Real Fiero Tech

My motor ('93 3.4 DOHC) started running super rich and has extremely high BLMs... something is confusing the ECU into thinking it's lean when it's not. I replaced the O2 sensor, can't find an exhaust leak, and have new spark plug wires. I'm now thinking it's a bad spark plug (I'm running AC Delco iridiums that I haven't checked in at least 25,000 miles; but they're supposed to last 100,000), or I have a dead injector.

The problem began around the time my alternator failed (the positive battery terminal stud came loose), so I may have had an overvoltage condition that damaged an injector coil. That would cause one or more cylinders to not fire, and the ECU would increase the fuel trim since the unburnt oxygen shows up as a lean condition. I'll be testing the injectors tomorrow.

So on the topic of fuel injector replacement...

Since new injectors are ~$62 each (i.e. too much) from a brand I've never heard of, I'm probably just going to get some used injectors and send them to Rich at Cruzin' Performance for cleaning and flow testing. I can't necessarily find a 91-95 DOHC in the yard, so what other options do I have for interchange without modifying the fuel rail or lower intake? I'm using the 91-95 DOHC fuel rail. I'm comfortable with going to up to 25lb/hr injectors. I have stock injectors now (20 or 21 lb/hr I think) but am making 15% more power than stock so I definitely don't want smaller ones.

I would prefer to not change the connectors but I can do that if necessary.

I can't suggest alternatives to pick up locally, I'm very tired atm and the thought of opening up the interchange again today makes me sick. However, I have in my reach six OBD 1.5 (94 I believe) TDC injectors. If you need, rifle me a paypal and I'll send them. While I may be lousy at wiring, you know how well I package items..

Do the 94 injectors flow the same as the 91-93?

Apparently the part numbers are different from '93 to '94. It anyone's guess what the differences are with sequential, I don't have any reference for flow. I quick scan doesn't turn up the same injector on anything else from 91-93. Want me to check the yard for a set of 93's?

The new 60*V6 search sucks btw.

Well I ended up testing the injectors today by hitting them with 12 volts and they all seeeeem to work ok... so now I'm running out of ideas on what might actually be wrong.

According to Fieromadman, all the injectors flow about the same, they just changed the design to make them quieter and try to solve the stalling issue.
If I still can't find what's wrong by tomorrow I think I'll buy your injectors, have them cleaned and tested, and toss 'em in just in case..

Have you ohmed out your injectors? Maybe something will show up with that.

Ground fault? I assume you listened to them with a stethoscope to make sure they are working.

Have you logged and graphed PCM data to see if anything else jumps out at you?

Have you actually checked spark to be sure you're hitting on all 6? Bad coil is more likely than a bad injector. Pulled plugs? I know it sucks on a TDC, but I've done it on a Northstar, so kwicherbichin.

I also have a set of 94-95 injectors for sale. They were working fine before I upgraded mine. The flow rates should be about the same from year to year.

The Dark Side of Will wrote:Have you ohmed out your injectors? Maybe something will show up with that.

Yes they are all at +/- 0.1 ohms. The complete set in parallel from (+) pin to (-) pin on the mini-harness agrees with the measured values +0.5 ohms. I did not verify the wiring resistance under load. It's possible that there is a connection problem there, though last time I had a problem in that circuit it resulted in a no-start condition since the voltage sagged too much during cranking. I will do some more testing.

The Dark Side of Will wrote: Ground fault? I assume you listened to them with a stethoscope to make sure they are working.

They all share one ground so if it's a ground fault none would work. I did not check them with a stethoscope but I individually pulsed them with 12v to confirm that they do open and flow fuel.

Have you logged and graphed PCM data to see if anything else jumps out at you?

No, unfortunately I don't have a laptop that will datalog right now. My scanner will only log @ 1hz for like 10 seconds, so it's not useful for that. I did scroll through all my data to make sure things looked reasonable. I fixed a vacuum leak to my MAP sensor and now get the same stable idle MAP as Blue Shift's car (I scanned it a couple days ago).

Have you actually checked spark to be sure you're hitting on all 6? Bad coil is more likely than a bad injector. Pulled plugs? I know it sucks on a TDC, but I've done it on a Northstar, so kwicherbichin.

Since it's wasted spark, a bad coil means running on 4 cylinders. It doesn't sound or feel like that's the case. I've had many bad coils and its pretty obvious (in sound and power) when it turns into a Quad-4. I did pull the rear spark plugs and they looked beautiful. Smooth and slightly tan, no damage or oil. I am going to pull the front plugs this evening after my engine cools down. Hoping to find a busted ground strap or something...

Series8217 wrote:

The Dark Side of Will wrote:Have you ohmed out your injectors? Maybe something will show up with that.

Yes they are all at +/- 0.1 ohms. The complete set in parallel from (+) pin to (-) pin on the mini-harness agrees with the measured values +0.5 ohms. I did not verify the wiring resistance under load. It's possible that there is a connection problem there, though last time I had a problem in that circuit it resulted in a no-start condition since the voltage sagged too much during cranking. I will do some more testing.

Just double-checked and my ohmmeter has a 0.5 ohm offset so the wiring checks out great.

The Dark Side of Will wrote:

Ground fault? I assume you listened to them with a stethoscope to make sure they are working.

They all share one ground so if it's a ground fault none would work. I did not check them with a stethoscope but I individually pulsed them with 12v to confirm that they do open and flow fuel.

I checked them with a stethoscope now.. couldn't get a reliable diagnosis. I can unplug the whole front bank of injectors, the motor will clearly run on 3 cylinders, but the steth sound doesn't change; the pulse from the other injectors firing in unison is far louder than a single injector even with the probe pushed directly into it. Too much vibration traveling through the fuel rail.. (keep in mind this is a batch fire motor).

I also tried unplugging and plugging back in the front bank injectors. I couldnt really tell a difference at idle by just unplugging one at a time (any of them) but with the front 3 unplugged it was definitely rough, and I unplugged one at a time then unplugged it and tried the next one; the idle sound was different each time.

I tried monitoring the integrator while unplugging injectors but I don't have a heated O2 sensor so it drops out of closed loop at low idle, especially with less than 6 cylinders..

Just finished checking the front plugs.. they all look good. One of them broke loose easier than the rest, but not enough that I think it was an issue.

I also plugged them in and laid them on the intake while I cranked the motor... they all sparked nicely.

did you check the spray pattern on the injectors?

Nein. Don't have a rig set up to test the fuel rail off the car.. I'm going to get another set of injectors and have them tested and cleaned by Cruzin Performance and just put those in.

I finally found an exhaust leak today, at the EGR port right next to the O2 sensor.... fixed that and nothing changed... wtf.

I'd tell you just to cap off the EGR, but you can't do that because you're in the PRK.

I guess you drove it around after you fixed the EGR leak? Any other EGR leaks? Broken EGR tube, for instance?

My EGR is blocked off. I don't have EGR on my short-runner intake.. it was the blockoff plate on the rear manifold that was leaking.

I had an injector fail on one of my Z34s. It surprisingly didn't do what you described, it just idled and ran rough. No SES light, not noticeably rich either. OHM'ed the injectors and found one bad.

Series8217 wrote:According to Fieromadman, all the injectors flow about the same, they just changed the design to make them quieter and try to solve the stalling issue.
If I still can't find what's wrong by tomorrow I think I'll buy your injectors, have them cleaned and tested, and toss 'em in just in case..

He's right, and the newer style significantly helped the stalling issue. So make sure you get 94-97 ones. 96-97 were the same, I think they flowed 1# higher maybe.

Series8217 wrote: They all share one ground so if it's a ground fault none would work. I did not check them with a stethoscope but I individually pulsed them with 12v to confirm that they do open and flow fuel.

I could be wrong, but IIRC, they all have separate grounds, the power is the shared wire. The ECU just grounds the injector to fire it.

I hate to even suggest it, maybe cam timing? Bad gas?

I replaced the lower fuel injector o-rings yesterday along with the fuel pressure regulator JIC, but the fuel pressure reads fine on the gauge.

Aaron wrote:I had an injector fail on one of my Z34s. It surprisingly didn't do what you described, it just idled and ran rough. No SES light, not noticeably rich either. OHM'ed the injectors and found one bad.

He's right, and the newer style significantly helped the stalling issue. So make sure you get 94-97 ones. 96-97 were the same, I think they flowed 1# higher maybe.

I don't have any stalling issues..

Series8217 wrote: They all share one ground so if it's a ground fault none would work. I did not check them with a stethoscope but I individually pulsed them with 12v to confirm that they do open and flow fuel.

I could be wrong, but IIRC, they all have separate grounds, the power is the shared wire. The ECU just grounds the injector to fire it.

They fire at the same time; the ECU is supposed to have two injector drivers.. IIRC there's something weird about it; only one injector driver actually on the PCB or something.. Anyway, they all fire together as a batch, the grounds are shared at the mini-harness.

I hate to even suggest it, maybe cam timing? Bad gas?

I've gone through two tanks of gas so I don't think so, unless they're putting 50% ethanol in there now. It's California, so you never know. Cam timing possible but unlikely and doesn't fully explain this behavior.

Series8217 wrote: I don't have any stalling issues..

Nor do you want any, don't put in 91-93 style if you do change them

Series8217 wrote: They fire at the same time; the ECU is supposed to have two injector drivers.. IIRC there's something weird about it; only one injector driver actually on the PCB or something.. Anyway, they all fire together as a batch, the grounds are shared at the mini-harness.

I'm sorry dude, lack of sleep affects thought process, I was thinking SFI.

I've gone through two tanks of gas so I don't think so, unless they're putting 50% ethanol in there now. It's California, so you never know. Cam timing possible but unlikely and doesn't fully explain this behavior.

Haha, although likely being land of the nazis, I doubt it. Plus I was just out there a week ago and my SRT ran great. Clicked off a 12.7. Although most wouldn't explain the symptoms, and would throw a SES lamp, it could very well be sensor related. Maybe try throwing in the ones you have extra of?

Checked the torque on your exhaust manifold bolts?

The Dark Side of Will wrote:Checked the torque on your exhaust manifold bolts?

Haven't checked that, good idea though. I'll check tomorrow.