MNSmith

Hello, I'm new to the forum. This seems to be the place to find answers!

1998 Chevy C3500, 7.4L, 4L80, 163,000 miles. Developed a sudden misfire. Came home from work, ran fine, hard misfire in the morning. Put my reader on it and got the random misfire code. Figured I do the quickie, plugs and wires. No change. Put the scanner to it. P0300, looked at the cylinder misfire count. #3 and #4, misfire. In 2013, I had thrown 8 injectors at it. Of the 8 I purchased, 2 were bad. So, I put 2 of the good GM's back in there. I guess I didn't have time to wait for 2 more Bosch's to be shipped to me. Back to the present, I pulled the upper plenum and sure enough, #3 and #4 were the old GM injectors. I had the two from 2013, so I put them in there. Still a #3, #4 misfire. Did a harness wiggle test. Ordered 8 new injectors. This weekend, I installed the new eight. New regulator, new fuel line, O rings, new fuel rail. Figured at 160,000 miles, it was about time for the regulator. Reassembled. Still a misfire on #3 and #4.

Verified spark at three and four. Started thinking the drivers were gone in the ECM. Checked all my grounds. All look good. Connected the scanner and did an injector balance test. All injectors pulsed when demanded. 1,2,5,6,7 and 8 have an exhaust temp of 350*, 3 and 4 somewhere around 280*.

I'm think of putting my compression tester on it tomorrow. Maybe grab my vacuum gauge and fuel pressure gauge as I seem to have bubbles at the schrader. Doesn't matter in my mind as 6 out of the 8 are getting plenty of fuel.

Thoughts?

FFDP

iTrader: (2)

Compression test is a good idea and I'd check the cap/rotor too.

B00sted2007

Hell, seems like you’ve got everything covered! Lol!!! Seriously! I’ll be waiting on the compression test results.... seems strange it just happened so sudden though! Really leads you to think something electrical, not mechanical!

MNSmith

That's what it strikes me as, electrical.

Didn't have time to do a compression check today. I did, though, replace the cap and rotor. #3 and #4 fire like they should now. #2 and #8 are now misfiring. The distributor shaft doesn't feel worn, but I'm leaning towards the distributor being the issue. Once warm, it is also hard to start. Went to put the pressure gauge on the schrader and found I didn't bring the right connector home with me. Ah well, tomorrow's another day.

MNSmith

Okay. Update. Distributor in the mail. When I was trying to fire it, it seemed hard to start. Put the pressure gauge on it. At key on, reached a max of 25 psi and dropped almost immediately when the pump relay kicked off. I know it's not the pump as that I installed new back in March and put plenty of miles on it. It's quiet, but it's pumping. What have I changed recently? The injectors, the fuel rail and the regulator. I'm going with a bad regulator. I can put the old one back in or try another new one. I'll opt for another new one. Either way, I wasn't going to pull the upper plenum to do the distributor as I didn't have to. Now, since I need to do the regulator, looks like I'm going to pull the plenum.

1FastBrick

iTrader: (17)

You need more than 25 PSI of fuel pressure for that engine. spec is 56-62 PSI

I don't care if you replaced the pump yesterday. If that gauge is accurate, Thats a Bad fuel pump.

MNSmith

A little more info for you. And this is why I'm not ready to condemn the fuel pump. Key on, the gauge rises to 25 psi max. Immediately falls to zero. That tells me nothing is held in the fuel rail. That is regulator. I mean, the thing won't even hold a pound. So, regulator first. Then we'll see what pump output is. AND what the regulator holds it at. But right now, the regulator doesn't hold anything! At the very least, a good regulator should have held that 25 lbs.

Basic fluid stuff. As flow increases, pressure decreases. If the regulator is completely open, the only restrictions I will have will be orifices and the size of the tubing.

I know what the GM spec is and 25 is nowhere close. And, since there is an immediate decrease in pressure once the pump turns off, unless all the new injectors are bleeding off, which I don't smell or see, the most likely culprit is the regulator. Assuming the gauge is accurate (Snap-on and new), I've got to do what it is telling me.

If it helps anyone feel any better, I have been doing this for 30 plus years. I have piece paper that says I'm a certified Master Auto Technician and another that says I am a Master Certified Heavy Truck Technician. But to me, all that means is that I take tests well. Unfortunately for me, all we buy in the fleet these days is Fords and Internationals. With a bigger emphasis on the heavies. The only time I get to work on my little ol' Chevy, is when it breaks. And sometimes I need to think out loud. Figured I started this thread, I would see it through to the end. One problem at a time.

MNSmith

Okay. Update. Fuel pressure regulator day. Out with the Standard version, in with a Delphi version. KOEO, jumped to 60 psi, held close to 55 psi. I think we have a winner. Gotta get my money back on that Standard unit.

Waiting for the distributor to show up.

MNSmith

Pulled distributor. Drive gear was really worn and there was a lot of slop in the shaft. Installed new distributor. Set "timing". Runs smooth. Took it for a drive around the block. Plenty of power. Looks like that did the trick. It's good to have my ol' truck back. Now I can get back to hauling the fuel altered around. Or the top fuel car. That is, after I reseal the trans pan that decided to spring a leak.