High idle speed and no more surging after intake rebuild

[82DMC12]

FWIW- It seems odd to me that the voltage droped over time, especially since the RPMs rasied too.

...not sure how accurate your gauge is, but it should be reading around 14.2V instead of 13V after all that time...
(May be related to the tach flicker as well??)

The gauge reads about 1V low for sure. When I had the binnacle out a couple weeks ago I was going to take the needle off and put it back on a bit higher but it wouldn't budge and I didn't want to break it. I just installed a new DPI alternator two weeks ago so I will double-check the volts when hot at the jump start post next time I'm working on it. The alternator replacement had no effect on this issue, I was really hoping I had something fried in the alternator causing weird voltage but I guess not. The alternator needed work anyway because the bearing was a little noisy (15 year old Hervey unit)

[82DMC12]

Another update -

1) I disconnected the grounds from the RPM relay, the idle speed microswitch, and the idle ECU and ran temporary grounds joined at the battery. I then started the car and it made no improvement in either the idle quality or the bouncing tach gauge so I don't think a have a bad ground. I also cleaned the ground that attaches to the frame under the jump start post.

2) Removed the throttle body from the car and cleaned the butterflys with alcohol prep pads and then put two dabs of hot glue on each decel valve. See picture below. As I was reinstalling the throttle body, I saw my worklamp shine through the butterfly plates and noticed one of the plates was not centered as well as the other. I slightly loosened the split screws just enough to get a bit of play and re-aligned the plate. I then started the car there was no improvement! So, I think I can rule out bad decel springs.

3) I decided to do another smoke test and found similar results as before when I did a smoke test - some leakage around the oil dipstick rubber tip, leakage at the lower crankcase where the dipstick tube fits into the engine, and a new one I didn't have before - a bit of smoke coming out around where the throttle butterfly shaft end comes out of the throttle body. Not too surprising since it seems there is a tiny bit of play in the throttle shaft when everything is out on the bench. I don't know if this is a problem or not, it could just be that as I seal up other places, the smoke is going to find increasingly small places to come out.

So, at this point I'm not sure where to move forward. I feel like I've checked everything that I can without just randomly replacing stuff. If I were to randomly replace stuff, I'd be looking at:

• Tachometer dash gauge - I replaced the board but not the face. Apparently there are magnets or some kind of dampening systems on the gauge separate from the circuit board. Maybe something happened to the dampener and that's causing my tach jumping. Totally unrelated to the idle coming up but maybe that would fix one issue? Once again I hooked up my Actron tach to the coil white/slate and checked the RPM's and the external tach isn't jumping around crazy so either the Actron has better dampening or the dampening in the dash gauge is bad. I don't know how else to monitor the white/slate signal unless I had an oscilloscope.
• Idle Speed Motor - I cannot come up with any proof that my idle motor is bad because it passes every test, but maybe something is wrong with it
• Ignition ECU - Just another place where white/slate goes, maybe something happened to the ECU when I had my Accel Super Stock coil hooked up backwards for the last 20 years.

PXL_20220618_162423582.jpg PXL_20220618_161810767.jpg

[Bitsyncmaster]

I was hoping that test of the deceleration valves would solve the problem. Anyway it reinforces that problem shows up when you see bad springs. I can loan you one of my idle ECUs if you pay for shipping both ways. I also have spare ignition and lambda ECUs but don't think that's your problem.

[82DMC12]

I was hoping that test of the deceleration valves would solve the problem. Anyway it reinforces that problem shows up when you see bad springs. I can loan you one of my idle ECUs if you pay for shipping both ways. I also have spare ignition and lambda ECUs but don't think that's your problem.

Thanks Dave, I appreciate that. I actually have a spare idle ECU that I borrowed from someone else but I don't know that it has been confirmed that it works perfectly either! I think the chances of two ecu's having the same weird problem would be very low. However if I look closer at the ignition ECU I might take you up on that.

I think you are right in that the springs have to look bad in order to actually be bad. There's also no way to adjust the tension on the springs unless you melt off the solder on the end caps. I'm not going to do that, I'm just going to leave the springs alone.

I'll probably order a new dash tachometer on Monday so I can at least give that a try.

[82DMC12]

Hey everyone, I think I have some good news.

I pulled my distributor out AGAIN this morning and reinstalled my old impulse coil and put a new "fly tach lead" on - this is the wire that comes off the impulse coil and goes to the plug that sits in the firewall (white and yellow Bosch two-pin plug). After buttoning everything back up and adjusting the throttle one more time, I did not have the jumpy tach OR the elevated idle! I have been working on the car and taking test drives for much of the day and none of my symptoms returned.

So, I cannot say for sure if the tach problem and the higher idle were definitely linked, but the jumpy tach was definitely caused by either a defective NOS impulse coil or my old fly tach lead was just breaking down.