I cannot for the life of me, get this 86' TPI 305 to stay consistent on the idle. I've tested, reset sensors, tested some more, replaced a few things. Car has always idled at 1500RPM since the day I got it running from the barn I found it. But, I drove it for 5 years like that and never an issue. It started this idle drop thing before I parked it to repaint it. It would die as I was slowing down, or sometimes while sitting at a stoplight. It started randomly... I hadn't changed any parts before hand. Then it started throwing the code 33 and 45(MAF bit the dust finally).
Now that I've got the bodywork done I started to track down the MAF and idle issue.
It'll start right up, runs about 2000rpm then slowly comes back down to 1350rpm'ish. If I try to adjust the throttle screw and reset the TPS, and relearn the IAC. The car will not hold an idle below 1200rpm.
And even at the 1350rpm idle it wants... occasionally it will still drop the idle down to low when shifting into gear (automatic car).
Every once in a while it'll act up just running in park. It'll start idling lower until it dies. But immediately will start right back up.
Here's a parts and test detail lists...
Smoke tested the vacuum lines (No leaks found)
Fuel pressure is 44-45psi not running. 38psi running. Only weird thing I noticed is not running... if they key is left on I hear a click and my pressure drops.
Timing is set to 8degree BD as per my hood sticker.
TPS is original. But reads smooth and correct for the car. Currently set at .054v at idle and I get 4.3v at wide open throttle.
EGR is original. Never had a reason to change it. Holds vacuum, and I can see the valve move when I test vacuum with my vaccum hand pump.
Distributor has all stock internals. But I put a new cap and rotor on it when I did the spark plugs. Those were o'rielly brand.

-AC Delco fuel pump. New 4 years ago (car was started twice a month and always kept barely any fuel in it). Drove about 1,500 miles before it was stored. NO issues then.
New AC Delco fuel pressure diaphragm at that fuel pump install. New AC Delco fuel filter.
-New AC Delco R43TS spark plugs, and Napa plug wires
-New AC Delco IAC (I also cleaned the throttle body and IAC port before the install)
-New AC Delco Coolant temp sensors (Changed both the ECM sensor, and the dash gauge sensor)
-New Spectre MAF (OE threw code 33) no code with this new MAF and it runs good. No check engine light.
-Rebuilt Bosch style fuel injectors. Factory replacement. All test good on ohm's.

 

Have you smoke tested the intake ? How low will it idle with iac commanded in and unplugged ?

 

Smoke tested, yes. I found nothing in the engine bay. I checked my EVAP canister and all.
With the IAC pugged in... lowest I can get the car to idle down is 1000RPM. But that is with the idle screw adjusted all the way in (just in enough to keep the butterflies from sticking). But when I set it there, the car will always drop RPM too low in gear and stall out.
If I set the idle while in gear. Lowest I can get it is 1000rpm. But then in park it jumps to about 1800rpm and stays there.

 

Did you pinch or cap off brake booster hose ? Really sounds like a vac leak

 

After texting one of my buddies that drag races. He mentioned I might have a vacuum leak on the intake pulling from inside the engine. Anyone know a good way to test this?
Only thing I can think of is block off the oil fill cap, and pcv valve and hook a vacuum gauge to it and start the motor.
I do get a little oil burn off on 1st start up. But I assumed it was just worn valve seals. Motor has been rebuilt 24 years ago, but only has 30,000 miles on it.

 

I’m not sure how much you will see past rings but could try plugging the pcv hoses and rubber glove over the tb and smoke test . Might not be to valid filling crankcase up with smoke. With it’s a vac leak or your tachs way off .

 

I've questioned that about my factory tach honestly. Can I test voltage or ohms from the tach signal wire at the distributor? If I have to pull this intake... It won't stay TPI. Not because I don't like the TPI. But for ease of parts and function. I've been a mechanic for decades on the side, and I currently machine, valve job, and build performance diesel heads.
I'm back from fixing dinner, and I'm going back to checking out my car.

 

nothing wrong with tpi it works fine, all parts are available for it gaskets, sensors. But good luck going carb .

 

I found a way to test internal vacuum leaks. I'm going to try and hook my gauge through the PCV valve though. I feel the dipstick is too low into the engine.

 

Also, only thought about going carb for if I want to add a cam, or things like that. I know it's sacrilegious to remove the TPI. My car currently is 99% stock. Still has the original catalytic convertor on it.

 

There is an 87.3% chance that is the case, and even if it does read correctly, no one should EVER use a factory tach to monitor or set parameters. That thing in the hole in the instrument panel is merely a general indication of whether the engine is running, and fills an extra hole where an actual instrument could be installed.

 

Finally got to drive the car a bit again today. It runs great!!! It still says 1350RPM on the tach at idle in gear. But, I'm calling it good and the factory tach is just off that much.
Now... the car will still randomly decide to drop RPM at a stop light. I drove for about 20 minutes around town and only once did it die. I wasn't stopped, I was rolling through a turn at a stop light. It started right back up in neutral.
I still have all my original distributor internals. Do you guys think I might have a pickup coil or ignition module acting up? Would that also cause the mis-read on the factory dictributor?

 

A tach that far off needs a tach board


https://ebay.us/m/7UEuUV

 

instead of vacuum apply pressure to the egr valve to close it and see if anything changes