Workhorse Occasional Power Loss

wesinls

wesinls

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Sometimes my motorhome (2008 with the Chevy 8.1) will lose a ton of power...typical grade pull would be say 45mph, goes down to 25.  And sometimes during the loss of power it will all of sudden re-gain the power.  It doesn't spudder or anything, just gets sluggish, then all of sudden wakes up and drives fine.

I replaced the fuel filter, spark plugs, plug wires, and cleaned the air filer (k and n banks kit)- the typical things that resolve the issue per what I've read, and hoping it's the fix...TBD if it resolves the issue, as i can't really tell I've fixed it until I'm pulling my enclosed up a grade.

Anyone else have similar symptoms and what was the fix for you?

 
Check your fly by wire pedal assembly. Friend had the same problem, it was hard to diagnose.

 
Rig up a fuel pressure gauge and watch it under load. My 8.1 i had in a 35’ a would pull mt. Springs grade on i8 w/trailer with buggy and 4/5 quads and never drop below 50 without foot all the way in it.

my guess is fuel filter, or weak pump

 
Mine does this but I get get a "Reduced Engine Power" message and a P-code.

I have a Scan Gauge 2 mounted and just clear the codes and we're back in business.... I loaded the X-Gauge for the Throttle Body so I can watch that.

I went down the rabbit hole of cleaning throttle body and trying to source a TAC module, but none of that resolved the issue. 
What I can tell you is there is definitely something going on between the DBW peddle and the TB sensor. On the scan gauge you can see the TB blade increase from 5-8% at idle to 25% or more and then not get to 100% when foot to the floor.

Next issues to chase are battery/ground, etc... Low Voltage or some odd item related.

Scary part of this "Reduced Engine Power" is while driving and you go from 65mph to 25mph in an instant. Trying to get off the road to reset the OBD2. Luckily most of the time it happens once I get off the HWY on an exit, in a gas station or in my driveway.

 
I had this happen to me as well, I kept thinking it had something to do with the fuel cap (smog).  It ended up being the Mass air flow sensor ( I think that is what it is called).  It is on the intake with a big screen.  Easy to change and not expensive.  It puts the engine into a safe mode of sorts.  You can pull it out and clean with contact cleaner, but just as easy to replace.

 
My friend had this issue and it was his fuel pump. 

 
My son is having the same problem and the code is saying telling us O2 sensor faulty  

 
The drive by wire being a possible issue sounds interesting - I've done all other checks / replacements mentioned (other than replacing a fuel pump but have confirmed good fuel pressure - but still plan to run pressure gauge while driving if the issue comes again)...

For the drive by wire issue - other than looking for obvious connection issues, how else to you troubleshoot that issue? 

 
wesinls said:
The drive by wire being a possible issue sounds interesting - I've done all other checks / replacements mentioned (other than replacing a fuel pump but have confirmed good fuel pressure - but still plan to run pressure gauge while driving if the issue comes again)...

For the drive by wire issue - other than looking for obvious connection issues, how else to you troubleshoot that issue? 
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He found it by process of elimination. Problem was he couldn’t find a new pedal assembly, had to find one at a RV recking yard.

 
This is kind of interesting.  Ive never really looked into the DBW system.  Looks like a bunch of OEM's use a dual TPS setup as a sanity check to ensure that the TB is not going nuts.  on the 8.1 the motor driver & TPS's are in one module through one connector.  @ $200 kind of an expensive gamble. 

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I cold see noise coming from the PCM on the 5-VDC reference causing a number of potential issues, which might explain why there so many folks say that "on mine it was the xxxxx".  I would think a scope on the 5V reference would be an easy way to rule out that as a problem. 

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