Limp mode

kdannyk

Well-Known Member
I have a 2005 trailblazer LT. 4.6 motor. One day it started running really bad. We had some -50 days and it ran fine and I away let it idle for 10 min before taking off and was really easy on it. I drove it to work last Thursday had no issue. When i started it to go home notice it wasn't running right put into gear and it would go into limp mode and die. The dash said low oil pressure I did have a full tank of gas and did put heat in 3 days before this started happening. I notice if I let it idle i have liquid coming out of the exhaust. i think it's unburnt fuel but not sure. Not sure where to began with trouble shooting. I check all fluids and they all look good. If i'm in neutral and go above 2000 rpm then it will go into limp mode and die.
 

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Wooluf1952

Well-Known Member
The liquid coming out of the tailpipe is water. All engines do that.
 
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kdannyk

Well-Known Member
I have not. Need to get a code reader. If the liquid coming out is water why is it so black? It's never been black before.
 

Capote

Platinum Supporter
I have not. Need to get a code reader. If the liquid coming out is water why is it so black? It's never been black before.
It's just soot from the exhaust pipe is all.
 

Wooluf1952

Well-Known Member
If the engine was running so rich that it was throwing out raw gas, you would get a CEL and the cat would probably be melted. :twocents:
 
There's 2 main systems or parts that will throw it into limp mode aka reduced engine power (orange down arrow) this is generally caused by the fan not working properly. In the under-hood fuse box you will see 2 black relays, the one towards the front of the vehicle operates the fan. You can swap those 2 relays to see if the symptoms change. The other black relay (these are technically called modules) operates the headlights. You should be able to tell if that is problematic if your headlights start acting up or if the engine starts running correctly. It is also quite possible for the fan clutch to be bad, but I've found most of the time the module to be at fault. The other thing that can cause REP is the throttle correlation. Sometimes the throttle body goes bad, generally TPS which is built into it, -or- the accelerator pedal assembly goes bad. The bushings in the accelerator pedal will wear out and if you push it towards the side it will temporarily loose connection and cause these problems. If by chance that is the issue, the accelerator pedal assembly must be replaced with oem parts, you're better off with a used gm part than a new aftermarket part here.
 

Chickenhawk

Well-Known Member
Was your check engine light flashing at the time? If so, it may indicate a misfire condition, sometimes caused by water from snow melting and pooling on coil pack #4. You really need to check and report the codes.
 
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kdannyk

Well-Known Member
I scan the truck for codes and a code P0068
Manifold Absolute pressure/mass air flow - throttle position correlation
 
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Mooseman

Moderator
The PCM thinks your throttle doesn't match what the MAF is reporting in airflow. Give your MAF a cleaning with a cleaner made specifically for it and clear the codes. Also check for leaks along the intake. Pay particular attention to the PCV vent hose at the bottom of the resonator box. Check for vacuum leaks.
 

DocBrown

Well-Known Member
Had this happen to my Sierra a few years ago, in sub-zero temps. Check your battery voltage. After sitting for at least 10 minutes the voltage should read 12.6 or higher. If it's lower you need a new battery. I put a new battery in my Sierra and my problems went away.

I think at 12.2 volts it's only operating at 25% or something like that. Wish I could find the chart. Anyway if the computer can't talk to the sensors because they don't have enough power, you'll get a flashing check engine light, and limp mode, because it thinks there is a serious problem. And the cold weather we had is hard on vehicles no matter how well they are maintained. If something isn't quite right, it will reveal itself in the extreme cold.
 
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kdannyk

Well-Known Member
The PCM thinks your throttle doesn't match what the MAF is reporting in airflow. Give your MAF a cleaning with a cleaner made specifically for it and clear the codes. Also check for leaks along the intake. Pay particular attention to the PCV vent hose at the bottom of the resonator box. Check for vacuum leaks.
Is there a write up some where on how to clean the MAF sensor?
 

TequilaWarrior

Well-Known Member
Is there a write up some where on how to clean the MAF sensor?
Every video / article I found is exceedingly generic and not specific to our engines.
It's a pretty simple part - carefully remove it and spray it down. If you're using the right cleaner you really can't use too much - just don't touch the sensor with anything (like a screwdriver / wrench). Let it dry thoroughly before reinstalling.
Since you're cleaning the MAF, you might as well clean your throttle body while you're at it. Also, wouldn't hurt to disconnect the battery cable while you do it - causing a TB relearn. There are walkthroughs available on this site for cleaning the TB and pulling the battery cable is part of doing that correctly.
Also, heed @Mooseman 's advice - check for vacuum leaks, especially check the vac line connected to the resonator. I believe there is a vac line that attaches at the TB, check that one too. When cleaning your TB, make sure you clean all the ports on it. I had one clogged with sludge but I don't remember if it was in my 4.2 or not. Your TB being exceedingly dirty could throw that code, BTW.
 
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kdannyk

Well-Known Member
Thanks for all the good info. I cleaned the MAF sensor and the throttle body but it didn't help. I did take the battery to part store to get tested and they confirmed that the battery is shot. I will get a new battery soon and see if that helps.
 
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kdannyk

Well-Known Member
I just got a new battery installed. Still having the same issue. I let the truck run for 10 mins and as soon as I gave the truck some gas it died.
 

budwich

Well-Known Member
what kind of reader are you using... some cheap ones only provide the "highest/latest" code and not all codes
 
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kdannyk

Well-Known Member
i'm not sure what brand of code reader it was. It was one from the part store. I believe it was Bosh. I work with GPS tracking company and I stuck one of your gps trackers on the canbus port and it got the same error code that the code reader got.

I attached the code down bellow.
 

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Mooseman

Moderator
Well, the MAF or MAP may be bad. Start with the MAP since it's cheaper to replace, then the MAF. I know it's parts shotgunning but without a high end diagnostic tool or something to monitor and analyze the data, not much else that can be done.
 

budwich

Well-Known Member
get a meter and do some measurements at the connectors for voltage, resistance and grounds for the MAF and TPS. They might indicate where more focus is needed.
 
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kdannyk

Well-Known Member
is there a difference between a maf and map? I thought I cleaned the maf but it was the Intake Air Temp sensor that I cleaned. I don't see mad at all behind the air box.
 

budwich

Well-Known Member
the MAF may be an integrated IAT sensor depending on your year. It is on the snorkel between the filter and intake / throttle body. Its a "long flat plate" inserted in the passage. The MAP is on the back side of the intake near the firewall that has a nipple that inserts into intake cavity.

cleaning the MAF usually means cleaning the "hot wire" resistor with a spray and hoping that it helps if the resistor is coated with any kind of "stuff".... not much else.

both units have some what simple meter tests to check some of the basics around the sensors. Those would be useful... along with checking wiring / contacts.
 
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kdannyk

Well-Known Member
I installed a new battery, clean the throttle body, cleaned he map and the air temp sensor and installed a new air filter. Trucks seems to idling really good and running better then ever but i'm still getting the P0068 code. I did have the battery disconnected and the code seemed to reflag. wondering if it needs to actually be cleared by a scan tool or if it needs time to clear on it's own.
 

Mooseman

Moderator
Just in case, unplug the cooling fan. It can sometimes go bad and throw the 5V reference out. If that doesn't clear it up, then take your pick of either the throttle body or the pedal sensor to replace first. For the throttle body, grab a used one from a junker.
 

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