Here is the problem:

Its a GM Suburban, 1999 5.7 engine - 150K miles

Idles rough, but runs fine when driving, hasn't affected gas mileage any and runs fine at driving speeds

Check engine lite came on,

Reads - Misfire #4 cylinder

Mechanic thought it may be injectors , changed them and still same computer reading (Wasn't charged for them )

Changed plugs and wires and rotor cap, ( wasn't charged for them, replace my ones back )

Did a compression check, found #4 cylinder was 10 lbs less then all the others, thinks it may be a sticky valve.
(Valve Job around $2400 )

The total for what has been done so far, $69.00 , since they didn't fix the problem, I was only charged for Diagnostic test.

He suggested that I run some Marvel Mystery Oil in the crank case and see if that would unstick it and he put some cleaner in the gas,

Any Ideas guys

New suburban lol

Just get a set of vortec heads from a LQ9/LQ4 and just swap the heads. The 317 casting number heads can be found at a scrap yard for $150. You can do the same with a set of heads from a 5.7 MPFI heads.

Doubt that 10# would make much of a rough idle.You may have a problem with your ECM. It may not be foring the #4 cylinder injector. Don't think the computer has the sensors to tell if a cylinder is low on compression or is missfiring since the engine still has a distributor but it would indicate if the injector is not being triggered. There are a set of lights that can check to see if the computer is activating the injector, Noob lights I believe is the name. With 150k miles on the engine putting on new heads or rebuilding those would probably cause a big increase in engine oil usage. b

Putting the marvel in the oil probably will not help a sticky valve. I would try some seafoam sucked in through a vacuum hose like you would do your boat to try and knock some carbon off the valves.

A 10lb difference in compression wont make a difference on misfire codes compared to the other cylinders! You either have an injector not firing or another issue like a broken valve spring or cam issue (Which I don't think)... Vortec's are bad about cracks in the heads... If someone has an updated scanner, then you can test the fuel pressure drop on each injector to see if that is the trouble! Hope that helps!

Fuel leaking in the plenum, stuck valve or valves, valve stuck do to carbon build up and guides get tight, or low compression can and will cause a misfire code on a cylinder. Went down this road several times. 10 pounds may not sound like much but it can and will cause a miss fire code. If you decide to drive it and want to try a fuel additive buy some Berrymans injector and carburetor fuel system cleaner. Darn good stuff. One other question, when the injectors was replaced was it with the new update kit?

Not sure about the injectors, they left the new ones in cause of the labor to replace them back with the old ones, good advice so far,

As I stated, it only runs ruff in idle, when you start driving it goes away and the gas mileage is not affected.

I didn't even realize this, but you my friend don't have a set of injectors. You have a spider injection system. It's a single main injector with 8 poppet valves. These are prone to leaking in to the intake manifold as the system is completely in side the intake manifold.

Realizing this, unless you were there at the time of the injector swap, and this system was In fact changed, I would question the value of the mechanic giving you the Information. Here is why I say that; when it comes to the GM MPFI/spider injection, it's a one piece system. You can't just change out some of the injectors, it all of them or none of them. One piece system.

So, negating what the mechanic said, I'm going with a leaking injection system, or an extremely worn out ignition system; possibly a combination there of.

if you have leaking injector lines or a bad fuel pressure regulator, you would have hard starts and you would see a puff of blue smoke. put a fuel pressure gauge on it and turn key to the on postion. fuel pressure should ramp up to 60-65psi and hold. if it drops, start checking for leaks under upper plenum. really need to see live data numbers and go from there

also sounds like a vacuum leak!

so Mark what your saying is they updated the poppet valves to new injector setup from the bulletin.it originally came with one big injectore that fired fuel to the poppet valves,the poppet valves were notorious for stopping up,so they came out with a replacement injector assembly to fix it.

if it still has the poppet valves that is what is causing the problem.

Otherwise.. A good decarb.

Guys, thanks, Guess I will be taking it to another Mechanic, I looked under the hood and couldn't see anywhere that anything had been taken apart and replaced. That really disappoints me, as they have always done good work for me and always fixed the problems on other vehicles I took to them. I really don't want to get rid of the Burb cause its what we drive on long trips for the comfort and room in it.

Just hate taking it to a GM dealer , they always seem to charge twice as much