Rough running 7.4 Vortec

[klittle1417]

Hi all,

I recently picked up a 99 2500 Suburban with a 7.4 Vortec. I bought it knowing it has some issues. But the frame and body are great so a few problems can be fixed.

The big problem is she runs rough. Has a rough idle, starts a little hard at times and has some valve train noise.

I'm trying to diagnose the rough running and see a few possible issues but hoping to narrow it down.

When I start her up it smells very rich and idles rough. I changed the plugs and a few, mostly drivers side, were black with some caked on crud. More crud than I have ever seen on a spark plug. They also smelled strongly of fuel.

It needed doughnut gaskets and exhaust flange gasket so I replaced those. When I took the exhaust apart I could see that the driver's side catalytic converter was empy. As in nothing in it. It did not look like the exhaust had ever been taken apart before I touched it. The passenger side cat was still intact. Until I cut it open and hollowed her out! I have also had a few chunks of the cat spit out the tail pipe before be messing with the exhaust.

What do you all think about this. Running rich on the drivers side? Caused by bad injectors? Bad FPR? I'm just not sure where to start. The old girl isn't on the road yet as I'm trying to get it running better or I'd have more information about drivability.

Thanks for any help you can give.
Kevin

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[DerekTheGreat]

The rough idle could be a bunch of things. Is it throwing any codes? If so it would be beneficial to see what they are and start there. Also, verify that the temperature sensor for the ECM is working properly. If it isn't signaling when the thing is warmed up it will cause it to run rich.. Rough idle could be due to intake gaskets but also burnt valves.. That's the case with my 350 powered K1500. Easy way to test for burnt valves is to take a damp paper towel or dry sheet of paper and hold it about 1 inch from the tail pipe while it's running. If valve(s) are burnt it will try to suck the thing into the tail pipe occasionally. Also, they sound funny.. chuff-chuff-chuff...

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[klittle1417]

I did forget to mention I had a P0131 code low voltage O2 sensor bank 1 sensor 1

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[aaronb]

The gutted cats are your main issue.
That will cause it run rich, and cause the post cat o2's not to read right and rich in the fuel mixture.
I had a similar issue with my 98 5.7 vortec. Gutted the cat without having the post cat o2's not tuned out. It ran rich, misfired, and fouled out the plugs and o2 sensors.
Replaced the cats, new o2's, and plugs and was back to normal.

 

[klittle1417]

I believe the rear sensor won't effect air/fuel settings in the computer

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[Justin S]

Yes, gutted cats should have no effect on performance, the rear sensors are only there for emissions purposes. I have a 98 454 and had similar problems and it ended up being caused by the crappy stock injectors and FPR. I had a plug like this before I had the issues.

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[klittle1417]

My plugs looked similar to that. Some were decent and some not. I should get a fuel pressure gauge and check the pressure before I buy injectors

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[Supercharged111]

Don't be surprised if the valvetrain noise is a bad cam. That's what mine was.