JasBass

Car is an 1989 L98 TPI with what I believe is an LT4 hot cam (PO installed and no records). Logged about 300 miles last weekend. I noticed inconsistent throttle response, almost felt like a dead spot in the pedal at times. I broke out the multimeter tonight and checked the TPS. It was reading 0.36. Loosened the screws and adjusted to 0.54. I checked the volt range and it smoothly rose and fell to/from like 2.3. Restarted the car and it immeadiately started to idle at 11-1300. Previously it was pretty faithful to 700-800. Did the usual IAC adjustment procedure and when starting the car with it unplugged, idle remained high. Set screw adjustment did nothing. IAC bit the dust or did I somehow manage to do something else stupid??

Car is on stock PROM tune (I know, not the best). PCM of NC is flashing a chip for me this week. Idle related to lack of tune?

Always, thank you

TheBlaster9001

Go back and re-check the easy stuff - did you put the car in diag mode before unplugging the IAC? Was the car in gear when you set idle with the screw? Also, you have to do IAC first, then adjust the TPS to .54. When you mess around with the set screw, you end up moving the TPS.

Try starting the car in diag mode first, then start with the IAC part. Make sure that the set screw isn't too far forward - if the car was idling high with the IAC disconnected, it suggest to me that there is either a big vacuum leak, or that the set screw is holding the throttle partially open. Without the IAC it should barely idle, like ~450 rpm.

If you have the car in diag mode, IAC off, and set screw backed all the way out, and it still idles high, then you've got an interesting situation.

vader86

What TPS voltage value when the engine is running on a scanner?

Otherwise go back and redo it. Stock RPM should be in that 700 range, so its going to constantly try to get down there. If its at 1300 its being given some bad information from somewhere.

JasBass

Currently assembling my scanning setup, so I don't have exact readout data yet. Next week or two on that. But from the meter it was reading 0.36. Honestly, I think i might have messed up the IAC reset process. All procedures tend to say key on, and not Key accessory. I'm assuming they want key accessory for the reset process?

Plan for tonight: Jump ALDL A+B, Key to accessory 30-60 seconds, unplug IAC, Key off, remove ALDL Jump. Key on, confirm/adjust TPS to 0.54. Key off. Back out idle set screw so blades are not propped. Key ignition and run engine to op temp >160 for closed loop and adjust set screw for min idle.

Anything I missed in the procedure??

Be gentle, i'm learning

TheBlaster9001

I'm pretty certain its key goes to on. Otherwise, the ECM doesn't get an on signal.

If I remember right, its A+B connection, key ON, IAC disconnect, remove A+B connection, start engine. Allow to warm up. You can also warm it up beforehand, makes no real difference. Next, chock wheels, handbrake on, put autos in drive, manuals in neutral (so you don't need blocks for a manual). If you have an auto, be very certain you don't open the throttle on accident, since it will be in drive. Back the set screw up until idle is at 400 rpm. It may try and die before you get it there, but ideally it will be around there.
Next, turn engine off, but don't put the ignition back all the way - just 1 click to turn the engine off. Reconnect IAC. You then put the key back to ON, and do the TPS adjustment. .54v, tighten hold-down screws. That should be it.

You can also refer to this write-up if mine is confusing, and feel free to ask for explanation. I've had to do this process several times, so I've got it memorized at this point.

http://home.earthlink.net/~cliff_har...djustment.html

JasBass

Right, key on to two clicks. There is 0% chance the car is going to idle at 4-500 with the cam in it lol. The question I really have is about the set screw. Who the heck knows where the factory setting was, the PO had some shady work done on the car (AKA why im even having to mess with this). So when doing this whole process, before starting, should I just back the set screw out until it's not advancing the blades and then do like 2 turns in? I just foresee the issue of having the car be able to idle until it is above 160.

TheBlaster9001

I would go more like 5 turns past backed off. My car has a mild cam, but had no problem doing 400. Get it warmed up first, that helped me. Once you have the IAC off and are setting min idle, you can continue to back off the set screw until it's at 400, or tries to die.

aklim

Could be the lack of tune but I'd check the basics first. What is the timing? What is the fuel pressure? Are the injectors standard size or what?

JasBass

Y'all, got it settled. I couldn't believe that thing actually idled at 400, it wasn't happy, but it idled lol. Have it set up to 800 right now until I get the tune in it on Friday, then i'll back it down to a healthy 6-7. I have a feeling whatever shady work had been done in the past meant that the IAC had never been properly set, they just fiddled with the set screw until it ran sorta ok.

When key on and the diag. jumped, before removing the connector, the IAC was making some tactile ticks. Is that normal?

Also, not sure what was up with the TPS, I absolutely could not get it to read 0.54 base like it did last night. It only went up to 0.47 which is in range with GM specs according to 89FSM. It went smoothly up to like 2.3 without any dead spots or large jumps. I wonder if the TPS is on its way out.

AK, I need to check base timing, thats on my list to do soon. My gut it telling me its a little under 6, but that could also be tune related. Injectors are Bosch D3 22/hr, and FP runs right on 43 with the vac removed from the reg.

Appreciate you all being patient with my young dummy self as I learn this stuff

aklim

Only way to change idle is in the ECM. It comes under desired idle or command idle or something like that .What you are doing is forcing the idle to be something it isn't. If you are going to set the IAC I would do it with a scanner and not the FSM way since it assumes much.

Normal.
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Are you sure it has a 5V reference?
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You have to check it out with a timing light and not your gut. It can affect rpm. Tuning only advances it from assumed base. My timing is set at 0 but it is written in the programming as to what to do for advance.

GREGGPENN

Start and run engine until it reaches operating temperature (closed loop)
Check and set ignition timing to 6 degrees before dead center (BDC) with tan EST wire disconnected
Check and set throttle position sensor (TPS) to .54 (+/- .08) volts at idle
Jumper terminals "A" and "B" on the ALDL
Turn ignition key on and do not start engine
Wait 60 seconds so that the idle air control (IAC) motor fully extends
Without turning ignition key off, remove connector from IAC motor
Turn ignition key off and disconnect ALDL jumper
Attach external RPM meter using Tach port or scanner using ALDL port
Start and run engine until it reaches operating temperature (closed loop)
Remove minimum idle air cap using awl if required
Adjust idle speed to DESIRED IDLE SPEED in either in park or neutral (With hotcam, I'd guess 800 for a stick, 550-600 auto)
Turn ignition key off and reconnect EST wire and IAC motor
Check and set throttle position sensor (TPS) to .54 (+/- .08) volts at idle
Reset ECM by disconnecting and reconnecting power at battery terminal (Not sure if you NEED to perform next 5 steps)
Depress accelerator pedal slightly
Start and run engine for 5 seconds
Turn ignition key off for 10 seconds
Drive vehicle to assist in ECM reprogramming

If you used a scanner, you should see 15-20 IAC steps If not, adjust idle set screw until it's in this range and reset TPS to .54V THIS IS HOW YOU INSURE IDLE SCREW IS SET WHERE IT NEEDS TO BE WITH YOUR CAM.


Note: The IAC allows air to (bypass TB) enter plenum WITHOUT pressing accelerator pedal. Getting IAC counts correct AND resetting TPS to .54V is the way to "set" idle-screw AND TPS so that ECM is able to control idle correctly. WHERE it idles at what temps, etc... is controlled by values in the computer. UNTIL you get these "mechanical" items correct, the computer is basically lost. I should add that timing is part of the "mechanical" calibrations necessary to correct operation. Some people monkey with timing. I wouldn't unless I did so via tuning parms (in the computer/ECM).

TheBlaster9001

This is correct, the ECM commands idle. It has a limit to how far it can contol idle, however, which is why it was idling at 1300 - with the set screw holding the throttle plates open, it'll have too much air to bring the idle down. Setting the IAC gets the TB set to an area when the ECM can handle the finer adjustments

TheBlaster9001

It can be tricky to get it to exactly .54. The car uses the inital voltage as 0% throttle anyway, so as long as it's within what the manual says, you are good.

I could have sworn my TPS goes up to 4.8 volts all the way open - you sure that you are depressing the lever on it fully? Your TPS may be dead at 2.3v maxed out, I don't think that's normal. Is the reference wire (the one you don't use when testing) carrying 5 volts?

GREGGPENN

It's not. Closer to 5V is normal.

aklim

I know mine does use initial voltage as 0% throttle but I think previous TPS units HAVE adjustable slots. Mine does not have slots Just 2 holes. I think the changes came with the switch to speed density

Could be the TPS is bad but maybe the reference voltage is not 5V?

JasBass

Derp moment. I was thinking it wasn't a 5v reference based on code 21 being set for a voltage greater than 2.5. But thats 2.5 with throttle closed... Sounds like the TPS has a problem. Quick, cheap fix, and maybe that will solve that throttle response issue between 3-4500 rpm where it feels like it falls on its face some. 45-5400 it'll pull ncie

Thoughts?

aklim

If you put the positive probe on the wires, and the negative on the battery negative, I am thinking that one is 5V reference and the other 2 will be about 0. One of them is ground and the other is the signal to the ECM. If you don't have 5V reference, I don't care what you do, it won't work. If you get only 2V, the signal is forever going to read lower than it should. Lets get this diagnosed before throwing parts, no matter how satisfying it is, at the thing.

JasBass

Just checked it, bottom grey reads 5.02v, others 0v. New TPS it is. I swear man, this poor car must have lived a hard life never being taken care of. I think i'm going to name her "Red Rescue"

aklim

Sounds like it is a TPS problem. As long as you make sure that the lever is not binding somewhere. I would hook it up and see if I can simulate WOT and get something closer to 5. If not, change.

JasBass

Yeah, all times I checked it, max was 2.6. Even looking this morning, blades wide open only got 2.6. Wouldn't surprise me if it was bad, or even if that was the right TPS on there. I cannot understate enough the years of bad mechanic work and poor maintenance i've had to undo on this car