Originally Posted by burned

You are not going to get a code for the misfire when the CEL is flashing.
Mine did the same thing when I got it up to 70+ mph or floored it up a hill (flashing check engine light).
I hooked up my scanner and watched the live data as I floored it up a hill. When I got up to 60 mph, cylinder #5 started to missfire constantly then stopped as soon as I let off the gas.


You may have the same problem. Not uncommon on these engines.

Originally Posted by Captain Hook

The fuel trims are still all over the place. If the numbers are positive, it means there's a lean misfire, (too much air or not enough fuel) and the PCM is adding fuel to achieve 14.7:1 fuel ratio. Usually a vacuum leak, or a restricted fuel injector nozzle. If they're negative, it's a rich misfire, (too much fuel or not enough air) and the PCM is subtracting fuel to lean out the mixture. This usually indicates a secondary ignition fault, (fuel not being burned) but it could also be an injector not sealing shut 100%. In the last freeze frame snapshot, the only one that sends up a red flag is LTFT B1, (cyls 1, 3, & 5). -6.3 is indicating that at least one of those 3 cylinders, and maybe more, is/are running slightly rich. Ideally, in a perfect world, all fuel trims should hang close to zero. Inexpensive things to try would be switch to AC Delco 41-993 spark plugs and check fuel pressure and leakdown. The leakdown test might spot a leaky injector. The other option for checking injectors is an injector balance test, but that requires a high end scan tool, and a very accurate fuel pressure tester. Without one of those big money scan tools, you're really limited when it comes to pinpointing problems like this. Just need to analyze the data you have, and make an educated guess

Originally Posted by serpico1

And what did you do to fix it?

Originally Posted by Captain Hook

Not surprising at all to have fuel pressure increase when you shut the engine down, in fact, it's perfectly normal. Checking fuel pressure at idle is the lowest pressure you should see. We're not concerned about the lowest pressure, we need to know the highest pressure. There are no "engine running" fuel pressure specs for this engine. All tests are made with the engine off.

In the underhood fuse panel, (on 1998 and newer), there is a vacant terminal next to the fuel pump relay. It is commonly referred to as the pump prime terminal. The circuit goes directly to the fuel pump.



Connect the fuel pressure tester to the service port next to the distributor.
Connect one end of a fused jumper wire to battery positive.
Connect the other end to the pump prime terminal.
When fuel pressure stabilizes, write down the reading.
Disconnect the jumper wire.
Wait 10 minutes and write down the fuel pressure again.
Post both readings.

The results will determine if a problem exists with the fuel delivery system. If there is a problem, they will not tell what or where the problem is; Further diagnosis is necessary, and we can explain how to pinpoint the problem.

Originally Posted by DimeBlazr

Fuel trims can be tricky to understand but they are a big help, really sucks that your code reader cant do live data. Heres some things to look at #1. Maf, remove it and take the 3 pieces apart so that you can actually see the element and clean it with the maf cleaner and an old toothbrush VERY GENTLY!, allow to dry and reinstall. This is the first component that will cause goofy fuel trims. If the maf has gunk built up on it it will tell the computer that there isnt a lot of airflow (when there actually is) so it wont deliver much fuel, this gives lean o2 readings and the the computer commands the fuel trims high positive numbers but still wont run well because it has mixed signals on what kind of load condition is actually present, thats why i had you disconnect maf, when you do that the computer uses throttle position, engine rpm, and manifold abolute pressure (vacuum) readings and calculates how much fuel and timing to give it based on predetermined values, it totally ignores the maf and O2 readings, its a good quick test to see if you have a maf or O2 sensor problem. However, i think your first problem may have been your air pump allowing unwanted air into the exhaust stream somehow which caused it to think it was lean and made it add lots of fuel which may be what hurt your original converter, it may have overheated or become partially clogged then. But now what you have may be an ignition system problem, your negative fuel trim number on bank 1 could be caused by a cylinder on that bank not firing and sending raw fuel to the o2 sensor which tells the computer its rich and commands the fuel trim negative to pull out the excess fuel, even though the other cylinders on that bank were running ok they were also commanded less fuel and now will make less power because they will be lean and they will also misfire, yes their lean condition would combat some of the non-firing cylinders rich signal but remember a lean misfire still means that some fuel wasnt burned during the mis and will also contribute to the rich o2 signal, the "false" rich o2 signal will command the fuel trim negative for that bank which can cause the P0300 misfire, in some cases on single bank engines the computer can actually lean it out on a false rich condition to the point that all cylinders are starved enough to cause it to stall. Another scenario is that you have an injector sticking open and thats causing the false rich condition.
Hopefully I didnt mess that all up and its understandable.
I say dissasemble maf and make sure its clean first, then if you havent done anything with the ignition system for a while i would look into that, plugs, wires, cap and rotor of good quality.

This picture is the maf from my blazer, it ran like crap before i cleaned it and was actually throwing O2 Sensor "LOW signal voltage" codes for both banks. Once cleaned the thing instantly ran like a champ.

Originally Posted by Captain Hook

Fuel pressure is too low, possibly causing both DTC's to show up. Fuel pressure must be within spec so the correct amount of fuel is discharged when the injector opens. If pressure is too low, not enough fuel is discharged and the engine will run lean.

Fuel pressure, when tested at the service port, is regulated pressure. It is not what the pump is capable of building. The reading is the lowest pressure of the fuel pressure regulator setting, or the maximum output of the fuel pump, (whichever is lower). Without further testing, a guess would be a faulty fuel pressure regulator. To find out for certain what the problem is, more testing is necessary. The next step is to find out if the pump is able to supply sufficient pressure. This is done at the fuel filter outlet. The nylon outlet line is disconnected from the filter, and the pressure tester is modified so that it will connect directly to the filter. All fuel pressure and flow must end at the tester, with no fuel allowed to the fuel metering block on the engine, (leave the disconnected nylon line dangling). While the pump is activated, the stabilized pressure must be 73psi to 108psi, and it must remain above 55psi for at least 10 minutes after the pump shuts off. Post both readings.

Originally Posted by Captain Hook

This will help with the adapter:

http://i561.photobucket.com/albums/s...ps5c876ad4.jpg


80MPH, (hmmm) 197F coolant temp, (perfect) perfect fuel trims, (something wrong here if it was misfiring) open loop, (something wrong here too, should be closed). Clear 'em out and do again

Is that the only data your scan tool shows in freeze frame?