Enkil

Sometimes when I turn my car on it'll shut back off. That's not too big of a deal, especially since it's getting pretty cold out around here; I don't expect it to stay started perfectly every time. However, when the engine is completely warmed up, it'll occasionally stall when I come to stop and the engine's running at idle. When I restart it, sometimes it runs fine, and other times the engine will rev up to about 2000 rpm, then stall again. If I give it gas, sometimes that'll prevent it from stalling, but then the RPM's will fluctuate from 2000rpm to 500rpm several times, if it doesn't just stall.

Here's the specs on the car itself:

1989 IROC-Z 305 TBI, everything stock except a crappy quiet muffler the guy before me put on it. The engine's only got 70,000 miles on it, and while a tuneup is probably a good idea, I don't know if that's what is causing it.

Also, I recently set the timing to about 8* BTDC. Soon afterwards I found a vaccuum hose that had come disconnected. So of course I plug it back in, and the timing seems to have changed. I know it's possible for timing to cause problems like that, but the engine has always done this. (Just not as much as now.) As far as I can tell, there is no ping. Never any check engine lights either.

Any ideas?

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89 iroc-z
everything stock, except for a mirror that fell off

Red Devil

Ok, (TBI right?)I'd shoot at either a mal-adjusted IAC or a TPS sensor. Also did you make sure you unplugged and then plugged back in the EST wire?

(A side question to everyone else... I one of my chilton's it says for '86-'92 cars to ground the ALDL after starting the car instead of disconnecting the EST. This work?)

[This message has been edited by Red Devil (edited December 10, 2000).]

ViciousZ

iTrader: (1)

Here's another vote for the IAC or TPS.

JP84Z430HP

Check the IAC and the TPS, along with the minimum idle speed.

For setting the timing, I always see what it says under the hood. This is sometimes just grounding the ALDL, sometimes it's disconnting the est bypass wire, and on a carbed car (LG4 for sure), unhooking the 4-wire connector. Most of the time it is the EST bypass wire on a fuel injected car.


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Working on:
'84 Z28 LG4 305
Added dual elec fans.
145 MPH IROC Speedo
Building 430 HP 350
using primarily GMPP parts.
Block is in shop!

Awaiting results from all 8 ASE tests.

Starting to look like the Kicker poster child!

Enkil

Yeah, it's a TBI.
Devil: Yeah I unplugged the EST. Under the hood it says to unplug the EST bypass wire before setting the timing.. and in Chilton's it says to ground the first two pins (I believe) of the ALDL... I've never tried the grounding. *shrug*

Now.. How does one go about fiddling with the IAC and the TPS? (Hell I don't even know what they are. *shrug*)

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89 iroc-z
everything stock, except for a mirror that fell off

Vader

Enkil,

A good basic outline is as follows:

Throttle Position Sensor (TPS)

Tools needed:
1. Digital Volt-Ohm-Meter (VOM)
2. Breakout jumper wires or probes (make your own)
3. AutoXray, Diacom, or similar scanner will replace the VOM and jumper wires.

Turn on ignition, but don't start the engine.

With a diagnostic scanner: plug in the scanner and read the TPS voltage. It should be 0.54Volts +/- 0.07 VDC.

Connect the VOM to the TPS electrical connector terminals ‘A' and ‘B'.

With a breakout jumper: Disconnect the electrical connector from the TPS. Install the breakout in-line, between the TPS and wiring harness connector. Connect the meter probes to terminals 'A' and 'B' on the connector. (‘B' is the positive connection, ‘A' the signal ground, or negative.)

With probes: If you have very slender probes on your VOM, you can back-probe the TPS connector while it is attached to the TPS. If you have made probes of large dressmakers pins or a similar item, you can back-probe the connector as well. Connect the meter probes to terminals 'A' and 'B' on the connector.

Turn on the ignition to read the TPS output voltage at the idle position. The reading should be 0.54VDC +/- 0.07VDC. The ideal is the center of the range, 0.54VDC for a stock engine. To adjust the output voltage, loosen the two Torx screws holding the TPS to the throttle body, and slightly rotate the TPS up or down, reading the voltage until it comes into specification. Tighten screws. Using the throttle lever, rotate the throttle to WOT (wide open throttle). The TPS voltage should be over 4.0 volts. Close the throttle again, and then slowly open it to WOT, observing the voltage reading. It should increase progressively and in a linear fashion. If it sticks or jumps or falls off at all while doing this check, the TPS sensor may be failing and could be a cause of stumbling and driveability problems.

After achieving the desired setting, turn off the ignition switch. Remove all jumpers or the scanner and reconnect the TPS connector as required.



Throttle Minimum Air Position

Tools needed:
1. Torx driver # T-20
2. Paper Clip
3. Small Punch
4. Tachometer

GENERAL NOTE: The engine should be at normal operating temperature before performing any adjustments. Never rely on the dash mounted instruments for diagnostics and adjustments. The oil pressure and temperature gauges and the voltmeter and tachometer just aren't calibrated accurately enough for diagnosis, but are a relative indication for monitoring the vehicle while driving.

For this adjustment, the transmission will be in DRIVE while you're under the hood. You will need to securely set the parking brake and block the drive wheels. It would also be a good idea to have an assistant hold the service brake while you perform the adjustments.

In order to successfully complete the adjustment, the IAC air passages and pintle need to be clean. The throttle plates and bores need to be clean as well. If this is not the case, you'll need to remove the air cleaner from TBI engines or the intake air bellows from TPI engines to gain access to the area to be cleaned. A spray-type carburetor cleaner works well for this. Cleaning the IAC passages on a TPI/MAF engine will set a DTC, but we'll be clearing that later. With the engine idling, direct the spray cleaner in to the IAC air passages and around the throttle plates. Shut off the engine and continue cleaning the throttle plates by opening the throttle manually. Once everything is satisfactorily cleaned, replace the air bellows on TPI engines. Many times, this alone can solve IAC/idle speed problems.

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.

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 450-500 RPM with the transmission in "DRIVE", 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 650 rpm in "DRIVE" (for unmodified cars). Idle speed in NEUTRAL or PARK is less significant, and will be higher.


Reinitializing the ECM

If you set a DTC during the procedure, the SES light should be illuminated on the dash. This ECM retains DTC data for the previous 50 engine starts, so the codes will eventually be cleared. If you want more immediate results, after shutting down the engine disconnect the negative battery terminal for five minutes. This will clear the ECM of all diagnostic trouble codes. Clearing the ECM also clears any data learned about your engine, and clears the radio presets. If you have a Delco-Loc or Theft Loc II radio, make sure you follow the procedure to unlock the radio protection before disconnecting the battery. This five minutes is also just about long enough to clean both battery cables. Reconnect the battery. When you first start the engine after clearing the ECM, the engine will operate with base parameters programmed into the ECM PROM. These parameters may not be optimum for your engine, but the ECM will enter a Block Learn Mode soon after the engine is warm and enters Closed Loop Mode. The ECM will write new data tables specific to your engine and will eventually rely on those tables instead of the base tables of the factory program. You can expedite this process by driving the car for 20 minutes under varying conditions to allow the ECM to initialize. Or you can wait and drive the car normally at your convenience. The BLM tables are constantly being updated as sensor input ranges change, but the greatest change will occur within the first twenty minutes of Closed Loop operation.

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Later,
Vader
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"No matter how hard you try you can't stop us now"
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85berlinetta

I had the random stalling problem when i had a mpfi 2.8l. It turned out to have a slight crack in the distributer cap, a little condistation, and that was my problem. A new cap fixed the problem. It only cost me a few bucks too.

soulbounder

I have the 305 TBI in my 91' bird and it was giving me pretty much the exact same trouble as you are describing. I brought it to a family mechanic (never a good idea) and he said it was the IAC valve. Well, he changed that and it worked fine for about 3 weeks. The problem reappeared. I brought it back to him and this time he hooked it up to a scanner and it showed a faulty MAP sensor. He changed it, I drove it home, and it gave me the same problem the next day. I took it somewhere else and finally the problem was solved. The ECM was not throwing any codes, but the thermstat turned out to be bad. He changed it and so far the problem has not returned. He said it was something to do with open and closed loop; hell, I don't know I'm just glad the problem is gone. BTW, my vote would go for the IAC valve too. Good luck.