I know that some engine control systems revert to a middle of the road timing or "limp mode" when there is a code in the system. does my 96 F250 5.8 do this? i am still woking on the O2 sensors and convertor replacement, and have been running it with the Check Engine Light on. I am concerned about having put 500 plus miles on it with the engine light coming on. It said O2 sensor was lean and 02 sensor was rich, so I figured it needs a sensor and i have a spare convertor off of my old truck, so i still have to do these repairs before I attempt to clear codes and check it again.

 

First, please post the actual code numbers you are getting. There's often a deeper explanation involved than what you will get from your code reader manual- and the same codes can be triggered for different reasons. In other words, it could be the sensor is bad, or it could be bad wiring making the computer think the sensor is bad.

As for the limp/failure mode, it's my understanding that this only happens when the computer itself is shot. I'm not sure how it works exactly, but I don't believe it occurs solely because one code is triggered.

PS, if your truck is a 96, it's more likely than not OBDII.

 

It's definitely OBDI, i scanned it myself. I don't really need any help interpreting codes, just curious about how the presence of codes affects the Engine's computer control. I realize it would be limited as it still has a mechanical distributor, but curious anyway.

 

In the short term, running with stored codes won't hurt anything. Some would say if you don't notice any major differences in performance, there's no reason to fix what's not broken. Others would say you should take care of any issues right off the bat.

I take the stance that I want my engine to be as efficient and smoothly running as possible at all times. If this means replacing a sensor even if I don't want to pay the money for it, I will.

Now with the o2 sensor, I'd say the primary thing you would notice is going to be MPG's as that plays a primary role in the computer controlling the air/fuel ratio, and determining when to open and close the EGR valve.

But again, in the short term it shouldn't hurt anything. Over the long term tho you'll probably see some performance issues develop.

 

In the short term, running with stored codes won't hurt anything. Some would say if you don't notice any major differences in performance, there's no reason to fix what's not broken. Others would say you should take care of any issues right off the bat.

I take the stance that I want my engine to be as efficient and smoothly running as possible at all times. If this means replacing a sensor even if I don't want to pay the money for it, I will.

Now with the o2 sensor, I'd say the primary thing you would notice is going to be MPG's as that plays a primary role in the computer controlling the air/fuel ratio, and determining when to open and close the EGR valve.

But again, in the short term it shouldn't hurt anything. Over the long term tho you'll probably see some performance issues develop.

 

Thanks- I agree with you about trying to keep them running smoothly and efficiently. The 50 bucks on the o2 sensor doesn't upset me. The truck smells and seems like it's running a bit rich. Got 12.8 mpg running 300 highway miles last weekend. A costly road trip.

I guess with a mechanical distributor, what else can the computer control other than fuel air mixture? I always thought the EGR only came on when the engine is warming up... Is that true?

 

The computer still has full control of the timing. The EGR should only come on when the engine is up to operating temperature while under cruise/part throttle conditions.

 

As far as the subject of Limp Mode information the PDF below will tell you all about it.

 

where is the obd port? i have the same truck and can't find it anywhere? is it the EEC test thing on the side of the fender under the hood?

 

Yes, it should be under the hood on the driver side of the engine bay, up near the firewall.

 

Ryan has some good info here: Ford Fuel Injection » How To Run a Self-Test