A/F Changing When Engine Is Warm

[ZBass28]

Trying to chase down yet another problem...getting really tired of this crap.

Engine runs nicely for about the first 15 minutes, keeps target A/F at idle right where I want it and runs nice and smooth. Then things just go haywire.

I can drive around at low throttle cruising and after a while the A/F will start to lean out. The proper running A/F of 14.0 has now somehow managed to jump to high 14's, low 15's. Same thing if I come to a rest at idle. Engine will be slightly surging up and down, A/F hovering in the high 14's/low15's. Adding fuel barely helps, but you can smell the unburned coming out.

Fast forward 10 minutes later when you are driving and those part throttle cruising numbers that had to be corrected earlier for being too lean have now decided to show running too rich, like in the 12's. Same with idle, come to a stop and now the A/F is in the high 12's.

This will keep going back and forth. What the heck is the problem, I've tried lots of things to correct this and nothing is working. Bad 02 perhaps?

 

[SouthernCal GN]

Are all your onboard sensors working correctly? Bad O2 would want to add fuel, as it's seeing a constant lean condition.

What ECM/chip are you running?

Heck, for that matter, what car are you talking about?

 

[ZBass28]

All sensors seem to functioning perfectly.

Does seem like it's wanting to add fuel, but then it will switch back and show rich condition after I make the necessary changes from the previous data I was using.

Car and system are in the sig. It's a '96 Z28, supercharged LT4 350 running an older version of FAST (circa 1999-2000).

I just tried switching to a different ECU table from a few months ago when the car was running decently, same thing. Runs great for a while then starts to run like ass. Popping out the exhaust cruising in 2nd or 3rd gear at 2000 rpms or higher, surging, etc.

 

[SouthernCal GN]

go to fullthrottletech.com- good site for FAST info.

 

[norbs]

Sounds like exhaust leaks in the system. Any minor leak anywhere in the system will drive the wideband crazy. Maybe things are warping when hot and causing leaks? Your ve should be adjusted hot and after a good drive, not a 5 minute warm up. Also if you are runing about 40* timing at idle it will surge. If you a/f is to lean it will surge. Keep at it.

 

[ZBass28]

I'm thinking it has to be a leak somewhere too, I can't figure out what else it could be.....

What about a small leak post 02? That really shouldn't affect anything, right?

Timing at idle is set at anywhere from 20 to 24*

 

[protosy]

I'm thinking it has to be a leak somewhere too, I can't figure out what else it could be.....

What about a small leak post 02? That really shouldn't affect anything, right?

Timing at idle is set at anywhere from 20 to 24*

depends on how close the leak is to the o2... something like 2' should be fine but 2" could be causing issues.

i had very similar behavior on my fast a few years back. it ended up being a combination of moisture in the connector and exhaust leaks. once that was sorted, i was able to dial it in consistently; before it would be pig rich or way stupid lean depending on its "mood".

 

[HighPSI]

Where is your air temp sensor located?

 

[ZBass28]

Air temp sensor is located in the aluminum elbow just before the throttle body (if you are familiar with LT1 engines)

Ever since this has been happening, which is recently, I have been thinking there has to be a vacuum leak or an exhaust leak. So far, I have found neither. Tomorrow I will throw the car on stands and completely remove the y-pipe and check all gaskets, and then closely check the headers.

I just don't get how one minute it can be reading perfect, then 10 minutes later be showing horribly lean, then I make the corrections and then 10 minutes later the same set of circumstances shows running awfully fat....with an exhaust leak you would expect it to just stay lean, right?

 

[bobc455]

I just don't get how one minute it can be reading perfect, then 10 minutes later be showing horribly lean, then I make the corrections

What corrections are you making? Are you changing the VE table

What is your target A/F, and what is the actual?

Is your WB in closed-loop? How much % correction does the WB have to make?

-Bob C.

 

[turbo_bu]

If your VE table is close, turn off the WB correction and see if it still does it. If it stops doing this, then try to find where you have an exhaust and/or intake leak.

 

[ZBass28]

Car runs in open loop up until 2000 rpms, so the idle problems shouldn't be affected by anything the 02 is doing, but it will run perfect, then lean when warm, then rich some time later, then lean...etc.

Bob, let's say car is supposed to run at 14.0 at 2000 rpms around 45-50 on the MAP. Things will be fine til running for a while, A/F will start reading lean bouncing around at 14.9 to 15.4. I'll make the necessary corrections to get things back on track. Drive for another 5-10 minutes and all of sudden that same area will now be showing that its running at 12.9 to 13.4. Make corrections and the whole thing just keeps happening. The corrections I've seen look to be anywhere from 7% to 11%. All corrections made to the VE.

 

[Chuck Leeper]

What's the fuel pressure doing when this is occurring??

 

[ZBass28]

Fuel pressure stays steady while this is happening.

I removed the y-pipe, checked all gaskets, flanges...no signs of any leaks. Checked all header bolts to make sure none were loose or backing out, checked gaskets as well and everything looks good.

 

[bobc455]

This is a weird one for sure.

Do you have a lot of enrichment according to the air- or engine-temperature? If the intake air temp is changing and you make a lot of correction for this, it could be screwing with your settings. (This is why I think the air- and engine-temp corrections should be according to a factor that changes the target A/F ratio, not one that changes the PW directly, since this will fight against the closed-loop).

What about your after-start? Is the decay super slow?

-Bob C.

 

[ZBass28]

I have no corrections for air or engine temps in the spots remotely near where the problems occur. The only time I have any sort of enrichment is in the very cold areas.

After start is same as it's always been for 7 years, this is really the first time I have seen this problem manifest like this. Is it just possibly a messed up 02?


Edit: I did just remember one other thing it was doing since this has started happening. In an attempt just to see how the engine would run in areas that I know are correct, I would hold the engine rpms at a certain MAP value...let's say 3rd gear, 3000 rpms between maybe 60-70 kpa on VE table. Car will be smooth and then you can physically and audibly hear the engine change. If you look at the correction it swings from pulling 11% fuel to running at 0% to 0.8% correction and then swings back to pulling 11%. You can feel the engine surge and pulses the car forward as this happens. This also has never happened before. I know there is no way the car should be pulling 11% fuel at that spot.

 

[bobc455]

Any chance you have a weak coil that could be heating up and misfiring?

Do you have a PCV that might be clogged? I don't think there is any EGR on your motor (but maybe I'm wrong?)...

-Bob C.

 

[ZBass28]

I'll have to look into the coil scenario, who knows, that could be a possibility.

EGR has been removed a long time ago. I'll double check PCV but everything looked good at first glance.

 

[ZBass28]

Haven't driven the car again yet, but did find another thing that has been happening for a while now.....

I took off the y-pipe and brought it to get checked out and welded if needed. Nothing looks like it's leaking. I disconnected the entire intake tract from the blower to the throttle body and noticed that the blue rubber connector from the discharge of the blower to the first tube was wet with oil. This has happened before in the past. If I look into the blower I don't see any oil, it's dry. If I look into the tubing it's dry as well.....where the hell is this oil coming from???? My throttle body was kinda black and sooty, but dry, not wet. My intake manifold is definitely oily wet looking inside, but has been ever since the engine was built. Reversion??? Any connection to the car running bad once hot? I'd think it would always run poorly if this had something to do with it.

 

[Lonnie]

Next time the car is surging at idle, try connecting it to a battery charger.
This will increase & stabilize the voltage. see if the idle smooths out.

My LT1 did this & also a 5.0 Mustang I built did the same thing with a FAST system. You need an alternator that will provide stable voltage.

Often the cooling fan (my LT1 varied the AF by 1 point when the fan kicked on) or even the headlights can effect the battery voltage enough to change the AF reading & make the idle surge if the alternator output varies a few tenths of a volt at low rpm.

Put a voltmeter on the car & see if there is a correlation.
Try this, it is easy & may help to solve your problem.

HINT to FAST.... add injector opening tables to the system so we can program for varying charging system loads. The factory puts them there for a reason.