1998 C1500 Cylinder 8 Misfire,

[Gonzaga Lobos]

This is my very first post. I figure I needed to post my story and findings since I've been trolling this website for months now.

I have a 1998 C1500, V8, 5.0L, 305, Extended Cab, 91K miles with the original engine. In April, I drove this from NM to CA. I got about 22 miles to gallon. Truck in general is in fantastic shape. While pulling into my neighborhood and into my driveway one day, I noticed the engine misfiring. I looked around and noticed the smell of gasoline underneath the hood. I noticed there was a pool of gasoline on top of the fuel injector assembly, right next to where the 2 fuel lines enter. I pulled off the electronic connector that connects on top of the assembly and there is gas in there as well. That wasn't good.

One thing that I did notice was that my fuel pressure goes from 58 PSI down to 50 PSI and stays at 50 PSI with the fuel pressure rail check, engine on and it doesn't fall below 50 PSI for a while after shut off. I don't know if a fuel pump change would fix that since all I read on other forums is 60-66 PSI is standard for these trucks. But is 60-66 PSI for the older fuel injectors? OR the multiport injectors? OR does it matter if the truck seems to be functioning normally with 50 PSI?

I do know that eventually I will have to replace the fuel pump, because I think that the sender is going out or something. When I turn the key to 'on', the needle for the fuel gauge on the dash goes WAY past 'F', then goes back down to where the needle should be in about a second or so. But that's another story to be told in the future once I get the new injectors in there.

I know that this is was a long post. I feel like I should contribute to this awesome forum, since it looks like this website will be bookmarked on my laptop from here on out. I even replaced my entire A/C system, minus the evaporator and compressor by just reading some posts on here. Much more to learn. I appreciate it, everyone.

 

[Schurkey]

Fuel pressure is supposed to be something like 62 psi during the prime cycle--and it may take more than one prime cycle to achieve max. pressure.

With the engine running, the fuel pressure is reduced because there's vacuum acting on the fuel pressure regulator. My '97 manual doesn't give a spec for pressure with the engine running, it just says it'll be lower than the prime cycle.

Good job finding the resistance problem with injector #8.

If you have stepper-motor gauges, it's fairly common for them to go full-range and then display the "real" figure. I don't know if you have stepper-motor gauges. If the gauges read accurately after the initial turn of the key...I don't think I'd worry about it.

 

[Gonzaga Lobos]

Just a follow up......it's been a while. To re-cap...

OBD II, Random Misfire. Cool! I figured it was the fuel injectors, so I look up videos on how to remove the plenum, remove and replace the fuel injectors with GP Sorensen Fuel Injectors, 800-1513N. Everything was great until about a couple of months ago. I started it up one day and the CEL popped up and gave me the Cylinder 8 Misfire code, P0308. Great. So I changed plugs/wires, ignition coil, compression test, MAP Sensor, Coolant Temp Sensor, cap/rotor. I realized that it would only misfire during start-up when the engine was warm. Every now and then, it would give a slight backfire at a cold or warm start up. It would always seem to start up without fail. About a week ago, while the engine was cool, I checked voltage coming out of the fuel injector connector, everything checked out. I also checked the resistance on the injector assembly for each injector, 12.6-12.8. Each injector has 2 prongs. Those checked out too.

Yesterday, I decided to check out the resistance on each injector only after I drove around for a while to get the engine warm. When I did that, I got an 'OL' on the multimeter on Cylinder 8, with the rest of the 7 injectors showing around 14.4 or something. I waited about an hour to re-check the Cylinder 8 injector. It went from 'OL' to 90.7, then worked its way all the way down to 16.8 or something within that hour. Those numbers got lower and lower as the truck engine cooled down. I reattached the intake, fired it up and it started up without misfire. Needless to say, I will remove these injectors and send it right back to AutoZone for a full refund, 3 year warranty.

Got my refund.

Fast forward through all that, and after breaking some injectors from the new set of 8, I finally got it up and running. No misfires, no codes showing up. You do have to be super careful when inserting or removing those plastic injector tips into the head/hole. When I removed the AutoZone brand injector unit, those came out super easy. But with the new injector unit I bought, those tips were stuck in there for good. There are videos on how to get those injector plastic tips/tubes out if you need to take them out. It was a pain. I almost ran into some issues, but ended up removing a few of them. Be super careful not to drop anything in those holes. Take your time. So far, so good. It randomly backfires after start up, sometimes, not all of the time. I don't know.