after intake gasket replaced, idle is up and down

[chriswilhelmart]

This is a weird problem, So i have a 1999 vortec suburban, 5.7 350. Ran PERFECT for years and years, until the other day the lower intake gasket finally blew. So i looked up the procedure and being a somewhat experienced mechanic, i dug down and got in there, cleaned up the internal valley under the intake, and took my time! I had heard of some issues like not putting the dist. back in right, so i took extra precaution in every step. To my surprise, it started right up first turn, as if nothing had been done to is in the first place, after everything was fixed and put back together. But all of the sudden the idle started going up and down, to the point that when im in gear, and im stopped, it looks like i have hydraulics under the rig because of its violent thrust forward and back. once i get going down the road everything is fine. I had an emergency about 60 miles away and ive been driving and hauling a small trailer back and forth for about 2 days now. but this is getting dangerous at stops.

I finally tried something that i was "sure" wasnt going to work. I turn the truck on, pop the hood, and disconnect the mas sensor plug from the air intake tube, which immediately fixes the idle, then i replug it in a couple seconds later, and woila, problem is fixed. it idles smoothly, it drives great, (has TERRIBLE mileage) but everytime i turn the truck off and back on again, like to get gas, i have to repeat this mysterious series of steps to get it to idle for me.

Ive looked and looked, all the connectors are together, i used the same parts as before, other than the gaskets. I cant find any air leaks, etc. and i need this thing up and running correctly today! So if anyone can help, please get in touch!

Thanks,
Chris~

 

[zora]

is your service engine soon light on? You are sure you have no vacuum leaks, you have checked with carb spray or similar procedure? did you disconnect the battery when you did the gasket change?

 

[T-Max]

You said "it drives great, (has TERRIBLE mileage)" and those two things don't exactly go together. Your mileage should be somewhere in the 15 mpg range. I don't know what you mean by "TERRIBLE mileage" but if it's much worse than before the problem arose then it's just not running properly and your problems go beyond a rough idle.

It may seem like the work you did is responsible for the problem, but it might be totally unrelated.

I know I'd need a lot more information before I could make any suggestions. If you read my prior thread -

http://chevyforum.org/chevy-forums/showthread.php?t=29961

You'll see that my problem turned out to be nothing more than a bad CTS. But it took a lot of work to finally pinpoint that as the source of the problem. The truck's ECM (computer) gets information from many sensors and any one of them could be feeding bad info. The CTS is a primary suspect as it often goes bad. If you read this post from my thread -

http://chevyforum.org/chevy-forums/showpost.php?p=47462&postcount=23

you can see a quick way to check to see if it's bad (just try disconnecting it).

The first thing you should do is check for any bad codes. But note that codes are not always set, as was the case with my bad CTS (it never set a code).

Checking for codes is simple -

http://www.chevytalk.org/fusionbb/showtopic.php?tid/273660

BTW, I wouldn't count on solving the problem "today" unless you're going to take it to a shop that has all the necessary equipment to diagnose things in short order.