Reduced Engine Power...

[Blackstar]

I realize this has been covered numerous times and I have read several posts concerning this. My problem seems, to me, a little different. Got home from work last evening and my '02 Envoy SLT was working perfectly. Started it up this morning and it sounded like my fan clutch had locked up. REP light and Check Engine light are on.

I replaced this fan clutch probably 5 or 6 years ago and it has worked fine ever since. Don`t remember the brand name. This time it`s acting a little funny...

Drive to work is all down hill for about 1.5 miles. On the way home tonight, on a level road it seemed like it didn`t want to shift out of second gear. I stopped to let some traffic pass and started out again with the same result. Goosed the throttle and it the calmed down a little and changed into 3rd at about 30 mph. Got up to about 40 and still didn`t shift into 4th. Engine revving at about 2500 at 30-35 mph.

Checked transmission fluid and it is at proper level. Truck has about 130,000 miles (235,000 km).

Cooling fan is very loud and when I tried to stop it with a rag I couldn`t stop it. Clutch is probably shot but why would it run in a lower gear and rev way to high.

I have a request out to a Buddy to borrow his code reader so I will get back ASAP with any logged codes.

Thank you for any halp and advice.

Glenn

 

[xavierny25]

Getting your code read is going to give us something more to go off.

Have you cleaned or checked your throttle body? The butterfly might be sticking do to build up on it. That's one thing to check.

Having that fan running at high speed is only going to keep the engine temp low and the pcm is going to just add extra fuel to burn and eventually clog up your cat.

 

[Blackstar]

I`m working another 12 hour shift tomorrow but hopefully going to have the code reader then. Hopefully will get to it Monday morning. Curious, would the fan clutch cause this other problem. This just happened this morning at start up. No symptoms beforehand.

 

[northcreek]

You know that your fan clutch is shot, so I would change that before I get into it too deep.

 

[Mounce]

Sounds like basic actions of REP as far as speed and shifting goes, not sure about fan though. It's in the name, it's going to greatly reduce any input and action.

Codes will tell the tale if present.

 

[mrrsm]

Since you probably do not have access to a Tech2 Hand Held Scanner to try and Command the EVCF to shut off and on ...on purpose... Try this Alternative Method instead:

First, Unplug the EVCF at the EVCF Connector on the Fan Shroud. Next... Run the Engine up to 2,500 RPM for only (2) Two Minutes.

If the EVCF Lock-Up mechanism disengages as temporary malfunction as a result of this action and then behaves itself after that... at least it will buy you some time while you back-order a replacement EVCF Unit. If you end up having to finally re-do this R&R as suggested by @northcreek... seek out @Matt 's Updated Excellent How-To perform the job of loosening the EVC "Jesus Nut" using a short length of Linked Chain attached to one of the Water Pump Black Serpentine Carrier Pulley Bolts while the other length of the chain loops through and is locked to the Driver Side Upper Front Engine Hoist. Its EZ-PZ after that.

If this repair action does not solve the REP problem... then look to your Throttle Body next, (as in ...FUBAR) along with the Throttle Body Wiring Harness (as in ...Hidden Broken Wire(s)) AND the TB Electrical Connector (as in ...Unplug-Spray-Out with CRC Electrical Connector Spray-Re-Plug) for the likely suspect(s).

You should also include your Accelerator Pedal Sensor (as in ...this is a Dual Signal Sensor with differential High and Low Voltage Limits that if Out Of Range... will make the PCM go Crazy if the ACC is FUBARed and hit the REP Panic Button as a result) while you are at it for the other likely culprit on this REP Problem.

 

[Mooseman]

The fan clutch has been known to short internally and screw up the 5V reference for the throttle body. hence the REP. Unplug the fan and see what happens. Either way, sounds like your clutch is done but try also pulling the fan relay as it may also be shorted and keeping the fan locked.

 

[Blackstar]

Thank you for all the suggestions. Getting ready to head out to work. I will have a look at some of this if I get a chance today during my breaks.

Again, thanks. This is a great forum.

Glenn

 

[Blackstar]

Well. When I started the Envoy up this morning to go to work everything was back to normal, except the service engine light was on. Started to drive away and got about 50 feet when the Reduced engine power light came back on. The cooling fan was back to normal, spinning quietly.

While at work today I had a chance to read up a bit from my repair manual and checked all the power connectors that could be causing my problems. I also swapped out 2 relays that are the same. The PCM and the HDM. Started her up and everything is as it should be. Drove around the block and all is well. I am now even able to stop the fan with a rag, like in a video I watched. Service engine light is still on and I will have the code reader tomorrow to check which codes are set.

I can understand why maybe a dirty connector or bad relay could cause Reduced Power but how could this have affected the cooling fan?

I will check back in tomorrow after I check for codes.

Thanks...

Glenn

 

[Mounce]

You swapped the black relays? If so the Hdm relay is headlight driver module, the other relay is actually the fan clutch relay.

 

[Blackstar]

Yes, I realize that. Didn't get a chance to check the headlights yet. But could any of this cause the clutch fan to lock up?

 

[Mounce]

Yeah if relay went bad and stuck on. Or computer noticed erratic loss of control of fan speed and commanded full lock, not sure how it handles fan relay failure, it's pretty rare.

 

[Blackstar]

Good day to you all. Well, this morning I went out to start the Envoy to go fetch the code reader and the Reduced Power light was on again. Took the Wife's Acadia to retrieve reader.

Hooked the reader up and got five stored codes. No pending codes.
P0452- evap system,
P1221- fuel pump secondary circuit,
P1271- Accel pedal sensor
P1635- 5 volt reference circuit
P1481- Cooling fan speed.

I don't know if any of these codes were old and maybe not related to this current problem. So, I fired her up and drove around the block a couple times and she ran great, didn't set any codes.

Is there a known problem with these older vehicles concerning wiring and which specific connectors I could check?

Do these codes show anything wrong with a sensor that could act up at times and work perfect at others?

Looking forward to replies... because I'm lost right now, as to where to look to remedy this...

Thanks Guys...

Glenn

 

[mrrsm]

Start with this ACC Sensor Problem… as IT may have started this PXXX cascade and left the TB and FPS reacting to commands from the PCM to cause the Reduced Engine Power situation. This is an Inexpensive Fix … BUT also a REAL PITA to perform:

P1271 = Accelerator Pedal Sensor (How to R&R the ACC Sensor/Pedal)

THIS Video demonstrates HOW and WHY the ACC Pedal Sensor Fails:

P1221 = Reduced Engine Power (Perform THIS Expensive R&R Only if Absolutely Necessary!)

P0452 = Fuel Pressure Sensor

P1635 = 5 Volt Reference Circuit

This Video will explain what @Mooseman addressed regarding the necessary PCM constant 5 Volt Reference Signal and how certain Module Failures (EVCF) can affect other Modules (TB) on the SUV OBD2 Network. Try pulling off the Three Large Electrical Connectors from the PCM and spray them out well with CRC Electric Circuit Solvent (NOT Brake Cleaner); being certain to first re-align their Green Metripack Rubber Grommets inside of the Connectors and then re-attach them to the PCM… but not with too much force.

P1481 = EVCF Cooling Fan Circuit (Scannner Danner Diagnoses it and solves the issue in this Video...Sort of)

https://www.youtube.com/watch?v=EEJdurlvlyA

 

[Mooseman]

The fan clutch has been known to short internally and screw up the 5V reference for the throttle body. hence the REP. Unplug the fan and see what happens. Either way, sounds like your clutch is done but try also pulling the fan relay as it may also be shorted and keeping the fan locked.

Have you tried the above? It is a known and documented issue with the fan clutch sometimes when it fails. You can run it with the fan unplugged, especially when it's cool out and not using the A/C. Just keep an eye on the temps.

 

[Blackstar]

Haven't tried any of this yet because of my 12 hour work shift. Like I posted today, the fan only locked up that one time. I have a lot of options here. I think on my next day off I will look into the Accelerator sensors.

 

[Mooseman]

I hear ya on the shifts. I work 10 hour day and night shifts. In fact, just finished this morning.

 

[Blackstar]

We used to run 12 hr shifts, day and night but this winter they cut out the back shift. Man, I could not get used to 12 hour back shifts. Tired all the time, no energy. Now we work 7am to 7pm 7 days a week so I only get weekends every now and then. The weather isn't helping me with the Envoy either. It's been either rain or snow with near freezing temps and windy and we're a month into spring. Nice today but not feeling well after splitting a bunch of firewood... still cold at nights.

 

[Eric04]

Brother, try unplugging the fan first. I know it doesn't seem logical that it would be connected to the other issues but when it goes toward the grave it causes havoc within the 5v reference system.

 

[Blackstar]

Will do Eric. Thank you.

 

[Blackstar]

So, I was back in the Envoy this morning to go to work. Busy couple days so didn't get a lot of time for checking problems. Started it up and got the REP light right away. Got to work and out of curiosity, I shut it off and restarted and REP light was out but Service Engine light was still on. Ran perfect on the way home.

Checked for codes and only one was set, none pending. P1635- 5 volt reference circuit. Checked back on MRRSM's post and will try cleaning the connections on the PCM and then hope for the best. Thank you MRRSM!

This Video will explain what @Mooseman addressed regarding the necessary PCM constant 5 Volt Reference Signal and how certain Module Failures (EVCF) can affect other Modules (TB) on the SUV OBD2 Network. Try pulling off the Three Large Electrical Connectors from the PCM and spray them out well with CRC Electric Circuit Solvent (NOT Brake Cleaner); being certain to first re-align their Green Metripack Rubber Grommets inside of the Connectors and then re-attach them to the PCM… but not with too much force.

Glenn...

 

[Mooseman]

DID YOU PULL THE FAN CONNECTOR???

 

[Blackstar]

Haven't had the chance to get at that yet Off for the next 3 days (after todays shift).

 

[Blackstar]

Update. It's been raining steady here for a few days but I did get a break in the weather and checked all the electrical connections under the hood.(all but the accelerator sensor). All looked perfectly clean. Ran well with no codes.

Went out this morning (it's still raining) and when I started it up I had the "Check Engine" and "REP" again. Shut it off, unplugged the fan electrical connector and there was no change. Had to go into work for a few minutes and after shutting it down and restarting after a few minutes it started up normal with just the "Check Engine " light on. After a couple minutes that light went out also.

When I was checking the electrical connectors the other day I noticed that the coolant level was low. Haven't had a chance to pick any up yet but am now wondering if a low coolant level could set some kind of domino effect and set codes for the "5 V reference"? Either that or could it be cold, damp or wet weather causing a short somewhere?

 

[Mooseman]

No. Completely unrelated. You'll have to grab the manuals at look at the schematic. Must be either a bad sensor or something in the chain.

 

[Blackstar]

Update... Just after my last post, Apr 29, I was on break at work, nobody around, so I got a jug of distilled water (had no anti freeze) and topped up the coolant... after topping up the rad and reservoir and running the Envoy up to temp it took almost a gallon of water.

Coincidence?? I haven't had a problem since then. No codes, no REP, running like a champ. I didn't post in the last few weeks because I didn't want to jinx my good luck.

Thanks for all the advice and ideas...

Glenn

 

[mrrsm]

Some more proof and emphasis can be seen below as suggested by @Mooseman back in Post #15 for us to be concerned about the behavior of the EVFC ...or...EVCF.... if you prefer... in situations where the 5 Volt DC Low Reference Circuits on a wide variety of Modules can rear its Ugly Head. The Car Doctor updated his YouTube Channel only a month ago with THIS interesting "Reduced Power" Case Study:

The Car Doctor’s List shows a Whole SLEW of Codes ‘clustering’ on a 2003 Trailblazer disabled by a “Reduced Power Mode” Problem:

U1000 - Loss of Class 2 Network Communication
P0452 – EVAP Pressure Sensor Signal Low/Short
P1221 – TPS Performance – Fuel Pump Circuit
P1271 – APP Sensor 1-2 Correlation Error
P1635 – 5 Volt Low Reference Circuit “A”
P1481 – Cooling Fan Speed Sensor Circuit (GROUNDS OUT WHEN SHORTED)

NOTE:

The (CKP) Crank Position Sensor ) ALSO Provides a 5 Volt DC Reference Voltage back to the EVAP Sensor and a Shorted APP Sensor can also affect the 5 Volt reference in the Vehicle as well:

Chevrolet Trailblazer GMC Envoy Reduced Power Mode Problem:

I’m not certain that the “Longer Bolt” Method being suggested here involves performing any less work during the Fan Assembly R&R of the Fan Clutch… but as we all learned from @Matt 's "Link Chain R&R Method" many years ago... its nice to see a ‘New Wrinkle’ like Banging on the A/C Fan Shroud while measuring the 5 Volt DC Reference on the GRAY Han Harness Connector while watching the DVOM -DMM Readings Bouncing Up & Down ...as being thrown at an ‘Old Problem’ with these EV Fan Clutch Issues ...From Time To Time… :>)