Limp Home Mode

[scott y]

With regards to the limp home mode, which I have never seen or witnessed in action, just how effective is it? You may read in the owners manual what it will do, but I'd really like to hear some input from those that have been there. Don't get me wrong I've no desire to be there I just find it interesting

[Cadillac Jim]

I've never seen it or used it, other than a quick snatch with the videocasette that came with the car, the owner's manual, and the FSM - and a Car & Driver road test. What I do know about it is

• If you use it for less than 50 miles, it won't void the warranty.
• It's intended to get you and the car to safety when the radiator is punctured or other total coolant loss occurs.
• It works by cutting off four to six cylinders by not firing the fuel injectors, and it staggers which ones fire to control heating over the eight cylinders to prevent overheating of any one of them. The pumping of air by the cylinders that don't fire helps to cool them.

The Car & Driver people of 1997 were quite enthuisastic about the Northstar and, combined with the chutspah they were known for at the time and the fact that nothing like the limp-home mode had ever before been seen, they drained the water out of their GM loaner car and ran 0-60 mph and quarter-mile times on the limp-home mode. I don't recall the times, but I believe that they were reasonably respectable under the circumstances. Their verdict, from memory:

• The car has about 50 hp in the limp-home mode.
• There was no perceptible change in the drive train or performance once the coolant was replaced.

All that said, it's there for you if needed but I personally don't recommend that you use it so long as you have a cell phone and AAA. It's a last resort intended for use where personal safety is a consideration, such as a blown radiator on the Bay Bridge or when threatened with a carjacking or other safety issue. It's right up there with driving with one or more flat tires, except that you can guarantee ruining a wheel and possible undercarriage damage by driving on flat tires, and the limp-home mode apparently won't damage the car if not used excessively.

[joeb]

i wonder what inputs the computer needs to go into limp mode? low coolant level, weird temp readings? how does computer detect coolant level? if i unplugged level sensor, would car go into limp mode? there is only 1 level sensor? so many folks here say they have headgasket issues and overheat motor. makes me wonder why computer does not go into limp mode to prevent overheating?

[scott y]

Those are all good questions joeb, thanks for that write up Jim.

[Texas Jim]

i wonder what inputs the computer needs to go into limp mode? low coolant level, weird temp readings? how does computer detect coolant level? if i unplugged level sensor, would car go into limp mode? there is only 1 level sensor? so many folks here say they have headgasket issues and overheat motor.

makes me wonder why computer does not go into limp mode to prevent overheating?

I have wondered that for a long time...

I wish I knew the answer to THAT one.

[caddypete]

That is something that has me confused. I had my caddy lose all coolant and was driving under 50mph it never went into limp mode, it would display "low coolant", "AC off" "Engine hot idle engine" and when it was getting close to overheating "stop engine, engine hot"

[CarlaValentine]

That is something that has me confused. I had my caddy lose all coolant and was driving under 50mph it never went into limp mode, it would display "low coolant", "AC off" "Engine hot idle engine" and when it was getting close to overheating "stop engine, engine hot"

My guess is that it only triggers limp mode when the coolant level is depleted and AFTER all the warnings have come onto the dash...kind of as a last resort if stopping the engine isn't an option.

[Ranger]

Coolant level has nothing to do with it. It goes into limp home mode at 265 degrees I believe and alternately shuts down one bank of cylinders at a time by disabling the injectors allowing the engine to pump cool air through the cylinders to air cool it.

[CarlaValentine]

Coolant level has nothing to do with it. It goes into limp home mode at 265 degrees I believe and alternately shuts down one bank of cylinders at a time by disabling the injectors allowing the engine to pump cool air through the cylinders to air cool it.

Well that settles that! I assume most people wouldn't let it get THAT hot before shutting the engine off.

[BodybyFisher]

For the record, I would NEVER use the limp mode unless it was of life threatening need.

Its great and I am sure it works but there is no reason to tempt fate, if you get that message shut engine off, shut it off why overheat an engine that badly

If it were used you would immediately need to change the oil, transmission fluid and thermostat

[caddypete]

Yep I guess noone would ever get to use the limp home mode. Once the DIC says to shut off engine, Nobody is going to continue driving. And who in thier right mind would let the engine get that hot and keep going?

[BodybyFisher]

Yep I guess noone would ever get to use the limp home mode. Once the DIC says to shut off engine, Nobody is going to continue driving. And who in thier right mind would let the engine get that hot and keep going?

My cousin drove his 96 with no coolant from Manhattan to his home on Long Island, the water pump belt came off and it boiled over if I recall

[CarlaValentine]

Yep I guess noone would ever get to use the limp home mode. Once the DIC says to shut off engine, Nobody is going to continue driving. And who in thier right mind would let the engine get that hot and keep going?

My cousin drove his 96 with no coolant from Manhattan to his home on Long Island, the water pump belt came off and it boiled over if I recall

I can understand those circumstances. The cost of a towtruck for a drive that long would be ridiculous!

[MrEldo97]

I think it fries the motor and the damage doesn't show up until much later.

[Cadillac Jim]

I think it fries the motor and the damage doesn't show up until much later.

That would explain why using the limp-home mode for less than 50 miles doesn't void the warranty. Wait...

[BodybyFisher]

The interesting thing about this however was the estimate that it only produced 50 hp in limp mode. I wonder what the temps hit during limp mode?, does it stay at 265 degrees? I am sure that producing 50 hp produces a lot less heat. This commercial impressed me back in 95

http://www.youtube.c...h?v=Nd5iFFTjgM8

[WarrenJ]

When the water pump let loose on my '95 Eldorado I drove about 1 mile in limp home mode. Believe me, the car lets you know it's distressed! Aside from the bells and dash warnings, you need to keep the pedal nearly all the way to the floor to maintain 50 mph.

Baby went another 30K trouble free miles before her Viking funeral.

The system shuts down PAIRS of cylinders at a time on a rotational basis by closing the injectors and defeating the associated coil packs. The momentarily disabled cylinders are cooled by the fuel-free air pumped through them. Because of its ignition coil pairing scheme a Northstar cannot shut down an entire cylinder bank's ignition without simultaneously shutting down the other bank. Oh . . . and it doesn't run smoothly.

Regards,

Warren

[Cadillac Jim]

The Car & Driver article based the horsepower estimate on the 0-60 mph and quarter-mile times. This would be about 18 seconds 0-60 mph and 25 seconds in the quarter mile. The car would apparently still go 70 mph or perhaps a little more.

[CarlaValentine]

The Car & Driver article based the horsepower estimate on the 0-60 mph and quarter-mile times. This would be about 18 seconds 0-60 mph and 25 seconds in the quarter mile. The car would apparently still go 70 mph or perhaps a little more.

I don't ever want to be the one to test that idea. And Warren, I can imagine it doesn't run smoothly at all. Having coil after coil go out on me, I'm pretty familiar with the feel of two dead cylinders and it isn't fun...at all.

[joeb]

i looked at a deville this fall that had overheating issues. was 40 miles from my house. i considered driving it but eventually passed on buying it. $100 tow was a safer choice than additional engine damage.

[Ranger]

The system shuts down PAIRS of cylinders at a time on a rotational basis by closing the injectors and defeating the associated coil packs. The momentarily disabled cylinders are cooled by the fuel-free air pumped through them. Because of its ignition coil pairing scheme a Northstar cannot shut down an entire cylinder bank's ignition without simultaneously shutting down the other bank.

Warren, are you talking about the pre-2000 engines with the "ignition coil pairing scheme"? What about the 2000+ engines that use the ignition cassettes? I don't know this for a fact, but I am not sure that only 2 cylinders at a time would cool a "dry" engine enough to be able to drive it. I seem to recall the Guru saying one bank at a time, but I could be wrong.

[WarrenJ]

Warren, are you talking about the pre-2000 engines with the "ignition coil pairing scheme"? What about the 2000+ engines that use the ignition cassettes? I don't know this for a fact, but I am not sure that only 2 cylinders at a time would cool a "dry" engine enough to be able to drive it. I seem to recall the Guru saying one bank at a time, but I could be wrong.

Yes. I thought the OP was talking about a '94 STS. I see now he also has an '02. I did some quick research, but wasn't able to verify the cassette system uses the same pairs. However, I seem to recall it does. After all, why reinvent the wheel (or ignition system) any more than necessary? It would be interesting to know if the cylinders are paired in the same way or not. I would guess they are.

I wasn't suggesting only two cylinders at a time are inactive. In fact, precisely half the cylinders are inactive over any meaningful time interval. Each and every cylinder fires on alternating compression strokes and is starved for fuel and spark during the intervening compression strokes when it would normally fire.

I shudder to think how the engine would run with banks alternating. I think the engine would shudder also. Been wrong before though.

Regards,

Warren

[Texas Jim]

I hope I never have to give a FIRST HAND report on it...

[Ranger]

I shudder to think how the engine would run with banks alternating. I think the engine would shudder also. Been wrong before though.

That does make sense. It's probably in the FSM, but I'm too lazy to dig it out and research it.

[Cadillac Jim]

The yahoos at Car & Driver were impressed, though. Their review of the Northstar was unusually gushy, with predictions that it would find its way into the Corvette (well, maybe the XLR...) and their entries in just about every event for years after that was an STS. But they never again mentioned the limp-home mode.

The injectors can be cut off in any pattern desired, including by banks, since they are fired directly by the PCM. Whatever pattern that GM uses, it's probably something like every other cylinder for a few revolutions, then switch to the other half but again every other cylinder. It would feel like a four-cylinder with a repetitive glitch every few revolutions.