all of a sudden engine running really really rich

[Joel Hauser]

I have a pretty much stock 1996 4.6L engine in my roadster. About 2 weeks ago I was out for a drive when all of a sudden the car started running terribly, like it was misfiring. I was able to get home, but had to keep the engine revving at much higher RPMs to keep it from stalling.
The exhaust smell was overpowering. I pulled the plugs and they were all carbon fouled black. Since the easiest thing is to throw money at the car, I bought a new rebuilt ECU (I had some suspicions that he ECU was bad because it stopped mating to my SCT tuner). I installed the new ECU yesterday; it made no difference; the engine is running terribly, won't idle, and smell awful. The new ECU from Flagship One is supposed to have a new stock tune on it.
I'm going to do a smoke test to check for a vacuum leak, which I believe is the most like cause of a rich running engine. I've previously installed a new MAF, 02 sensors, new plastic intake manifold, throttle position sensor, idle air control valve, fuel pressure regulator and just about every thing else that hangs on the engine. I have the stock mustang throttle body (with the mustang on it).
What gets me is the engine was running great, then all of a sudden it is running so rich it barely runs.
I don't think reading the codes will be particularly useful; I don't have rear O2 sensors, CATS, or EGR, so I always have codes.
Anyone have any suggestions before I start wasting more money on parts?

[Al_C]

just a question: what changed from when it was running well until it wasn't? The new stuff you put in (MAF, O2 sensors, intake manifold, etc) - was the engine running well after those things were changed? I'd check the MAF first, then the O2 sensors, but that's just me.

[Joel Hauser]

thanks Al C; that's the funny thing, I hadn't changed anything. The car only has 2500 miles on it, and all the new sensors and parts were installed many hundreds of miles ago. I don't think its the 02 sensors. It's my understanding that they don't kick in until the car is warmed up, and the car is running badly when cold and warm. I have another MAF; after I check for vacuum leaks, if there aren't any I'l swap the MAFs to see if that helps.

[Railroad]

I think there is a vacuum operated fuel pressure regulator on these engines. Seems like it is mounted on the fuel rail. If the diaphragm goes bad, it lets unmetered fuel go into the vacuum/intake source.
Sorry I cannot be more specific. Long time ago issue for me.

[Joel Hauser]

Railroad, the regulator is exactly where you describe. My regulator is new, but I'll definitely check it to see if it is leaking. I believe when they go bad the fuel will leak onto the vacuum side. Thanks.
Joel

[Joel Hauser]

I did a smoke test using a homemade smoker. There was a little smoke coming out of the black plastic vent on the Idle air control (IAC) valve. I installed a different IAC I had laying around. The car runs a little better, but won't idle. I reinstalled the old ECU, and the car runs terribly, very rich and misfires. So I've put the new ECU back in. I haven't checked the fuel pressure regulator yet, but I don't think it is bad; there isn't any gas on the vacuum side.
I've emailed Ron Francis wiring and asked if he will program the ecu.
Any more ideas out there?

[bobl]

The spark plugs may be fouled, so it may still run poorly even with the problem resolved. I'd replace the plugs and try it again.

[Joel Hauser]

Bobl, I thought of the that, so I cleaned the plugs with a harbor freight plug cleaner. I'll take another look at them. Thanks

[AC Bill]

You may have to adjust the new IAC to meet the specs, as far as the idle goes. I'd doubt that is the main issue though.
My first thought was the MAF, as Al had suggested. You had an air cleaner installed, right?

[Joel Hauser]

139957139958
After driving less than a mile, the plugs are fouled with carbon. I haven't tried swapping the MAF yet. The air filter did not fall off.
I wish Edelbrock made a carb conversion intake manifold for the 1996 SOHC engine...but they don't, so I'm stuck with fuel injection, which I'll never figure out how to fix.

[Norm B]

Is any plug or plugs more fouled than the others? The reason I ask is, I had a similar issue on another project I am working on and it was two injectors stuck open. If you have a mechanics stethoscope listen to each injector while the engine is running. Should be a very audible click every time the injector opens. In my case all the plugs fouled because there was so much fuel going into the intake it was getting to all the cylinders. The computer couldn't reduce the flow from the other injectors enough to compensate. The O2 sensor readings were all over the place.

HTH

Norm

[rich grsc]

Have a close look at the O2 sensors wire. Make sure they don't have a bare shorted wire. I had one shorted right where it exited the body of the sensor.

[Jeff Kleiner]

Ring out the MAF wires and if they check out try a known good meter.

Jeff

[AC Bill]

If your system is all working well, EFI is pretty hard to beat overall, but I will admit it is harder to diagnose an issue.

Have you checked for fault codes? If not, borrow, or invest in an OBD reader, you might learn a lot, rather then simply guessing.

[Joel Hauser]

I think I have good news. (how self absorbed have I become, when a successful repair of my toy sports car makes the "good news" column in these hard times?) As AL C, Jeff K and others suggested I checked for continuity, ground and power between the ECU and the MAF. All was good. I installed a "new" old MAF, and it started right up. I went for a short, brisk drive -it's 28 degrees here in up state NY. No miss fires, no rich exhaust smell, no stalling. The car did not get up to operating temperature, but I'm hopeful this is the fix I needed.

Thanks to everyone for your input and thoughtful suggestions. The MAF was actually one of my last thoughts, as it was a new part installed only 1000 miles ago.

Now, back to work. I'm a legal aid lawyer helping poor people with real problems.

[Jeff Kleiner]

Sounds like you’re on the right track Joel! A lesson that I’ve learned many times in my 45 years of doing this stuff is that “new” does not necessarily mean “good” ;)

Jeff

[Joecobr]

Good to hear, Joel! Hopefully, it will run well for a very long time! Happy New Year!