Rapid thumping noise
 

More often than not, when I turn the ignition Off: I hear a quick-time rap/rap/rap coming from the drivers-side lower dash area. It's like it's coming from the brake pedal, but it's not moving. I'm thinking some kind of vacuum motor rapidly cycling? Everything works ok afaik. At a loss to understand this. I haven't been using the AC mode....too cold....using only the Econ for Heat. Any ideas/help/suggestions would be appreciated. Thank you.

Check for codes and what TPS is in P01. Then re-post.

Cold engine/no codes/P.01=3.3-3.7 at idle. The rapping noise is from gas pedal movement. I recently installed a new ISC. Went through the complete procedure from the FSM before hooking up the electrical connector. I did notice an odd thing after the installation. Upon easing up the throttle, the idle would be around 950rpm. I would 'burp' the throttle and it would idle down to 750rpm, where it should idle. I thought something was 'hanging' with the isc plunger. It wouldn't idle down to that 750 by itself, only after a little 'burp'......but sometimes it does? The engine(new)/computer doesn't have a lot of miles/time on it....260mi/5hrs. Thanks slowroll for the response

Possible weak return spring. It happens. If you run ISC back so it's not contacting throttle lever, what does TPS read? Does closed throttle / ISC nose switch indicator light come on when gas pedal is released?

Spring seems ok, but it wouldn't break me to replace it. Engine running/not running to read the TPS when not in contact, and read what....voltage or P.01 degree? I don't know of any A/B nose switch indicator light.....?
When I was doing the installation procedures, I think the P.01 wasn't to spec(I should have written the value down...didn't), but the next step was to read the voltage at the test point. The voltage was in the ballpark and the procedure was that no adjustment was necessary. The OEM isc plunger had thread-lock on it and was damn-near impossible to move. The new isc plunger can be moved(screw'd/un-scrw'd) with one finger.....I don't know if that is/will be a problem or not. It's always in contact with the throttle lever so it shouldn't move/change, but then again.....
I'm also hearing a slight octane-knock which REALLY bothers me.....

In earlier post you refer to FSM, do you actually have one? If you don't say so,and I will try to walk you through it.

First thing I bought back in 1997!

In section 8E on-board should be described along with status lights that come on ECC head while in on board.

8E is about windshield wiper system. OBD 1 I've never seen any status lights on the ECC displays except for the telltales.

Try 8D or look in index for Computer Diagnostics. There is a page that will show ECC head and what the lights mean when in diagnostics. If ECM see ISC nose switch closed the low light should should come on, when gas pedal is depressed light should go out.

ok....finally figured that status light out. Never had a code in that area, so I hadn't known about the 'off' light in the CCP being used as a diagnostic tool. I did that particular test and the Off light stayed lit. Tried manuvering the ISC plunger and determined the new ISC is defective, in that it was hanging....once bottomed, it wouldn't return properly. Brought it back and have another for replacement.....Wells brand. Had I known the brand beforehand I'd not have purchased it, but maybe this one will be good. I'll deal with it tomorrow and let you know how this works out. Thank you for the good help, I appreciate it very much.

Upon further review: I have the new unit, but decided to take the OEM unit apart. Cleaned the corrosion from the electrical parts and NOW I'm asking what grease should I use? Choices I own: white lithum/blue HT wheel-bearing/black chassis/regular dark yellow wheel-bearing/vasoline. The '87 grease was hard and caked. Electrically the old unit checks out fine.

All better now. Installed new unit and it's performing correctly. Old OEM unit would not test good everytime. It would occasionally, then it wouldn't, so I trashed it.

My ISC is causing problems very similar to yours.
 

My ISC is causing problems very similar to yours. I had rapid thumping. idled to high and not coming down. In the error codes it also says the E26 error code. How much is a ISC? Where do they sell them?.... and is it hard to install with out a professional mechanic to help out?
Talk to you soon,
Jason Micallef