Rough running 7.4 Vortec

[TexasAggie]

Howdy folks -

I picked up a 1999 Suburban K2500 last week and have been going through it getting her ready to tow my travel trailer. A few days after I got it it started stumbling bad at speed. This weekend I changed the plugs, wires, cap and rotor (cap was a huge PITA to swap out). It is running MUCH better (#7 plug wire was burned through completely at the boot), but I've still got a stutter all through the rpm range. It's almost not noticeable when driving around (blends in to the road noise and feel). When sitting and revving though, it's vibrating all through the rpms.

What should I check next, and how can I help isolate the issue? I'm guessing it's ignition related, but not 100% sure.

Thanks!

 

[Supercharged111]

The 454 Vortec is also notorious for fuel pressure regulators and injectors failing.

 

[454cid]

Check fuel pressure, although I would think if it were low you would have hard starting and you didn't mention that.

Have you had a scanner on it, to check for codes?

I wonder if your distributor gear is worn. I still have my original, but I hear that they can wear and cause problems.

Clean grounds? An important one is right by the thermostat.

 

[TexasAggie]

Starts right up every time, hot or cold. No CEL and no codes (I checked to make sure the light was working).

I checked the disti when I had the cap off and it seemed nice and tight. I'd rather not pull that thing out, but I'll do it if I have to. I read a lot about the disti wearing out and fuel pressure, but unsure how to go about checking all of it. Where is the port to check fuel pressure? I can rent a gauge and check it this week if it's easy to get to.

How much can someone tell by plugging into the computer on this thing? What GM softare/scanner do you have to have, and can it pretty much pinpoint the issue?

The engine bay on this thing is incredibly clean for 180k miles. I'll check the grounds though to make sure.

 

[stutaeng]

Um, weird it's not throwing a CEL...

Basic scanner with live data is what you want. Maybe the fuel pump doesn't have enough flow under load. You can monitor the fuel trims when it's acting up. Ideally they should be under 10%, but below 15% is still okay. Above this and you will get a code. If both banks are lean, very likely fuel delivery problem such as fuel pump.

With same scanner you can check other things, like MAF, MAP, spark advance, ECT, O2 readings and other invaluable data that will help you diagose the problem.

Otherwise a fuel pressure guage with a long hose stuck on your windshield. Again, monitor pressure when it's acting up.

On these issues you really have to catch the thing in the act...if you know what I mean.

 

[Dravec]

Fuel pressure port is at the very front of the intake plenum, looks like a tire valve with a cap on it.

 

[TexasAggie]

Um, weird it's not throwing a CEL...

Basic scanner with live data is what you want.

Any preference on which one?

 

[454cid]

I'll just mention the fuel pressure guage since other's have already addressed many of the question you responded with after my post. I've got the Harbor Freight fuel pressure gauge. It's only about $15 last I knew. I did have to make a "gasket" for it so that the included hose would seal to the fitting on the gauge. It also doesn't have a pressure relief button, but it does get the job done for not much $$$. A parts store may have a bit better one. If it's not too much money, I'd opt for a new one instead of the one that they've loaned out, and then keep it.

If you don't get a gauge with a pressure relief valve, you could always pull the fuel pump fuse and let the engine die... that will get rid of a lot of the pressure. Maybe throw a rag over the connection while you unscrew it, so as to contain any spray.

Oh the grounds... I don't know what you've got for tools, but I like to wire WHEEL that kind of thing and not just use a hand held wire BRUSH. I've used a cheap plastic bench grinder that I've run over to the truck, and I've also used a very cheap little Harbor Freight Dremel type tool (but much lighter duty). I've not done the truck recently, but I've started buying FluidFilm and would likely spray a little on the grounds once I got the back together. I doubt this is the main problem, but it won't hurt to be sure of it.

 

[TexasAggie]

I actually have a fuel pressure gauge already, so I'll check that out tonight. What should it read at idle and under load?

The reason I'm thinking ignition is because it's doing it all the time - barely noticeable at idle, but very noticeable when revving in neutral. Doesn't get worse under load (almost gets drowned out by the road noise).

 

[C30454]

You need a scanner, and look at misfire data to confirm that what you are feeling is actually a miss. And if you do have a miss, it should be able to pick out which cylinder, and then you can narrow it down from there. Also know that aftermarket distributor caps are HORRIBLE, and prone to crossfiring into adjacent cylinder pathways inside the cap.


Tom