What is limp mode? - Ford Mustang Forum
 
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post #1 of 15 (permalink) Old 02-10-2009 Thread Starter
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What is limp mode?

Ok, I've heard a lot of conflicting stuff on this topic. I've heard that our cars don't have limp modes, and I've also heard that they do and that it can be triggered by anything that causes a CEL. I've also heard everything from a sluggish throttle to a complete engine shutdown and drift blamed on limp mode.

Does anyone know exactly what limp mode is, if we have it on our cars, what causes it, and how to know if you're in limp mode?
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post #2 of 15 (permalink) Old 02-10-2009
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We do indeed have a limp mode. It's there to protect the engine from damage due to a damaged computer, sensors or cooling system problem. I read somewhere that it shuts down half of the cylinders.
http://www.allfordmustangs.com/forum...limp-home.html

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post #3 of 15 (permalink) Old 02-10-2009 Thread Starter
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From that thread:

Quote:
After getting some gas this morning I turned onto my street and my car began running really sluggish. I saw that a wrench icon had just popped up on my instrument cluster. It's says it is the electronic throttle control and that my car was put into "limp-home" mode.
I don't own a 2005+, so I don't have any wrench icon. Is that the S197 CEL? And if so, how does the car report that it is in limp mode? He said that the car says it was put in limp mode, but there's no limp mode light, of course, and I don't think there's a code for it. If the throttle sensor was going bad, seems like he could have just been having bad performance because of that and was assuming it was a sort of safe mode.
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post #4 of 15 (permalink) Old 02-10-2009
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Quote:
Originally Posted by ztpkng View Post
From that thread:

I don't own a 2005+, so I don't have any wrench icon. Is that the S197 CEL? And if so, how does the car report that it is in limp mode? He said that the car says it was put in limp mode, but there's no limp mode light, of course, and I don't think there's a code for it. If the throttle sensor was going bad, seems like he could have just been having bad performance because of that and was assuming it was a sort of safe mode.
My bad, I thought you had a 2005. I was in the wrong forum. Sheesh.

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post #5 of 15 (permalink) Old 02-10-2009 Thread Starter
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Well, thanks anyway.

I've just heard so many things attributed to "limp mode". Basically, it comes up when someone says they have a CEL and their car isn't running right. Is it possible that the cars are running poorly for the same reason the CEL is lit... a malfunction? If the car puts itself into some sort of safety mode, I'd like to know how to identify it.
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post #6 of 15 (permalink) Old 02-10-2009
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The non-S197 cars don't have this "limp home" function...

Exactly.

What happens when our cars throw a CEL also depends on WHY the CEL is flashing. Most of the time drivers know that you can get it to go away if you disconnect the negative terminal on the battery... For a while.

But what the car does is reset to a basic tune setting, shedding any custom or performance tunes, and losing its basic "learning curve" air/fuel map. It then takes a while to re-learn, during which period it will run a little rough, but otherwise still function.

Once this period is over, it will be back to something close to the original situation, DEPENDING, of course, on WHY that CEL was triggered.

If the problem recurs, the PROCESS starts all over again, repeat ad nauseum.

The most common situation involves the eternal "removing the cats and putting in an offroad mid pipe" problem. The car has 4 O2 sensors in the exhaust. It actually USES the front 2 sensors, and therefore these should be re-installed. Sometimes a harness extension is needed, but this should be done anyway.

The back 2 sensors just monitor the cats, which if they are removed means these sensors are just an annoyance. They can be written out by a handheld tuner (SCT, Diablo, etc), or they can be handled with MIL eliminators.

There are dozens of OTHER reasons why a CEL will trigger and the car throw troublecodes. Read the codes and get back and we will see what is causing them.

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post #7 of 15 (permalink) Old 02-10-2009
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Question

Isn't "open loop" considered "limp" mode on our cars?

For instance, when your car has a sensor fail, the computer changes from closed loop function to open loop function but the car then runs off stored Keep Alive Memory (KAM).
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post #8 of 15 (permalink) Old 02-10-2009
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Originally Posted by grn 00 gt View Post
Isn't "open loop" considered "limp" mode on our cars?

For instance, when your car has a sensor fail, the computer changes from closed loop function to open loop function but the car then runs off stored Keep Alive Memory (KAM).
Its what I was referring to when I went through all that O2 sensor stuff. Its just not called "limp home" like the 05's.

Its also not like many other cars (such as my Mercedes) who have "limp modes" all over the place, including the stability control system, the transmission, and of course the engine controls.

In the case of many of these systems, like the one I mentioned in my ML500, getting the tranny into Limp Mode means you are LITERALLY limping along in first gear until you get to a garage that can repair the car. Same thing with some cars and their emissions controls, they will shut down and allow the car to barely perform, almost undriveable, until the car is repaired.

The open loop backup (closed loop refers to the fact that the Mustang uses sensors within a "closed loop" to make ongoing changes within a certain set of limits to minimize pollutants, for instance, or maximize gas mileage, whatever the closed loop program is designed to accomplish) and its blah performance is the reason why we always recommend keeping the front sensors intact (and the closed loop process working) - the car just runs better with that way. But with our GTs, the car is not totally compromised in the open loop state, at least not as badly as some other vehicles. LOL, I've seen guys running that way for months, not that its a very bright thing to do.

With the 2005's and their "limp home" error message, I'm not sure how compromised the car becomes while it is "limping" around.

But OK, if we want to say that the open loop backup is a "limp mode", I could see that. Shoot, it might even be good psychology. Usually when I see this discussion, its because some member has slammed an offroad midpipe on his car, disconnected or thrown away all the O2s because some mastermind told him the only thing they did was turn on a CEL, and was now wondering why the car didn't seem to run properly.

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post #9 of 15 (permalink) Old 02-10-2009
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Quote:
Originally Posted by ztpkng View Post
Well, thanks anyway.

I've just heard so many things attributed to "limp mode". Basically, it comes up when someone says they have a CEL and their car isn't running right. Is it possible that the cars are running poorly for the same reason the CEL is lit... a malfunction? If the car puts itself into some sort of safety mode, I'd like to know how to identify it.
If the Check Engine Light comes on and you have NOT been making modifications of some kind, your next step should always be to pull the codes and see what they tell you. And yes, the CEL (in that case) is USUALLY because something has malfunctioned. MOST of the time this will be something minor, a vacuum hose or wire. The next most common cause ("natural" cause, ie, one not caused by a human being, usually the owner, changing things around), would be O2 sensors, which do age and finally fail. There are also a lot of other sensors (just as grn correctly indicated) that can trigger these codes, some of which can be tricky to interpret (cam sensor trouble codes on these cars are often in error, since a failing alternator has been implicated in many such cases).

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Simply put:

Open Loop - WOT - Pulls parameters from a fuel map based on engine speed and feedback from select sensors.

Closed Loop - not WOT (more specificially, less than 30% throttle typically and on the lower end of the powerband) - uses real time data from the O2 (and other sensors) to adjust fuel trims and spark.

There is ALOT more to this, including all kinds of weird hybrid modes, but the above is super oversimplified.
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Open Loop at lower power levels without the information the EEC needs to do its job would be a pretty darn good (and yes, oversimplified) defnition of a "limp mode" symptom to fit the narrow confines of this particular discussion.

All makes perfect sense, though I don't think this will help the OP unless he does a lot of homework. Arcane terms like "stochiometric" spring to mind, LOL.

Perhaps I've been doing this stuff too long. I may be unfair. Maybe I should start assuming that the problem is not the "obvious usual", but instead an opportunity to get more members to understand the secrets of air/fuel mixtures!

But I suspect just convincing everyone of the merits of keeping a few of the original sensors still working will have to do.

Good simplification, Quick4.6, and grn00, you were correct as well.

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Quote:
Originally Posted by Quick4.6 View Post
Simply put:

Open Loop - WOT - Pulls parameters from a fuel map based on engine speed and feedback from select sensors.

Closed Loop - not WOT (more specificially, less than 30% throttle typically and on the lower end of the powerband) - uses real time data from the O2 (and other sensors) to adjust fuel trims and spark.

There is ALOT more to this, including all kinds of weird hybrid modes, but the above is super oversimplified.
Correct,
I was actually refering to "forced" open loop due to a failed sensor where the car stays in open loop until the fault is corrected and code(s) are reset regardless of thottle positon.
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Open loop can also be caused by an electrical problem in the ECU power circuit or any sensor circuit. I had such a problem not long after installing the KB on my 'vert. The 3 position switch for the multi-tune chip had a fault in it which kept shorting and sending my ECU into open loop and stalling the car, at WOT and full boost. Quite unnerving actually.

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post #14 of 15 (permalink) Old 02-11-2009 Thread Starter
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Thank you very much for all the feedback!

Currently, I do have a CEL, but it's just the PFS and I'll be replacing it this week. I was not worried about my car, specifically, but I had just read another reference to "limp mode" elsewhere on the forums, and decided to ask the question.

I can see how open loop could be considered our "limp mode", but when the term is thrown around forums, it is generally meant as a computer-inflicted performance slump that is designed to keep the engine safe until you fix whatever is wrong with it. I've read that it actively retards the timing and fuel. However, it sounds like the consensus is that our cars are not dialing back performance due to random CELs but that they might stay in open loop if part of the feedback system is malfunctioning. That takes a bit of the mystery out of it.
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post #15 of 15 (permalink) Old 02-11-2009 Thread Starter
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Quote:
Originally Posted by BlueStreak03 View Post
Open loop can also be caused by an electrical problem in the ECU power circuit or any sensor circuit. I had such a problem not long after installing the KB on my 'vert. The 3 position switch for the multi-tune chip had a fault in it which kept shorting and sending my ECU into open loop and stalling the car, at WOT and full boost. Quite unnerving actually.
I'm not sure what you mean.... isn't the car supposed to be in open loop at WOT? Why would this cause it to stall?
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