98 Tahoe Engine Problems

My 98 Tahoe with 155,000 miles started idling rough and using oil last month. Prior to then, it ran very good. Although it does not smoke on start-up or while running, the tail pipe is coated with black soot. It uses a quart of oil every 250 miles! I've read the following codes in the last two weeks: P0420(cat eff below threshold bank 1), P0174 (system too lean bank 2), P0171(system too lean bank 1) and P0305 (Cyl 5 misfire). Compression is 175 - 180 on all cyliners except # 6 is 155 - 160. Seven plugs were dry, but looked blacker than I would have expected for 30K miles (GM OEM Platium). Plug in cyl 3 was very oily. I changed all plugs which improved idling somewhat, but cyl 3 was really bad again in 500 miles. I've had the Tahoe on the road for 500 miles round-trips in last week and it ran great - plently of power and good gas mileage (16 or so). I've changed the oil every 3,000 in this one and it's never been hot. I changed the intake gasket last year to repair a coolant leak. The PCV was changed three months ago. Also, I had a noisey lifter on the passenger side so I ran a quart of marvel mystery oil through a couple of oil changes ago and that eliminated the clicking. Since I've got low compression in #6 where the noise was coming from, I think I may have a valve problem on that side, but I don't know what would be causing the bad fouling of plug number 3 and excessive oil useage unless I've got a ring failure. Could that be the problem even with 180 psig compression in that cyliner? May be too much info here, but any help would be appreciated. I'm afraid I may be headed for the reman engine store!

before resorting to a reman engine i would replace the valve seals. if the oil control rings are stuck/broken/worn out you will still have compression but oil will seep through.

We had just about the same problems...once the dealer changed out the catylac? converters everything went back to normal. Good luck.

I agree with the valve seal replacement suggestion. I did them on my 96 Z71 5.7L at 155,000 and now I don't use but 1qt over a 6,000 mile interval. Before I would use a quart or more every 3,000 miles. By the way, do you only use your truck for short trips consistently? You may have a lot of sludge built up in your valve cover areas and in the oil pan. Short trips are hell on a vehicle, make sure your giving enough time for the engine to warm up and boil off any condensation in the oil which leads to sludge formation. -Chris

Blue smoke on start up after vehicle has sat for a while = worn valve seals or guides.

Blue smoke on engine decleration when you rev the engine up and down = worn piston rings or cylinders


On a high miler engine your further ahead to spend the money towards a rebuild instead of patching it up and spending it again down the road.