1996 GMC Z71 - 220k - Rough Idle/Hard Start - New to Me

[96CK]

Well, I suppose this is as good a place to place an introduction as any.
To start off with, it feels good to be back on a forum, I haven't been on one since I had an LS1 powered F-Body.

Alright, so I just purchased this truck as a fixer to daily driver project. They just don't make them like they used to.
It has the 5.7/350 motor, not sure of the transmission (one problem at a time)

Current big issue is the hard starting, rough idling.
I've done some digging and ordered the MPFI "conversion" just from everything I've seen this HAS to be the root cause of it all.
I averaged 15mpg for 300 miles driving from Brownsville Tx, to Houston Tx, It idles rough, doesn't start right away (one start, rough idle, dies) then fires right up and idles rough, has NO power under acceleration, etc etc etc. All of these seem to align with a failing/failed CSFI.
I have changed the fuel filter and I'm getting ~55psi at the Schrader valve underhood with an almost immediate drop in pressure once the truck shuts off.

So, I plan on replacing that this weekend.

While I'm in there what might be some other things I should look at/into?

I plan on pulling/cleaning all or most sensors, checking the distributor cap/rotor, plugs seemed fine like a motor that's fuel starved (running lean), air filter needs changed.

~Ross

You must be registered for see images attach

 

[OutlawDrifter]

Sharp pickup!

Probably wouldn't hurt to do plugs/wires/cap/rotor since they are of unknown quantity. It would be a good time to check the distributor shaft look for excessive play.

I'm also a big proponent of doing ALL of the fluids when I buy a used high mileage rig, front to rear. Don't skip the power steering, brake fluid, both difs, xfer case, etc.

 

[96CK]

Outlaw - Thanks for that, you are correct, sure can't hurt!

Well, I only listed the core problem here lol.
Can't get 4wd to do anything, currently leaking at the rear main (or it seems), A/C blows hot Texas air in my face, Suspension needs love...... lots of love..... I need to undo some poor wiring choices (Hot wiring brake lights to trailer brake, only), you know "project truck" lol.

I'm absolutely going to drain all fluids, get all those covers off and do some due diligence.
I imagine there's a "fluids" post, but do these trucks love/hate any particular fluids in general?

 

[HotWheelsBurban]

Well, I suppose this is as good a place to place an introduction as any.
To start off with, it feels good to be back on a forum, I haven't been on one since I had an LS1 powered F-Body.

Alright, so I just purchased this truck as a fixer to daily driver project. They just don't make them like they used to.
It has the 5.7/350 motor, not sure of the transmission (one problem at a time)

Current big issue is the hard starting, rough idling.
I've done some digging and ordered the MPFI "conversion" just from everything I've seen this HAS to be the root cause of it all.
I averaged 15mpg for 300 miles driving from Brownsville Tx, to Houston Tx, It idles rough, doesn't start right away (one start, rough idle, dies) then fires right up and idles rough, has NO power under acceleration, etc etc etc. All of these seem to align with a failing/failed CSFI.
I have changed the fuel filter and I'm getting ~55psi at the Schrader valve underhood with an almost immediate drop in pressure once the truck shuts off.

So, I plan on replacing that this weekend.

While I'm in there what might be some other things I should look at/into?

I plan on pulling/cleaning all or most sensors, checking the distributor cap/rotor, plugs seemed fine like a motor that's fuel starved (running lean), air filter needs changed.

~Ross

You must be registered for see images attach

If it's an automatic, it's most likely a 4L60E. These are good transmission for a half ton IF you don't beat on it and change the fluid and filter periodically. I have one in my '99 Burb behind the 350 that's got over 200K on it and still going.
For oil, I run Quaker State 10w30 or 10w40 depending on what time of year. 10w40 for summer. As long as you stay with name brands you should be good. I use QS because that's what Dad taught me to use, and we've run it in all of the vehicles in my signature (most GM trucks with 350s) and others that are not listed. Works for me so that's why I use it. I also use AC oil filters because they have more filtration media than the cheap brands.

 

[OutlawDrifter]

Outlaw - Thanks for that, you are correct, sure can't hurt!

Well, I only listed the core problem here lol.
Can't get 4wd to do anything, currently leaking at the rear main (or it seems), A/C blows hot Texas air in my face, Suspension needs love...... lots of love..... I need to undo some poor wiring choices (Hot wiring brake lights to trailer brake, only), you know "project truck" lol.

I'm absolutely going to drain all fluids, get all those covers off and do some due diligence.
I imagine there's a "fluids" post, but do these trucks love/hate any particular fluids in general?

Use your favorite flavor or brand that meets the spec for each fluid and you'll be fine. Synthetic won't hurt anything across the board on the oils.

 

[HotWheelsBurban]

And welcome to the forum, from a little bit south of you in Houston!

 

[Schurkey]

Current big issue is the hard starting, rough idling.
I've done some digging and ordered the MPFI "conversion" just from everything I've seen this HAS to be the root cause of it all.

There's more than one cause of hard starting, rough idling.

But it seems that the MPFI conversion is a great starting point. If it were me, I'd pull the entire intake manifold off, so that the manifold-to-head gaskets can be replaced.

I have changed the fuel filter and I'm getting ~55psi at the Schrader valve underhood with an almost immediate drop in pressure once the truck shuts off.

That's not good. Is your pressure gauge accurate?

While I'm in there what might be some other things I should look at/into?

I plan on pulling/cleaning all or most sensors,

Connect a scan tool, verify ALL the sensors before you waste time doing work to them that they may not need.

O2 sensors get lazy with age/mileage. Pay special attention to them--waveform or cross-counts to assure they're responsive.

As long as you have the scan tool connected, look for "codes", and look at the misfire counts for each cylinder, transmission shift adaptions, fuel trims, etc.

checking the distributor cap/rotor, plugs seemed fine like a motor that's fuel starved (running lean), air filter needs changed.

Distributor gears seem to be a weak point on Vortec engines. The bushings in the plastic housing are known to fail. The screw-holes where the distributor cap is secured are also weak.

Can't get 4wd to do anything, currently leaking at the rear main (or it seems), A/C blows hot Texas air in my face, Suspension needs love...... lots of love..... I need to undo some poor wiring choices (Hot wiring brake lights to trailer brake, only), you know "project truck" lol.

We'll be here with advice. Post in the proper sub-forum.

do these trucks love/hate any particular fluids in general?

GM has bulletins on using synthetic gear lube in axles built before a certain date. The synthetic lube degrades the crappy silicone sealer they used at the factory. Could be that the crappy sealer is long-gone, and therefore not a real worry any more. Permatex makes a "special" silicone sealer specifically for use with gear lube. Or use a paper gasket on the cover.

If you have G80 locking differential, you need special "Grape Juice" rear axle lube. Expensive.

You WILL need a scan tool to bleed the ABS unit. Fair chance the ABS hasn't been properly bled since the truck was built. REALLY GOOD chance that the rear brakes are the horse-crap 254mm (10") leading-trailing drums, so the park brake cables better not be seized, and you'll need to use the park brake regularly to keep the rear shoes adjusted.

That's the only sore spots in terms of lubricants that I know of.

 

[96CK]

There's more than one cause of hard starting, rough idling.
But it seems that the MPFI conversion is a great starting point. If it were me, I'd pull the entire intake manifold off, so that the manifold-to-head gaskets can be replaced.

Any recommendations for gasket sets? I've been out of the "working on my car" game for a while, like back when Jegs, Summit and LMC were catalogs..... I'm not "old" but it's been a few years......
I'm kind of eyeballing this, or will I need a more comprehensive kit for removing the entire intake assy?

1996 GMC K1500 Fel-Pro MS901312 Fel-Pro Intake Manifold Gaskets | Summit Racing

Free Shipping - Fel-Pro Intake Manifold Gaskets with qualifying orders of $109. Shop Intake Manifold Gaskets at Summit Racing.

www.summitracing.com

That's not good. Is your pressure gauge accurate?

I'm pretty sure, It's a new gauge literally purchased it for this one purpose/truck. I have friends who have some though, I suppose I could bounce my numbers using their gauges.

Connect a scan tool, verify ALL the sensors before you waste time doing work to them that they may not need.
O2 sensors get lazy with age/mileage. Pay special attention to them--waveform or cross-counts to assure they're responsive.

So, I had one O2 code, but I also had an exhaust leak at that joint flange in front of that mid O2 sensor, fixed that, haven't re-scanned yet still have an SES for the "ignition control module" something to do with voltage, but dug deeper into that and basically came up with, it may not be that module. which I purchased, replaced, fixed nothing, returned.... It was a hail Mary just due to the simplicity of "plug and play"

As long as you have the scan tool connected, look for "codes", and look at the misfire counts for each cylinder, transmission shift adaptions, fuel trims, etc.
Distributor gears seem to be a weak point on Vortec engines. The bushings in the plastic housing are known to fail. The screw-holes where the distributor cap is secured are also weak.

I'll take a look at all that while I'm in there, I mean, it does have 220k on it so it's very possible I'll find more and more gremlins the deeper I dig.

 

[Schurkey]

Does the injector kit come with the upper manifold gasket? If not, you'll need the full set not just the lower.

I would probably buy the Fel-Pro set that includes the upper gaskets. A guy on this site that I've learned to trust doesn't like those Fel-Pros, though. Maybe he'll chime in and provide a part number for the gasket set he likes.

Consider valve cover gaskets. The newer center-bolt covers seal way better than the perimeter-bolt covers ever did...but as long as you're in there...

If you're lucky, your "rear main leak" is either the rear seal on the intake manifold, or the oil pressure sending unit behind the manifold.

 

[Shadowmyrock]

Well, I suppose this is as good a place to place an introduction as any.
To start off with, it feels good to be back on a forum, I haven't been on one since I had an LS1 powered F-Body.

Alright, so I just purchased this truck as a fixer to daily driver project. They just don't make them like they used to.
It has the 5.7/350 motor, not sure of the transmission (one problem at a time)

Current big issue is the hard starting, rough idling.
I've done some digging and ordered the MPFI "conversion" just from everything I've seen this HAS to be the root cause of it all.
I averaged 15mpg for 300 miles driving from Brownsville Tx, to Houston Tx, It idles rough, doesn't start right away (one start, rough idle, dies) then fires right up and idles rough, has NO power under acceleration, etc etc etc. All of these seem to align with a failing/failed CSFI.
I have changed the fuel filter and I'm getting ~55psi at the Schrader valve underhood with an almost immediate drop in pressure once the truck shuts off.

So, I plan on replacing that this weekend.

While I'm in there what might be some other things I should look at/into?

I plan on pulling/cleaning all or most sensors, checking the distributor cap/rotor, plugs seemed fine like a motor that's fuel starved (running lean), air filter needs changed.

~Ross

You must be registered for see images attach

That's my exact truck but doesn't look nowhere near that good right now but it will once I get the body work done but I've had it for like 24 years I love my truck you have the 5.7 is what year is that a 98 looks good that's the new 50 truck right there I'm not selling mine