Suddenly running super rich, no codes, starts great then dies after 5 seconds.

[Stinger]

So my Mustang went from running great (Lito tune) to running super rich last weekend. Parked it in the garage running great, got in it the next day and it started, did the 5 second idle up and as it settled down to the ~1000 rpm idle it stumbled and died. Restarted it and same thing. Restarted it again, gave it some throttle to rev it up and as soon as I let off the throttle, it died again.

So I stopped and scanned for codes and there were no codes (which is weird). So I determined it's running super rich (smells like gas, burns eyes if you go behind the car right after revving it up, condensation that blows out of exhaust smells like gas).

I recorded a few datalogs and sent them to Lito to see if he could spot anything but we didn't make any progress.

It runs PERFECT during the initial startup and the 5 seconds after it starts which should eliminate any mechanical issues.

Here is a video of multiple startup attempts. First couple I don't give it any throttle, just let it do it's thing. After that I start screwing with the throttle to keep it running/rev it up and clear out the excess fuel.

https://www.youtube.com/watch?v=IjoHqdl4MtE

At 46 seconds into the video the ECU resets after the car shuts off (you can here it beep and says engine temp warning and check fuel cap in the cluster).

Just after 1 min mark of video you'll see it drops below 300 rpm and then back up to 1000 rpm a few times, that's not me giving it throttle, it's just nearly stalling and then catching it self momentarily before going back to running rich.

With no codes and apparently no mechanical issues, I don't even know where to start with diagnosis. Anybody ever had similar issues? Thanks.

 

[retfr8flyr]

Have you tried reloading your tune? Some times the tune file just gets whacked and reloading will correct the problem.

 

[irishpwr46]

do you have delete plates?

 

[Stinger]

I didn't reload the old tune but Lito did create a new tune for me to try which was just the old tune modified to force closed loop after 5 seconds. Made no difference.

Delete plates, yes.

 

[Stinger]

Is there any way to test the fuel rail pressure sensor? Lito replied back that the fuel pressure doesn't look normal. I'd like to be able to test it before spending $100 on a new sensor and finding out it's not the problem.

 

[JeremyH]

Sometimes the diaphragm breaks and you get fuel in the vac line, but they can fail with no physical symptoms as well. I have a spare for sale in the classifieds.

 

[Stinger]

Sometimes the diaphragm breaks and you get fuel in the vac line, but they can fail with no physical symptoms as well. I have a spare for sale in the classifieds.

Thanks, made an offer in your for sale thread.

 

[Maka]

Was this figure out yet? I'm getting something very similar. Mine idles all over and bounces from
Rich to lean

 

[Stinger]

No, not yet. This thread is like 12 hours old.

 

[Maka]

Sorry lol just trying to figure my car out I've replaced a lot and it's been like this since I bought it 3 weeks ago. I can't move the car.

 

[Stinger]

Well, new FRPS installed and it initially ran no better. Unplugged the FRPS, fired it up and it ran bad for a few seconds so I revved it up a few times and it seemed to clean out and run better than it has in a week. It was lean at idle (18:1) and would go into a normal range when revving (13-14:1) but amazingly ran better than with the FRPS plugged in. So I shut it down and plugged the FRPS back in and it now ran better (though far from perfect). Still took a couple revs initially to clean it out and get it to hold a steady rpm or to idle without dying. Once I did that though, it at least wasn't shutting down after a short time of idling anymore.

Only difference was after all of this run time, the engine was warm (coolant around 140 degrees) so I'm not sure if it is truly better, or if it just runs better warm than cold. I'll know tomorrow when I do a cold start.

If it runs better tomorrow on a cold start but still takes a few revs to clean out. I'm going to suspect the plugs are fouled from all of the misfiring during diagnosis last week. If it runs like it did last week on a cold start, I'll know we're back to square one as nothing has improved.

 

[skwerl]

Was watching youtube videos a few minutes ago and saw this-

 

[irishpwr46]

when was the fuel filter last changed?

 

[Stinger]

Been datalogging and looking at ECT and IAT values and such so it's not the ECT.

Fuel filter was last changed about 10k miles ago (car only has 40k miles on it).

 

[Stinger]

Started it today on a cold start and it ran the same as it did last week so the FRPS swap changed nothing.

Going to have to pull the plugs and hope none of them are stuck..not looking forward to that!

 

[StangPhD]

Could it be a couple fouled oxygen snesors?

 

[Stinger]

I don't know. Lito doesn't report any "weird" readings from the o2 sensors (and it seems to do it even in open loop when they are ignored) so I doubt it but anything is possible at this point.

I did a boost leak test over the weekend and didn't find any leaks. It's a blow-thru MAF so I only checked for leaks after the MAF since nothing before the MAF would effect it at idle. I also pulled the MAF and cleaned it. No change.

 

[irishpwr46]

could it be a bad engine temp sensor?

 

[Stinger]

Engine temp appears accurate in the datalogs.

So here is a weird development. I pulled the MAF to see if it was dirty (not a spec of dirt or oil on it, looked brand new). So I pulled the inlet pipe to do a boost leak test (no leaks and the throttle body was super clean as well, no oil residue or anything), I pulled the front driver's side coil so I could take a look at the spark plug. Pulled it out (wasn't stuck) and it was black but not fouled or filled with carbon or anything so I put it back in. It was like 2am when I got done so I figured I'd avoid waking the neighbors up and wait until today to fire it up. Started it up today and it ran perfect. Shut it off, fired it back up, same thing, ran great. So I let it warm up, drove it around the block a few times, still ran fine so I went about 1 mile from home and floored it a few times. Pulled hard, AFR was as expected (low 11's/high 10's) and everything seemed fine.

So it seems to have magically fixed itself just like it magically broke itself. While this is good (it now runs properly), it's also bad because 1) I don't know what the problem was and 2) it makes me nervous about driving it far from home for fear of being stranded and not knowing how to fix it.

I tried wiggling the harness around everything I'd messed with (MAF, Coil) while it was running to see if I could make it sputter or stall but it didn't every stumble at all during the test.

 

[irishpwr46]

do you have any wires that run close to any of the belts/ pulleys?