who knows about the code content of the limp mode?

What I mean is that the other day after doing the only things I could without scanning to my ride (I turned off the EGR flag and messed with fan temps and lock up speeds). I put my chip into the moates holder backwards. the result upon start up was an instant trip into limp mode and the SES light on. before I realized what was going on I was treated to an incredible idle and although the tip in of the throttle was a bit poor, it had a "snap" to it I had never expected. A responsiveness after tip in that caused to sound incredibly clean. I didnt rev it very high as soon as I realized the light was on but it was really healthy sounding the couple times I did give it a little brap.

the car is the 1992 B4C with AUJP on the stock 730 ECM, and the 412 I built with the huge cam.

My question is; what are the properties of running in limp mode that could have made my car run so well?

 

True limp mode, no doubt about it. There was no code being run, the redundent fuel device (RFD) was in control. This is a hardware device built into the ECM. It uses the values in the NETRES (the two chips under the knock filter on the MEMCAL) to run the fueling. This is a pseudo SD mode that runs a tad on the rich side.

The ignition system was running in backup mode. That is base timing until 1600-1800 RPM where a set amount of timing is added (20 deg??). The ignition module handles this functionality.

RBob.

 

that is a huge help and a bunch more info than I even imagined would be available. any more specifics or input is greatly appreceated.

The 1600-1800 must be the area where the "flub" is felt. I suppose I could experiment to see with a timing light if possible what is going on too. after the 1600-1800 when that "set" timing comes in is where she runs real nice. I think this might seem silly but it could help give me an idea of which way I have to go.

 

I thought that it relied on the A/C generator effects of the pickup reluctor for inducing some timing advance, and that the EST stayed dead. At one time it was taught that the design was developed with the EST so as to be able to isolate the ignition in the event of a processor failure, but this data goes back to the CCC days, so it's real dated.

 

Not silly at all. Put a timing light on the engine and open the EST (bypass) connecter. Start 'er up and watch the timing as the engine RPMs are brought up. You'll see the timing jump.

Part of what you are getting is that the limp mode typically runs the engine on the rich side. With a modified engine the limp mode fueling could be richer or leaner, and/or have different curves then the re-tuned chip has.

This all may be changed with the addition of external resistors. I believe the '747 NETRES has been hacked & documented.

RBob.

 

Unless, you have a real need to do this, I'd suggest just not even running the engine in limp home mode. It really is just that, just enough function to get out of traffic. Lots of cars would never make it HOME in that mode. Running that far retarded that rich ain't will at least carbon things up for you.

If you really wanted to work out what's going on, this is really something to do on an ecm bench.

 

Where can one find the hacked and documented info? I'm curious to see the inter-operation of the network resistors with respect to the redundant fuel device......I remember seeing some snips of info on the DIY-EFI archives, but nothing in great detail. Does anyone have info on the RFD chip itself?

 

I've had to drive home 90 miles from the strip in limp home mode in my vette. Not fun.

 

I am new to the world of chip burning, my brother and I are starting to get into it a little. I ran my car that way, with no sensors at all ...o2,maf, nothing, for about 9 years, and it ran 14.0 in the qtr!maybe I'm just lucky , but about 3 months ago I decided to hook every thing back up and now it is running like ****!!!!!!so here I am trying to learn how to burn a new chip to get back to the performance I had in "limp home mode".

 

So let me ask,,,,In limp home mode we all know the car is basically running minimum parameters to get by.

What do you think a motor about the size of his car would run in Limp Home Mode vs Regular mode? For example: If it turned 12sec in the 1/4 with it 100% in limp home what are your guesses it would turn?

 

That is a loaded question, I will tell you it will run faster than in limp home mode,however,what if we had a way to run the tuned port on our own parameters with out any sensors(tps,o2,maf,etc..) that would open up a whole new can of whoop-***! It has to be possible! If GM can make one set of paremeters to get it home , than why can't we make the same parameters make it kick a$s. Just a thought don't take it too serious it may be the coorslight talkin' too ya! DUNNO? After all I am ignant when it comes to this stuff.

 

Definately the coors light talking Bro!