Really starting to think the issue is elsewhere. Tommorow im gona run a known good hose straight from the tank to the pump eliminating the fuel supply grom tank to pump. If i still have the same issue then im gona switch out to a known good pump and then i should know fuel supply or pump issues. Sounds good in theory lol!
Oh and its in an 88 freigjtliner. I jave no way of knowing how much Frankenstein work had been done by the previous owner and i think is some of what the problem is!
Found the issue. The hose that goes from the filter to the pump! The inside liner would collapse and cause a restriction. When it was off and under pressure it would be wide open. In tje system under low pressure draw from the tank it would collapse. Found it by going with known good hose from tank to pump. Then it was just a process of finding out which hose was bad.
Sometimes the best way to find a problem with a system is to eliminate as much of the system as possible. Then add each part back till problem returns.
Yep in the end thats what I had to do. That one was real tricky cause the only tje way the fault would show up was on the truck under load. The part in your hand looked just fine. With the new hose to the tank it ran like it should. Then just swaped hoses with known good hoses till I found it. I did put a vacume gauge in the line and knew I was looking for a restriction and not air leakage. Plus it didnt have the tell tail indicator where the throttle hangs and its slow to come down to idle like you usually get with a air leak. Any one of us could have looked at that hose a hundred times over and not have seen the issue lol!
Fuel suction problems, seem like the worst possible problem to diagnose no matter what. Whenever one comes my way it is like an immediate groan. Had one come to me repeatedly last year, undid the hillbilly stuff and changed just about everything. In the end the customer could get it to run a while by pumping the primer. He wanted to rent one of mine while I really tore into his, but I let that slide because I saw how he took care of his own equipment. Never did get to figure that one out. Had another very experienced mechanic much older than me calling and asking similar questions some years before. All I could say, just check the basics.
had a guy bring me a 4700 ihc years back with a dt466 and it would not suck fuel unless it was drawn out the bottom of the tank. the standpipe in the tank must have had a tiny pinhole. he didn't want me to spend much time on it so we just hooked a fuel line to the bottom of the tank at a convenient place ( there was a pipe plug ) and that cured it