Rough idle - cyl 5 misfire - burnt valves?

[White99]

I have a ’99 Suburban with 150,000 miles on it. It is basically stock.

About a month ago I started having problems with rough cold idle. It then got worse and started missing when I accelerated at highway speed (onramps, hills, etc). I took it to Autozone and they pulled the codes for me. It showed a Cyl 5 misfire and Oxygen Sensor FWD Bank 1. So I replaced the oxygen sensor and did a tune up (plugs, wires, cap and rotor). This fixed the misfires at highway speed, but did not help the rough idle.

I then cleaned and recharged the K&N air filter and cleaned the mass flow sensor. My wife insisted that it seemed to her like a fuel injector problem (Yes, she knows about such things). So I ran fuel injector cleaner through it (1 tank with GumOut and 2 tanks with Seafoam). That didn’t help either, so I suspected the poppit valve in the CSFE for cylinder 5, but I don’t know how to check that.

Over the last 2 weeks the rough idle has gotten MUCH worse. Yesterday I took it in to a reputable local shop for a diagnostic check. The computer analysis showed that there is still an issue with the oxygen sensor and the misfires on cylinder #5. The manager of the shop suggested that replacement oxygen sensor (Bosch) may not be functioning properly and that maybe I should replace it with an AC Delco. He also suggested that it may not be getting power, but the fuse was OK. The misfires on #5 cleared up when the engine RPM got above 1200 – 1500.

They found that the porcelain on that spark plug was blistered indicating that the cylinder is running hot. One possible explanation would be that the cylinder is running lean, but they determined (I don’t know how) that it was getting proper fuel flow. The plug also showed signs of fouling from burning oil so they replaced the plug with one that foul. The plug has something on it that actually sets the plug back out of the cylinder a bit.

They also found the compression on #5 to be 20 – 30% low (120 vs 150) when the engine is warm, but OK when the engine is cold. They said this is a sign of a burnt valve and they did an air decay test. This test showed that both the intake and exhaust valves were leaking and presumably burnt. They estimated $1500 - $1700 for a valve job.

The truck ran worse than ever on the drive home.

My questions are:
- how do test the oxygen sensor? I can test for power at the connector, but what signal should I see?
- If the compression is good when the engine is cold why would the idle be bad?
- Why would burnt valves cause that cylinder to burn oil? Does the oil run down the valve stem and leak past the valve?
- I can’t afford $1500 for a valve job! I have moderate mechanics skills, how much of this can I do myself? Is it reasonable for me to pull the head and take it somewhere to have the valves done? What are the “Watch Outs”?
- If I do the valve job what else should I do at the same time? Are there things I can do to improve performance and/or fuel economy?
- I don’t feel that the shop identified a root cause. The manager simply said that he sees a lot of burnt valves in Chevy 5.7L’s. Should I still be looking at poppit? What else would cause a single cylinder to run lean?

Any suggestions would be appreciated!!!

[MT-2500]

Low compression will make it miss.
What were the compression readings?

[aztecf]

These are notorious for sticky valves, 2 cans of Sea Foam in the gas tank, one can in the oil, and if needed one can poured slowly down the brake booster vacumm line with the engine speed at or above 2000 rpm. You will be amazed by the results.

Jeff

[maxwedge]

That decarbonbizing may only be temporary as the valve guides are machined for too little clearance, TSB is out on this and so far the chemicals are not a long term fix, if that is the problem to begin with, usually this miss occurs under load at higher speeds.

[White99]

MT-

The compression was around 120. The shop thought they should be 150 - 160.

[MT-2500]

If all cylinders are even no problem there.
It is when you get uneven readings or more than 20-25 lbs of difference that you will get a engine run ruff or miss.
I would suggest a injector balance test.
And pull valve covers and check for a weak valve spring ,sticking valve guide or worn cam lobe.
And on the 99 check the camshaft retard setting {dist position} with a scanner