Ok, have had our 1998 GMC K1500 for a few months now and recently it started running rough. Idled really rough and just didn't seem to have the get up ...

Ok, have had our 1998 GMC K1500 for a few months now and recently it started running rough. Idled really rough and just didn't seem to have the get up and go it had when we first got it. Now granted the old 5.7 has 210,000 on it so it may be just a bit tired and the fact the folks we bought it from used it as a secondary vehicle and we started driving the heck out of it might have something to do with it. Took it to the shop and the mechanic didn't even bother to do any kind of troubleshooting, just started throwing parts into it, a set of plugs, new cap and rotor and new plug wires along with a new fuel filter and it isn't running or idling any better than it did before they did all that work to it, then they said "oh you have a misfire on cylinder 4" and threw another $200 worth of work into replacing a fuel injector, still no improvement in running and the gas mileage has gone wayyyyy down lately. Now the thing belches black smoke whenever I stomp on it and a friend of mine who has worked on everything from A10 warthogs to 66 Fords is 99.99% sure the catalytic converter is clogging up on it.

Fast forward to now, I say to heck with it and start researching the issues on the internet like I should have done in the first place. Once I get the catalytic converter replaced I was checking out different videos on YouTube about the whole "Seafoaming" the engine to see if that may help out a bit. My question is, are there any issues in treating the engine with Seafoam with the mileage as high as it is, and not knowing much of the service history prior to us owning it? Don't want to end up causing any damage to the thing.

Thanks for any advice, I appreciate it very much!

I will tell you this: Converters do not go bad on their own very often. They usually go bad due to a misfire condition allowing raw fuel down into them causing them to overheat. The old small block engines like the 5.7 were notorious to have intake gasket leaks, which can cause a misfire. To answer your question, using the seafoam will not hurt anything.

One thing about seafoam, do NOT put in gas tank of vehicle with old fuel tanks. You can put it in crankcase, but you will have to change oil within 50 miles. But what helps most of all is running it through booster lines. That's what makes your truck do the seafoam smoke out. I don't think seafoam is going to help your situation if your truck is still running rough after all those parts. But it is a cheap thing to try. can is 6 or 7 bucks and you only need a third of the can to run through brake booster lines. Just running though brake booster line will clean out hoses, clean combustion chamber out, and the exhaust fuels helps clean o2 sensors and cats. I seafoam everything, old and new. If you need step by step, let me know. I can do it it my sleep

The mechanic down the road did say he thought he could see a small oil leak at the intake gasket, how big of a pain is it for a shadetree mechanic to change a intake gasket? I'm tired of paying that guy good money and not getting any tangible result for something I may be able to fix myself.

They are not too hard, however, you will need a diagnostic scanner to get the distributor placed back in the correct orientation (the camshaft position sensor is in the distributor). A code reader will not be able to do this.

Intake gaskets aren't hard to change. Pretty sure you just have to take off throttle body and intake arm then the actual intake and a few other this and thats. You will need a torque wrench or a well trained hand to torque it down. Sounds like you're running pretty rich if your blowing black smoke. Are you currently running on all 8? Intake leak should cause you to run lean.

The check engine light has been on for while, even before we bought it according to the friend we bought it from, sounds like I need to get it to AutoZone and let them scan it. The mechanic down the road however said that it wasn't showing any error codes when he hooked his scanner up, although I was always under the assumption if that check engine lights on there should be some type of code showing.

I'm going to go out on a limb and say that if the intake gaskets have never been changed they are leaking. They may not be the root cause of your problem, but changing them will not be a waste of money. I'd reccomend using the more expensive ones ( http://www.amazon.com/Fel-Pro-MS98000T- ... B000CH0C0Y ). They are reinforced with metal, rather than like the factory ones that are reinforced with plastic.

All I sure do appreciate all the info and advice here, these things always seem a bit daunting at first but there seems to be a good group of folks here that can offer guidance as needed.

Thanks Again

pour the sea foam in a plastic cup and use the hose from the brake booster to syphon it into the intake.it should smoke pretty bad until all the carbon is removed. then change the oil .if you have had any valve trian ticking this should cure it. I did this to both of my fathers trucks and all of the valve noise was gone soon after. you could spray some carb cleaner around the intake and listen for a change in the idle to help find the leaking gasket. good luck.