OK, First off, I have a 1990 ZF6 Coupe. Modifications include; deleted air pump, deleted egr, hedman headers (no egr fittings), and 3" exhaust w/ x - pipe, bullet cats and magneflow mufflers, and an adjustable fuel pressure regulator. These mods were done about 4 years ago, except at that time the exhaust behind the headers was a dual 2.5" system without cats. Back around that time I adjusted the timing and fuel pressure to target a high 800mV O2 sensor and 0 knock timing retard.

Fast foward to this past weekend when I replaced the clutch and flywheel with stock LUK parts. No issues 'til this mornings trip to work. I noticed the average fuel economy reading start to decrease about 45 mins into my 3 hour drive. I normally see 23.1 mpg, but it steadily decreased to 20.0 mpg by the time I arrived at work. I also noticed an increase of "popping" in the exhaust, and near the end of the drive when the road was clear to accelerate, black exhaust. The SES light came on, but it usually does on the highway because I removed the egr.

During lunch, I will attempt to pull the codes, check the resistance of the fuel injectors and check the fuel pressure (I need to buy the multimeter and fuel pressure guage).

I had a long weekend putting the clutch and flywheel in and a long drive back this morning. I'm looking for some feedback to check if i'm heading in the right direction, some other checks I can do to diagnose the issue, and people's experince if they've had a similar problem.

Thanks,

 

I had a simular condition when an injector failed. It was working part time. It was only detectable by listening to the injectors with a mechanics stethoscope. A section of tubing held to ear and placed on injector will work. Listen for a steady "tick-a tick-a tick-a" at idle.

The one I had that failed went "tick-a tick-a tick-a-pause-tick-a pause-tick-a tick-a-pause". There is not supposed to be a pause. During the pause the injector is stuck and will flood that cylinder causing the rich condition.

This may not be your trouble but until you pull codes and post them I thought it may be worth checking.

 

It could also be a faulty O2 sensor or faulty connection, of if the weather was cool you could need a heated O2 sensor.

Before I converted to heated sensors they would get very cool, the car would dump more fuel, cooling it off further, causing it to dump more fuel, until the car would barely run and it would burn your eyes. If I could get it to run at a sufficiently high enough RPM to warm up the O2 sensors so that they actually responded, it would run fine again until it cooled off.

If you have access to a scan tool it would be useful to watch your O2 sensor and fuel trims. You might be able to determine where the problem is without ever popping the hood.

 

Thanks guys. I went out at lunch to attempt a Datamaster scan. Unfortunately, I didn't have the drivers for my Belkin USB to Serial adapter on my laptop. I used the paper clip method and got one code, 44 (low voltage O2). Now I'm wondering if I plugged the O2 sensor back in I'm fairly confident I did, though. I hope it won't do too much damage waiting 'til the weekend to replace it. I don't have any place to change it, or a jack to lift the car.

Thank you for the tip on listening to the injectors. I'm fighting a rough idle, smoothe high rpm condition. A lot of the rough idle was fixed by the replacement of the dual mass flywheel, plus I believe I hear a spark going to ground through the plug wire. The car has 140k miles, so I'm sure there's a ton of stuff that needs replacing, but I do want to make sure I replace stuff that needs replacing.

I've been thinking about a heated O2 since I did the headers 4 years ago. I may instead try the WB O2 hack for the ECM. Or, one of the WB kit's has a simulated NB output, so I wouldn't have to hook up two sensors.

 

I finally got Windows to cooperate with the Belkin port converter and was able to capture some data today during lunch. The car is not staying in closed loop, the O2 volts are low and so are the O2 cross counts. I'm thinking it's time for a heated O2 sensor. If anyone has any tips, or other things to look for, let me know.

 

Having the same problem... not sure if it's the injectors or the wideband sensor...

But I think it's going to be an injector.

It just started for me as well.... JUST when I thought I had the damn thing together.

 

I had the same problem. I had a leaking injector, which didn't solve the problem. Heated O2 sensors did the trick. Check our casperselectronics for prewired heated O2 sensors.

 

Thanks for the casperelectronics tip. They have a bunch of stuff! I'd buy that kit, but I need to make sure I fix the car this saturday. i'm going to have to run around and piece something together myself. But, the picture provided on casper's website, plus the C4 TechTip are helping a lot. It looks like I have to find an O2 sensor first, then I can get the connectors to create a harness.

 

I just used that tech tip and realized that my ~2 yr old fuel pressure regulator pussed out on me! Damn thing is holding the right pressure as long as the pump is on, but is leaking fuel into the manifold through the vacuum hose.

And it was an aftermarket unit (Holley).