I hope yall can help me out here:

Im driving on the freeway the other day and I step on it to pass and the truck has great power all the way up to 80-82 mph. When I hit this number I hear what sounds like an electric switch popping back and forth coming roughly from where the PCM is mounted and the truck depowers like I am hitting the governor.
I instantly became concerned and pulled off at the first station. I filled up, let her cool a bit, and got back on the road. Same scenario, same results.
This time, I pulled to the right lane and dropped down to about 70. Everything was fine again. Went through this about 5 times or so, then I was home.
She has great power through all the gears, just wont get passed 80.
EGT's pre-turbo were right at 1000, boost was 15-17 PSI. Didnt pay very close attention to the Tach, but generally should have been around 22-2400 RPM.
I added gauges recently in hopes to get a tuner soon, but no other electrical has been done.
Other than that, in the last 6 months, I shimmed the fuel pressure and unplugged the wastegate actuator. No other mods.

What do yall think, is the PCM on its last leg? Will a Tuner take care of my problems, or will I need a new PCM before?

Thanks again for taking the time to read this.

 

Sounds like the speed limiter, thats what it does. Most trucks are set at 98mph. Try unplugging both batteries over night.

 

Thank you, I will give that a shot.
In the past it has not kicked in until 95 or so.
Definitely scared the crud out of me when it happened at 80.

 

something don't sound right at all, so here's a bump to the top.

 

Well, I disconnected the batteries overnight and it fixed the problem temporarily.
Out on the highway again this weekend. Had the same problem, the first time it happened at 75 mph, then again shortly there after at 65 mph.
I pulled off the highway and turned the truck off. After coming back to it half an hour later the issue has again resolved itself. This was a week ago, no problems since.
Any ideas on what could be causing this?

(One thing I did notice this time is that I was getting a slight vacuum sound in the cab when this happens. Everything is tight on the turbo plumbing, Could this be turbo flutter? It almost feels like its defueling. I apologize for my ignorance, I have never experienced turbo flutter.)

 

Just a shot in the dark but maybe this has something to do with it. It is set to dump excess pressure from the turbo to prevent turbo damage.

 

I don't think its the problem in this situation Jim. Many of us (me included) run with it unplugged with no issues.

 

Ok like I said shot in the dark

 

There are some relays mounted behind the dash, the fuel pump relay being one of them.
Something is making it cycle, like possibly a loose ground wire or a poor connection somewhere.
It might take some real work trying to find the culprit.

 

I replaced the fuel pump about 2 years ago, I will check all of the connections to make sure something hasnt worked loose. Thank you for the heads up on that.

Any reason it would just occur in 6th gear at high speed?

 

Sorry, not a clue. I have a six speed myself and the only electrical connection that I can remember is the reverse light switch.
That obviously has nothing to do with being in 6th gear.

 

Ok, i have a little more to go on now hopefully:
I noticed an intermittant power loss at lower speads the other day.
It comes and goes, and does not last long.
I am wondering if this may be the issue others have had with the wire harness under the valve covers.
if this was coming lose and knocking out a cylinder or two at a time, would this cause the same symptoms?

 

meant to add, the noises are gone as far as I can tell.

 

I don't think it would cause a clicking under the dash board, but it will certainly cause a loss of power.

 

I have never had this happen to me in my 6-speed. I would try checking under the valve covers