Does anyone have this problem? When I turn on the rear defrost it significantly alters the idle (lowers it by about 400 rpm). I am thinking this is not normal.......is it????? One cold night a few weeks back it basically stalled the car if you let off the gas until the car got warmer.

 

I've noticed something like that on my car, though not to your extent. IIRC my rpms actually rise a bit. Even my turn-signals cause a slight raise in rpm's. I think it has to due with a drain on the charging system which in turn affects the ignition system. I could be wrong there though.

Matt

 

As you put more load on the electrics in the car, like turn on the rear heater elements, or lights etc then the load on the alternator increases, and thus requires more energy from the engine to turn it over. If the engine is not cleverly managed then this will cause a drop in the revs as it's not being given more gas etc to create this extra energy.

However some more modern engine management systems will actually realise that more load is there and tweak the engine's settings until it can provide that power, and in this case can sometimes increase the resulting rpm. Not sure how "clever" the tpi cars are, as to whether they do the more modern technique of increasing the rpm to cope or not.

 

The tuned port and throttle body models should compensate with the IAC control, you don't mention what engine or fuel delivery you have. Carbed models use an idle load compensator. You have a problem either way, a 400 rpm drop with even a full electrical load should not give you the problem you describe.

 

it's an 85 FB with a 2.8 v6

Could this maybe mean the alternator is on its way out??

 

Ahem, when you turn on the defrosters it engages the AC compressor. They did this to make sure the AC compressor gor run and lubed during the time of year when AC isn't used.
It sounds like your ECM isn't up-compensating when the compressor kicks in.
The electrical load of running the defrosters is no more than running in the "heat" position and the blower motor isn't enough of a drain to drastically change the idle.

 

I beg to differ, the AC compressor is run in defrost to rid the car of moisture, i.e. frost on the windshield. Even when its cold the refrigerant is cooler and moisture will condense on the evaporator in the heater box ridding the cabin of humidity.

 

No need to beg.
Think of it this way, what if the car doesn't have A/C? What rids the car of moisture then?
But if you lived in a northern state and the A/C pump didn't get run for 9 months of the year straight, what would the effect be on the pump?

 

if you don't have a/c then heat is your only weapon against the moisture. It just doesn't work as efficiently.

 

Sure it does. I have never had A/C in my car, never had a problem with moisture either. Not even in the deep south where it is now.

And at either rate, the AC compressor is probably what is dragging his engine down.

 

He is talking about the rear electrical defroster or so I thought. It is probably the biggest electrical hog in the car. That being said, the alt when required to supply large amounts of current will require horsepower. 1 HP=746 Watts. However in that case even if it draws 20 amps, at 12 volts it is only 240 watts. The easy way to tell what is going on is to turn things on one by one and watch the RPM level. If it does not change or remains stable and when you get to the rear defroster it takes a nosedive I would suspect a very high current situation, maybe more than the alt can supply. The V-6 models do things a little differently than the 8's. For instance if the A/C comp is on and high pressure is sensed from the power steering the A/C comp is momentarily turned off to take load off the engine. The IAC simply cannot compensate for the horsepower drain on a 6 cyl engine. To drop 400 rpm is significant, you have an interesting problem. You mention on a cold night until the engine got warmer, was it OK after it came up to temp? This problem has to be analyzed step by step. Perhaps you do have a problem with the IAC. Cold engines tend to stall if they are not choked as in the case of a carb. FI engines use an intake air sensor, coolant temp sensor and internal programming in the ECM to provide proper rpm and fuel delivery with a cold engine or when it's in "open loop". This ones gonna take some detective work, get back to me if you can pin things down. Hope this helps some, Dan

 

Quite right, he did say REAR defrost. I missed that part. While it does take quite a bit of juice to run it, I don't think it in and of itself would be able to drag the engine down that far, even if you had the headlights, stereo (unless you are running a home stereo in there) and blower motor.
Sounds like a short somewhere or maybe a very weak alt.

 

yeah we are talking about the rear defrost (if I remember correctly, the AC only comes on with the front defrost). But anyway, that cold night it would just about stall it. I think it actually did once. Once it was warmer, it still affected the idle just not to that degree (still 2 or 3 hundered rpm at least.

If i remember correctly, a couple of times I have also noticed a slighter variation in the idle when turning the headlights on too. ANyone elso notice that?

Back in late summer I cleaned out the TB and IAC thoroughly so hopefully that's working efficiently.

Thanks for the replies, keep the ideas coming.

 

Yes, turning on the headlights does affect idle, for less than a second. The ECM should see the drop and command the IAC to retract and raise the idle.
Have you checked your IAC? If you can scan the engie while running, take note of the IAC steps, turn on the headlights and see if it changes, if it does, try the rear defroster and see if it moves again. It may be stuck in the bore and unable to retract to raise the idle.

 

unfortunately I don't have access to an X-Ray. After the idle drops, it does eventually come back up a bit but it certainly isn't instantaneously or anything. There is a definite lag. That cold night though it didn't come back up much, it just kept chugging. That would have been with the lights on too though.

 

Well, you could always try a shot in the dark and remove the IAC and clean it and the bore it sits in and see if that helps.

 

I would agree with Morley, pull the IAC and look at it. They get gummed up beyond belief. I recently did a major job on my 89 and mine was so gummed up I just replaced it. Any load on the engine such as alt throttling up will change the rpm and the IAC is supposed to provide compensation. Make sure the throttle body pintle area is also clean and free of deposits. Even if it's cleaned they sometimes get sluggish from age. Also, check the voltage at the battery and see what's going on there. I am snowed in here, I'll give it some more thought.

 

Actually I cleaned the IAC and the TB thoroughly back it the late summer. THey were spotless when they went back in.

 

battery voltage is good too....over 13 volts when running. Battery was new this year too.

Danno - you mentioned about sluggish IAC's. That is interesting.....how would one know if it was sluggish, is it supposed to set a code (can't remember the code for IAC off hand)? How much are they new, are they expensive?

 

You'd need a scanner to spot a sluggish IAC and then you'd have to know what to look for. It won't set a code if it moves slowly or even not at all.
They cost about $25-$30 for a new one (aftermarket) and about $50 from GM (local dealers)