intermittent loss of engine power and throttle control... no codes set

Hello.  I am the new owner of a 2006 Avalanche (1500, LS, 4x4).  I have been experiencing some strange behavior with the truck.  I have searched this site and the web, but have not found anything exactly like what i have been seeing.


My symptoms seem similar to what i would imagine the Reduced Engine Power or LIMP mode would be... with a few caveats.

1. i do NOT get the Reduced Engine Power message on the DIC
2. i do not get any codes stored after this happens
3. it will fix itself after a few minutes, without the need to restart the truck first
4. the door locks seem to activate repeatedly if i try to accelerate while in this mode (??)

I will try to describe what is happening.  This has been occurring a couple of times per week, the rest of the time, the truck drives great.  While driving, i will suddenly not be able to accelerate past 2k RPM.  When the engine hits 2k, the engine light will flash once, the doors will lock themselves, the speedometer will fall off (then recover).  if i try to accelerate through 2k, there is just no engine or throttle response at all.  The truck will stay in this mode for a few minutes before coming out of it all on its own (sometimes longer than others).  During this time, i will notice the A/C gets warm and the cruise control will not activate.  I have also noticed that the Average Fuel economy gauge will drop drastically each time this occurs... like from 16 down to 8 or 9 after a single instance.  The fuel economy on the truck seems to be normal (13-15 mpg at fill-ups), but the gauge definitely thinks otherwise.

the first time this occurred, i was cruising on the highway.... so i was able to continue driving like this for 15 miles at about 60 MPH, other times, i have been on local roads.  if there are hills, then it is nearly impossible to continue on.

i got excited to find that the braided ground strap from the engine to the firewall was very loose... but i have tightened that up and the issue has still continues to occur.

since this is so intermittent, i fear that a trip to the dealer will be useless and expensive since it likely wont happen when they drive it.  Any suggestions on where to start looking?  any ideas on what else i can try to capture the next time this happens to try and narrow this down more?

Two things come to mind: check for vacuum leaks, and check for proper fuel pressure. Fuel pressure is not monitored by the PCM, so if the fuel pump is failing, it won't set codes. Of course, usually, they just stop one day, but I have had them fail intermittently before. The fuel pressure regulator is adjusted by engine vacuum, so a vacuum leak would affect fuel pressure.
It does sound like lack of sufficient fuel flow to the engine.

Thanks,  I will check into those things.  I agree that the behavior of what the engine does seems like fuel pressure.... but what would cause the door locks to continually "lock" themselves... the AC to shut down, cruise control to shut off, etc...  those additional symptoms had me leaning toward something more electronic related.

I agree with you that with the power loss happening at the same time as these electronic issues it does seem to indicate an electrical problem.

You mentioned the speedometer falling off and then recovering.  Does it recover before the other problems go away?

yes.  i dont think i explained that part of it very well in my original post.  The speedometer drops out right as i hit 2k rpm... then is immediately fine again unless i accelerate up to 2k again.  the time that this happened on the freeway, the speedometer was fine for the entire 15 mile drive... so long as i kept it under 2k rpm.

i grabbed a quick video when it happened the other day... not sure if this will share properly... but give it a try... it shows what the dashboard does when this happens.  the clicking sound you hear is the doors locking.

https://photos.app.goo.gl/iXmbS4f99ey2GANa6

Very likely a bad ground. Check your battery cable for corrosion both at the battery and where it connects to the block. Also check the ground strap from the drivers side of the backside of the block to the firewall. Your symptoms are possibly related to a bad ground under the drivers seat at the frame.

You will likely need to pull the ground cables at the different locations and clean them up as well as the place connected to.

I suspect your power issue is the fuel pump loosing some power due to a poor ground and should be easy to see with a pressure gauge.

I believe this thread will be helpful:

http://chevyavalanchefanclub.com/cafcna/index.php/topic,103136.0.html

I noticed in your video that beyond what you described earlier the gear indicator is also rapidly jumping from D towards P and then back.  I suspect you've got a problem with either the pressure switch in the transmission or the Park Neutral Safety Switch on the side of the transmission.  If the computer thinks you've put the truck into reverse when driving down the road it's going into abuse mode and will pull power.

The good news is that both of these switches are easily replaced and aren't very expensive parts.

hmm... good catch.  Maybe that is also why the doors lock each time?? I think they are set to lock when the car goes into gear.... will search for some threads on those components as well.

yes, I suspect that going in and out of park, at least the computer thinking it is, is causing the doors to lock and unlock.

Ok. I started by checking the grounds that are located just under the drivers door. They looked good, but I went ahead and cleaned them up anyway. The truck is very rust-free since it had been in Florida up until a year or 2 ago. That did no good and the problems continued.

Then I saw some posts and videos talking about another ground that can be seen just to the passenger side of the engine block, under the hood. It is a single black with yellow wire that is outside of any wire loom. I guess that it usually runs back behind the engine to ground somewhere. However, mine was run to a stud on the engine itself (previous owner??). I tried to wiggle that wire and the ring connector just snapped off. Immediately, the engine stopped and the dash lit up with messages about Reduced Power, Service Stabilitrack,etc... I could not restart the truck until I reconnected that ground.

I attached another 18? of wire and extended the wire over to ground with the large braided ground strap on the firewall. So far, the issues have not returned. It has been about 3 days. I will give it about a week to be more confident.

I have posted some pics below

https://photos.app.goo.gl/wn5qC5g9A175eTDJ6