1999 5.7 350 vortec P0304 cylinder 4 misfire

[SableSlayer]

Isn’t my intake supposed to be sealed tight on the throttle body ? I did my smoke test and right away smoked started pouring out of the here ?

Did you get this sealed up yet?

 

[socal k1500]

Coming out of the air-intake hat where it seals to the airhorn? Or coming out farther below and I just don't see it?

A new gasket/seal between the hat and the throttle body airhorn would resolve that leak--or maybe just tightening the fasteners.

Ok so I tightened hat but it makes no difference very loose.

Did you get this sealed up yet?

I popped the orange gasket out of the Air intake "hat" that sits on the throttle body. it seems this gasket is shot and doesnt seal anything. I ordered some O-rings on amazon that people say fit in the slot where the gasket sits and help seal the "hat" on the throttle body. These O-rings are heavy duty and rated up to 250f so they shouldnt degrade easily.

When i tighten my "hat" using the thumb screw it tilts slightly and is very loose on the TB. So for the mean time i flipped the gasket upside down the bottom of the gasket has more meat on it and still has a proper square edge and i put some grease on it, this helped it seal a little better.

After doing all this i then took off the intake and then sealed the throttle body and wedged the throttle plate open. and contined the smoke test to the manifold but i couldnt see smoke leaking anywhere in the engine bay.

I put a new ECT and IAC in. The idle issue seems a little better but the truck still has the cylinder #4 misfire when going up a hill under load.

The shop who installed the injectors for me wants to take a look at their work to make sure everything is proper because the owner assured me the injectors should have fixed the cylinder 4 misfire. I promise to keep you guys updated because i know misfires on these engines can be a PITA to diagnose

 

[socal k1500]

You're never going to see a small vacuum leak with a cheap dampened gauge.

You MIGHT see the effect of a small vacuum leak using electronic vacuum sensors. Maybe. The one on my 'scope can match intake-valve-open vacuum pulses to the correct cylinder creating it; but I've never used that to confirm an intake gasket leak.

Years ago, they were selling ultrasonic microphones for detecting vacuum leaks and window-seal air leaks. But you'd likely have to cram the microphone inside the intake runner, or work it in from under the manifold. Totally impractical.

A smoke-test can often confirm very small leaks. If the gasket is leaking on the bottom-side, you'll have to blow enough smoke in there to fill the crankcase before the smoke will come out the oil-fill cap opening after you remove the "710" cap.

If you don't know how old the intake gaskets are...time to pull the intake manifold. The Vortec engines had tremendous problems with intake gaskets when they were new. I thought the gasket was "improved" to solve that issue; but I'm hearing on this forum that it was never "really" dealt with--better, but still not "fixed".

I've lost track--have we completely ruled-out injector and ignition problems? Valve springs, cam lobes good?

The new injectors could be an issue, But i doubt it because the misfire happens exactly like it did with the old injectors. The valves have not been checked and i suspect my issue could be the valve guide issue GM had a TSB about with cylinder #4

 

[SableSlayer]

Ok so I tightened hat but it makes no difference very loose.

I popped the orange gasket out of the Air intake "hat" that sits on the throttle body. it seems this gasket is shot and doesnt seal anything. I ordered some O-rings on amazon that people say fit in the slot where the gasket sits and help seal the "hat" on the throttle body. These O-rings are heavy duty and rated up to 250f so they shouldnt degrade easily.

When i tighten my "hat" using the thumb screw it tilts slightly and is very loose on the TB. So for the mean time i flipped the gasket upside down the bottom of the gasket has more meat on it and still has a proper square edge and i put some grease on it, this helped it seal a little better.

After doing all this i then took off the intake and then sealed the throttle body and wedged the throttle plate open. and contined the smoke test to the manifold but i couldnt see smoke leaking anywhere in the engine bay.

I put a new ECT and IAC in. The idle issue seems a little better but the truck still has the cylinder #4 misfire when going up a hill under load.

The shop who installed the injectors for me wants to take a look at their work to make sure everything is proper because the owner assured me the injectors should have fixed the cylinder 4 misfire. I promise to keep you guys updated because i know misfires on these engines can be a PITA to diagnose

I'm really leaning towards it being your intake manifold gasket. I chased all this5 numerous times before doing the intake gasket since it had been done ten years prior. See what that shop wants to do it while they have it in there again. Go with the felpro or mahle metal gasket upgrade.

 

[SableSlayer]

I'm really leaning towards it being your intake manifold gasket. I chased all this5 numerous times before doing the intake gasket since it had been done ten years prior. See what that shop wants to do it while they have it in there again. Go with the felpro or mahle metal gasket upgrade.

 

[SableSlayer]

You can see in the photos where the gasket failed. The failure of those gaskets cause internal vacuum leaks that you won't see with a smoke test.

 

[Schurkey]

A smoke-test can often confirm very small leaks. If the gasket is leaking on the bottom-side, you'll have to blow enough smoke in there to fill the crankcase before the smoke will come out the oil-fill cap opening after you remove the "710" cap.

The failure of those gaskets cause internal vacuum leaks that you won't see with a smoke test.

Well, you should see the indication...AFTER you've blown enough smoke to fill the crankcase.

For the record, Vortecs aren't the only engines to fail those silicone-beaded intake gaskets.

This old gasket set was under a ZZ4 intake manifold--the original 6-bolts-on-each-side design from '55 to '85-ish.

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[socal k1500]

Well, you should see the indication...AFTER you've blown enough smoke to fill the crankcase.

For the record, Vortecs aren't the only engines to fail those silicone-beaded intake gaskets.

This old gasket set was under a ZZ4 intake manifold--the original 6-bolts-on-each-side design from '55 to '85-ish.

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I also forgot to mention I have these codes that keep popping up , idk if they are related maybe my ecu or harness is bad ?

 

[SableSlayer]

I also forgot to mention I have these codes that keep popping up , idk if they are related maybe my ecu or harness is bad ?

Being that you have all those codes check all your electrical connections again and grounds. It's hard to diagnose a bad ECU. If you can change those intake gaskets yourself it could save you a lot of money that way being that they are so cheap and narrow out that part of it being that such a common issue. I would lean that direction before the ECU as I found some of those "U" codes can get very common. I've seen them across Fords and Chevys. Also if it does end up being the ECU I would look heavily into the upgrade to the gray box ECU's rather than the black box That is currently in these trucks

 

[socal k1500]

Being that you have all those codes check all your electrical connections again and grounds. It's hard to diagnose a bad ECU. If you can change those intake gaskets yourself it could save you a lot of money that way being that they are so cheap and narrow out that part of it being that such a common issue. I would lean that direction before the ECU as I found some of those "U" codes can get very common. I've seen them across Fords and Chevys. Also if it does end up being the ECU I would look heavily into the upgrade to the gray box ECU's rather than the black box That is currently in these trucks

Same hill same exact misfire, I’m taking it to the shop Monday, I’ll let them know about the lower intake gaskets