Hey guys, my pops truck started misfiring badly the other day while he was driving. He called me so I drove over to where he was to check it out. I brought my scanner with me to look at the CEL and I got a code for "random misfire"

The truck is a 2000 CCSB Sierra 2500 4x4 (classic) with the 7.4L big block and has 170K miles on it. It could barely idle and was popping from the exhaust. He was only about two miles from the house so I was able to park my truck and limp his truck the rest of the way home.

So far, I've changed the spark plugs and wires as a starting point and there was no change. What else should I look for?? I've heard the 7.4's are notorious for the distributer shaft going out on high mileage trucks. I'm thinking that's the next step, but I want to be sure before I tear into it... From what I've read on the various forums including this one... its not a fun job... Advice??

 

It's not a fun job and it has to be right on dead nuts as there is little to no adjustment if you are off 1 tooth. Intake manifold had to be removed and you will need new gaskets.

Be sure to Check your fuel pressure. Pump could be dieing.

Cap and rotor may also need to be replaced as I did not see that mentioned. Make sure you get the correct rotor too!!! ask me how I know... :-(

 

Order a replacement distributor from eBay, they have a nice one that's about 45 bucks and it's made of aluminum. Kinda a pain in the *** to change but it's doable with the upper intake plenum off. While you are in there Swap out the injectors for some ls1 stock injectors (direct swap) and replace the FPR. The factory 454 injectors suck. Made a huge difference on my truck. But the eBay distributor is cheaper than you can get a cap and rotor for at the local parts store.

 

Ok, yeah I figured I had to remove the manifold since I can barely see the distributer back there. I will check the fuel pressure also. Thanks.



Good to know... I'll look into that distributer and injectors. I did hear that the injectors sucked too, lol. Thanks!

 

I'll beg to differ guys; I've done a couple of Vortec 7.4 distributors with only removal of the coil and module bracket at the rear of the plenum. No, it's not fun, but it is 100% do-able. Just lay across that plenum and get busy. Be careful you don't stab yourself in the chest with the miscellaneous hardware in the area.

You can check for the obvious signs of wear first by popping off the cap and try moving the rotor side to side. If it doesn't budge, try turning it either direction until it pops up, and move side to side again. If you can make it move, you've got significant wear. I'll go ahead and say at 170K if it's the original distributor, it's almost surely worn. Not saying for sure that's the cause of your problem but it easily could be.

Guess it also bears mentioning you need a scanner that can read cam retard angle, if you remove the distributor. You want to make 2 marks, distributor body position and rotor position, and re-stab it (or stab the new one) as close as humanly possible to those marks. Fire it up and hold RPM's to at least 1,000 for a correct reading and adjust to within +/- 2 degrees of zero.

Before all of that though as said, you should confirm fuel pressure first, and if you haven't done cap and rotor, do them. The Vortec caps can fail internally and you'd never know it. The cheap new ones are notorious for it within a pretty short time period so spend a few bucks to get good parts. RE-USE your original cap screws, especially if the new ones have threadlocker on them. The plastic distributor body is crazy easy to break the tabs off for the screws...I can bet you a dollar it's already got cracks, if not broken. If you do strip the holes or break the tabs, it's not the end of the world. If the distributor is otherwise OK, you can actually get a nice tight fit of the cap by looping strong zip ties around it between the wire terminals to down under the distributor body. Thanks to the crossfire design of the cap it will keep the zip ties from slipping off sideways.

Injectors and FPR aren't really a bad job other than having to fight the coupling nut on the EGR tube. The "backup nut" portion of the fitting has flats that are too narrow to hold it in place with a regular wrench as it will bridge the gap and hold the coupling nut, too (on most trucks, anyway.) Channel-Lock makes a narrow jaw adjustable that works like a champ as a backup wrench to keep the fitting from turning, while you use a regular wrench on the coupling nut. I used a 2x4 scrap of wood against the valve cover to rest the backup against while breaking the coupling nut loose.



Be sure when you do the FPR you replace the short piece of vacuum line and use fuel/vapor rated line so it doesn't crap out prematurely. You can't get to this line to replace it later without pulling the plenum again. When removing the injectors from the fuel rail be stupid crazy prying off the little clips; it's easy to break the nub off the rail that they push onto. Again, zip ties can rescue it, but avoid if possible.

Richard

 

I agree, it can be done, How ever it is difficult to see and work back there. At Least removing the top makes it easier to see what's going on.

I had an issue with 1 clogged injector from the vehicle sitting. So I pulled it any way to service them.

If you do work on the manifold get the gasket set...

 

Oh, you wanna actually SEE what you're working on instead of using braille? I just hate doing extra work, and was able to get it done laying across the intake. You're on target though if it's got injector or regulator problems, you're going in there anyway.

Richard

 

Well it sat for almost 2 years so the injector was gummed upped from the shitty gas...

 

What do you guys think of this kit?? Looks like its got everything I need...
https://www.fiveomotorsport.com/bosc...ht-tune-up-kit

 

ls1 injectors can be had for roughly 100 bucks on ebay, and the FPR is like 30-50 bucks.
part number for injectors is 12555894.
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