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weird idle coming off power; coasting


bryanb

Well-Known Member
TRS 20th Anniversary
Joined
Jan 25, 2009
Messages
93
City
Western Montana
Vehicle Year
1989
Transmission
Automatic
Been having idle problem, but I cleaned out the throttle body as best as I could, and now its been better, but this is a side effect.

After driving at a constant speed, and letting off the gas to begin to brake for a stop, the engine speed will come down to around 1000 rpm, and then 'bounce' for a few seconds. it will fluctuate between 1000, and 1200, and then calm down.

What might I look into here. I took the throttle body off, and cleaned it, and the idle air bypass valve out, and reinstalled, and the slow idle is better, but like I said, this new problem seems to be a side effect...
 
You repalce both gaskets?

Double check everything.....make sure everything is connected right, etc.

If it started happening after you cleaned stuff, then there's a 99.99% chance it's something you messed with.
 
I did not replace the gaskets.

The one between the throttle body, and the intake, and the one between the throttle body, and the idle air bypass.

I did very carefully pull them off with a sharp putty knife, and put them back on. They were not ripped at all, so I thought I'd just reuse them.
 
Easy way to check the gaskets is to spray starter fluid around them while the truck is idling. If it idles up then the gasket is leaking and needs replaced. In that aspect, would also double check all your mounting bolts and make sure you tightened them all down.
 
All the bolts looked good. Didn't actually test for a leak at the gaskets though.

its almost like if i let go of the throttle quickly, its bounces when it gets to the bottom. (the pedal does not move like that though) I did hit the linkage on the throttle body with some WD-40... Is it too slippery now?
 
Also, this will NOT happen while in park, or in drive with the brakes held. And, when it does happen, it will stop pretty much right after I hit the brakes.
 
When applying the brakes could the vacuum from the booster be affecting vacuum for something else changing the idle.
 
The throttle is not to slippery. As posted earlier you need to check for leaks look over what you pulled off to clean the TB. Spray all around and try to find a leak did you mess with the pcv. You know for $10 )you should have got new gaskets. :). Keep us posted
 
I think I might have fixed this.

After having this issue, and learning when it occured, and establishing a pattern, I found that it would ocur when the speed passed around 35 on the way slowing to a stop. It seems that the tranny wants to downshift? as I slow down, and it was revving the engine at that time, usually at 35 but at other speeds too.

I changed the tranny fluid, and filter (quite plugged) this weekend, and after2 short test drives, the problem as seemed to have gone away.

What is the tranny doing when slowing down?
 
if its an auto its suppose to be free spinning or in a neutral state... with you fluid being bad it can change the functionality (big word!!) of the trans such as Slipping, down shifts, and so on... keep us posted on if it starts again
 
That's good if changing the tranny fluid fixed the issue...but it sounds exactly like what my Tempo was doing...turned out to be a bad MAP sensor...the one that is on the firewall just above the heater box...if you have one, try changing that if the symptoms come back...
 
Turns out the tranny work didn't fix anything. Its still confusing, though since it never happens with it in neutral, only in drive.

I have the gaskets now, and want to try to take the TB off again (haven't had time or good weather as of yet), and look at it and the IAC. Is there a way to test the IAC off the car?

it seems to only happen when the engine is up to operating temp, hence me thinking it was fixed during the first test drive after changing the tranny filter.

And only happens when the engine rpm hits 1000 coming down from something higher, after letting off the gas. (then bounces between 1000, and 1200) Sometimes, if I let off real slowly, it won't happen.

Sometimes I'll get it to happen again, when downshifting, and it again comes down to 1000 rpm from something higher.

What gets me is that it never happens in neutral, only in drive.
 
Throttle body thoroughly cleaned, and new gaskets installed.

No change in the problem.

I did notice something with the Idle air bypass. The electric motor or whatever on the end of it, rattles when shaken, much like a new PCV valve. Should it? I wonder if there is play in that little motor or something.

Napa says its not available as a separate part, and the whole works is over $90. Thats big bucks considering the condition of the rest of the truck :) so don't want to spend it unless there is a real good chance thats the source.
 
I had similar issues and it was a vacuum leak. Try shaking the throttlebody/IAV while the truck is idling. You can also apply make-a-gasket to the mating sides of the gaskets to be sure they seal well (keeps them in place during install too).

Barring that, disconnect the plug on the IAV and let it idle. Your truck should run without it hooked up, I believe ti should drive too (albeit with a high idle). See if the problem is present with the IAV disconnected.

Also, check if your TPS is giving weird signals at any point.
 
Map sensor...the one on the firewall...passenger side...it's gotta be there because my truck had one...if it's not there...then that's the problem...
 

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