Vortec 7.4L misfire

[Falcon_LS]

The engine was rebuilt about a week ago on this '97 Suburban, but there's a misfire on cylinders 2, 7 and 8 (verified by a GM Tech 2). The misfire is worst on cylinder 7.

The tech reckons it's either the distributor cap or the fuel injectors, but I was wondering what else could be a likely cause. Worthy of note, this is an export vehicle and does not come with catalytic converters or oxygen sensors either. The exhaust reeks and idle is slightly rough, although not too bad. The truck shivers as you take off though.

This has been an issue before the engine was rebuilt. The plugs and wires are all new, as is the fuel filter (all ACDelco). The MAF has been cleaned, and tested ok and there are no codes stored in the PCM.

 

[Boomer]

This has a distributor and the gear at the bottom of the shaft will wear out, making the teeth very thin. This can lead to the symptoms you have. Pull the distributor (mark it first) and check this. If worn, replace the gear and see how it runs. Many folks overlook this on GM engines.

 

[AVB]

It could also be from the cam retard offset.

 

[Scott_Tucker]

Why was this engine rebuilt? Was it because of the misfire?

 

[c502cid]

Considering what cylinders are misfiring, I'd place an internet guess on the intake manifold not being sealed up or incorrectly torqued.

 

[morepwr]

I had a coil go bad in a 5.7 version of this truck and it caused a weird misfire pattern.

 

[NateDN10]

Originally Posted By: Scott_Tucker
Why was this engine rebuilt? Was it because of the misfire?

There was an earlier thread on (I believe) this same vehicle. Massive overheat.

 

[flacoman]

I vote for fuel injector, exhaust stink sounds like a rich mixture,and an overheat will hurt them. If the rail is exposed, you can move # 7 around and see if the misfire follows.

 

[c502cid]

Not that I really care if I'm right or wrong, but it's a rebuilt motor and the misses are on on three cylinders of the four corners on the intake. The computer could be pumping fuel to make up for the massive leak. Spray some carb clean over the corners and rule it out before you start doing other things, it's the easiest thing to check of anything you have to diagnose.