Changing a timing chain...

TeamGreen · April 19, 2002

My engine seems to vary about 500rpm or so on and off when at idle and I think its because the timing chain is loose/worn. Well I have never actually changed one but have done some of my own work. My question is has anyone change one? How hard on a scale from 1-10 is it?

95Yukon · April 19, 2002

One way to check it is by pulling the distributor cap off, then turn the engine over slowly (by hand) in both directions and measure the amount of slack it takes when the crank rotates before the rotor moves in the other direction.

If it needs one, I'd say its about a 3-4 on a scale of ten if you have some mechanical knowledge. Drain coolant, remove water pump, remove vibration dampener (possibly the hardest part of the whole thing if it is stuck and you don't have a puller) then you may need to loosen some of the front oil pan bolts to get the timing cover off (not sure on the newer engines) remove and replace the gears and chain making sure to line up the marks, then put it all back together.

Normally, a worn chain will make the cam run "late" valve timing, but I don't know if that would cause it to vary it's rpms like you described. What engine is it, and how many miles on it? It may just need an idle control valve and/or a good throttle body cleaning.

TeamGreen · April 19, 2002

The engine is a 350 and has about 110k miles. Where are the idle control valve and throttle body located?

95Yukon · April 20, 2002

take off the air cleaner assembly and look on the passenger side rear corner of the throttle body for the idle control valve. (the throttle body is the injection unit you have where a carb normally sits on top of the engine (what the air cleaner attaches to)

There is a wire connector on the idle control valve..take it off and unscrew the idle valve assembly. (its about a one inch wrench, but you can usually just break it loose with a large pair of channel locks and unscrew it by hand) When you take it out, it has a plunger with a rounded tip. Clean that off real good with some carb cleaner spray and also in the hole it goes into in the throttle body. Also spray some down the throttle body and into that area on top where the injectors are.

Put the control valve back in, tighten it so its snug, hook up the wire harness connector, and start it up. It will idle real high right at first until you shut it off and restart it. That is normal and part of how it recalibrates itself. Once it is idling normally, spray some cleaner down the throttle body with it running (you may have to keep it at high idle so it doesn't stall) then put the air cleaner back on. If anything, you will have a clean throttle body, and it will most likely fix the varying idle problem as well for the price of a can of Gumout. (I recommend the Valvoline brand throttle body cleaner spray, actually) it works faster and gets it really clean.

If there is a lot of junk on the valve when you clean it, it may have been the problem. If it looks worn around the tip of the plunger, you may need to replace it. I think they are around 25 bucks if you need a new one.

Hope this helps.

95Yukon · April 20, 2002

And by all means, if you are not completely comfortable doing such a procedure, take it to a repair shop, and have them do it or give you an opinion on what the idle problem is.

Since I do all my own work (as a former mechanic) I tend to get carried away and forget that not everyone can be comfortable doing such things, but the main thing is to be safe about it and consult a manual if you do it. I consider that particular procedure a basic maintenance thing, and very simple.

Just hate to see anyone get ripped off by a garage or dealer. In this case, they might try to sell you a new throttle body. (plus labor)

TeamGreen · April 20, 2002

Thanks for the help. I have changed my water pump before so it doesn't look like its that much harder. I will try using some carb cleaner on the TB first.

TeamGreen · April 27, 2002

Well I took out my idle control valve and cleaned it with the valvoline tbi cleaner. It wasn't really that dirty though and my trucks still not running right. It stalls sometimes when I pull into my garage, it changes rpm at idle and when I give it steady gas too. Have any other suggestions on what it might be? Thanks