AFR at 12-13 on decel?

[GrimJack]

This just recently started to happen, and the only thing I've done recently was change the plugs. I don't think it happened at the same time, but maybe...

Checked for codes, nothing there.

On deceleration, car is in gear, foot off the throttle, my AFR used to sit at 20+. Now, the AFR bounces around 12-13, and the engine stumbles and burbles a fair bit. If I push in the clutch, it goes back to 14.7.

Any ideas?

Mods: RC550s, MAFT Pro with MAP, Aeromotive AFPR, Innovative Motorsports LC1 wideband... and a bunch of other things that I can't imagine should impact this at all...

 

[jetjock]

Start by checking where you get code 51 Grim. Either press the pedal in diag mode or bring idle up to appx 1700 and slowly drop it 100 rpm at a time while watching the tach and listening for fuel cut. If you don't see it or it takes more than a tiny amount of pedal to get 51 check the TPS adjustment and/or the throttle plate. Check the dashpot too. Could also be in the speed sensor but you should get a code for that.

 

[quake]

This just recently started to happen, and the only thing I've done recently was change the plugs. I don't think it happened at the same time, but maybe...

Checked for codes, nothing there.

On deceleration, car is in gear, foot off the throttle, my AFR used to sit at 20+. Now, the AFR bounces around 12-13, and the engine stumbles and burbles a fair bit. If I push in the clutch, it goes back to 14.7.

Any ideas?

Mods: RC550s, MAFT Pro with MAP, Aeromotive AFPR, Innovative Motorsports LC1 wideband... and a bunch of other things that I can't imagine should impact this at all...

did you use the same plug type and gap?

 

[shaeff]

going out on a limb here, but would stuck/leaking injector(s) cause this?

 

[spoolint78]

I think most cars hit 20+ a/f when you let off the throttle, and then they drop back down to 14.7. Can't recally, haven't driven my car in a while.
As for a stuck injector its possible, but i wouldn't think it would be the culprit, never really heard anything about rc's having problems, especially in the mk3 community.

I would check for codes, and reset the ecu.
Double check that you have no fuel leaks (especially if you have a dual pump setup in the tank, it could be leaking back into the tank, and not making it to the rail).

I would also check your tune on the maft pro, as well as vacuum leaks, more specifically the line going to the fpr.

 

[mkIIIman089]

going out on a limb here, but would stuck/leaking injector(s) cause this?

Seems unlikely, if it were leaking I imagine it would affect more then only decel.

My initial guess was going to be what jetjock has already said.

I also have my doubts that a vac leak on the FPR would cause anything like this. The injectors should be off completely above ~1700 revs... nevermind.

 

[jetjock]

^ It's tune! It's codes! It's fuel!

Now you know why they call it "shotgunning"

 

[MmmBoost]

its most likely the tps' switch that says when its closed.

 

[hottscennessey]

How much vac. you pulling on decel, grim?

 

[sneakypete]

As for a stuck injector its possible, but i wouldn't think it would be the culprit, never really heard anything about rc's having problems, especially in the mk3 community.

funny, i thought the same thing until i experienced that exact problem. i had a bad miss, then one day it was fine. 2 days later the miss was back. changed plugs wires and test the coil and all was well. i knew it was number 3 so i pulled the injector plug, and sure enough it was the culprit.

 

[GrimJack]

Update: I hate it when my car fixes this stuff without any input from me. I can't get it to happen anymore, and I STILL haven't changed anything. One more symptom that I forgot at the time of the original post - when it does this, it idles high as well, about 1500rpm instead of the usual 700.

Start by checking where you get code 51 Grim. Either press the pedal in diag mode or bring idle up to appx 1700 and slowly drop it 100 rpm at a time while watching the tach and listening for fuel cut. If you don't see it or it takes more than a tiny amount of pedal to get 51 check the TPS adjustment and/or the throttle plate. Check the dashpot too. Could also be in the speed sensor but you should get a code for that.

Thanks JJ, that's where I was looking as well. I get code 51 with VERY little throttle - should there be more of a gap before that shows up?

Everything appears to be working as designed right now... of course, that's probably why the problem went away. Grrr... I'll catch the little beggar sooner or later, next time it happens I'll run through the same steps.

did you use the same plug type and gap?

No and Yes, but I don't think this can have anything to do with it, because the problem is intermittent.

going out on a limb here, but would stuck/leaking injector(s) cause this?

I don't think so - if it were, my idle AFR would be whacked too.

...as well as vacuum leaks, more specifically the line going to the fpr.

Already checked that, as I've seen a rich condition in the past (on another guy's car) where a torn membrane in the AFPR was sucking fuel into the intake through the vacuum line.

A *LOT* of fuel. But again, that should affect the AFR globally (well, as long as you stay out of boost) instead of just on decel.

^ It's tune! It's codes! It's fuel!

Now you know why they call it "shotgunning"

LOL! To be fair, I did ask for ideas.

How much vac. you pulling on decel, grim?

-20 something, my gauge only goes that far.

 

[Dan_Gyoba]

Dunno what you're running for engine management, so I'm going to make guesses based on TCCS, and the fact that you changed plugs.

My instinct on this is screaming "TPS!"

More specifically, it's saying that you're getting an intermittant connection failure from IDL to E2.

Normally, when you let off the throttle, IDL and E2 connect, and the ECU cuts fuel until RPM comes down to below (IIRC) 1000 RPM or so. It does this any time IDL and E2 have a connection at about (again, IIRC) about 2000 RPM. In this case you're going to go way lean, but since this is a fuel cut condition, that's normal.

If this is not happening, then on decel, the TCCS will still be supplying fuel, though the TPS position should still have it in open loop mode, without O2 sensor feedback, which is going to be rich.

This also means that RPM won't drop as fast, which would explain your high idle. This will happen even at high vacuum.

Another thing that triggers this for me is that you changed your plugs, which means removing the 3000 pipe, or whatever you've got there for your IC piping. This means that you could have affected the wires and/or TPS connector when you were in there.

It's worth a try...

 

[jetjock]

I'm stickin' to my original diagnosis (pretty much same as Dan's) here. It sounds like an intermittent IDL connection. The high idle further implicates it.

You might want to try driving around in diag mode for a few days and watch to see if 51 goes away when the throttle is lifted. The only problem with that is you'll need a bit of time before getting on the throttle again as once a code is begun displaying it will continue until finished even if the problem has gone away. Fwiw there's no problem with driving the car in diag mode. It doesn't change how the engine operates at all once it's off idle.

The other thing you could do is wire a light or LED to ID and +12. That'll give you instant notification of what it's doing and if you hook it up at the ECU it'll also check the wiring.

To check fuel cut itself follow the procedure in the book ie; remove the TPS connector, short IDL to E2, open the throttle to about 2500 and hold it there. If everything is working you'll know it.

Or just go for broke and install a new TPS. Btw I've see fuel regulators fail that way too. Sometimes it's just a small leak in the diaphragm. I make vacuum testing them with a hand pump a habit these days.