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99 Suburban 350 ?'s

pronstar

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350 Vortec Gen I SBC
Found this vacuum line under the hood - any idea what this end is supposed to connect to?


IMG_8358.jpg

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Also, I get a Cyl #7 misfire code when I'm at WOT and high rpm - like over 4K rpm.

Initially it was accompanied with just a slight knock, so now I run mid-grade 89 octane. No knock, but still get the code. Same with 91 octane.

Runs great, and I don't feel the miss, but it throws a code in this situation only. If I drive normally no codes at all.

Plugs, wires, distributor cap & rotor are all new.

Any thoughts?
 

milkmoney

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U know how i know u can get ur hand down in there and reach that hose in a small compartment?
 

HANDY

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I'm at work so I can't download the illustration on here, but if you have an e mail address I can send you the GM illustration ?
 

pronstar

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I'm at work so I can't download the illustration on here, but if you have an e mail address I can send you the GM illustration ?
robbobster at mac.com
 

pronstar

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What's the function of it?
Heater works pretty good...
 

3queens

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miss fire could be a intake valve seat wearing out
very common on that engine
recheck wires and #7 spark plug for damage on porcelain or crack
swap out a coil with another cylinder and see if cylinder changes # for missfire too
 

pronstar

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miss fire could be a intake valve seat wearing out
very common on that engine
recheck wires and #7 spark plug for damage on porcelain or crack
swap out a coil with another cylinder and see if cylinder changes # for missfire too
Intake valve seat I can live with...runs too well to tear into it.

I'll pull the plug and check it out

I've got a distributor/cap/rotor so can't swap a coil...?


You guys rock.
Seriously.
 

Blue

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Great engine shitty intake gasket/coolant combo. Also the poppet style injectors aren't the best but mine were doing fine at 214k
 

3queens

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Intake valve seat I can live with...runs too well to tear into it.

I'll pull the plug and check it out

I've got a distributor/cap/rotor so can't swap a coil...?


You guys rock.
Seriously.
roger that
been a while since ive sold a cap and rotor :thumbsup

no coil swap test --it works or it dont with only 1
was thinking later model

good point on the intake gasket on those too

spray brake cleaner around where head meets block on intake
see if idle changes if so you have a leak

or swap injector position to see if miss fire follows
 

SBMech

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There is a GM TSB for that scenario, after double checking your basics (cracked plug, high resistance or shorting wire) it boils down to the valve stem sticking or rotator not working properly. Recheck the tune up stuff, add a can of 44K fuel injection cleaner...just for shits and giggles.

Valve job fixes it.



Heater valve, connects to solenoid on firewall. Behind the intake hose. :thumbsup

Misfire TSB
Subject: SES Light and P0300 When Towing, Cruising Uphill or on hard Acceleration - kw 4.3, 5.0, 5.7 accelerate cruise cylinderhead DTC L30, L31, L35, LF6, LU3, P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307 P0308 # PIP3081 - (07/23/2004)
Models:
The following diagnosis might be helpful if the vehicle exhibits the symptom described in the PI.
Condition/Concern:
The vehicle may exhibit a SES Light due to a P0300 and misfire. If the misfire is related to the information below, it will typically happen while cruising uphill, pulling a trailer or on hard acceleration and then stop misfiring shortly after returning to an idle. Typically, cylinders 3, 4, 5 or 6 will be the ones to experience this.
Recommendation/Instructions:
If the P0300 SI diagnostics did not isolate a concern, the following may help:
Remove the valve cover, valve springs and valve seals on the effected cylinder or cylinders. A small wire tie or rubber bands can be placed in the valve stem keeper groove to prevent the valve from falling into the cylinder. Rotate the valve while moving it up and down in the guide to see if it binds. If a binding valve is found, remove both cylinder heads and use either of the following methods to increase the stem to guide clearance:
^Send the head out to a machine shop and advise them to increase the valve stem to guide clearance to 0.002 inch per guide.
^Hone the guide with a 9 mm hone, such as Snap On BCG249, until a clearance of 0.002 inch is obtained. This will usually take about 4 strokes of the hone.
Reassemble the cylinder heads using new valve seals.
Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed. If these steps do not resolve the condition, please contact GM TAC for further diagnostic assistance.


Another misfire TSB


  • ENGINE CONTROLS - MIL ON/MISFIRE/MISFIRE DTC'S SET

Bulletin No.: 06-06-04-046
Date: September 12, 2006
INFORMATION
Subject:
Information on Engine Misfire MIL/SES Light Illuminated or Flashing DTC P0300, P0301, P0302, P0303, P0304, P0305, P0306, P0307, P0308, P0420 or P0430Models:
1999-2007 Cadillac, Chevrolet, GMC Full-Size Pickup and/or Utility Truckswith 4.8L, 5.3L, 5.7L, 6.0L or 6.2L VORTEC GEN III, GEN IV, V-8 Engine (VINs V, C, T, Z, B, 3, M, 0, J, R, U, N, Y, K, 8 - RPOs LR4, LY2, LM7, L59, L33, LC9, LH6, LMG, LY5, L31, LQ4, LQ9, L76, LY6, L92)
with Active Fuel Management(TM) and E85 Flex Fuel
If you encounter vehicles that exhibit the above conditions, refer to SI for the appropriate DTC(s) set. If no trouble is found, the cause may be due to an ECM ground terminal that has corroded with rust over time. Inspect the main engine wiring harness ground terminal (G103) for this condition.


The wire terminal (G103) attaches either to the front or to the rear of the right side cylinder head, depending on the model year of the Full Size Pickup and/or Utility Trucks. If the ECM ground terminal has been found to be corroded, then follow the service procedure outlined in this bulletin to correct the corrosion issue.


Remove either the nut or bolt securing the main engine wiring harness ground terminal (G103) to the right cylinder head. Refer to the above illustration to determine where the ground is located on the vehicle (1).
Remove all rust from the ground terminal, the cylinder head and the retaining nut or bolt.
Position the main engine wiring harness ground terminal and install the nut or bolt.
Tighten:
Tighten the retaining nut or bolt to 16 N.m (12 lb ft).
Apply some type of electrical moisture sealant to protect the harness terminal from further corrosion.
 

pronstar

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Let's say you were me...truck runs great.

Went thru all of the aforementioned, and the problem turns out to be valve related.

Would you bother fixing it, or just clear the trouble code every few weeks?

I'm leaning toward the latter, but I'm lazy
 

wsuwrhr

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Let's say you were me...truck runs great.

Went thru all of the aforementioned, and the problem turns out to be valve related.

Would you bother fixing it, or just clear the trouble code every few weeks?

I'm leaning toward the latter, but I'm lazy
Fuck the trouble code anyway.

If the car didn't have the codes, would you even know?

Brian
 

wsuwrhr

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Take a compression reading....

Maybe even check the adjustment on that one valve, I know it is hyd lifters.....but you never know.
 

wsuwrhr

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I think a leaking intake should cause air to come out of the intake, or some kind of goofy intake noise.

An leaking exhaust would cause more of a miss but not noticeable noise.

Brian
 

Blue

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I think a leaking intake should cause air to come out of the intake, or some kind of goofy intake noise.

An leaking exhaust would cause more of a miss but not noticeable noise.

Brian
My leaky intake gasket had a path from the waterside to intake track and it sipped coolant for a longtime until it started misfiring. Cost me $48 for the new version of gaskets, few hours and a sixer later it was done. Loved that motor we beat the piss out of it and it never skipped a beat. I say run it
 

SBMech

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Let's say you were me...truck runs great.

Went thru all of the aforementioned, and the problem turns out to be valve related.

Would you bother fixing it, or just clear the trouble code every few weeks?

I'm leaning toward the latter, but I'm lazy
The only real problem with that is:

1: It won't fix itself, we're talking a valve stem that is sticking/binding under hard extended load, if that is what comes out of it all after a close reassessment of the misfire...it might just be a plug or wire breaking down under load.
To be honest, it usually sets a P0300 random multiple misfire code for that valve issue, unless it really is sticking/binding on just that cylinder. Usually it's multiple valves under extended load scenarios.

2: 99 was a shitty year for cams....the camshafts go bad on #1 and #7 lobes. Combine that with a binding valve....you do the math..:skull

A weekend of drinking and you could throw some decent heads on it and be ahead of the game....they are not that expensive if you do it yourself. :)
 

pronstar

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Since it's not COP, how does the ecu knows that it's #7 ?

I get that it won't fix itself, but to Brian's point, if it didn't throw a code I'd have no idea there was an issue.

I think this motor will outlive us all, but I have been eyeballing an HT383e plug-and-play crate motor for shits and giggles
 

SBMech

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Since it's not COP, how does the ecu knows that it's #7 ?

I get that it won't fix itself, but to Brian's point, if it didn't throw a code I'd have no idea there was an issue.

I think this motor will outlive is all, but I have been eyeballing an HT383e plug-and-play crate motor for shits and giggles
Crank sensor. It can tell when a certain cylinder is not pulling it's weight.

Between the cam/crank sensor phasing it can tell easily that there are misfires. Use a decent scan tool and you can see the counter...:thumbup:

You might get another 100k out of it, or it could eat a cam or a valve next 100 miles, no way to tell without a teardown, and if you are going to all that trouble, you are fixing it lol
 

Cray Paper

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I wouldnt worry about. I bought a 98 GMC brand new and daily drove it for about 90K miles before we bought our travel trailer. On our first trip with the trailer, the same thing happened. That truck never once had the CEL light come on and it only happened when we were pulling the first large hill with the trailer in tow. I panicked, stopped at the first auto parts store I could find and puled the code. Miss fire on cylinder #7. I cleared the code and it never came back. I replaced the intake gasket at about 110K and gave it the first tune up then sold it with about 145K on it to my younger brother. It now has over 300K on it and runs great. Doesn't leak a drop of oil, gets the same fuel mileage as new and still leaks antifreeze from the intake manifold just as it did when it had around 25K on it and 10K after I replaced the same gaskets.

I would run it till it doesnt run anymore.
 
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