7 replies to this topic

[WestCoaster]

1998 GMC K1500 5.7 liter VIN Code "R", CA emissions, 64680 miles, 3.73 final drive, automatic trans. Truck runs normally on flat and level and up grades by itself, but upon towing trailers uphill, CEL comes on and engine performance begins to drop off. Scanner indicates random misfire #4 with codes P0300 and P0304 in memory. Primary and secondary ignition checked, and cap, rotor, wires replaced. Primary and secondary fuel components from in-tank pump to intake crab and injectors diagnosed to have no problems. Problem persists. Preliminary TSB #1539013 mentioned in Motor Age magazine indicates in-spec but overly tight exhaust valve clearances causing misfire upon heavy load conditions. Borescope inspection of all cylinders and exhaust valves show no significant carbon buildup. Small ring of deposits seen on valve stem-to-guide interface. Anyone experiencing this problem and what is solution? Next step for me is to teardown and inspect for tight exhaust valve clearance on #4 especially. Help?

Rich

[TEE]

Try Seafoam

[WestCoaster]

Try Seafoam :thumbs:

<{POST_SNAPBACK}>

Have tried Seafoam, BG44K, Champion FIC, 1,1,1-TCA, Marvel Mystery oil and all give temporary relief. This leads me to believe that some deposits on the valve stem are further reducing an already tight clearance and is exacerbated when hot under loaded conditions. Injector spray patterns are perfect as seen under stroboscopic light. Borescope magnification of 100X shows unusual longitudinal scratches on the exhaust valve stems when compared to my 96 Chevy that has 155,000 miles on it. This further leads me to believe tight valve clearances exist. Tearing down one head tomorrow. Will let you know.

PS Observed an intake gasket coolant leak and small oil seepage on one head gasket near oil galley anyway. Needs to be done.

[ChevyTech007]

You could have an intake gasket leaking vacuum on #4. Be sure to examine the rubber seal on the intake gasket on #4 when you tear it down.

[WestCoaster]

You could have an intake gasket leaking vacuum on #4. Be sure to examine the rubber seal on the intake gasket on #4 when you tear it down.

<{POST_SNAPBACK}>

Will check for that but seems like fuel trim and O2 and CAT monitor data look too good for vacuum dilution. Think there's any substance to the tight valve clearance problem?

[TEE]

You could have an intake gasket leaking vacuum on #4. Be sure to examine the rubber seal on the intake gasket on #4 when you tear it down.

<{POST_SNAPBACK}>

Will check for that but seems like fuel trim and O2 and CAT monitor data look too good for vacuum dilution. Think there's any substance to the tight valve clearance problem?

<{POST_SNAPBACK}>

Yes yes yes....a leaking intake will cause all kinds of problems.
I still say, "what a poor design on intake gaskets"

[WestCoaster]

Had the wife pull off both heads, and send them to the machine shop. (She's an ASE Master Tech) They found #4 exhaust valve almost stuck in the guide,....very difficult to rotate or move. Explains the longitudinal scratches in the stem. The valve isn't rotating like normal. Guide to stem clearance was 0.001" or less. Hot tanked the heads, new guides, honed to 0.002" clearance to the valve stems, valve grind done and head reassembled and should be good to go for engine reassembly tomorrow or the next day. Valves were incredibly clean as are the pistons. Good maintenance and premium fuel and oil products paying off dividends. Will find out soon enough if this solves the problem. I'll let you all know.

[WestCoaster]

Test driven today with 5400 lb trailer going up a 6 1/2 percent grade on Interstate 80. No CEL for a change! More power, no drop off in performance. #4 exhaust valve was really tight, and two others were not quite as bad. The raw clearances were in spec, but with a small amount of carbon and sludge inside the guide, the valves were having a hard time moving freely. Guess they need heavier valve springs if they want those tight clearances to work! I've heard of many people having some kind of towing or loaded condition problem like me. Now, I wonder if GMC will show corporate courage and customer goodwill by helping me out with this repair bill. Just glad to have it back to normal.....so far anyway.