thedesertvette

Background on Vette: 1985 Z51, auto, stock, I bought it from the original owner about 2 years ago. The engine was torn apart and literally in boxes, the PO was in the process of putting a 383 in it but never got it in. It had been sitting on jack stands for 8 years. I put the engine back together and it runs great except for the intermittant stalling. About twice a month, usually in stop and go traffic, it will just stall but start right up. Since I have had it, I put in the following new parts: Plugs, wires, cap, rotor, all new gaskets from the heads up since I had to reassemble the engine, new IAC, flowmasters, CTS, new tires, filters, oil, fuel filter, etc (all the usual maintenance stuff). It has 150K miles on it, I have put 25K of those on it in the last 2 years.

So it runs great and I love cruising around in it but it has had this intermittent stalling at idle. I completely torn down the TB and cleaned it as well as a new IAC. Set the base idle according to the procedure, checked the TPS for .54V, checked the timing (in drive with connector by the PB booster disconnected). It still was stalling intemittantly so returned the IAC and got another one, same symtoms. Fuel pressure is fine, checked that.

Today, I was at the gas station and it stalled, try to start it and it would only run if I gave it gas. It would not run unless I gave it a little gas. I drove to another store and had to have one foot on the brake and one on the throttle at every stop to keep it running. I thought this was a good thing, hard failure and I will be able to figure out the problem! No such luck, by the time I got home it was running fine again!!!

I have checked for codes, there are none. I have searched for vacuum leaks and found none. I am at a loss as to what to check for as the problem is not there now. Any ideas what to do next? Could it be the EGR sticking?

Thanks for your help!

mcm95403

My 91 has done that as well. I see you have a new IAC. On mine, if I unplug my IAC and then re-connect it it seems to solve the problem. I think in my case it's just a flaky plug. I have a new one but haven't had the chance to change it yet.

Hope that helps!

Frizlefrak

You found no vacuum leaks....but how many inches of vacuum at idle? I'd like to know if you have a leak perhaps in the intake, not a vacuum line.

Yes, your EGR may be hanging up too.

I would remove and thoroughly clean EVERY connector on the car, particularly emissions / sensor related. The ECM is shutting the engine off because it's getting an erroneous reading from a sensor and believes it is idling too high. This could be caused by faulty sensor, faulty connection, or erroneous vacuum signal.

Check operation of MAF sensor too.

Report back with what you find out.

thedesertvette

Guys,

Thanks for your quick replys! I will clean all the connectors, I have some connector cleaner I will spray them with and clean them out good. I did spray the MAF wires with MAF cleaner several times over the last 2 years. One of the things I did try was to pull a vacumm on the two vacuum lines that go into the side of the TPI manifold. Pulled 15 inches and it stayed there, didn't bleed off, but I didn't measure the vacuum, I will do that in the morning.

Thanks again!

Cliff Harris

Some connectors have dielectric grease in them. If you find some, don't clean that out. It's there to keep the connector waterproof.

Frizlefrak

Different car, but same scenario....had a big vacuum leak in plenum gasket. Ran like crap, stalled, A/C wouldn't engage, overheated, you get the picture. I checked everything you've checked to no avail. Finally put a gauge on it.....12" of vacuum, and a rocky reading. A gasket set later, runs like a top.

These cars are very sensitive to vacuum. I think you'll find it there or in a flaky connector.

thedesertvette

Frizlefrak, I measured the vacuum at the rear most port on the passenger side of the TPI manifold (think it goes to cruise control and inside) and measured 16". It was rock solid, no bouncing. My vacuum gauge is old and may be off a little.

I cleaned all the connectors: TPS, IAC, temp sensors in front of intake, pollution stuff in front, temp sensors in heads, egr controller. Didn't get to the air temp sensor under TPI manifold or connectors that go thru the firewall or the ones at the ECM.

Did I miss any other connectors that I should try and clean?

Like I had mentioned, the car usually runs great and it could be another month before it stalls at idle again. If the EGR was sticking, would it set a code? Forgot to mention that the O2 sensor is new and I had cleaned that connector when I installed it about a month or so ago.

Thanks for your help!

Frizlefrak

Intermittent problems are the worst....if you could just get the damn thing to do it all the time you could fix it, right?

I suspect cleaning your connectors will solve the issue. One final thing to check is all grounds on the car. EFI is also very sensitive to grounding properly.

thedesertvette

When I reassembled the engine 2 years ago, I made sure I cleaned the half a dozen or so grounds that connect to the back of the drivers side of the block/head. Also cleaned the ground that goes on one of the thermostat cover bolts. I will check them again, are there others I should check?

Thanks!

Red89'-L98

Give a look at the fuel pump, mine used to do the same thing. Fixed the pump and have not had a problem since and it has been almost a year.

Just a suggestion.

thedesertvette

Veryevilkitten2, I have checked the fuel pressure and it is good. What was the problem with yours? Is it possible that the pump is intermittently shutting off at idle? Since it happens so infrequently, there is no way I can have the pressure guage on all the time.

Thanks for the suggestion!

Midnight 85

I'm wondering about the module inside the distributor or the ignition module on the passenger side right next to the blower motor. If you have a service manual I believe there are checking procedures. If you don't have a manual get back with me and I'll check mine.

Midnight 85

One more thing to check, unplug the tan spark control connector and drive it around, if it does not die while driving with that connector unplugged then it is probably your pickup coil in the dist.

Red89'-L98

My car would die every once in a while then when i started it up it would start fine. Then it got to where it would die more often and i would have to give it gas to start it or at idle i would have to give it gas for it to stay running. After that it got to where every time i started it i need to give it gas and I HAD to give it gas to keep running. I checked my fuel pump first thing and replaced it because the ethanol in the gas destroyed it.

Frizlefrak

OK, one more thing to check.....your fuel pump relay. If the relay is dead it will cause prolonged starting times, as the ECM has to rely on the oil pressure switch to close the backup circuit to energize the fuel pump. Consequently, if oil pressure were to fall below about 4 psi (say at low idle), the FP circuit would open (assuming a dead relay) and the car would die.

Remember....they are wired in parallel, not series....so if the relay is working correctly, it will run with zero oil pressure.....well, until it seizes anyway

This is an unlikely scenario, but possible. If you can't find the issue any other way, tape a fuel pressure gauge to the windshield and drive the car until it replicates the problem. Note pressure at the time of the event. An intermittent fault in the FP and / or regulator is possible. Would be good to rule this in or out.

thedesertvette

Midnight 85, I do have the FSM and will see what it says about checking these parts. That reminded me that I forgot to clean the connector by the heater fan motor so I did that. I didn’t clean the connector to the distributor that lays on top of the trans, too hard to get to.

Veryevilkitten2, I don’t see how the pump could be bad if it runs off-idle but who knows!!! I did have a problem right after I got the car running, it had not run in 8 years and the short hose off the pump had gotten soft and split. I replaced the short piece of hose and all was well. Your description of the problem sure sounds like mine though. After cleaning all the connectors, I will see if it stalls again and if so, I will replace the pump. I see Autozone has Airtex, are they any good?

Frizlefrak, the car always starts right up fast. I bet it starts in less than 1 revolution LOL. Also, when you turn on the key, you can hear the pump run for 2 seconds or so. Man, I would hate to have the pressure gauge taped to the windshield for a month LOL.

I did check the vacuum line to the fuel pressure regulator and it was not wet with gas so I don’t think the diaphragm is bad.

I appreciate all the help guys. I will see if cleaning all the connectors fixes the problem. I will let you know either way as I hate threads that just end without a solution LOL.

Frizlefrak

OK, so your FP relay is working. Keep us posted....I'd really like to know the resolution on this one.

Midnight 85

Personally I would not use an Airtex pump, my first choice is Walbro followed by AC Delco. I'm having a real hard time believing it's a fuel problem but I could be wrong. I'm betting it's electronic, and I'm betting it's inside the dist.

Frizlefrak

Ensure the ignition module has a nice thin coat of heat sink grease. Another thing that's easy to overlook.

jan-erik

HI.
I have an 85 z-51 manual 4+3 and had the same symptoms. As You have changed a lot of possible causes there is one main left. If you have a faulty EGR valve this is exactly the symptoms. One easy way to check if the valve holds the vacuum. remove the vacuum hose to the valve and apply vacumm with a vacuum pump. The valve should hold the vacuum (decreasing) in 20 seconds. If it goes faster to 0 change the valve.
The valve pintle can be stuck or there may be carbon deposits on the valve seat or pintle. I would have changed the valve if suspicion. You said the car haas been standing for 8 years and the chance that there has come corrosion on the valve pintle and seat is large and then the valve does not close properly. If you get too much EGR the idle suffer and the engine may stall.
Another thing is to do the minimum idle air adjustment to ensure that the idle without IAC valve( it must be closed) and without the ignition adjusting. Lars Grimsrud have a procedure for doing this. Think that the idle then should be adjusted to about 450 rpm and I would adjusted it to 500 as the IAC is a bit slow. For me this worked.
Here is the procedure:


Setting Base Idle Speed (using 89 GM Helms Manual)


If this has not been done on your car before, a pressed-in plug will cover the adjusting screw. It’s located on the driver’s side of the throttle body. Remove this plug by using a small punch. Impact the plug (lightly) on the edge of its face and this will allow its removal.

1. Start the car and let the engine warm up to its operating temperature. (It is suggested that you make sure the timing of the engine is set to specification prior to doing this procedure.) During this step, the engine should be in closed loop operation and all electrical accessories INCLUDING COOLING FANS should be off. Shut off the engine after it is warmed up.

2. Turn ignition switch to ON, but do not start engine. Adjust the Throttle Position Sensor to .54 +/- .08 volts using a digital meter. There are 3 wires stacked vertically on the TPS. You will need to be able to measure the voltage between the two top wires. You can either buy a special harness connector that breaks these wires out (from Mid America costing $12), or gently pierce the insulation of the wires with the pointy prongs on your voltmeter. You can also stick a paper clip into each of the two top locations of the connector and clamp onto the paper clips to measure the voltage. The adjustment is accomplished using a Torx driver and loosening the two screws that hold the TPS to the throttle body. Then rotate the sensor either clockwise or counter clockwise (with the ignition on and engine not running) to obtain the .54 volt specification. Retighten the two TPS Torx screws to “lock in” the adjustment.

3. With the IAC connected and the ignition “OFF,” insert an Auto Zone tool (free upon request from Auto Zone) into the ALDL diagnostic connector terminals “A” to “B.” (These are the upper right two holes in the ALDL connector.) This grounds the ECM and places it into the diagnostic mode.

4. Turn the ignition to the “ON” position without starting the engine. Wait 30 seconds before doing step 5. Doing this forces the IAC pintle to the closed position (closes air passage) within the throttle body.

5. With the ignition still in the “ON” position, disconnect the IAC connector at the IAC valve. This connector has four wires and is located on the passenger side of the throttle body. This action prevents the ECM from controlling the idle speed using the IAC valve.

6. Turn ignition switch off!

7. Remove the Auto Zone tool from the diagnostic connector.

8. Disconnect the EST timing connector located just behind the driver’s side valve cover. This is necessary to prevent the ECM from attempting to adjust the timing as you adjust the idle speed. This will be a brown wire with a white stripe and easily separates using a small screwdriver to raise the locking tab. Failure to do this step will allow the ECM to control the idle and you will not be able to properly adjust the base idle.

9. Start the engine. The idle RPM will be very low, and you may have to coax the engine using the gas pedal or your hand and the throttle cable to keep it running. I find it useful to insert a small screwdriver between the end of the idle adjusting screw and the throttle body to keep the car running at this low idle point. HOWEVER, REMOVE THE SMALL SCREWDRIVER BEFORE MAKING THE FINAL IDLE ADJUSTMENT!!!

10. If your car is an automatic, set the parking brake and put the transmission in park. If your car is a manual, leave it in neutral.

11. Adjust the idle speed screw to obtain 425 +/-25 engine rpm. This will require a Torx tool to adjust the idle screw in the throttle body. As you adjust the idle screw to obtain the desire idle speed, you are changing the TPS voltage setting. The TPS will require re-adjustment in step 12. Make sure the cooling fans are not running while you make this adjustment.

12. Shut off the engine, re-connect the IAC and the EST timing connectors.

13.Turn on the ignition switch and do not start the engine. Reset the TPS to .54 +/-.08 volts as stated in step 2. Turn off engine.

14.Reset the IAC pintle position by:
a.Depress accelerator pedal slightly.
b.Start and run engine for about 5 seconds.
c.Turn off engine for 10 seconds.
d.Start engine and check for proper idle operation.

15. The idle speed of the engine should be in the 600 – 700 range per the ECM programming with all connections re-established and engine at operating temp.

You are now in adjustment on base idle speed and TPS output. Start the engine. It
may take a few minutes for the car to “catch on” or learn its new settings.

Revised by Bob Lamb 10/03 from a tech article written by Lars Grimsrud 10/01. These changes were required to align the Grimsrud article to the Helms GM manual for an 89 Corvette.

Hope this will help
Jan-Erik