FredR

For whatever reason it sounds like the idle contact is not making every time. When this happens the revs will sink until the motor stops or the static opening in the ISV holds the idle. When the a/c is running if the contact is not made the additional load on the crank will cause the motor to die. At a steady idle the alternator only makes about 50 amps which is barely enough to run the fans so the battery supports and the voltage drops. When the idle speed drops below normal the alternator is probably going to be turning out squat at the depressed revs.

Remember what i advised you about the throttle cable - the adjustment is critical- if it is set marginally this could explain why it is hanging up sometimes.

Speedtoys

The alternator could put out zero, his healthy battery will still run the car.

FredR

Indeed but the voltage will dip to 12 volts and even less depending on how many amps are running at the time and this is exactly what Paul described. When the engine "recovered" the volts came back up but when the engine dies completely, as it will with no idle contact made and the a/c running the volts do not come back up. Nothing wrong with the ISV and most likely nothing wrong his new switch.

What Paul can do if he has not done so already is to put a few drops of sewing machine oil on the bare throttle cable where it enters the outer sheath, work the throttle some and then repeat the process about 4 or 5 more times to help work the oil throughout the length of the throttle cable..

Speedtoys

I get that, yes, the gauge (which we agree we should never be using to determine health) went down as alt output goes to 0...and comes back. Not a big surprise.

The car is dying for reasons other than a dirty corroded path to an uncalibrated (from age and corrosion) gauge is showing a "really low" voltage just as the car dies.

That it fires right back up, tells me to ignore most everything alternator or battery health related.

Id ask the OP what the entire history of cleaning -every- power point, ground, and pod connection was before going down this road.

A 400 amp alternator wont keep the car from dying...or connected to shore power the whole time.

dr bob

Rolling idle contact check:

Set the digital display to show miles-per-gallon. Driving the car, the LH will close injectors when the engine RPM is above about 1300 .and. the idle switch shows the throttle is closed. When that happens, the displayed MPG number goes to 90. Watch that display as you drive, and you should see 90 MPG displayed as you drop down towards a stop with car in gear and clutch engaged. If the display doesn't do that, there's a very good chance that the idle switch isn't working correctly.

The idle switch true .and. engine speed trigger the idle stabilizer valve into action. The ISV bleeds additional MAF-metered air around the throttle plate when the idle speed goes too low.

Un-metered air (vacuum) leaks into the engine can cause erratic idle characteristics. The crankcase vent system draws functionally-inert combustion gasses from the crankcase via vents in the cam covers and a vent in the oil filler neck. If there's an air leak into the crankcase or that vent plumbing, the engine will run lean until the oxy sensor detects the problem and richens the mixture for you, even if the idle stabilizer valve is working correctly. Mike77 shares that he fixed a hole in the oil filler neck as part of solving his erratic idle issues. Tired hoses, failed connections, and stoooped stuff like a loose oil filler cap will contribute to the problem.


This stuff is north of simple, but the diagnostic steps to find the problem aren't. The idle switch function demonstration shared above is super easy. For intake leaks there's a little more fun to be had. The intake pressure test rig that Dwayne details in his intake-refresh instruction is perfect for finding if there are leaks. A smoke machine might make those easier to localize, but the basic see-if-this-is-the-problem starts with the intake pressure test rig. Start with a visual and audible inspection of course; you may be able to see or hear a leak before pulling out all the tools.

merchauser

dr bob, and fred: just tested the idle contact switch 3 ways: audible, check, with instant fuel gauge, and foot off reading 90mpg, and with hammer. all good. fred, when I did that prior work, I replaced all the throttle cables, so they are good

dr bob, a few months ago, I did a full intake refresh, and smoke tested: no leaks. again, my idle is rock steady, except for this load issue, which is intermittent. while testing my AC readings, I held the car at 2k for a good 5 minutes on three occasions, and each time I lifted off the pedal, the car died. each of those times, when I turned on compressor and fan on high, volt gauge dropped below 12 and stayed there. just went for a drive, now that car is cooler, and performed same test in the garage. gauge never dropped below the middle of 12 and 14 which is my default, as I added load. this issue in intermittent? only ground I never tended to was the battery ground strap. anything there to see? are there upgraded battery ground straps?

Jason89s4

The battery ground strap--without any debate--is the most important of all grounds. It is quite literally the mother of all grounds.
You can buy a battery ground cable at any auto parts store, of good quality and gauge, for $12-15. You don't have to leave it on if you want to go back later a put the expensive "flat" cable on, but with all the testing you are doing, why not eliminate this easy...and common issue?
See post #13.
And yes, bad grounds can be intermittent and load dependent.
Jason

FredR

Paul,

I was under the impression you had replaced the earth straps- inspect yours- lift the heat shrink at each end and inspect the braiding where it is soldered to the lug- chances are it is shot if still the original item. If it is still original also replace the engine strap [rear passenger side of the engine].

Roger does an after market version that is a bit chunkier than the stock battery earth strap..

If you replaced the throttle cable then theoretically it should not be an issue - are you sure the cable is moving freely at the end of its travel? Have you checked the idle switch contact status when the motor dies? Until you know for sure what is causing your issue do not eliminate anything based on assumption.

merchauser

lol: I was meticulous in attending to grounds, but gave no thought to the battery strap. I cleaned up the block to chassis ground, but thinking I should replace that one as well.

throttle cable is moving freely, but I will keep idle contact switch on my mind

Jason89s4

If it is the original style, wound copper with no insulation, you can remove the engine to body ground strap and soak it in white vinegar overnight.
Corrrision will go and it will look brand new. (Don't do this with insulated wire.)
Jason

SteveG

I don't think this is the same as braided which is OEM on all I have seen; I do not know if this makes any difference? I haven't seen Roger's improved version, but the Porsche item when last priced by me wasn't more expensive, so I'd surely go with that. This would be the ONLY item that Porsche supplies that isn't more expensive.

Also another picky point, but "clean if needed" is spacey. Depending on climate and when was it last done? -- I would hit all with 000 paper or emery and De-Oxit everywhere, THEN retighten.

Stan : How did you miss this? You are slipping.

FredR

I doubt the idle switch will be at fault but given the info provided and the fact that it is one of the last things you worked would put this on top of my possibility list.The cable may well feel OK but the question would be whether the adjustment is suspect such that it normally works ok but in some circumstances it simply does not travel fully. Correct adjustment of the cable is critical and there is little scope for error without inducing a problem at either the idle position or the full throttle position.

merchauser

your post and thought is not in vain; I am keeping it high on my list

long drive this AM and no issues at all. most notable, is that when I have my problem, as I add load, radio, AC, lights, fogs, etc., the volt gauge progressively drops and ends at 12 or slightly below (at idle) today, the gauge stayed in the middle between 12 and 14 and never approached 12. ideas???

FredR

I have not seen anything in your thread that suggests you have an electrical issue and what you have documented can easily be explained in terms of what happens when the idle switch is not engaging as it should. That does not mean that an electrical problem of some kind is not occurring but the chances of such seem low to me.

A few points that you may not appreciate:
1. You have a 110 amp alternator- at idle it probably produces no more than 50 amps- if that.
2. With the a/c running, both fan motors are running as is the blower and between them they alone will consume the running amps at idle yet alone whatever else you throw into the mix.
3. A fully charged battery should indicate 12.6 volts under no load- meters that indicate the state of charge measure voltage [nothing else] over a narrow range and by the time it indicates 11.5 volts it is down to 10%.
4. To start the car the battery must have about 40% if charge in it.
5. The alternator is rated for 110 amps and not too surprisingly, if you add up the amp rating of the things that can be expected to run simultaneously they come to about 100 amps or so.
6. Once the alternator cannot deliver the required load the battery will make up the deficit and once the voltage drops to 12.6 volts the battery is then kicking in.
7. As the battery delivers more amps the voltage drops as it delivers them. If the alternator belt snaps the voltage will immediately drop to about 12 volts, maybe less.
8. As you start the car depending on the temperature the battery will chug out around 300 amps to run the starter motor, energise the starter motor solenoid and provide power for the ignition, computers, coils and display etc- the voltage will drop to around 10.5 volts during this activity.
9. Drive around with everything switched on at low revs [circa 2k rpm] and your battery will likely be playing a role. Stick a powerful sound system in the car and it will struggle that bit more until the motor runs at or close to 2800 rpm.

On balance the system works OK but everything has to be in good order. Create problems by having defective earthing or having degraded wiring between the battery and the main bus in the central electrics and the problems multiply. If you still have the original wiring between the starter motor and the hot post/ABS chances are it is shot electrically speaking.

Alan

Is the car converted to R134a? does it still do this if the AC is turned off?

Alan