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

 

In my 1st post in this chat I mentioned this "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."
Should I lose my pressure after around 10 seconds? I remember reading a posts somewhere that a person had to add a "Check valve" on their fuel supply line because theirs went bad in the fuel pump.
I changed my ignition module in my distributor, and put in new wiring harness for the distributor. But My car still has the Idle drop & stall randomly. I have noticed now my car doesn't start right away... it turns over for about 5 seconds before fire up.
Fuel pressure still tests the same.

 

From my understanding the valve in the fuel pump is supposed to hold pressure and is probably going bad. To confirm this pinch off the Supply line right after you prime it and watch the gauge. If it holds then it means your injectors/Regulator are all good and not leaking down and it confirms the fuel pump valve is bleeding pressure.

 

Could also be the fuel pressure regulator. It should hold pressure for a lot longer than 10 seconds

 

The regulator is cheap. I might try and change it just for giggles. But I just put a new AC delco one in 4 years ago.

 

You can figure out which it is by pinching off the supply and return lines one at a time and seeing which case has the pressure loss issue.

 

Check the EGR passages in the lower base plate/intake. The valve could be working great when applying vacuum to it, but the EGR passages inside the intake could be narrowed/clogged up which could cause the drivability problems you’re experiencing.

 

I replaced my cheap Chinese OSIAS fuel pump with a Delphi. No change (I just wanted to make sure I had a better pump anyways).
BUT..... I think I found my vacuum leak!!! If I spray directly at the TPS shaft I can stall the car using brake cleaner.
Is there a seal or bushing rebuild kit for the throttle shaft?

 

https://www.facebook.com/share/1NySU...ibextid=wwXIfr

 

I did this re-bush thing recently...

https://www.corvetteforum.com/forums...-throttle.html

 

I have a similar issue with my 86….the car will start and idle at about 1200 RPMS on a cold start (normal I think?) and will gradually work its way down to about 780-800 RPMS and stay there until I shift it into drive. When I do the car will jerk hard I’m assuming because of the high RPMS? It will then go to 680 RPMS. Then the car will drive fine no issues. Every once in a while at a light the RPMS will surge just a bit and quickly recover. Now if I drive it for a while and get it warmed up then park it, as soon as I start it back up it will be back to 780-800 RPMS until I shift into drive where it drops to the 680 RPMS and jerks hard but drive fine??? I’ve replaced everything IAC etc even the throttle body with no change??? No vacuum leaks. Had it at two reputable shops and they couldn’t figure it out? Any suggestions? Could it be an electrical issue? What are internal engine vacuum leaks and how would you test for that? Valve stem seals are new. Car drives fine just this slight high idle issue that I can’t solve???

 

That is pretty normal. The torque converters in these cars are very low stall and the TH700R4 goes into gear very harshly. The later 700s had updates to help with going into gear so hard but I don't know if it was every fully remedied as my 89 TBI car SHOULD have the updates and still almost squeaks the tires on a cold start.

The high idle you're experiencing is just the "choke" parameters in the ECM warming up the motor and the catalytic converter.

 

Thank you for the information. I agree my RPM readings aren’t too far off but sometimes it just seems the car should warm up sooner and after ten minutes or so drop below 750??? Plus as it’s coming down I do notice a very slight shake of the needle at times. I think I always equate these cars to how they would have come from the factory and I can’t imagine them shifting into drive so hard when new??? I guess I could live with it granted that the car drives and shifts fine. I also got spoiled with my 91 GTA which never experienced any of these issues.

 

Get an ALDL cable and read the actual RPM from the ECM. There's a good chance you have some error in your factory tach as mentioned earlier as well. I use ALDLDroid on my phone to data log some. It will export to TunerPro RT so I can run histograms and other analysis on it. You can watch your IAC counts and other data to help diagnose it if there does seem to be an issue.

 

That is something I definitely need to do.

 

Not proud... But I remembered when I was a pipefitter, I used TFE paste and it never hardened, and never melted.
I just need to get to a car show this weekend. Next week I have time to pull it apart and put new throttle shaft bushings...if I go that route.
Car will now Idle at 1000rpm in park and 750 in gear. But it still will randomly stall at any random time. I noticed tonight it is running rich as I can smell and see the fumes. I'm going to have to think about injectors again. I put rebuilt Bosch (newer style 19lbs) in 17 years ago. But, I think one is hanging open randomly. I tested and they were all at 12ohm. But I can't test them when the issue happens. No check engine light.

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I’m a sprinkler fitter! I know the paste! As crazy as this sounds it seems as if it was a common issue with the 86’s???? My mechanic swore a new throttle body would solve the issue and I got a great one from SPR performance. Again the car runs great I just think the idle is a little too high but why I don’t know???? I had a compression test done on the engine and it was fine. It’s like there’s a small vacuum leak but no one can find it????

 

Getting better as I keep digging into it.
Is there a specific PCV valve for the TPI cars? I put a new one on it before I parked it (4 years ago)... But the car will Idle lower and run if I vice-grip close the vacuum line going to my PCV valve.
Only reason I found this out was I was hearing a vacuum leak once it was warmed up... and could not find it. I even pulled the plenum so I could check the EGR, and IAT sensor to make sure I didn't have a leak there.

 

They're cheap enough, swap it out with a new one. That could also indicate a leak in a valve cover gasket or even the ends of the intake manifold where they join the china walls.