My 1998 Z71 5.0 starts and runs great....until it's in a long pull, like a mile long hill. Started this about a month ago. Runs strong until 3/4 or so of the way up. It begins to miss, sometimes almost to the point of stalling. New improved fuel spider a year ago, recent fuel filter change. Because it felt like a fuel issue, I ran my fuel pressure outside an zip tied it to a wiper arm. 55 psi the whole time. Been a while since I tuned it up, so installed new plugs, wires, cap and rotor. No change. Rarely sets a code, when it does it's always P0305, but this is obviously more than 1 cylinder missing. I am attaching a screen shot of what my scan tool is showing for fuel trim. I don't understand exactly what fuel trim is, but there seems to be a big difference between bank 1 and 2. Am I correct in that thinking, and if so could this be a cause of the problem?

Much thanks.

 

Fuel trim is the PCM’s response to oxygen sensor input - positive fuel trim means the PCM is adding fuel (opening the injectors longer) and negative fuel trim means the PCM is removing fuel.

Short term is a direct response to the upstream (pre-converter) sensor and long term is the overall or average adjustment of fuel trim over the course of a drive. Ideally the fuel trim (short or long term) stays at zero, but -10 to +10 percent is normal.

The difference between bank 1 and bank 2 suggests a problem with the bank 1 upstream sensor, injector wiring, or a mechanical problem affecting only bank 1. This could be something such as an intake or exhaust leak isolated to bank 1 upstream of the sensor.

That said, if you’re certain this is happening on multiple cylinders, I’d recommend either replacing the ignition coil or testing it the next time the problem is present.

 

Thanks for the reply. There is only 1 sensor, and it is on the bank 1 exhaust. I feel certain it is multiple cylinders due to the truck at times almost stalling. I don't believe in "throwing parts" trying to resolve aa problem, but I am seriously considering replacing the coil and see if it makes any difference.

Thanks again Gumby

 

If it truly only has 1 oxygen sensor, then either it was originally equipped to meet some oddball emissions standard, or the other sensors have been deleted and some kind of tuner or programming has been done to allow the PCM to compensate. By 1998, OBD 2 protocols were required by law, and the PCM can’t make the adjustments necessary to meet emissions without 4 sensors.

Additionally, there wouldn’t be any fuel trim readings (they would be pegged full negative, full positive or listed as NA - I can’t recall), because the PCM remains in open loop (default air/fuel mixture) until the sensors are heated adequately to provide accurate readings. Until then it ignores O2 input.

I’m not one to just throw parts either, but it’s been my experience that ignition coils are quite difficult to accurately test (at least with a single coil system). Resistance checks across the terminals are often unreliable unless it’s a direct short. I know they tend to start malfunctioning when they get hot, it tends to be more of a problem under load, and the older the coil the more likely it is to happen. In terms of parts, they’re cheap enough that it’s worth a shot before moving on to more involved diagnosis, IMO.

Other components affected by heat that may not be throwing codes are the cam and crank sensors, and the ignition control module. I can provide the necessary info to test them if you’re interested.

 

I appreciate your input Gumby, and I stand corrected, there is an O2 sensor on bank 2, it's just not as visible as bank 1, and I've not had occasion to look before. The coil appears original, and with 220,000 miles and 20+ years I'm going to invest in a new one. And please do send the testing info.

 

Sometimes it hard to see the forest for the trees. I've chased this miss for a month now, spent a lot of money and time. I woke up this morning and watched a you tube video on O2 sensors and fuel trim. The guy doing the video said something that made a light bulb flicker in my head. He said don't think of "lean" as not enough fuel, but to much oxygen.
The truck has had an annoying exhaust leak that was steadily getting worse. Turn out the leak was where the pipe connects to the exhaust manifold. 6" above bank 1 O2 sensor. I remembered from my drag racing days using vacuum created in the exhaust system after every cylinder fires for crankcase venting. Cut to the chase, fixing the exhaust leak stopped 02 from being drawn in and fooling the 02 sensor. A trip up my "test" hill and the truck pulled smoothly all the way driving normally. I circled back and started back up, the time pushing it pretty hard. A slight miss developed at the very top, and quickly smoothed out. I figure the computer is relearning the new data it receiving.

 

Hopefully it’s resolved for good then. The real test will be pulling a fully loaded trailer up that hill on a 90+ degree day.

 

My thoughts exactly. My 18' flat bed trailer is heavy by itself, Gonna try it empty first. The short term fuel trim numbers are much better, #1 1.56 & #2 2.34.

 

Hi, Jim&Doani: I was about to post the suggestion about checking your exhaust system for a couple of things like the Catalytic Converter, other obstructions or leaks, but you already found the solution. Good for you! And thanks for posting the update. I had this type of problem in the same situations and it turned out to be my Catalytic Converter (water accumulation over years of sitting too much, and the inside developed an obstruction).