tach / engine speed hanging at +1,500 RPM's

[68 & 00 GT]

Ok my car was bucking on the highway only when I let off the throttle. In fact if I pushed in the clutch it would usually die out, so I had to restart the car on the highway by letting out the clutch in 4th. OK so I replaced my IAC, (w/ a new Ford one) and while I was at it I bought some B12 chemtool, pulled my intake plenum, and cleaned it out completely w/ the TB still attached. It was nasty black all in side. Put it all back together, and it idles flawlesslesy now. But it still bucks a little on the highway when I take my foot off the gas, and some at cruising speed - not near as bad as before, but when I push in the clutch it doesn't want to die at all, but also now the rpms drop to about 1,500, and then go up, and down 200-300rpms above, and below 1,500. Then after a few seconds will go to normal idle ~700rpm's. Any ideas ? What other sensors could cause this ? Seems to accelerate just fine w/ some decreased throttle response at very low rpms - Oh and no engine light

Considering all the waiting to install parts I have I do not need tuning issue right now

Thanks -

 

[Ben99GT]

Sounds like a vacuum leak to me. You might want to make sure the plenum gasket is seated right, it's tightened down enough and all the vacuum lines are connected and so on.

 

[68 & 00 GT]

hmmm.

I check on that. I wonder if I'm loosing metered air somewhere after I pulled my Anderson cold air off the TB ?

 

[68 & 00 GT]

Guys I dont get it. I can't see putting my Novi on this weekend like I planned until I get this fixed. I pulled my plenum again last night, and re installed it to be sure I didn't miss something when I put it back on after I cleaned it. Still no change. It bucks at cruise, and decelleration, and the tach hangs around 1,500 for 5-10 seconds then falls to a perfect idle. Starts perfectly, idles perfectly, and accelerates perfectly. I'm stupmed. Has to be an air, or vaccum leak I guess. How do I track it down. Isn't there a chemical that I can spray on the fittings to see if the engine revs, or bogs to find a leak ?

Any suggestions please ? What about the EGR sensor, or some other sensor ?

 

[Ben99GT]

Yes, you can use ether (starter fluid) to help find a vacuum leak. Spray the ether wherever you think the leak might be with the engine running and if the engine starts revving up you have a leak.

If you just had a MIL it would make diagnosing the problem much easier.