View Full Version : Limp yo-self home, N*!

01-14-04, 06:23 PM
Ok , so the Northstar V8 has a limp home mode, the question is... has anyone encountered this? Could you explain the details on what it feels like/ what happens when it goes into limp home mode? Any lights come on or does it say something to the effect of limp mode in the DIC?

01-14-04, 09:20 PM
I experienced it in my wife's aurora when the water pump belt broke. The temp went up to about 275 and then the traction control light came on. There were bells and a change oil now message was displayed. The power dropped significantly and the engine had vibration.

After stopping a few times worring about the temp, I drove 5 miles limping home. Not fun but I gets you there.

01-15-04, 10:59 AM
The loss of coolant "limp home" mode is automatically initiated by the PCM if it senses extraordinarily high coolant temps based on input from the coolant sensor in the cylinder head.

The coolant temp sensor location was developed such that it senses coolant temp circulating thru the head when it is "wet" or covered by coolant. Should the coolant level fall even slightly in the engine the sensor will be uncovered as it is near the highest point in the head coolant jacket. If uncovered, or "dry", the sensor is designed and positioned such that it will sense the rapidly rising temperature of the exhaust port wall, thus responding to high "high metal temps" as well.

Before this happens the low coolant warning should be on and two different coolant temp warnings should be displayed.

When the PCM senses that the temp sensor is so hot that it has to be 'dry" and uncovered it switches in to the "limp home" mode to keep the engine running without damage so that you can drive to a safe place.

The limp home system works by turning the fuel injectors off to four cylinders allowing them to stop running but they still pump air thru the cylinders to cool them down. Those four cylinders will be shut down for a predetermined number of cylinder firing events while the other four provide power. After the predetermined interval those four cylinders start firing as the injectors are reactivated and the other four cylinders are turned off by disabling the injectors so that they can cool. The two groups of cylinders shut down in sequence are 1-7-4-6 and 2-3-5-8. The cylinder group operation is regulated not by a timer but actually by counting the number of firing events each group sees. This was much more effective at protecting the engine than a simple timer. In limp home mode you will notice that the two groups switch much more slowly at idle and then rapidly at higher speeds because of this.

There are other things done with the idle speed, fueling, spark advance, etc. in concert with the cylinder cut out but the main function that makes the limp home system work is the cylinder cut out. Remember, the valves are still operating so the cylinder still pumps air thru it...making it an internally air cooled engine when in limp home mode. This is often confused with the DOD (displacement on demand) system upcoming on GM engines and the older V-8-6-4 system on the 81 Cadillacs. They are not the same as the Northstar limp home mode. In DOD the valves are actually disabled so that they stay closed to disable the cylinder. Just turning off the fuel to disable a cylinder causes huge pumping losses that are immediately evident trying to drive the car. With only 4 cylinders running and 4 dead but still pumping the engine has the net output of about 1.5 cylinders as much of the power goes into the pumping loss of the dead 4 cylinders. In DOD, the gases trapped in the cylinder act as an air spring returning the power used to compress them to he piston on the down stroke.

If the engine needs to implement the limp home mode it can safely go 50 miles at 50 MPH with no damage. This has been tested and documented by several unbiased (actually they were biased...they figured it would not work and tried to prove it...and ended up proving that it DOES work very well)

If limp home mode is ever used the engine oil should be changed immediately after when the cooling problem is being fixed. The oil gets extremely hot when the engine reverts to limp home mode as the oil takes over part of the task of cooling the engine.

Hope this helps.

01-15-04, 11:07 AM
Thank you, bbobynski. This is the best Limp Home Mode explanation I've ever seen..

01-15-04, 05:55 PM
Hence the friendly "Change Oil Now" message...