I've been having issues with my Sierra Denali for week or so. It is a bone stock with an lq4/4l65e. it progressively seems to be getting worse. The issues are:

Low power at high RPM. As soon as it hits about 4,000 RPM's it acts like it hits a wall, there's no power at all, it sounds like its misfiring bad or pulling timing. This started out of the blue one day. One second it ran fine, the next time it hit 4,000 RPM's it ran like crap. Spark plugs and wires are fine, engine runs smooth as butter at lower RPM's. It may be a little unresponsive to quick throttle inputs, but I've never driven another 99-02 truck with the corvette style DBW throttle so that may just be normal?

Softer than normal shifts, the 4l65e has a shift kit and only about 10k miles on it. It always shifts really nice and firm, but since I started having this issues it's been shifting softer than normal. Not anything that worries me, the trans may just be breaking in, but it is definitely noticable.

Alternator Stops charging at high RPM: This is actually a new issue i've noticed, but since I replaced the alternator the alternator quits charging at around 4,000 RPM's, like someone flipped a switch. I *think* it may have something to do with the loss of power? It goes from 13.9 volts to 12.6. The alternator I installed is used, it has great voltage at lower RPM's. My old alternator was a chinese copy of an AD244. The used one I installed is a Delphi AD244 that worked totally fine on the vehicle I removed it from, although I never revved it to 4,000.

Hard starting, worse when cold. The truck doesn't start right after sitting for any amount of time. It will fire once or twice and then die, cranking speed is fine but you have to crank for while. below -20 the truck almost never starts. hot starts are totally fine.

The only code the truck ever throws is p0420, and its hit and miss. If you unhook the battery it may not come back for months. One time it threw a code for bank 1 sensor 2 low voltage, not sure if that would cause any of these issues.

So far I have replaced the MAF sensor, and cleaned the throttle body. Cleaning the throttle body seems to have helped the starting issue a little bit, but it's still not right. I will be testing fuel pressure as soon as I have the opportunity. Could the issue just be a plugged cat, or bad fuel pump? Or could it be somehow related to the alternator?

 

 

Just rolled over 311,000 today, AFAIK, the only thing that's been replaced is the trans and alternator. that's why I was thinking cats or fuel pump.

 

The springs for the alternator's brushes are bad. This causes the brushes to bounce and kills output. I will bet that your power loss and bad alternator are the same problem. I learned about this with my previous injectors. The data for them was incomplete in the areas of lower voltage. My alternator went bad and I installed a rebuilt one. I started having a high rpm cutout, and when I logged a run, I found the engine went super lean at 5000 or so rpm. This coincided with a voltage drop at exactly the same rpm. The re-builder's response was that no one else ever complained. He was kind enough to order a new alternator minus the cost I paid him for the rebuilt unit. I won't go with rebuilds anymore if a bad set of brushes might melt my engine. You should be ok with a rebuilt with a stock engine and tune.

 

Stock engine and tune for now. Working on it very slowly, but I'd like to go 80e and turbo eventually.
I've never had an alternator failure like this. Any specific brand of alternator that is recommended? if not I'll just order a Delphi and hope to get another 300k out of the alternator.

 

I thought Bosch was a good brand when I bought it, I am not so sure now. This alternator has been trouble free, though. Go with the Delphi

 

 

No, That's on the list as well as getting the cats hollowed out. The truck has good fuel pressure, at idle its roughly 50 psi and it responds to revving well, but with a load on the engine it's a gutless turd. As of now it won't rev over 4500 AT ALL with a load on it. It acts as though you've hit the rev limiter.

 

Today I changed out the fuel filter and alternator and replaced some loose hoseclamps I found on the chinese "cold air" intake. They stripped out, not sure how. Last time I had it off they tightened up fine.

One of those things has completely fixed the issue. Not sure which one. The fuel filter isn't original, but the fuel that drained out wasn't pretty.... Feels good to hear her hit the limiter without complaining again. Hopefully shes good for another 311,000 miles!

 

Thanks for letting us know what you found! Good info.