i have a friends truck im trying to figure out it has black smoke on acceleration the vacuum is good the turbo is working 10 psi peak drops to about 6 psi the fuel wide open is 63mm on the scanner wondering if there is any other cause for the smoke it is a 98 1500 p/u with the s motor:help:

Black smoke would indicate unburnt fuel at/near combustion tempature, most commonly seen with low boost where the O2 content has been reduced to the point of burning inefficiently/incompletely and being sent out the pipe in that state.
If a condition occured such as additional oil in the chamber or possible injectors not atomizing fuel the burn could look the same.

If it isn't a stuck EGR valve, turn the boost way up and it will go away. (at least mine did) I did everything else I could think of and nothing helped.
I just turned it down today because of code 78 to 14 - 15 psi peak, 14 at 2500 rpm and 12 at 3500 rpm (I need to redo my boost fooler). The EGTs went up about 250° post turbo up my test hill at the same speed and I get smoke from a dead stop again.
I think I am going to crank the turbomaster back in to where it was.

Its definitely and EGR system related problem...what a terrible design, I truly feel sorry for those owners out there with 98+ 'S' engines, the 'S' is for 'S'ucks.

i pulled the egr had a look at it all seems to be fine there opens closes the only thing i dont know is if it opens and closes at the correct time this is a 100% stock 98 s motor the only thing that has me puzzled is the fact it started with the bad tank of fuel it was poping and sputtering and he told me it was backfiring like a gas motor and has smoked ever sinse i checked the pump timing with a mt 2500 put it into set timing and set to 3.5* then did tdc ofset and it went to -.84 that is where it is now but the smoke stayed the same

Have you checked to see if the gasket in the center of the intake, under the EGR is there?

it is there but mabey it got damaged when the engine was backfiring will have to check that out

If it started with the bad fuel then I would bet that it is the injection pump. Mine started shortly after I put a used IP on it because the old one died from bad fuel and it still had a little in it. (the WIF light came on a couple times after I replaced the IP)
It has been over 20,000 miles since mine started it.

Black smoke with good boost, seems like a bad injector is letting out more fuel than it should and/or bad atomization, especially after a bad tank of fuel. How long ago was your bad tank of fuel? Have you run any injector cleaners through your truck since then?

I put a new set of injectors in and it didn't make any difference.

Just for good measure, I would change the fuel filter. Not your problem, but can't hurt.

New injectors, good boost, black smoke. Possibly a cracked exhaust valve, if your not building enough compression in one or more cylinders you'd be pushing through unburnt fuel (black smoke). Check your compression.

Hate to even ask this one, is the air filter clean?

98 S engine check for MAF functioning properly as well, I replied to a similar question in post last nite on a K1500 http://www.dieselplace.com/forum/showthread.php?t=30878 (http://www.dieselplace.com/forum/showthread.php?t=30878)

EGR valve spring is calibrated to normal oem Boost, and exhaust backpressure in the turbine - requires vacuum assist to open at those oem levels.

Backfiring will blow the valve open, as will exceeding oem Boost levels.

Backfiring can damage the plunger and spring.

Should check spring holding force, and total seating when valve is closed

First check, always, is air filter - plugged intake path can, and usually does, result in black smoke at factory Boost levels.

New injectors, good boost, black smoke. Possibly a cracked exhaust valve, if your not building enough compression in one or more cylinders you'd be pushing through unburnt fuel (black smoke). Check your compression.I had my head gaskets replaced a few months after changing the injection pump. They sent the heads somewhere to be pressure tested and they were good. It didn't make any difference to the smoke with the new head gaskets either.

I'm gonna keep throwing out ideas. If the truck was backfireing there is the posibility the the doughnut gasket between the upper and lower intake manifold for the EGR passage blew out, which could be letting exhaust gas through your intake at all times, reguardless of EGR's condition or position. This also fits the conditions mentioned as when the engine is warming the EGR would be normally open, but then when you hit operating temp the EGR is supposed to close, in this case the EGR would be closing but that wouldn't stop the flow of exhaust gasses. You only have to pull the upper intake to check this so you'd only be out a $13 gasket.

Donut gasket is a good point, mine was bad because it rusted away little support was being given to the gasket retainer portion that was gone.

Donut or "Mystery gasket" went twice on mine. It appears to never be reusable.

The part number is DELCO Part # 219167 or GM Part # 10191428

Sorry, I spelled donut wrong. I never needed to know how to spell it to eat a few.):h

The EGR valve is N_E_V_E_R open, unlesss decelerating, or load conditions call for, or is bad.

Don't believe it? Crank the engine cold, use fingers to open the EGR valve, then post results here.

Or, use your hand vacuum pump on the diaphragm to open the valve at idle.

I put a new fuel filter in the next day after the bad tank as well as some conditioner and ran the next dozen or so with the standadyne stuff in every other tank. The K&N air filter is clean. We'll re-inspect the EGR and plenum gaskets and maybe put a new EGR on.

About a half an hour down the road after filling up the backfiring started and it sounded like the timing was off on a gas motor.I has to limp it home, and on the way it sounded like I had spun a bearing, clanging and banging, but after adding the conditioner it came back to normal except for the black smoke, which is always there especially when accelerating Censored . Thanks for the input and will keep trying.

i fixed dishfixer's truck today the problem was mystery gasket fel apart i guess with the backfiring

:thumb: :clap: And on to the next one....