ISC headaches

Longhood

Well-known member
working on an FL80 with an early ISC ,8.3 CAPS engine CPL 2689. It shows fuel issues, low power, but more like the electronics aren't sending the signals, shows one code that will never clear 128 152 7 SID, boost erratic or intermittent.
I did the battery connection check and clean already.
I have an OTC 3418 code reader, never have trouble clearing codes on other applications.
the 8.3 also dies sometimes after about 10 minutes of idling, (not a idle shutdown) then acts like it has run out of fuel, on restart. bit the lift primer pump makes good pressure at key on although it does take longer to build to bypass pressure after the engine dies. doesn't die all the time.
suspecting ECM failure, but I don't want to just throw parts at it till something sticks.

Any ideas??
 

Truck Shop

Well-known member
The best I can say is set-up a remote fuel source to bypass the tank and fuel line to lift pump. I would make sure its getting fuel. What happens at
a steady 2200, does it die any quicker? It's a strange one for sure. But I have had problems with those lift pumps in the past. Also something moving
floating in tank, intermittent block of fuel?

I just went eight rounds with a 6TS cat that had similar issues, it had even been to a KW shop. After many hours checking things I found two old tube
sending units in the bottom of the fuel tank. On one of them the reading strip had come out of the tube and was stuck between both old tubes, with
the tank pick-up tube right over it. It would idle fine then die or die while driving, and sometimes suddenly recover and run fine for a few minutes or
100 miles. it just depended on when it would float up and block off the fuel supply.


Truck Shop
 

Longhood

Well-known member
I am going to try a separate tank, to the primary filter to see if the vacuum readings change much, but it shod good pressure at the gear pump when it dies, engine speed doesn't seem to make a difference, at first we though it was the cruise control circuit, because it didn't die when the cruise was shut off and the pedal was blocked for higher idle speed, but it still acts up.
 

rzucker

Well-known member
The best I can say is set-up a remote fuel source to bypass the tank and fuel line to lift pump. I would make sure its getting fuel. What happens at
a steady 2200, does it die any quicker? It's a strange one for sure. But I have had problems with those lift pumps in the past. Also something moving
floating in tank, intermittent block of fuel?

I just went eight rounds with a 6TS cat that had similar issues, it had even been to a KW shop. After many hours checking things I found two old tube
sending units in the bottom of the fuel tank. On one of them the reading strip had come out of the tube and was stuck between both old tubes, with
the tank pick-up tube right over it. It would idle fine then die or die while driving, and sometimes suddenly recover and run fine for a few minutes or
100 miles. it just depended on when it would float up and block off the fuel supply.


Truck Shop
Had a Peterbilt awhile back that had about a dozen of the foil seals from anti gel bottles stuck in the pickup tube. Blowback with high pressure air found that one.
 

Longhood

Well-known member
Well I think I finally got this wrapped up, I built a manometer and used inches of diesel (cause dyed diesel is easy to see in a clear tube and it won't cause any problems like coloured water) I found my fancy fluid filled vacuum gauges to be very in accurate to put it mildly.
In hindsight inches of fuel makes good sense, because it gives exact measure compared to tank draw, within about 2 inches in this case, and that also allows for possible measurement errors, I saw it was close and moved on.
post water separator added a bit more restriction, but the difference between lift pump outlet and post water separator was huge, like 60 + inches, and then it would drop measurably when I opened the valves between post WS and lift pump out (the lift pump only runs for about 1 minute and then the gear pump takes over, as Truck Shop mentioned above, )
the gauges didn't really show much difference , but they read in inches of mercury, which is a lot less sensitive than inches of fuel. I also changed out the boost sensor, which must have been internal shorted, although it still doesn't explain why the code wouldn't clear when the sensor was unhooked, unless baseline is a resistance reading, which seems to be classified. I really don't see why cummapart decided the lift pump/ fuel manifold needed to be on the back of the engine, since most of these engines are in mid size set back axle crap wagons :badi but in a perfect world the crap wagon designer would get to fix their brilliant ideas, not old worn out wrench benders.
 

Truck Shop

Well-known member
i like that inches of diesel idea, glad you found it. And about those crap wagons, good description.:bdh

Truck Shop
 

theironoracle

New member
Need px9 help

My kenworth says "exhaust (scr/def) service required". What do I do? Is there a def filter in the system? Plugged def injector? 2014 t370 first time this has come up. Thanks TIO
 
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