5.7 l TBI idle problem, now smokes

This engine had an idling problem where it would idle @ 1800 rpm. I changed the TBI to manifold gasket and the idle is much better, but still about 200
rpm too high. Now there is a new problem, There is a wisp of light blue smoke when first started, which turns to grey then a rich, thick brownish grey smoke when pedal is pushed, which is worse when you let off the pedal. I changed the PCV valve and some of the smoke is better, except for when you let off the pedal. Any suggestions ?
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Your valve stems guide seals are worn. The wisp of smoke is from oil that seeps by the guides on start-up. This motor is about as complex as a brick and the guide seals are a fairly easy job if you are mechanically inclined and have the time and tools. I think they switched to ceramic in '91 or '92.
The extra 200 RPM could be from a dirty seat on the IAC valve. I believe in an earlier post you said you replaced the IAC (?) but you also need to clean out the inside of the orofice where it lives. Any dirt or grime or build-up inside the throttle body can prevent the IAC pintle from seating fully and will let a little bit of air by, which can raise the RPM slightly. You may also need to check for leaks on the TBI base gasket that you just replaced.
You mentioned in a previous post that you've been a firefighter for 20 years - I have also. I owned an '88 C1500 Silverado with the 350 and put almost 200,000 miles on it before I traded it in back in '97. It ran just as strong when I traded it in as when I bought it. I miss that truck - it was easy and cheap to work on and the 350 gave you plently of notice to fix small problems before they got out of hand.
Cheers - Jonathan
The grey and brown smoke when driving sounds a lot like burning oil, which may go away when you change the valve guide seals.

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bad valve seals or stuck rings also. Before you tear into the engine, try something that works occasionally. Get 4 quarts of oil, a quart of Risolone oil additive and a new oil filter. Also get a quart can of Motor's 5-minute engine flush. Follow the directions on the can of engine flush (add it to a COLD engine and run it for ONLY 5 minutes; then drain). Do a filter and oil change after running the flush through (add the Risolone too) then drive the truck about 25 miles at highway speed.
IF it was stuck rings, you may feel a difference in engine response (I say IF because, while this works, it may not be your problem). I used this method on my own 1990 'Burb because it was smoking when I bought it in 1997 (89,000 miles) and the smoke ceased after the flush. I've used this on numerous vehicles since and it works more often than not. But, it may take longer to see results or it may not work at all.
If you need to change the seals, the FelPro P/N for the O-ring seals is SS 5102 (for al GM v8's from 1955 to 1992) Also get a set of umbrella seals FelPro P/N SS 10058 (these are for a Caddy V8 1965 - 1967) and install them along with the O-rings. Other umbrella seals may work but these are what I've used.many times. To install the umbrella seals, just slip them over the valve stem and push them down as far as they'll go.
I bought the valve seals soon after I purchased the truck but haven't installed them yet because the flush cleared the problem up. With 190,000 miles on the odometer, the truck uses less than a quart of oil every 4,000 miles and runs great. BTW I run the engine flush through every two years just to keep things kosher.
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% There are two classes of pedestrians in these days of reckless motor traffic - the quick and the dead. ~ Lord Dewar 1933 ~
Climbing into a hot car is like buckling on a pistol. It is the great equalizer. ~ Henry G. Felsen 1964 ~
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This thing just won't play right ! no matter what I fix on her , she still idles @ 1000 -1100 rpm. I have had the following things changed or fixed: 1. TBI to manifold gasket 2. IAC unit 3. ECT sending unit 4. O2 sensor 5. Flushed entire cooling sytem 6. Flushed and serviced Transmission 7. All vacuum lines 8. Checked timing 9. Purged cooling system 10. Complete engine tune-up and service 11. New fuel filter About the only thing I haven't changed is the paint. The idle returns to normal when the tranny is placed in drive, then back up when in park.The van runs and drives very good on the road, just nobody can figure out the stupid idle problem. Two other possible culprits mentioned: Motor mount or Thermostat. Where do I go from here ? I'm driving it 450 miles to Florida tomorrow {I hope} Should I worry about the idle, or just drive the old bat ?
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