Is my computer bad or my O2 sensor? 1996 Ford Ranger

Tiny
PCMENTOR29
  • MEMBER
  • 1996 FORD RANGER
  • 4.0L
  • 6 CYL
  • 4WD
  • AUTOMATIC
  • 180,000 MILES
Using OBD Wizard with an ELM scan tool, my Short Term Fuel Trim (Bank 1, Sensor 2) stays at -1 all the time. In fact the value, min, mean, and max values are all -1 as you can see in the attached image. Could this mean the ECM or computer has a problem? Could this be the reason Cyl No 6 has a constant misfire? You can also see there are values for the Short Term Fuel % trim - Bank 1 which are a little negative or lean condition. Could Bank 1 Sensor 2 O2 sensor be bad? Thanks for your help.


Old question:
My problem is I get p0306 and sometimes p0174. The #6 cylinder won't fire. It has good spark, compression and the injector checks ok with opening and holding, ohms, and noid. I was reading a troubleshooting guide on this engine and tried to hook up my laptop and ELM scan tool. I tested spraying some carb cleaner to test the O2 sensors and they seemed to respond. From this picture can you tell me why the STFT No 2 is -1 across the board? Thanks.
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Monday, June 30th, 2014 AT 11:10 PM

11 Replies

Tiny
WRENCHTECH
  • MECHANIC
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You don't need to be concerned with the short term reading at all. It's the long term that matters and both of those are showing a lean condition. Other than doing an injector balance test, there is no way to know if the injector is working properly. The resistance reading does not give a clear picture. I would check it carefully for any other causes of a lean condition like vacuum or intake snorkel leaks.
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Tuesday, July 1st, 2014 AT 3:09 AM
Tiny
PCMENTOR29
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Thanks for the reply. I tried it on another car and got same issue so I will ignore it. I was planning to change No 6 injector. I have a reman injector and the plenum gaskets. The Plenum gasket set I got has the two full sized gaskets and the 4 or so rubber seals. Can you tell me what to do with the seals? I pulled a used Explorer 4.0 injector from the junkyard which gave me an idea on how to do it but I did not see the rubber seals. A while ago I tested No 6 injector plug with a NOID light and it passed. Also I verified the ohms were good(comparable to good cylinder) And I hooked up a gas pressure gauge and did not see any loss of pressure and I hooked a test plug on the injector and saw I could pulse with 12v and the pressure went down like it should. Should I change the injector or are there other tests I can perform? I will try again to watch the O2 volts as I spray carb cleaner around the intake gaskets. It get up to 110 deg here in the day time so I may wait till dark. When I first started on this two years ago cyl No 6 misfire I remember pulling the spark plug and it appearing wet. Is it possible the injector is somehow allowing too much gas in to the cylinder to make it not fire? Thanks.
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Tuesday, July 1st, 2014 AT 5:13 AM
Tiny
WRENCHTECH
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Before you go changing parts you need to first somehow verify whether it's spark or fuel that is triggering the miss.
What brand plugs are you using?
What does the plug look like when removed?
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Tuesday, July 1st, 2014 AT 7:23 AM
Tiny
PCMENTOR29
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I don't know the brand plugs. I have a jumper wire and plug for the injectors. I was going to an injector balance test as you suggested, thanks. I will pull the spark plug tonight and report on appearance.
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Tuesday, July 1st, 2014 AT 3:02 PM
Tiny
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On the injector balance test good cylinder drops 8 - 11 psi with 10 pulses of 12volts. Bad cylinder drops 6-8 psi with 10 pulses. I did the tests 4 times on each of the two cylinders. I had to start the engine each time to get max psi on the gauge. Is a 2-3 psi difference significant?

I also did a LTFT fuel trim test. On bank 1 LTFT was 9% at idle, 9 % at 1500 rpm, and 11% at 2500 rpm. On bank 2 LTFT was 20% at idle, 14% at 1500 rpm, and 14% at 2500 rpm. Fuel pressure is 30 psi running. Could a dirty MAF cause such bad numbers? I have MAF cleaner spray I could use. Could a coolant temp sensor cause this? I tried spraying carb cleaner all around the engine while watching both O2 sensor voltage. It didn't change.

I also checked the DPFE(EGR device)When I applied vacuum the volts changed from.5v-4.9v like it should. When I applied vacuum it went down slowly. Is this a problem? Also the EGR with vac applied roughens the idle. The EGR diaphragm holds vacuum. When running at idle there is a very small vacuum(1 inHg) on the vac pipe which activates the EGR. Is this normal? I haven't pulled the spark plug yet I think it would look wet. Thanks for your help.
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Thursday, July 3rd, 2014 AT 2:45 AM
Tiny
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It appears the injector balance test has exposed some clogged injectors. That's not a great amount but could be significant during times when a rich mixture is called for. If it's not too much work, you may want to try moving those injectors to different cylinders for testing.
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Thursday, July 3rd, 2014 AT 2:52 AM
Tiny
PCMENTOR29
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I have new set of plenum gaskets. Should I leave the old gaskets on while I change the injectors in and out? Then use the new ones when I finish?
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Thursday, July 3rd, 2014 AT 3:13 AM
Tiny
WRENCHTECH
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As long as they stay in one piece with no damage you can re-use them
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Thursday, July 3rd, 2014 AT 3:17 AM
Tiny
PCMENTOR29
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My main concern is #6 is a dead non firing cylinder. Can you give me the steps to diagnose it? Thanks
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Thursday, July 3rd, 2014 AT 9:43 PM
Tiny
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The same steps that you are already taking. First determine if it is spark, fuel or compression that it is lacking. Once you know that, then determine the cause. If you have already narrowed it down to fuel and your injector balance test shows a problem, then it's time to either replace or move injectors around.
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Friday, July 4th, 2014 AT 6:00 AM
Tiny
PCMENTOR29
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I didn't realize the 3 psi difference was important enough to tear it apart and change the injector. My LTFT2 is at +21%. Is this because of an vacuum or air leak? Or is the injector so clogged it's lean and not allowing combustion. But if the injector works within 25% of normal would this happen? I want to see if the EGR is leaking air into the plenum. How can I do this? Also I sprayed carb cleaner around the injector to see if the o-ring was leaking. It didn't max out the O2 gauge I was using. I did notice when with engine off that when I moved the injector I heard gas escaping. Could a bad injector o-ring cause this lean condition? Thanks for your help
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Monday, July 7th, 2014 AT 9:29 PM

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