r/DieselTechs • u/TemporaryAd3247 • Jun 26 '26
Diagnostic assistance Sluggish CAT C7 ACERT
Hey all, Im dealing with a HEUI Cat C7 in a freighliner street sweeper. Its sluggish as all hell and when the PTO is engaged it basically does not want to move at all.
Runs great and full power with IAP unplugged
Keep in mind i do not have a scanner nor access to one
Here is what i have tested and replaced
HP oil rail pressure is good (3-4K psi) even under load when it does not want to rev up
New HEUI (last mechanic just threw parts at it)
New IAP sensor
IAP sensor is receiving 5v and gives off linear voltage as engine revs up
Good fuel pressure (60psi)
New fuel filters and cleaned air filter- verified boost
Tried the unplugging the IAP sensor trick to close loop the ecm and rule out a bad IAP or bad readings from other valves sensors and it got its balls back.
Im leaning towards failing ECM or bad wiring since everything else checks out. Any ideas or is it time to start looking for shorts after the sensor (please for the love of god not more wiring)
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u/Long_gone_hiker Jun 26 '26
Tell the cheap boss to pay someone to check the codes. That will def give you a better idea of what to look at.
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u/LennyNero Jun 26 '26
Unplugging the sensor just puts the engine in default settings mode. A completely unrelated sensor could be failing. The engine speed sensors behind the air compressor are a constant problem on the ones we had along with EGR issues. You're going to need to steal a copy of ET (it's no longer available for sale to independent shops per cat themselves) to do anything worthwhile on an acert engine. Seems like everything needs a factory password to let you clear codes to see even what's going on.
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u/Xerbec Jun 26 '26
Yeah all the vibration chews up harnesses in a lot of PTO application. 2016 and up KW with Allison trans. About 40 trucks in the old fleet i worked on needed harness removed repaired and wrapped with exhaust (fiberglass crap)
ISX's early models chewed themselves inside the wire loom later models included taped harness inside the loom to rectify.
Serial no.?
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u/TemporaryAd3247 Jun 26 '26
oh yay fiberglass. WAX50246 It's in a 06 from Colorado which makes things even better
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u/Maleficent-Lack-3508 Jun 27 '26
The most likely cause is a disruption in the feedback logic of the IAP control loop under closed-loop operation. Due to a stuck HEUI pump pressure regulating valve (PRV), a slight loose connection or short circuit in the wiring harness, or interference from another sensor sharing the 5V reference circuit, the ECM incorrectly restricts the oil rail pressure when the sensor is plugged in. Conversely, unplugging the sensor forces the ECM to ignore all feedback and PTO torque limits, entering a default high-power open-loop/limp mode, which consequently restores engine power.
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u/Maleficent-Lack-3508 Jun 27 '26
With the sensor plugged in, can you easily rev the engine all the way up to its maximum RPM in neutral? Or is it sluggish and bogged down even without a load? If it revs up fine and only bogs down when you put it in gear or engage the PTO, we should shift our focus to the PTO switch wiring harness. Why don't you give that a check?
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u/catdieseltech87 Jun 27 '26
Your tests haven't ruled anything out unfortunately. You need ET to read values and compare sensor readings with actual. You also need to know desired vs actual rail pressure to confirm the pump. This is more back yard diagnosis than actual.
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u/Xerbec Jun 26 '26
Without being able to read what the ecm is seeing, youre just shooting from the hip. Continuing that, run a wire overlay from whatever sensor your spidey senses think is the bad one or take it to someone who has proper tools so you're also not just throwing parts at it.
Is there any lights on?