Here is my last thread about getting her running again.https://www.corvetteforum.com/forums...n-t-start.html


Recap:
New intake and valve gaskets
new FIC injectors
new fuel filter
new plugs
IAC replaced last year
New MAF
New ECM

I've had a process to get my 85 to running well again following the FSM and suggestions from the last thread and now I'm so close, but one lingering issue with my idle. When I come to a stop and push in my clutch the idle fluxuates up and down a few times (570 to 850) before settling to 650rpm's where it idles real nice. I know the FSM says 450 for an 1985 but it just seems to low. Just did the IAC adjust and my TPS reads .54 but I'd really like to get the idle right, any ideas?


thanks

 

That is too much of a fluctuation to be normal. I suspect there is an air leak but being that it is an 85, not sure how to scan it without a Tech1 scanner. Assuming you got it right, are you sure the timing is at 6 degrees? How do you know you set it to 450? As to it being too low, what do you want to do with it? You cannot alter command idle speed.

 

I've double checked the timing and its at 6. I followed a TPI idle procedure that says its for 1985-92 and it says 650, which is what I used instead of the 450 in the 1985 FSM. It just seems to idle smoother there. I suspect you are right on the air leak but I can't seem to find anything. I've replaced some brittle hoses with new ones and everything else looks ok. I took good pictures before pulling the intake and believe all hoses are hooked up correctly.

 

What are you trying to say there? This is an 85 and if the FSM says 450, that is what I would use. So how does it say 450 in the 85 FSM and 650 for the 85-92 FSM? Kinda contradictory to me.

Again, how did you know you have it at 450 or 650 or whatever? Are you sure the tach is accurate? I'm not.

 

I'm sure the 450 is a misprint. 650-750 seems more reasonable.
I'd go back and make sure the IAC is clean and reset properly.
Check the intake system for leaks and bolt torque as well.

 

Maybe it means that we set it to 450 with the IAC closed and disconnected and when connected, the IAC will increase the idle speed to 650. Don't know. Wasn't there.

Regardless, the question is how he achieved the 450 or 650. Guessing by engine sound? Seeing what the tach says and the tach probably isn't accurate?

 

I don't think 450 is a misprint. The procedure SHOULD say that you bottom or manually plug the IAC (so zero air gets by the IAC...ALL air comes past t-plates and PCV), then set to 450. THEN you enable the IAC and it should hold the idle at "spec" which should be ~650 in gear. May require resetting by driving over 30 mph.

I agree that fluctuating RPM likely = lean mix (air leak).

 

A little clarification, yes I am going off the tach in the car, what would be the best way to know for sure? I can see the numbers fluxuating and I can hear it as well.

Its a 4+3 transmission, my 1985 factory service manual says 450 RPM for a manual set with the IAC unplugged, Lars paper too. I tried that and it seemed to not idle as smooth and it stayed at roughly 450rpm's after setup. Somewhere, the FIC site possibly, I found instructions showing the 1985-1992 idle set up that shows setting idle at 650 with the IAC disconnected and it pretty much stays at 650 after the idle set up. Its pretty much the same paper as the Lars paper (aren't they all slight variations of his paper?) except for the idle setting. Other than the jumping when I first push in the clutch as it seems to be finding the 650 it idles real smooth at 650 after a couple of seconds.

I'll recheck for any air leaks, sounds like the best way is spraying some carb/injector cleaner for possible leaks.

 

Are you bottoming out the IAC BEFORE starting the adjustment? Or are you simply unplugging it while idling and then doing the adjustment? Also, is your IAC even working? It's odd that if you set the idle at 450, then enable the IAC....it keeps running at 450. Something isn't right there...




"Locate the ALDL connector under your dash panel, in the driver's footwell area. Remove the plastic trim cover (if it is still there).

Cut and form a paper clip into a "U" shape. Insert the clip ends into the ALDL in the 'A' and 'B' sockets.

Turn on the ignition, but don't start the engine. This will force the ECM into its diagnostic mode. Wait 30 seconds to allow the IAC pintle to fully extend. Under the hood, remove the electrical connector from the IAC, then turn off the ignition and remove the paper clip jumper from the ALDL. With the IAC pintle fully extended (closed) all idle air will be controlled by the position of the throttle plates. Some manuals indicate that the EST bypass connector should be disconnected for this procedure, while some make no mention of it. While timing is a factor in idle speed, the EST should only operate as a function of engine RPM, temperature, and detonation sensor inputs. To remove all doubt, disconnect the EST bypass connector is your car is so equipped. Some TBI and V-6 engines do not have this bypass connector, and therefore must be set with no regard to the EST system. The EST can be bypassed on some cars by grounding the diagnostic terminal at the ALDL and continuing with the procedure, but the fuel mixture will be skewed to the rich side, affecting idle speed as well. In any event, the minimum air position idle speed range is wide enough to allow for some variations. As always, it is best to consult your service manual for the exact procedure for your system.

Locate the Torx screw on the left side of the throttle body. It may be equipped with a protective metal cap from the factory. This was intended to discourage adjustment. If the cap is present, use a small punch to knock it out. Once the screw is accessible, start the engine and place the transmission in DRIVE. Adjust the throttle stop to obtain 400 RPM with the transmission in "DRIVE" on an automatic transmission car, 450 in neutral on a manual transmission car, rotating the Torx screw clockwise to raise speed and counter-clockwise to lower speed. Once the idle RPM is set, place the transmission in PARK and turn off the engine.

Re-connect the electrical connector onto the IAC. Start engine. Idle speed should be governed by the ECM at approximately 600-650 rpm"


.

 

Problem is you have an 85 and it isn't easy to scan. I have a tach on my SnapOn Timing light and verified that with what the ECM sees. BTW, before sending it out for calibration, the tach would be 300 over what was read by the ECM at higher rpms. Lower, who knows.

I suppose without the timing light or scanner, you may have to find a way to see if the ECM is agreeing with the tach. I have no idea why but the 91 Firebird I had was spot on till 04 when it was wrecked. The C4 and others have reported it, doesn't seem that accurate.

I guess you have to spray something. I would be leery of ether.

 

Who knows. It might be doing something but the gauge might not be accurate. That is another possibility.

 

I'm sure the tach is way better than good enough...at idle. If the guy hears the engine surging...and the tach needle is bouncing....I think it's fair to say that "tach accuracy" ain't the problem. Engine shouldn't surge on clutch-in.

 

Yep, I followed this process.

My fuel pressure pops right up to 37 but doesn't hold very long, maybe a minute or 2 then it's back to zero.

 

That shouldn't matter when it's running. As long as it has good pressure when running, that's what it needs.

Do you have an air leak between the MAF and the TB?

 

You set the idle to 450 with the screw but after it will be 600 when the computer manages it. Dont set it to 650 first its too high.

 

Ideally (for best comfort and stall prevention), the rpm should hang a bit on the high side (~900 rpm) during coast down with the clutch depressed/disengaged early. This rpm is not really controlled by the ecm, but is a function of the IAC throttle following and warm park position logic + offsets and historic IAC values.

Once the vehicle speed falls below 1.8 mph, PID control should reduce the rpm towards the target and then control to the target (700 rpm is typical idle target for 4+3 cars at warm/hot coolant temp).

From the description, this sounds like the surge is occurring during the coast down phase before PID control is active. I would suggest increasing the min air adjustment so that a comfortable margin above the target can be maintained during the coast down phase, while still allowing PID control to reduce rpm to the target at standstill. This may require some trial and error.

Rpm stability during coast down is at the mercy of the spark advance table (load vs rpm). Your current min air adjustment is likely causing you to enter unstable territory.

 

Can that be adjusted without the ECM being reprogrammed? If you open the butterflies more, won't the ECM shut the IAC down to compensate? If you increase it till the IAC cannot close itself any more, haven't you removed the IAC essentially either partially or wholly as far as control goes?

 

So a little more information. Car runs much better when its warmed up. Its a little hard to start at first when cold. It won't stay running on intial start up. But starts up and runs ok, not great, on the second start just a few seconds after the first time. Once its up to normal running speed it runs great except for the slight rpm movement when slowing down and getting down to idle speed. Starts up fine when hot.

Also, my plates were expired and I need to get emissions (in Colorado) and it passed with flying colors. The emissions guy told my son, who took it for emissions, it runs cleaner than a lot of the newer vettes. Looking at the report it could have passed for passenger cars 2000 or newer.

 

I would be checking the Coolant temp sensor, based on that last post.

 

Thanks, been gone for the weekend. I'll check that next.