94 ATX Rough Idle, running rich on entire front bank? --- 137, 177, 181 codes

[timmay316]

This past Thursday my 94 ATX started acting up. On the way to work in the morning it started bucking a little, but it would come and go and after a quick stint on the freeway it seemed to go away. I thought maybe it was my imagination and let it go at that. I got in the car to leave that night and it took a few more cranks than usual to start, and when it did it would barely idle and sounded like it had massive cams (really low, lumpy, rough idle). I had no other choice than to limp it home, and in the lower RPMs it was really sluggish, but once the secondaries opened up it seemed to clear itself out. I have a catless y pipe, so I'm not worried about clogged cats, but the exhaust does smell of unburnt fuel at idle.

I checked codes when I got home and it spit a 177 and 181. I poked around a little, and the MAF is good (tried swapping it with a known good one, no change), and there are no vacuum leaks.

I'm really not sure what to think, I did a complete 60k minus changing the CPS about 15k miles ago, and up until now the car ran great. At the time I also changed both O2 sensors and the fuel filter. In fact, just last weekend it made a 450 mile round trip with no hiccups, and got 27.5 mpg.

Could this be a symptom of bad CPS (never been replaced)? My only other guess at this point is the O2 sensors went bad again... Please help!

Thanks
-Erik

 

[timmay316]

Tried replacing the cam sensor tonight, and it seemed to help a little, but the lumpy idle is still there. Also, I ran the KOER tests and got a 137 code (front bank rich), and after the cylinder balance test, cylinders 4, 5, and 6 were all found to have problems. I am at a loss at this point, I really don't know where else to look. Bad fuel pressure regulator? HELP lol.

 

[ViPER1313]

Random thoughts:

My guess is that the timing belt skipped a tooth on the cam and I would look at the timing first. I don't think it would be the coil because they share 2-cylinders on opposite banks. DIS - maybe?? Do your plug wires have a really bad arc??

 

[SeanMc]

I fixed my rough idle by changing out my coil pack.

 

[hawkeye18]

It does seem very odd that only the front bank is acting up. With these cars, if it's a spark issue, you'll usually lose 1&6, 2&5, or 3&4. It sounds like fuel might be your issue, but damned if I can tell more than that. it would be very odd for three injectors to all stick open.

With the car idling, unplug the spark plug wire boot at either end of one of the front cylinders. Is there any change? Repeat for the other three. If there is no change, or the change is the same for all three, then it's not the spark. during the cylinder balance test, IIRC, the computer cuts fuel to each cylinder (as it does not have spark control of each individual cylinder), so if that's throwing codes the fuel system is where I'd look.

Remember, the balance test isn't looking for a specific RPM when it cuts out each cylinder. It's looking for an RPM drop. If the RPMs don't drop when the ECU cuts fuel, then something is wrong.

 

[SeanMc]

You have your wires switched around. http://www.shoforum.com/showthread.php?t=89091 for a diagram on the wire routing.

 

[timmay316]

You have your wires switched around. http://www.shoforum.com/showthread.php?t=89091 for a diagram on the wire routing.

I wish it was that easy lol. My plug wires are hooked up just like in that diagram.

Just for giggles I tried swapping the coil pack. Again, no change. I also disconnected the plug wires one at a time, and every one made the RPMs drop except #4 and #3, even when I disconnected both the idle stayed the same (I used the old screwdriver trick and even though there is no change in the idle, both wires are getting some spark). So I guess I found my problem cylinders, and the fact that those two are both not firing would normally lead me to believe that the coil pack was bad. But like I said I swapped the coil and nothing changed. I threw an Ohm meter across those two wires and both checked out fine, and I even pulled those plugs, cleaned them, and swapped them with other cylinders and the problem didnt follow the plugs, it stayed on cylinders 3 and 4. So I am once again at a loss, the only other thing I can think of is that both of those injectors are either not firing or are stuck open. I find it hard to believe that two injectors would go at exactly the same time though. This is starting to get ridiculous lol.

 

[SASHO91]

I'm having the same problem on my 91' as well.... http://www.shoforum.com/showthread.php?t=91475&highlight=fuel+codes
Nobody seems to have a deffinite answer.

The only thing I can figure is an injector is sticky and only doing it a certain rpms...

 

[timmay316]

That's exactly it, even free revving the car it cleans up at higher RPMs, but down low it just falls apart.

 

[SeanMc]

DIS? Faulty wires? Plugs gapped incorrectly? Possibly a faulty wiring harness. Check the plugs and see if they are full of fuel (should be a sorta gray-ish color).

A while ago i had an issue where I was getting no power to my o2 sensors, misfiring, etc. I checked the wiring harness from the computer to the o2 sensor with an ohm meter, and found I was getting no power. Changed the wiring harness, bam, everything cleared up, including the injector that had stopped firing.

 

[timmay316]

DIS? Faulty wires? Plugs gapped incorrectly? Possibly a faulty wiring harness.

Plugs are all gapped correctly, I double checked today. As far as I can tell the wires are all good, they arent very old, all Ohm out fine, and as far as I can see there's no arcing (sprayed with water). Would/could a bad DIS clear up at higher RPMs? I am starting to wonder about wiring, but so far I havent found anything frayed and I've "wiggle-tested" everything I can get my hands on.

 

[itwonder]

... I also disconnected the plug wires one at a time, and every one made the RPMs drop except #4 and #3, even when I disconnected both the idle stayed the same ... So I guess I found my problem cylinders, and the fact that those two are both not firing would normally lead me to believe that the coil pack was bad. But like I said I swapped the coil and nothing changed.

Cylinders 4 and 3 share the middle coil position, which is coil 2. Since you've ruled out the coil pack and plug wires, there is likely a problem with the C2 signal from the DIS. Try a new DIS and/or check the wiring harness. The C1, C2, and C2 signals from the DIS to the coil can be probed with an LED type 12V tester. The wiring diagrams are in the How to forum under the Firing Order sticky.

 

[timmay316]

Cylinders 4 and 3 share the middle coil position, which is coil 2. Since you've ruled out the coil pack and plug wires, there is likely a problem with the C2 signal from the DIS. Try a new DIS and/or check the wiring harness. The C1, C2, and C2 signals from the DIS to the coil can be probed with an LED type 12V tester. The wiring diagrams are in the How to forum under the Firing Order sticky.

I thought 3/4 shared the bottom coil position? The only question I still have is why does it clear itself up at higher RPMs? I would think a bad DIS would cause problems at all RPMs, or am I wrong?

 

[itwonder]

I thought 3/4 shared the bottom coil position? The only question I still have is why does it clear itself up at higher RPMs? I would think a bad DIS would cause problems at all RPMs, or am I wrong?

I was referring to the location on the SFI wiring diagram. Physically, the plug wire terminals for 4 and 3 are at the forward end of the coil pack.

To clarify, cylinders 4 and 3 share the middle, Coil 2, position as shown on the SFI DIS wiring diagram I pointed you to at http://www.shoforum.com/showthread.php?t=69573
In that diagram you will see that the coil is controlled by the Coil 2 or C2 signal from the DIS Ignition Control Module, pin 11. There is another figure there that shows the pin location for C2 on the coil pack connector.

Make sure DIS grounding is correct.

 

[timmay316]

Thanks for linking to those diagrams, I looked right past them.

Just an update, I tried swapping the DIS module for a known good one. Again, no luck. And so far the wiring harness is checking out fine. How likely would it be I just have a bad ECU?

 

[timmay316]

Had a buddy of mine bring over his Snap-On scanner tonight, and got to watch some sensor voltages in real time. The rear O2 is acting really odd, seems like its almost dead, stays pretty much stays at a constant .75 volts under any conidition, occasionally fluctuates between .65-.9. I know this isn't helping matters, but is there any chance this O2 sensor being "stuck" at a constant voltage is what's causing my rough idle?

It seems to make some sense, the only time the car runs rough is at idle and under low load (ie, cruising at a steady speed) when it would be in closed-loop, but like I said before under heavy acceleration the car runs great, when it would be in open-loop (unless I'm mistaken here). No O2 reference=runs good. I am going to swap it out tomorrow either way, but I guess I was just wondering if a bad O2 could really make the car idle this poorly.

 

[timmay316]

O2 didn't help. I think whatever is causing the misfire is just sending so much raw fuel into the exhaust it killed the O2 sensor. I want to try swapping the ECU, but I was wondering, are the ATX and MTX ECUs different? I've got a friend that has a good computer out of a 94 MTX, but I don't want to swap it in if its not compatible.

 

[Kens1992mtxSHO]

MTX and ATX ECU's are different.

 

[timmay316]

Damn lol. Thanks for clearing that up though. So anyone in the SE Wisconsin area have an ATX ECU?