Alright guys, so here's the deal. I have a 1999 GMC Jimmy SLT, and since I bought it back in September I've made a few posts on here regarding a multiple cyl misfire that was never really properly diagnosed. I live in northern wisconsin so in the winter up here, its extremely cold. It will run like dog crap if the weather is above 10 degrees Fahrenheit. Stumbling at higher speeds, PCM kicking the trans out of overdrive sometimes, hesitation and horrible fuel mileage. Heres what I replaced: plugs, wires, cap/rotor, ignition coil, bank 1 sensor 1 O2, post cat O2....now heres some things the mechanic i took it to found. He said he has no idea what may be causing the misfire, but when they were using the scan tool monitoring the system, he said the 3 O2 sensors were all reading very odd. I do NOT have a catalytic converter, so the O2 reading on that was ZERO. Is that correct because it only monitors the cat efficiency, and there isnt one there? the front passenger O2 was reading like 125 or something and didnt move. I forgot what he said about the driver side one i replaced. The fuel trims are all pretty high also, and the missing is likely because the plugs are fouling out. Also while they looked at it, cylinders 3 and 4 were both misfiring...is that a coincidence that those two are the two middle cylinders? My question: he thinks that if I got a catalytic converter and patched the exhaust leaks after the 2 upstream O2's, this would create more backpressure and make the O2 sensors read normally.....what do you guys think?

 

Pretty much have to start at ground zero here.

What is the fuel pressure with the key on, engine off, fuel pump running, (don't start the engine)?

What is the fuel pressure 10 minutes after the pump shuts off?

Does the SES, (Service Engine Soon) light come on and go off when you turn the ignition to the run position, (don't start the engine).

Is the SES light on while driving?

Does the SES light flash while driving?

What DTC's, (Diagnostic Trouble Codes) are in memory?

After the vehicle has been off for 10-12 hours, and before you start it, connect the scan tool and turn the ignition to run, don't start the engine. Check the ambient temperature, check the ECT, (Engine Coolant Temperature) sensor and the IAT, (Inlet Air Temperature) sensor. Write down all three temperatures and post them.

Post all 8 fuel trims when the engine is at operating temperature.
Need to know if they are positive or negative:
Idle:
LTFT banks 1 & 2.
STFT banks 1 & 2.

2000 rpm:
LTFT banks 1 & 2.
STFT banks 1 & 2.

Plugs, distributor cap & rotor all need to be AC Delco, (it does make a difference).

A good catalytic converter will not affect exhaust back pressure.
The B1S3 O2 sensor, (behind the converter) will read ZERO if the sensor is gone. If the sensor is there and the converter is gone, the sensor should indicate an average of the 2 front sensors.

When you diagnose problems, you have to assume the last guy didn't know what he was doing That's why we need all this data. Start with checking fuel pressure and leakdown and post the results.

 

^^Very good advice above... interested to see what the ECT sensor is doing. Also What did the old plugs look like? Does the engine currently burn any oil?

 

1) fuel press. key on engine off= 61psi
2) engine running= 58psi. I didnt mention that I did the MFI conversion so it's got a new spider in it as well
3) SES light does go on and then off when key turned
4) SES light has come on a few times with the P0300, it recently flashed about 4-5 times and then stayed solid. Next day, it was on but within 10 minutes of driving it went away and the engine ran pretty good. (250 mile trip back home)
EDIT: 5) DTC's in memory are the P0300 and P0147 (i replaced B1S3 O2 sensor, the one that now reads ZERO) and I also replaced B1S1 O2

Now I don't have one of those fancy scan tools that can read live data, but when I read freeze frame data from the misfire code the ECT was 192F.

All I know is LTFT and STFT were positive, don't quote me on this but the LTFT was ~20 and STFT was ~9...maybe vise versa but I cant view it now as the code went away. These were stored at ~2300RPM

My mechanic said B1S3 was just reading zero when he viewed live data, could this be because the O2 sensor is faulty without the cat there? Weird if you ask me! :P

Yeah, the last guy DIDNT know what he was doing. I've had some baaaaad luck with buying used vehicles...maybe this is a sign I'm just going to buy new

KM346, he said #3 Cyl plug was fuel fouled and when he replaced it and I came back to have him check it, he said it still looked good. All the plugs look good at the moment. And no, it doesnt burn any oil. Thankfully! lol

Let me know what you guys think! Sorry I couldn't provide more info for you guys, hopefully what I provided will give you a little more of an idea.

 

On the fuel pressure:
When you initially turn the ignition to the run position, (don't start the engine) the fuel pump is activated for ~2 seconds and shuts off. Fuel pressure must be checked during the 2 seconds when the fuel pump is activated. After the pump shuts off, turn the ignition off. After the pump has been off for 10 minutes, read the pressure again, (don't turn the ignition on). We need to have accurate numbers for both of these tests.

FYI: GM does not publish any "engine running" fuel pressure specs for this system. All tests are made with the engine off. Engine running pressures are worthless.

If fuel trims are positive, the P0300 is being caused by a lean misfire. This lets out the ignition system as a possible cause. If the fuel trims are negative, (rich mixture) the ignition system is still a possible cause. This is why accurate fuel pressure numbers, positive or negative, are so important in the diagnosis. They tell us what to look for, and what we can rule out.

The P0147 is a problem in the B1S3 O2 sensor heater, or the circuits for it. If the tip of the sensor is in the exhaust flow, it should show a voltage reading, even if the converter is gone. If it's just dangling, zero output voltage. Whatever the problem is, it's not affecting engine performance. B1S3 monitors the efficiency of the converter, that's all it does. This is something that can be diagnosed after the P0300 is repaired.

#3 plug being fuel fouled is another problem. Theorhetically it should set P0303 if it's completely fouled. Fuel fouled plugs would cause fuel trims to be negative, and most likely set a DTC indicating a rich condition.

To get an accurate diagnosis, we need accurate data for ECT, Ambient and IAT temperatures, fuel trims, fuel pressure, and leakdown pressure. Without the data, we can only guess what the problem is.

 

My understanding is that the upstream O2 sensors were reading a false lean condition, therefore just dumping more fuel into the engine. The B2S1 was staying the same and didn't change at all at 125 (volts? or fuel trim not sure what he said) B1S1 was reading normally and B1S3 was just 0....I'm not sure, but you are correct, I don't want to play the guessing game any more with this thing...costs too much money! I also just think its weird that on some days it really bad, and some days you dont notice it at all. Intermittent issues will be the death of me!
What I will do is on Friday when I replace my cat I will take it to the shop and have my mechanic who does all the "professional" work that I cant do, and have him scan it and give me all those readings.

with these trucks, its a love hate relationship! I will let you know what those readings are and we will go from there.

 

If/when an O2 sensor is dieing, or dead, its reaction time is slow, or voltage output is low, or zero. Either way, the PCM will interpret it as a lean condition and increase the injector pulse width. This will richen the mixture, and show positive fuel trim numbers, but the O2 sensors will still report lean. The PCM will increase pulse width as far as it can, fuel trims will go max positive, and fuel mileage goes right down the toilet. A good scan tool can check the O2 sensors to make sure they are reporting and switching correctly. B1S1 and B2S1 both need to be good for fuel trims to be accurate. Fuel trims need to be accurate to help diagnose the problem.

Since it seems to be temperature related, you'll need to leave the vehicle with him overnight. The vehicle must set 10-12 hours without starting it. This gives both sensors time to stabilize with ambient. Connect the scan tool, turn the ignition to run, (don't start it) write down temps for IAT, ECT sensors and ambient temperature.

 

That makes sense, Hook. I'm old school and don't understand how computers work so I like, you know, hate computers

 

Diagnosing the computer control system on a vehicle is no different than anything else: Once you understand how it works, it's not so difficult to figure out why it doesn't. The PCM is a very powerful, invaluable piece of diagnostic test equipment. It's not there to intimidate you, it's there to help you.

 

I'd like to learn more about it, which is why I'm going to a tech school for this sort of stuff.
On a lighter note, I will post my results as soon as I can