Greetings all,

Just FWIW, this issue is related to a 84 C4, but as it is the same engine as an 82, and as I hang out here more due to also owning a 75, I'm asking this question here in additon to the C4 section. That and and I think more technical input is available here.

So the story is I recently found a decent deal and picked up a 1984 4+3, Z-51 as a driver while I'm working on the C3 this year. The 84 has 74K miles. Came w/lots of documentation/receipts and it appears to have been fairly well maintained by it's two previous owners.

The crossfire injection is new to me and I need to understand why it wants to idle high, approx 1100-1200 RPM when warm, and 1400-1600 when cold. It occasionaly gets down to 900 when warm, but more often than not when I lug it down in gear by dragging the brakes to about 8-900 RPM and then push in the clutch, it jumps back up to 1100-1200 and stays there, at least for awhile.

Due to this when in traffic it wants to "drag me along" at a high idle setting when I want it to actually drop down to normal idle speed in gear. As it is I can't creep along in traffic w/o holding it back via the brakes or constantly riding the clutch in and out.

What items/processes do I need to check/troubleshoot to address this issue?

TIA.

Regards...............George

 

Most common cause for a high idle is a worn set of tB's, wiggle the throttle shafts, if they have play false air is getting in there, GM really was cheap using a steel shaft in an aluminium housing without any kind of bushing. You can use the synchro and idle set screws to turn the idle down. It's a bit of a tricky operation and I feel that the procedure in the GM manual isn't quite what it could have been, they drill the housings to remove the caps and you don't need to do that, same as removing the synchro screw, it's easy to just break the weld and leave the "thimble" screws in place, makes adjusting it by hand a lot easier.

It can also be a vac leak, usually the plenum gasket or a set of sticking/dirty IACs. If the gasket is leaking spray some wd40 around the lid, if the idle changes you have found your leak.

84 are the only year for metal brake boosters, check for a rotted out one that would be a pretty nice leak, then again you'd notice that in the braking department.

 

There is a common problem with the plenum gasket leak. The next problem is ususlly the idle air controls (IAC) which stick. There are two IACs and they must be working properly with properly balanced throttle bodies. There is a elaborate proceedure for balancing the throttle bodies that Bubba doesnt know, so he just turns some screws. Look at the linkage between the throttle bodies and see if he has removed the factory installed welded cap on the linkage adjustments. If he has, you will need to readjust the balance and the minumum idle before you do anything else.

 

Heres a qusetion for you. If I take the throttle bodies to a machine shop and have bushings and a new shaft installed, what kind I expect by way of improvement? Assuming my gaskets are good with no vacuum leak.

 

I personally dont think you will see any difference. There is plenty enough range of control in the IACs to take care of that small of a leak. It could change the balance and minimum idle adjustments a little.

 

Thanks Twin_Turbo and wombvette. I will check the TB shafts and also check for vacuum leaks. Will also check the TB linkage for any "Bubbaing." I think not based upon the cars history, but one never knows.

I'll post results when I know more. (or be back w/more questions)

Thanks again!

 

You really need to send any TB work to Dan the boring guy. He is the shiznit's when it comes to any work on your throttle bodies...
http://www.swko.net/~lionsden/crossfire.htm

 

You are forgetting about one thing, the loose shaft is not a cosntant vac leak, as the shaft moves or virbrates the leak gets larger or smaller, same when you wiggle it by hand. This confuses the engiune controls by varying map signals and the iac's will go crazy, also there's a good bit of idle control in the IACs but not near enough to handle a good vacuum leak, if they're clsoed they're closed, you can't close a hole further than fully closed now can you

If you can have the housings machined for bushings then by all means go for it. You could even have the housings bored, thing the throttle shaft and do some other tricks to get some more flow out of them.

 

I would bet on a vacum leak from the upper plenum. Usually these leak on the left rear corner. If you have access to a scanner check the IAC counts-if they are at 0 you have a vacum leak.

 

Wait a minute twin, old boy. Are you saying that if I have a ,more or less, round hole with a smaller, more or less, round shaft in it, and I shake the shaft around a bit that the area of the misfit changes with position of the shaft. My old geometry professor would like that one. I agree with you really that the wear is a bad thing, but I dont think that it makes a great difference unless it is a whoper of a worn shaft.

 

I know what you're getting at, that the "open" space does not change as it is the "area" of the hole in the housing minus the "area" of the shaft cross section, however you forgot one thing and that is what part is above and what is below the throttle plate, so if you pull the shaft up the opening will be under it and it's directly open to the vacuum under the plate If you have an erratic idle that seems to drop down to an incosistent rpm after revving it may very well be the shaft.

 

Aint this a fun discussion? I understand what you are saying, but, at idle, the plates are sucked down so all of the extra leakage is above the shaft. His question was about idle, and, at a steady state idle, the leakage around the shaft is at least constant and if the wear is not so much that it overpowers the IACs ability to adjust, it would make little if any difference in idle quality. I think that if we are talking about off idle operation the wear could cause uneven lifting of the thrttle plate and may cause some erratic throttle response there.

 

This is a good discussion as my 82 has some idle issues. The shaft feels loose and is detectable with closed plates by jiggling the linkage. Shaft moves, plates don't. It feels as though the shaft is worn and in need of replacement. But in doing so, it seems like a good idea to have the machine shop put bushings where none previously existed. Doing so should imrove over all idle and throttle response, assuming there is no vacuum leak coming from where the mounting plate itself meets the intake. But it's an 82 so I already know this is already an issue. Beyond those mods, what other things should I be doing since I will have the TB's off anyway?

 

This is a great discussion.

My 82 this year suddenly has developed a slow to go to normal idle. Maybe its my perception and it was cold (50F) when I first ran the car early this last Saturday.

Would the laptop/win ALDL program show IAC counts as well as coolant temp sensor readings?

How much info does this program give you?

Brent...

 

No I mean idle, you'd think that there was enough vacuum to pull the paltes down but there isn't as they are spring loaded and they will be all over the place resuting in a different idle rpm after you mash the throttle. A vacuum will have good suction but if you crack the restriction open just a tad most "sucking" power is lost, try it w/ a vac. cleaner, hold your hand on it and then just slightly move 2 fingers apart to create a slit, you'll notice that the suction will be greatly deminished. I have witnessed this idle problem many times, set the idle and after some full throttle stomps it's too low or too high again, all those times it was a worn housing and a loose shaft. The IACs have some adjustaility when it comes to idle but not that much, they are around pos #22 at proper idle, if you have a leak there's not much you can regulate down with them.

 

So in effect we agree. With a seriously worn shaft there will be some idle, and or, off idle degradation.

 

This is exactly what Jim (elkabong) showed me when we were setting mine after a manifold swap.

It would never settle back to the same setting after blipping the throttle once or twice...