Vortec 7400 rough idle

[montess85]

Just bought a 1999 GMC 2500 Crew Cab Short Box with the 454. The truck has about 90,000 miles on it. When driving it home I noticed a slight rough idle that I didnt notice on the test drive. It drives down the road fine and doesnt hessitate or stumble. Its not really bad but you can feel it shake a bit on idle and low rpm. It idles about 500rpm when in gear. The previous owner said it had a hard start issue recently and he chased the problem down. He said he replaced the MAF, Fuel pump, And injectors. After the injectors the hard start stopped. There is no check engine light on either. Any Ideas?

 

[dr_goodwrench66]

Any place you can go to look at the misfire counter with a scanner?

 

[montess85]

I might be able to get my hands on a scan tool

 

[454cid]

Hopefully, he replaced the FPR, too. I'm kinda surprised he replaced the MAF. I still have my original at over 200K miles.

My truck has always idled a bit rough, like what you describe.

 

[montess85]

Yea not sure about the FPR. I hope he did since he was there. Not sure why he changed the MAF either. Im going to try to get a scan tool this week to check some things.

 

[454cid]

Yea not sure about the FPR. I hope he did since he was there. Not sure why he changed the MAF either. Im going to try to get a scan tool this week to check some things.

Is the new MAF the original style or the new style? I think the new style might be the same MAF that the LS engines use, with an adapter for the wiring.

 

[montess85]

Not sure

 

[dale_gribble]

Mine idles like crap and pretty much always has. Here's the thing, our trucks only have so much ECM adjustment away from stock when we begin modding with better flow updates. The MAF (volts vs. airflow) table really needs to be adjusted to compensate for flow mods, specifically the CAI in your case. In some cases the truck can compensate for the changes and in some cases it can't.

If you get a hold of a scan tool, see what your LTFTs 1 and LTFTs 2 look like. That shows how much fuel the ECM is pulling or adding based on the MAF reading.

I am combating my truck with this same issue right now. I'm hopefully getting an updated tune with updated MAF cal and it should help.

 

[454cid]

Not sure

You didn't do the work?

 

[montess85]

The truck is bone stock. The previous owner had the work done so im not sure. The mods listed below are on my Tahoe. Im going to try to get ahold of a scan tool.