My Blazer has been surging at idle when cold. In the process of trying to figure this out, I have replaced the IAC, the MAP sensor, cap(AC Delco), rotor (AC Delco), wires, plugs, and the engine temperature sensor (the one in the passenger's head). The vacuum reads steadily at about 19 when idling. The fuel pressure test read 54 when running and 59 (without losing pressure) with the car off.

I'm trying to go through different systems to narrow down the problem. What is the IAC voltage supposed to be? I checked the MAP sensor and the TPS and they both had about 5 volts going to them. The IAC has 12 volts going to two of the wires. Does that sound correct? Does anyone have any other ideas as to what the problem could be?

Like I said, it does this when it is cold. It starts up immediately every time, and it seems to idle fine for about 2 seconds, and then you can hear the idle start to get heavy and then it starts bouncing. It will surge for about a minute or so before it seems to stabilize. In addition to bouncing, it seems to miss a little. The exhaust is really rich, and several times a minute, there is a little pop from the exhaust pipe. Until it is at operating temperature, the engine will dip down and almost stall when you first give it gas or shift in to gear. When it is warm, it idles fine and it revs up fine until around 1700/2000 rpms. After 1700/2000, the sounds changes (a little bit of a flutter sounds, maybe) and it runs rougher.

I don't know if this has anything to do with the problem, but my ABS light comes on randomly, usually when the car first starts, but it doesn't always stay on. Sometimes it turns off when you put the car in D or R. Today it stayed on all the way to work until I put the car in Park. However, when I revved the engine up over 2000, the ABS light came back on. At lower rpms it went off again and the came on again when I revved it over 2500.

My specific question is about testing the IAC voltage and other problem-related electrical circuits, but I would love to hear any other advice that you have. Thanks.

 

The IAC valve itself has two coils (4 circuits, 2 for each coil) and the PCM will pulse the circuits to get the desired movement out of the valve. By applying alternating ground & power to each of the circuits, the PCM can move the valve in either direction at varying speeds.

A hunting idle is typically a result of a vacuum leak. To diagnose a vacuum leak, start by disconnecting and sealing off each of the vacuum ports on the intake manifold (EVAP, brake booster, PCV, etc) and see if the problem persists. If it does, then you have a gasket leak somewhere. If the condition is not present, then there is a problem with one or more of the disconnected vacuum lines. Plug each one back in independently and see if the problem reoccurs. If it does, fully inspect that system for leaks or other malfunctions.

Testing the MAP and TPS sensors goes beyond testing for reference voltage. The MAP sensor should have an output to the PCM proportional to the vacuum level in the intake (click here for the voltages for a given altitude). The TPS sensor will have an output to the PCM proportional to the throttle position (~0.5V at closed throttle and ~4.5V at wide open throttle). Both should be monitored by 'back probing' the sensor return wire to the PCM. Both topics are covered in this thread specific to a '93 CPI engine, but a lot still applies to yours as well.

 

Is the check engine light on? If so, what DTC's are in memory? If the check engine light is not on, chances are the problem is not related to the OBD system. Other things to check are vacuum leaks, primary and secondary ignition, and engine mechanical. Your 95 "W" engine is equipped with Central Multiport Fuel Injection, (CMFI) and has several known problem areas and this might be a good place to start looking. Key ON, engine OFF, fuel pump running: fuel pressure must be 55psi to 61psi and must remain above 50psi for 10 minutes after the fuel pump shuts off.

 

Thank you for the in depth replies; the amount of time that you put into this web site blows me away...

My heater/vacuum actuator system is fine, and so is the brake booster line. I replaced the PCV a few months ago, but is there a way that I can check it? I know that if I remove the PCV hose, the idle goes up and if you suddenly plug the vacuum, the engine surge occurs.

As far as a check engine light, I haven't had one until today. A few months ago, after cleaning my EGR, the engine started surging at any RPM and barely running at idle; when starting it, you had to nearly floor the accelerator. I replaced the EGR and the MAP sensor and the problem went away, but the engine would still surge when cold. Well, the problem came back today, and it threw a couple of codes: 32 & 45. I think 32 is the EGR and 45 is running rich. The exhaust smelled pretty rich even before today, but it never threw a code. Obviously I will pull the EGR and see if there is anything clogging it, but could there be something wrong with the system besides just having carbon clogging it?

 

So I pulled the EGR and it had a nice chunk of carbon in it. The interesting thing is that the problems that it was experiencing because of the EGR were similar to the cold surge and rough running but a lot more extreme. I haven't checked the EVAP system yet, but could this cause a surge when cold? Are there any other sensors etc. that might be the problem?

 

Check fuel pressure and leakdown. Very easy to check, takes about 15 minutes.

 

Well, I did check that, but I'm not sure if the readings are too low. I know that they're borderline, but the car has no problem at all starting up. It fires up immediately. The pressure test read 54 when running and 59 (without losing pressure) with the car off.

 

Your fuel pressure readings sound good enough.

 

Well, I was reading through the different sensors in my Haynes manual, and it sounds like it could be a whole slew of things from the air conditioning clutch relay to the neutral start switch. One thing that I've noticed, and I have no idea if it's related to the cold surge, is that sometimes the ABS light comes on randomly, but it does it especially when you first start the car and it'll go off or come on when you put it in D or R. Could this possibly be an electrical circuit thing that is affecting the idle? Which sensors are part of the same circuits?