Greetings. I am new to this site and am very impressed by what I see so far. I am becoming frustrated with my 97 5.7. A few months ago I installed a new Delphi fuel pump. Shortly thereafter a misfire started. After replacing intake gaskets, plugs, wires, cap, rotor, and EGR Valve, I determined it must be an injector so replaced all with updated spider assembly. Truck ran like a new one for about 1500 miles. Now has misfire, but only when "warm". When starting from stone cold it will run perfectly for entire trip. When started hot, it also runs fine. But when starting after has cooled down a bit will misfire for a bit before smoothing out. Seems to be taking longer though now to smooth out then a couple days ago. Compression tested fine, all parts replaced with Delco, no codes, just flashing CEL when misfire happens. Any thoughts?

 

It's possible that other ignition components could be giving you problems. Ignition coil, ignition module, or pickup coil. All of those parts have a tendency to operate a bit odd given different temperature conditions. The only real way to know is to try to get the codes read. Although with the CEL just flashing instead of staying on, it may not be storing any codes.

Also, just thought about it, the camshaft position sensor, or the crankshaft position sensor could be suspect too. These both help the ECM determine what cylinder is ready to fire.

 

I just have cheap OBD II reader, but no codes stored as of yet

 

Did you do a leak down test? It may be the valve stem seal causing your problem. A leak down test will let you know if everything mechanically is fine. If you have air bubbles in the radiator, this means you have a cracked cylinder head. If so, you need to replace the head gaskets. If air is leaking from the exhaust or intake, it means you could have a bent valve or bad seat.

 

I don't think it is any of these as the misfire is so intermittent. It either runs extremely well, or has a very pronounced misfire. Today on cold start up, there was a misfire and flashing CEL for about 2 minutes. Then the misfire stopped as did the flashing CEL, and for most of my half hour commute which is mostly highway, it ran great. Then started misfiring again on and off for the last part of the trip. Whatever the problem is, it seems to be happening more often. I will dive in and start investigating tonight.

 

Plugged in OBD II reader on the way home today and got P0137 - O2 Sensor Low Voltage Bank 1 Sensor 2. Is the misfire causing the O2 sensor code, or is the O2 sensor problem causing the misfire?

 

A misfire wouldn't cause a voltage code. It's a problem with the circuit or a faulty sensor. I doubt this would cause your miss. The heads on the 96-early 99's had a problem with cracking. I've replaced the heads on 5 pickups in these years. All were between 160-180k miles