I haven't driven my GF's C4 much but I have a little lately.

It drives fine until the temps get above 225 and then the engine starts surging at idle. It also has stalled while it's surging.

I noticed that the temps get into the 240's also. the fans come on when the A/C is turned on.

I'm assuming that the the surging problem is heat related, but it must be triggering the problem as just getting hot wouldn't make it surge.

Thanks, for your help.

BTW: car is a 94 with 44,500 miles

 

Gary, I have Datamaster. We can hook it up and log some data, if you would like.
I also have TunerCat, and we can lower the fan on/off temps.

-Mike

 

That would be great I will email you with my phone "s

 

some pointers, but I suspect you all will be fine, check the radiator for debris, and make sure the IAC is functioning ok.

 

Sorry, I've got C5 shop manuals and know it pretty well but I'm C4 ignorant and only have the owners manual...

IAC is the...?

Now if you would have referred to the EBCM/BPMV, I would have been all over it.

Thanks!

 

I went through this with my '89. Replaced ECM, TPS, cleaned throttle body and I still has a problem. Finaly pointed to my injectors. You can pop the wire harness off of them and ohm them out to see if they are shorted. I can't remember what they should ohm out to. I do remeber they were all over the place. Some were 17 some were 14.

Maybe someone can help with the procedure. The car only had about 80K on it at the time. Replaced injectors and it's fine now.

As for injectors I went with Delco replacments, but a lot of people have recommended Ford injectors. I think I saw a topic a few days back on the subject. I would stay away from Accel, however. I put them in and two were bad, had to redo the whole thing.

Good Luck...
Christ

 

I went by Gary's today to log some data. Everything looked normal. BLM's were even better than normal, the best I have ever seen on a stock engine.

The problem seems to arrive when the engine is hot and the a/c is activated. The car's RPM's will surge and eventually kill the engine.

Any ideas on the problem? Bad a/c compressor? Bad compressor clutch?



-Mike

 

You might want to try a new O2 sensor(s) because alot of the times a lean idle will produce a surge.. unless your messing with a ford EFI but thats a different story.

 

The ECM is trying to correct for a problem that gets to cycling too far past optimum for some reason. A number of things can cause this. Fuel delivery, or unmeasured air getting into the system are two of them. Perhaps you do have an injector that's not putting enough fuel out, and the O2 sensor sends a lean signal to the ECM to richen the mixture by extending the open time, which produces too much fuel and then cuts the fuel back which starts the process all over again. A vacuum leak can let air in that is unmeasured by the MAF. This can create an unstable idle cycle. Does your year still employ an MAF sensor, or is it a speed density system? See if there is a leak in the plenum between the throttle body and the MAF sensor if your year still employed an MAF sensor? A vacuum leak might be in a line that is only visible to the manifold when the AF is engaged.

If you could data log you could see what conditions accompanied the surging, and get some clues as to what is causing the cycling. I had exactly the symtoms you described in an after market EFI setup. The O2 sensor delivered higher voltage at 14.7 than it should. At 14.7 it was reading .75 volts. This produced the same kind of cycling that you described, because at stoimetric, it was getting a rich signal from the o2 sensor. The ecu would cut back the fuel, and it would almost die. The mixture would get down to about 20:1 before the O2 sensor would cross over to below .5volts, and then the ECU would richen up the mixture and start the entire thing all over again. I solved the problem with this set up by setting the cross over voltage on the ecu to .7 volts. It runs along fine now. Of course I could have installed a new O2 sensor, but this works, and didn;t cost a penny. In my case, perhaps the problem is the location of my O2 sensor. I don;t know how programmable the factory ECM is, but this is the kind of info a data log can reveal.

 

How did you change the cross over voltage for the o2 sensors in the ecm? There is a parameter for this?

 

When you guys do find the problem, could you post it? Having the same hunting problem at idle on my L98. Thanks.

 

Might want to look at the IAC. It is commanded to add steps (counts) for A/C engaged, if it is slow to react then it may be causing your problem.

 

Alvin, we ran a datalog and it isn't lean at idle, it doesn't even go lean when the surging starts. As soon as the a/c is activated, the surging begins.

 

My after market ECU is programable with a lap top, and the crossover voltage is one of the variable that you can set. I don;t know if this can be changed on the GM ECM. Look at your data log, and see if anything is changing as the surge starts. Check the voltage from the TPS, (throttle position sensor)this could be fluctuating. and sending a signal for the IAS (idle air solenoid). I don;t know why the AC would set it off though. The AC would ask it to open slightly to accomodate the extra load on the engine, so maybe that is what is going wrong. Is the computer setting any codes? Again look for leaks in the hose that connects the MAF and the Throttle body. That will realy foul things up.

 

Warm the car up until it starts to surge, then turn i off, Disconnect your O2 sensor, Start the car back up and see if the problem is gone. This is the advice I was given on the forum and it worked. My o2 sensor was bad and it never gave a code. Good luck.

 

This sounds like a sticking pintle in the IAC to me. Cleaning it is a fairly straightforward and no cost procedure.......that would be my next step.

 

Good thought Caboboy. That IAC is a simple mechanical sliding valve that can get sticky. It certainly can't hurt to clean it. I would not rule out a lean condition on the basis of a data log. If the O2 sensor is bad, it will give a false A/F ratio. I would never have known my engine was going lean from the O2 sensor alone: I also have a wide band sensor, and a lambda gauge, which was giving me different info from the laptop which was reading the narrow band O2 sensor. Go to a shop with a tail pipe sniffer, and see what is coming out of the tail pipe. See if it agrees with the O2 sensor. If it does not, the O2 sensor is bad. If the sniffer is showing a lean condition, while the O2 sensor is sending more than .5 volts, then the computer thinks your engine is rich when it is lean, and will continue to try and lean it out. It seems that when these things go bad, they don;t just fail. They procede to put out the wrong voltage, which fouls eveything up to a fair-the-well.

You can measure the voltage from your O2 sensors directly. Get a pig tail that will fit the sensor, and measue with a good volt meter. It should be reading about .5 volts. Cool thing is if you could rig up a test harness that would let you keep the sensors in the system while you read them. If you unplug them, then it goes into open loop which likely has a default setting that is designed to get you to the shop when they fail.

 

Does anyone have pictures of:
1) The location of the IAC (I have a 91)
2) What the IAC looks like, or should look like if cleaned

If I am lucky enough to find it and remove it, what is the best way to clean it, and how do I make sure I put it back together correctly??

After reading various posts of surging idle and also of the engine dying, I think it would be a good idea to check out my IAC.

Thanks

 

If it's only happening with a/c, make sure it's got a steady a/c signal as long as the compressor is engaged. Best to observe the compressor so you can make sure it's not cycling (or if it is, make sure that the signal toggles off). Most Scanners show the signal as a "yes" or a "no", but I don't know how it's going to appear on the software you're using. When you have the "yes" verify a 10 to 15 increase in IAC counts as well as an increase in MAP voltage since a MAP handles the load on the '94. Operate the a/c on/off switch to make sure these items decrease when the a/c is off. You'll probably need to capture about 30 seconds of data to make sure everything is working as it should and you might also check for voltage spikes whenever the fans or compressor come on. If you have graphing capabilities with your software, that'll probably make it a little easier. Based on what you've posted, I'd guess that it's either the a/c pressure switch, programmer, or PCM, but that's only a guess.

 

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 brakes 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 into 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.

If this doesn't solve the problem, you may need to remove and clean the IAC stepper motor. If the IAC appears to be clean and functioning properly, continue with the adjustment procedure.

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Idle Air Control Cleaning

You can remove the IAC and service it. Remove the electrical connector from the IAC. Unscrew the IAC unit from the throttle body.

You can gently rock the pintle back and forth and allow the spring to extend it until it comes apart in your hands. Clean everything with lint-free cloths and a mild solvent. Harsh solvents can affect the insulation of the stepper motor coils. It's generally the dirt and buildup on this worm shaft that causes sluggish IAC operation.

When the worm gear on the pintle shaft is clean and dry, apply one drop of clean light oil to the shaft and work the pintle back into the rack gears of the motor by the same rocking motion. It takes a while to get the pintle back into the worm gears, but you'll get it. It is important to get the pintle fully retracted into the housing so that the pintle is not forced against the gears when reinstalling the IAC unit in the throttle body.

While the IAC is out, clean the air passages in the throttle body. The oriface in the TB where the IAC resides is the seat that the IAC valve closes against, and it can accumulate a lot of carbon, dirt, and debris. The easy way to do this is with carburetor cleaner and a small stiff brush.

When everything is clean and dry, replace the gasket if it is damaged, apply a little anti-seize to the threads, and torque the IAC to the proper specs. (13 ft/lb for '85-'89 , 30 in/lb for 1990-on.) Proceed with setting the TPS and minimum air position.

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Locate the Assembly Line Diagnostic Link connector under your dash panel. Remove the 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. With the ignition still on, go 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 400-450 RPM with the transmission in "DRIVE" on an automatic transmission car, 450-550 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 in "DRIVE" (for unmodified cars). Idle speed in NEUTRAL or PARK is less significant, and will be higher.



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.