2003 HD1500 "reduced power mode" back to the drawing board

[ifixedit]

Hi all,

Well, here I was a few weeks ago claiming victory figuring I had finally resolved this very annoying problem and it's back with a vengeance. Here's the story. 2003 HD1500 crew cab with the 6.0 engine that just turned 200,000 miles. A few years ago ONLY when it got to around 10 F outside driving along either in town at slower speeds or on the highway it would suddenly go into "reduced power mode" and I would have to pull over, stop, shut it off for a few seconds and then restart it and it would run fine but the check engine light would be on for the next 3 restarts. It would go a day to a week and would do it again but still mainly during only colder weather. Now it's doing it just about any time but always seems to do it when I am holding the pedal steady be it on the highway, in town, in heavy traffic with the A/C, just really no rhyme or reason to it but is getting worse. If I continuously move the pedal it won't do it. The scan code is always the same which is P1516 (TAC module). Another thing I have noticed lately is it starts to surge a little when I hold the pedal steady and then it happens, it also seems to shift a bit weird or if you let off the gas the tranny goes into kind of a coast mode like the converter is unlocking and the engine idles down, even at 60 mph. It also has never gone into reduced power mode until it is fully warmed up. Other possible clues, does it in the morning only after it has sat all night, mainly when there's a lot of dew or moisture on the truck, or after is has rained, so it seems something is getting wet but I've checked everything under the hood, all plugs and so on and have even used dielectric grease on them and there has been zero evidence of anything getting wet.

What I've done: Since February 3 new throttle bodies, one aftermarket new unit and 2 re-man AC Delco's, truck runs fine from a few weeks to a few months and the problem is back again. Flushed and refilled cooling system thinking it's maybe some kind of cooling issue. Replaced pedal position switch on the pedal assembly, replaced the intake manifold gaskets and the knock sensors and it's harness. Performed the tug and wiggle test on the wires going to the throttle body and the TAC (throttle actuator control module) with the engine running, can't get it to set a code doing this. Replaced and retightened all the grounds at the back of the engine block and cylinder head including cleaning and retightening the grounds under the drivers door on the frame. Yesterday I decided to buy and replace the TAC with a new one, no luck, did it twice this morning. In the past I found wires under the power box that had been chewed on by mice and repaired them. I've pretty much exhausted all the possibilities other than a stick of dynamite under the hood and my next move is to replace the MAF as I've read if it's giving either a faulty air flow or temp reading this could set the P1516 code and I also plan to replace the throttle body harness with the pig-tail replacement.

Other things that have been done. New alternator last winter, it was doing the P1516 thing before this so I don't suspect the alternator is an issue. Battery is 2 years old and reads almost 13 volts with the engine off so it doesn't seem to be a battery issue. I checked the battery ground to the block and it is tight.

This is driving me nuts as no matter what I do it seems to only get worse!

Anything you can throw my way that I may have missed would really be appreciated as I'm out of ideas for now!

Thanks!

[ifixedit]

O.K., since I know this problem is a tough one to crack, and I've yet to have a taker here with any ideas, I'm going to throw one idea of my own out and see if it makes sense to anyone, the more I think about this the more it makes sense to me anyways, so here goes. As I said, the problem started during the winter, usually when it got to a certain temp but over time has gotten progressively worse and the temps have not seemed to matter as much. I'm beginning to think the winter temp thing is just more conincedence and there's been another underlying problem.

 

I drive the same route to and from work each day, early on when it was cooler out it would take longer for it to happen, but interesting would do it at about the same location in the drive either to work or on the way home. The past month it's been going into reduced power mode sooner and oddly within about a quarter mile of the same location every time, temps in the morning have been about the same for the past few weeks.

 

Here's why I'm starting to think this is a clue. About 6 months ago, just once I had one other code that the O2 sensor, #2, bank 2 was bad, this is the O2 sensor behind (at the back of one of the coverters). The truck kept running and there were no other issues that day. I cleared the code and it has not shown up since. Tonight while driving it the check engine light came on but no reduced power mode, haven't gotten to the auto parts store to scan it yet but I'm betting it's the O2 sensor again. What I'm starting to think is one of the coverters may be plugged or is getting there and as I've read elsewhere a plugged converter can cause the truck to go into reduced power mode and set the P1516 code which seems to set for a number of reasons just to throw the truck in protective limp mode. One time when it was really hot out this summer (high 90's) it went into reduced power mode in heavy slow traffic and I had to pull over and sit to let the truck cool down for about 10 minutes before it would start and run again leading me to now think maybe one of the converters was hotter than hell as all other times it has always started right up again and ran fine until it happened again which was sometimes right away and sometimes a few miles down the road, or the next time I drove it.

 

My theory is perhaps one of the coverters is getting too hot, the warmer it is outside the faster the coverter heats up and the sooner the problem happens, the cooler it is outside the longer it takes for the converter to get too hot and is why I said here earlier that the problem of it seeminly only doing it in the winter is just coincedence as maybe the converter was starting to get bad at that time and it has slowly gotten worse over time. The only problem with this whole idea is you would then think it would happen every time I drive it which has not happened and I would also be getting a code about the converter which I haven't gotten so far either. Only other guess I have is a faulty MAF. Other than that a stick on dynomite and a few rounds of lead under the hood is going to be the next fix!

 

How common for a vehicle with 200K on it is likely for a converter to get plugged?

 

What do you think?

[Snow White]

I do believe you have a flaky temperature sensor, the one for the computer (i think there are two). My brother had one go in his 5.7 and it buggered the shit out of the computer making it run rich to the point it would quit..... I suspect yours isn't quitting because it's working sometimes but not all the time.... when it actually completely fails (if it ever does) it'll be the most mysterious problem unless you actually had a pc to watch the sending unit function. his just happen to say i'm always hot, please add more fuel lol.

 

I could see how the problem would be worse in the winter because it takes longer to get up to closed loop operation and dumps more fuel until it finally does.

 

i.e. change the temperature sensor, clear the cats, clear the O2 codes see what happens.... sounds to me it's running rich.

[ifixedit]

Thanks for the reply, it doesn't seem to be running rich as the mileage is the usual and until it does it's thing seems to be running normal, but then again hard to tell from the drivers seat so it could be running rich. All I know is if the truck "thinks" it's -30 or +100 outside and it's actually 45 F, it's gonna throw things off. it certainly seems to be something temperature and or moisture related and perhaps my plugged cat idea is a long shot or a portion of the problem. Just before it happens it acts like the engine is getting the wrong input information, the pedal gets weird, the engine kind of bucks or surges a little and then it goes into reduced power mode.

 

This morning was another kind of damp and foggy morning, we had some frost and the truck was soaking wet when I came out and started it. For the heck of it I first performed a voltage test, the battery was @ 12.65 volts, started it and was up to 14.77 volts so there's no battery / charging issue. I then drove it and made it about a half mile down the road and "bing-bing" reduced power mode, stopped, shut it off, restarted and running fine again. I made it to the auto parts store, we scanned it, and the usual code P1516 and another for the bank 2, #2 O2 sensor which I also figured. I picked up a pig-tail connector for the harness going to the throttle body and an O2 sensor and was on my way, I will install both tomorrow and see what happens. The guy at the auto parts store thinks there's a bad or weak connection in the harness to the throttle body and the moisture is somehow affecting it or going from cold under the hood to warm is causing a connection to expand and is causing the problem, I guess we will soon find out. The very first morning drive is the test.

 

Lately this has ONLY happened when it is wet or damp out, when it's warmer and dry, or in the middle of the afternoon after I've drove it a couple times that day the thing runs like a champ, no matter how hard I try I can't get it to do it. So it seems to be affected by either a higher humidity level, temperature issue, some water or moisture getting in somewhere or I don't know.

[ifixedit]

I may finally be able to claim success! I installed the replacement pig-tail harness to the throttle body today and way back, like 6" way back from the plug the yellow wire was bad. Just by chance I cut the wire off in the exact place where it was broke inside, the giveaway was the plastic just stretched in that spot, no wire connected inside the casing in that spot at all.

 

How in the world GM let so many of these trucks go out the door with bad wires in the harness going to the throttle body is beyond me! I hope they weren't being cheap purchasing made over seas wire or some second hand surplus stuff. I mean really, how many times have you bought a spool of wire and the copper inside has breaks in it? Totally ridiculous!

 

The true test will come tomorrow morning with a drive in the conditions where it has given me trouble every day for the last week. If it acts up again the entire yellow wire from the throttle body back to the TAC is going bye-bye for a new one.

[gmtech4]

Bad bad wire lol

[ifixedit]

Bad bad wire lol

Yeah, found a bad wire and the piece of junk still does it! Drove it this morning in it's most favorable (colder, damp) condtitions to act up and sure engough didn't make it a half mile from home and the "reduced power" crap lit up. Seems over the past year the most effective thing I've done is clean the grounds and contact area on the frame under the drivers side door. So I took a look at them again today after they hadn't been touched in about 6 months and they were looking a little cruddy again, so out came the grinder to clean up the contact area on the frame, a fresh supply of dielectric grease and we'll see what happens.

 

If it does it again, it goes on Craigslist and someone gets a truck pretty reasonably priced, the buyer just has to figure out what's wrong with it! Good luck!

[ifixedit]

Update: Tonight I went back to what I had done in the past and as I mentioned earlier, going over the grounds seemed to be the most effective in solving this problem. Earlier this summer I found a site that talked about the PCM maybe not being grounded good as the problem and performing a simple voltage drop test at the PCM grounds would tell you if you had a good ground or not. I don't remember the readings I was getting at the time but as I recall they were a bit on the high (unacceptable) side, so I had removed the PCM, took one of those stupid blue crimp on connectors (lesson learned, don't use one of those stupid blue crimp on connectors) and proceeded to connect it to one of the PCM grounds and run a new ground wire up to one of the lugs on the firewall. For two months the truck ran fine, then the problem started coming back here and there, then as of this past week has been going into REP daily again, sometimes multiple times a day. Tonight I got out the multi-tester, set so I could read both millivots and volts and performed voltage drop tests checking each ground (main engine strap, PCM ground) one by one. You do this with the ignition in the "run" position but with the engine not running. Put the positive lead of the multi-tester on the negative side of the battery connection and the ground on the tester to the grounds you want to check. Doing this you should get at least some kind of reading set to millivolts (I was getting anywhere from .017 to .185 depending on which ground I tested, all good), and with it set to volts you should not get any reading or no more than .12 per the info I had read.

Well, when I tested the ground I had ran from the PCM earlier this summer I was getting nothing with the tester set to volts or milivots which told me my cheap stupid blue plastic crimp on jobbie had failed and was not performing it's grounding duties as originally planned. So, I tore into the PCM, removed the large blue plug, took off the cheapie blue crimp on deal, bared about a half inch of the wires of two of the PCM's grounds (black wires with white stripes), took the bared end of the new ground wire and tightly wrapped it around both and soldered it all so the connection is now solid and is not going anywhere. I tightly wrapped up the new connections, put the PCM back in and ran the new ground to the same locations as the grounds under the drivers side door on the frame. Now performing the voltage drop test I was again getting a reading, but nothing set to volts and set to millivolts only a very low .002 which means pretty much a perfect ground and no voltage leaking through the ground.

I took the truck out for a drive and immediately noticed much smoother running and throttle response again like in the past, no weird trans shifting issues as it had been doing the past few days, especially when letting off the gas on the highway at speed, the engine braking that was happening is gone and it once again coasts nicely, so far so good. Not gonna claim victory just yet as this thing has surprised me a few times now, so will drive it for awhile now and check back later. Keeping my fingers crossed!

[XYZ]

I relocated both of the grounds that had been running to the backs of the cylinder heads. I moved them to a couple of body bolts on either side of the truck where there was much easier access.

[ifixedit]

So far so good. This morning was one of those perfect cooler, damp mornings where it would go into REP for sure. Drove the entire 28 mile jaunt to work this morning, I was just tempting the thing to do it. It went all the way without a problem.

 

Here's the info I found a few months ago if it helps anyone else, start reading at testing PCM ground basics http://easyautodiagnostics.com/gm/4.8L-5.3L-6.0L/electronic-throttle-body-4

[Snow White]

wow, a bad wire, that is unbelievable that you found it.

[ifixedit]

Well, I'll just say the wire in question I found going to the throttle body over the weekend was very suspicious looking, the plastic was stretched and it came apart very easily where I happened to be cutting, looks like it was broke inside as many have reported finding with the yellow wire, but was likely still making contact. A little more pulling and it would have failed. Turns out didn't resolve the problem though and finding and re-grounding the faulty ground to the PCM last night was quite simple and perhaps HAS resolved the problem as it's the first time in over a week the thing ran without conking out all the way to work. Again, the darn thing has fooled me more than once though and until I drive it for a week or two trouble free will I say ifixedit!

[XYZ]

The grounds and the wire harness to the throttle body were both in need of repair, so you had two things going wrong actually, both notorious issues with these trucks.

[ifixedit]

The grounds and the wire harness to the throttle body were both in need of repair, so you had two things going wrong actually, both notorious issues with these trucks.

Yep, I've covered all the bases now, new pigtail harness to the throtle body and a solid re-do this time of the PCM ground which will not lose contact as the cheap blue plastic connector did. Interesting, even fixing the grounds on the block and at the back of the cylinder head were not enough, telling me somewhere in the harness there is an issue and did not reslove the ground problem, only a fresh wire directly from the ground wires out of the PCM to the frame seems to cure it. Too early in the game though, it made the 56 mile round trip today for the first time in awhile with no issues at all and ran perfect. The slight miss I was noticing at idle in and out of gear is gone, tranny again shifts normal, has a bit more overall power, and no surging when holding the pedal steady @ 65 MPH, all good signs but I'm not taking it to the bank just yet. Will let er' buck for a week or so now and see what happens.

[James Lang]

I thank you for the information, my truck just told me the reduce engine power code. I was looking at many parts of the normal, have to look more into the wires and ground. Thanks again.