Upper intake manifold inspection

[jamesdking]

98 GMC Yukon 5.7 217000miles
P0304 on hills when gears drop and rpm goes up for a 10 sec period. And especially when it shifts back and rpm’s lower. Bucks and misses pretty badly. Stay off the gas for ten seconds and it goes back to normal. But it’s a flashing cel and I’m sure it’s not going to keep doing it forever sadly.

It’s done this for years and I’ve tried
Plugs
Wires
Swapping both
Distributer
Coil pack
Fuel pump and filters
Spider injectors and intake gasket
Grounds
Exorcism

Edit:the intake was replaced at 135000miles way before I owned it. Back in 04. This is only evident of the service records I was given in handwriting. It also mentions a miss at acceleration and hills if I can read it correctly. So this has possibly been a 20 year long issue!

So today I was bored and had a little starter fluid left in a can so I sprayed it around the intake manifold. Near the back drivers side I got a “poooffff” from the engine and a stutter. So I decided to take off the manifold to look at it for leaks.

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I don’t know what it should look like but there’s clearly a difference from front to back I would assume from the tube at the front circulating air and it having at least some exhaust gasses to blacken things. The back of it is oily but not particularly gassy so I didn’t suspect leaky injectors. And the number 4 is all black and sooty.

Last thing I noted is the gasket around the injector connection is looser than I could ever imagine it was if it was properly seated to the manifold. Like by a full inch.

Anyone wanna chime in with thoughts before I put it back together and avoid mountain passes for the rest of its life?

 

[RichLo]

That EGR is right next to #4 (P0304code) possibly dumping more EG's into the manifold under load? Plugged exhaust/cat?

 

[jamesdking]

I replaced the cats as well after doing most of the major tune up. They were cheep eBay cats but I wanted to make sure the cel light would work when it needed to as it was always on from a cat code.

 

[Schurkey]

You have it that far apart. Pull the manifold base off of the heads, and replace the commonly-failed lower gaskets.

 

[jamesdking]

I will do that.

 

[Vikingdude]

I can see it has the updated injectors, did the bracket get swapped out at the same time? If you leave the bracket from the old poppet style injectors in, the injector interface sits a bit low. Something to think about.

 

[Komet]

+1 it's probably the lower gaskets.

 

[jamesdking]

I don’t recall replacing the bracket but it was a couple years back. If it came with the new injectors I bet I did tho?

And as far as it being the lower gasket I’m fairly hopeful as I’ve done a ton of other things to bring it back to running reliably. And each step fixed a problem but the miss was there and has increased in frequency and severity over the years. It would surely be a relief if it was that all along.

Not to backtrack from good advice but is the intake soot normal looking to anyone else? Seems like it makes sense if that’s the egr inlet and all.

Thanks everyone.

 

[Schurkey]

Yes, that's where the EGR gasses enter the plenum. The EGR valve and plumbing are directly forward, and visible in the photo.

 

[GoToGuy]

It's a safer deal using a handheld propane, or carb cleaner squirt rather than dumping lighter fluid.