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Andy, Truck Diagnostic Technician
Category: Medium and Heavy Trucks
Satisfied Customers: 1661
Experience:  International and Isuzu trucks , International, Maxxforce, Cummins and Isuzu engines, Diagnostic and electrical troubleshooting specialist , Hvac and truck /trailer refrigeration troubleshooter
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I have a Cummins ISL 8.9 liter that has a hesitation problem

Customer Question

I have a Cummins ISL 8.9 liter that has a hesitation problem when accelerating after coasting. The insite software is telling me that the turbo is commanded to about 70% closed but the turbo actually goes to 95% closed. After a fer seconds the turbo will go to the commanded % closed and everything is fine. Any ideas?
Submitted: 2 years ago.
Category: Medium and Heavy Trucks
Expert:  Andy replied 2 years ago.
Hi there ! Sorry to hear you are having trouble! My name is ***** ***** I would like to try and help if I can ! Do you have an engine serial number along with any fault codes in memory?
Customer: replied 2 years ago.
Esn is 46715288. Software calibration is up to date (version 57). No codes. This engine can run for hours with no problem. Can pull long grades. I recorded 1 hour of engine data using Insite software. Egr and fuel pressure are all very close to commanded values. Dpf regens normally after about 100 hours of drive time. When I get the hesitation after coasting it is the vgt commanded to say 70 but the vgt jumps to 95. This ONLY happens after a long coast say going down a grade and then hitting the accelerator. Very intermittent. No correlation with Jake brake being on or off. Engine has 50,000 miles on it. I called Cummins tech support. They were out of ideas. The only odd observation is that when this hesitation happens, the inlet air temp voltage drops but temp reading doesn't change. My next step is to double check for a loose ground connection.
Expert:  Andy replied 2 years ago.
That is a good start . If the problem is intermittent and after a few hrs of operation every time , then we can suspect high resistance in the ground circuits could definitely be causing this.
Expert:  Andy replied 2 years ago.
Any luck with the grounds?
Customer: replied 2 years ago.
All grounds are solid. I was looking at the one hour drive data log and I did notice a pattern where voltage to the air INLET to turbo temp sensor to drops from about 2.3 volts down to below 1.5 volts. When that happens the turbo jumps up to 95% closed. I need to look closely at that sensor and wiring and connectors. There is no way that intake air to the turbo can be jumping that much in sensor voltage in just one second. If wiring looks good, then I will change that temp sensor. Very odd that a bad intake air sensor could cause at jump in the vgt. Ever see a problem like this?
Expert:  Andy replied 2 years ago.
No I have not however, this could still be a faulty ground causing this . Just because the grounds are solid does not necessarily mean that the grounds are clean. We should either do resistance checks from battery negative stud to every other ground on the truck ( alternator , starter , engine block , cab ground , frame ground) With absolutely everything completely powered down on the truck , there should be less than 1 ohm of resistance between each test point . Anything higher than one ohm indicates a dirty ground and must be taken apart and cleaned up because when there is power on and circuits are running , that resistance increases .
Customer: replied 2 years ago.
I know exactly what you are talking about on no resistance or voltage drops across grounds. I did verify that all grounds are good. In studying all the other sensor voltages, they are stable or change slowly; except for the air inlet sensor to the turbo. I can find nothing wrong with the wiring harness or connectors. So, I will replace that sensor and see if this cures the problem.
Expert:  Andy replied 2 years ago.
Ok so it is possibly shorting internally once it reaches a certain value. Good luck and please keep me updated on your findings .
Expert:  Andy replied 2 years ago.

Hi there ! Just looking for an update !

Customer: replied 2 years ago.
Hi Andy. Sorry for tge delay. I put on over a 1000 miles over 3 days and was all but convinced that changing the exhaust pressure sensor and updating the calibration software on the ecm solved my problem. Then on day 3, the hesitation happened again. Darn. The engine ran perfectly mostly at between 55-65 mph. Up and down grades using Jake brake no issues. Then I came into a construction zone coasting down hill to a narrow one lane bridge where I slowed down to about 40 mph and then when accelerating after the bridge, the vgt went to 95% closed for about 15 seconds and then went back to normal operations. So, in doing more reading, I did find out that 95% closed is the vgt braking mode. So, somehow that vgt is going into a braking mode when it shouldn't. About the only other thing I can think of is to remove power from the engine and transmission computers for an hour or so to have them reboot on the hunch that I have a trans - engine issue where the trans and engine are not communicating properly when a downshift is required under heavy acceleration. Any other ideas?
Expert:  Andy replied 2 years ago.

Do you have any fault codes in memory to help us narrow this problem down ?

Customer: replied 2 years ago.
Andy, this time it did get it to throw a code 2387. When I got the hesitation, I deliberately held down the throttle until the engine recovered. But all this code does is to confirm that the turbo position of 95% closed does not agree with the engine ECM commanded position of about 70% closed. That mismatch must exist for over 10 seconds before this code would be set. So, my holding down the throttle basically forced a code to be set. Now, if I was getting this code and mismatch between commanded and actual turbo positions at various road speeds while under various loads, I'd say OK, turbo or actuator is bad. It just seems way to odd for me to have the mismatch after coasting and then acceleration when the transmission must first downshift. Btw, no codes on the Allison 3000 trans.
Expert:  Andy replied 2 years ago.

I will send the troubleshooting ! That is what I figured you were going to have .

Expert:  Andy replied 2 years ago.

Here is the description :

And here is the troubleshooting :

Expert:  Andy replied 2 years ago.

so 50/50 chance that it is either the actuator or the turbo sector gear or turbo is bad . Follow the troubleshooting and it should guide you in the right direction .

Please let me know if you need anything along the way .

Please do not forget to rate my service to you at the end . Feedback is important and bonus is appreciated . Thank you .

Customer: replied 2 years ago.
Thanks for the links, but I already have that information and have run down that trouble tree more than once. What I need to know is why the vgt goes into a braking mode only on full throttle after coasting to a lower speed that requires a trans downshift. The turbo passes the actuator tests and turbo has smooth range from open/closed with actuator removed.
Expert:  Andy replied 2 years ago.

Are you performing these tests with a hot engine? I have been fooled by this before !

Expert:  Andy replied 2 years ago.

When checking sector gear for smooth range of motion it should be at operating temperature. I have encountered some units that were fine when checked at ambient temperature but when I put them back together and ran them to operating temp , would start acting up again . And so when taking them apart when hot , I discovered that the sector gear was indeed binding and no longer smooth . The fix was to replace the turbos on those and problem solved.