90 RS, Once warm, idle drops and dies

[Dave_Calif]

I have a 90RS, 305, 195K miles. I've owned it for years and it has been fine, but a month or so ago it started dying. It runs fine above idle, it runs fine as long as it isn't in drive, and it runs fine when cold. Basically once warm when you stop at a light it holds about 5-600 rpm then a second or two later you can tell it looks like it is smothering and drops the rpm and dies. If I drive with two feet up to a light, and just keep the idle up a bit it works.

I don't have a thermostat in it, but haven't for years because I always seemed to have overheating prolems.

A few years back it failed smog so it has a new cat, and ever since then it has had great smog results. One of the guys said he was amazed how low the results were with the miles on it.

I replaced the plugs last night, the EGR I thought was bad so replaced that, checked and changed some of the vacuum hoses (don't see any more leaks or marginal hoses), but the prob continues.

Any ideas/help would be appreciated.

Thanks, Dave

[Rally Sport]

What about your ignition timing? Maybe thats whats causing it to die or your ignition control module.

[Nate355RS]

At a stop it reves up to 5000-6000 rpm? That's pretty odd, but the stalling issue sounds like a fuel problem, probably the fuel filter. If the filter isn't it, I would check the injectors.

[Dave_Calif]

Originally Posted by Nate355RS

At a stop it reves up to 5000-6000 rpm? That's pretty odd, but the stalling issue sounds like a fuel problem, probably the fuel filter. If the filter isn't it, I would check the injectors.

Oops I meant 500-600rpm (normal). I fixed the original post. I was thinking the filter but I changed it probably 10-15k miles ago when the fuel pump went out.

[wrightz28]

I'm goin with fuel too.

TBI or TPI?

Other possibility is the TCC solenoid in the trans might have retired.

[Dave_Calif]

Based on your replies, research, and a lot of time I have eliminated a lot of things but it is still a problem. It does exactly the same thing, once hot, when you stop at a light it runs for a second or two then drops rpm and dies (hasn't changed symptoms)

But I have tried so many things and eliminated so much I second guess myself. I think it is the Knock Sensor (only because I can’t think of what else it can be), but here is why I think so:

I replaced the Idle Control Valve and adjusted, exactly the same thing. I replaced the EGR, same thing. I checked the timing and it was fine (0 degrees). Now with that said when I disconnect the wire (to check the timing) everything runs fine. When I hook up the wire it dies (when hot). Unhook it and its fine (did it multiple times). So bottom line is when it is running with that wire unhooked the problem disappears.

• I checked out the EGR and I really think it is fine (never engages when in idle and it isn’t supposed to).
• I kept looking for a vacuum leak over and over, but after about 5 minutes I unhooked it and you could tell there was still pressure so I think it is fine.
• Based on my findings I just can’t see it being a fuel issue.

Anyhow I am still looking to see if you think I am missing something? Any feedback is appeciated...

Thanks
Dave

[wrightz28]

If it were a knock issue you would get a DTC 43. And the timing is supposed to be at 0 with the wire disconnected. Connected it should 'bounce' on and off scale.

[Dave_Calif]

I'm not getting any codes, but only thought it was the Knock Sensor because that affects timing, and when the wire is unhooked the car runs fine thus the timing isn't changing.

The timing is at 0 degrees with the wire unhooked. When hooked up I can't see it, so it doesn't bounce in and out.

For some reason when the wire is unhooked the car idles just fine (I drove it around all day today). The check engine light is on, but it runs fine.

What else could be in the circuit that could cause it to run fine with the wire unhooked and consistently dies when hooked up (when warm)? Obviously (I think) something must be screwing up the timing causing it to die. I had the control module (computer) checked at Autozone and it ran the test 5-6 times. I guess that could still be bad. But I'm at a loss what else could be giving me this symptom.

I am fishing for what else could cause it when the wire is hooked up but runs fine when not.

Thanks...

[wrightz28]

If the ICM is indeed okay, which yes they can test fine one minute and bad the next, let's back up here.

Usually a temp related problem points at the ICM, but if it runs fine HOT with the ref wire disconnected, I question that.

How about the basics, plugs (what type ?) and wire condition. The coil wire.

i would also get a hold of a vacuum gauge, they are really cheap, and verify the the exhaust is not restriced. Although if it's bad enoug to stall the motor, you should see the clog area glowing.

[Blue Bowtie]

Since you've already replaced the IAC, you need to complete the procedure and adjust the throttle minimum air position.

1. Get the engine to full operating temperature, turn off all accessories like the air conditioning and rear defrost.
2. Shut the engine off, and remove the air cleaner.
3. Plug any disconnected vacuum lines.
4. Bypass the ESC (disconnect the "Set Timing" connector).
5. Connect a tachometer to the ignition coil - DO NOT use the in-dash tachometer for diagnostics, as they are notiriously unrelaible. They are more of a trim package, or way to fill a hole in the dash than an instrument.
6. Insert a jumper wire between the 'A' and 'B' terminals of the ALDL connector under the dash.
7. Turn on the ignition but DO NOT attempt to start the engine.
8. Wait at least 20 seconds with the ignition ON to allow the IAC pintle to fully extend and close off all IAC idle air.
9. With the ignition still ON, unplug the electrical connector from the IAC stepper motor.
10. Turn the ignition off and remove the jumper wire from the ALDL connector.
11. Block the drive wheels, set the parking brake, and/or have an assistant hold the service brake.
12. Start the engine, and place the transmision in DRIVE. You may have to hold the throttle open to get the engine to start and run.
13. Adjust the throttle plate stop screw on the throttle body to attain an RPM of 500-550 RPM in DRIVE.
14. Shut off the engine, reconnect the IAC electrical connector, and install the air cleaner.
15. Reconnect the EST bypass connector. The base throttle position is now properly set.

The throttle body should be clean before adjusting the minimum air position. Since your '90 has a fixed (non-adjustable) TPS, there is nothing more to do.

[Dave_Calif]

Thanks for all your help thus far, I keeping learning more and more about the problem but I think it still exists.

• When running without the wire connected I went through a lot of fuel (170 miles on a tank, yes I live in LA but...).
• The IAC was definately bad and some of my troubleshooting was with the original one still in. I now have the new one (IAC) in for good, and I think it is working fine (when I disconnect it, it will idle high until I do the adjustment).
• I adjusted per Blue Bowtie's instructions and I think I masked the problem. (base idle is higher)
• It runs OK now but I smell fuel, so I think by adjusting the idle stop I am adding more air and thus it still runs, so gas mileage will be poor and if I went for smog it would fail. I haven't run it long enough to be sure but just my gut feeling.

I am convinced (which changes depending on the day) that it is a fuel problem. I was thinking not enough fuel but I am thinking too much fuel instead (and adding air compensates). So I am thinking the O2 sensor now is a problem. I know I change my thoughts but every time I spend hours on it I learn new things that change my opinion.

Assuming too much fuel because I offset it by increasing the idle stop to compensate, other than the O2 sensor I would love your thoughts as to what could cause too much fuel?

As always thanks for your ideas. I have come a long way and keep thinking I am just about there.

Dave

[wrightz28]

The coolant temp sensor (CTS) on the front of the intake manifold is actually more of a key player in the fuel mixture tahtn the O2. If your haven't done the o2 in a while, it wouldn't hurt. The CTS is only $8 so probably not a major loss in replacing it.

[Blue Bowtie]

The oxygen sensor is not even involved in open loop mode (on initial start and until the engine is warmed, and at WOT). Fuel trim in Open Loop Mode is based largely on TPS and MAP inputs, in conjunction with the distributor reference pulses feeding RPM information to the ECM. The CTS and IAT/MAT sensors are used for some fuel offset trims. If the MAP is high (vacuum is low) the ECM will add fuel, presuming more engine load.

With the EST bypass wire disconnected and in Backup Fuel and Spark Mode, the ECM will use table values for fuel trims. If the engine runs acceptably under those conditions, something is altering the fuel trim (incorrectly) while the ECM is in Closed Loop Mode. That indicates that at least one piece of data sent to the ECM is erroneous.

As Wright indicated, the CTS can play a role in this. The MAP and IAT/MAT can also affect it. You can either replace the CTS, or test the resistance at a known coolant temperature, then compare it to the table below:



The IAT/MAT sensor should have the same resistance values at any given temperature.

The MAP sensor can be checked with a voltmeter, and either a vacuum gauge or vacuum pump. At a given absolute pressure (or vacuum) the output voltage should be close to the following table:



If those items check within acceptable limits, the distributor reference information is suspect, and takes us right back to the ignition switching/amplifier module and its wiring.

[Dave_Calif]

Thanks for all your help!!!

Last weekend I tried all your suggestions with no luck, so I reset myself and tried to start from scratch thinking something was overlooked.

The original IAC was definately bad, but a lot of troubleshooting was with that one in it. Then I had a new one and the symptom never changed. I got to thinking I am going to change it again because that is the obvious thing for what it was doing. So I got another one and it was acting weird but I did all the adjustments etc. and this weekend I drove it and it seems to be fine.

Today I manually cranked down the idle limit stop screw (as it was idling just a little high) and it was fine except when I popped it in drive (with load). It did the same last week but it seemed after a while it fixed itself. In any case the original problem seems to be resolved and working fine. When hot I can step on the gas and let it go real quick and the revs drop really low but it corrects itself and stays going.

In any case I think I am finally past this nightmare and thanks for all your input and ideas....

Dave

[Nate355RS]

Good to hear that you got it straightened out. Was it idling high even after you drove it around for a little bit? Because if you start the car up and let it get warm and don't move, the ECU stays in open loop which has a slightly higher idle. Once you start driving for a little bit it switches over to closed loop and the target idle drops back down. The prom is programed to change certain parameters in order to reach a target RPM, and the ECU and those parameter are based on a certain position of the stop screw. So with the idle lowered manually and you stab the gas the ECU is like "OK no throttle high RPMs, change settings to lower RPM" and with the stop screw adjusted to a lower idle the otherwise normal parameters in the ECU produce really low RPMs and then the prom is like "Woah, too low RPM, change parameters to compensate."

Sorry for the personification but you get the picture. The programming behind the IAC routines are pretty complicated and mediocre at best. When I had a TBI 305 some days my idle would be really high and some day's it would be a little low, probably to do with outside air temp and baro reading but not sure (I didn't dive too deeply into the IAC algorithms).

Anyways, good job on getting it working