Brett951

Apologies if this has been asked/answered before, the search page is broken (64 - Host unavailable)

I don't use my AC much, but it was 100 here yesterday and in northern Oregon that's damn hot. I'm used to all four cylinder cars going "ugghh" when you turn on the AC, but my 968 last night was beyond that. It blows cold air, but the engine would cycle like it lost all spark or fuel. I say cycle because it was very rythmic, like one beat no power, two beats with power for as long as I left the AC on. I did leave it on for awhile thinking maybe things needed to circulate since I don't usually use it. At one point I approached stop-and-go traffic and put in the clutch and the engine actually died. I let out the clutch and let the engine restart and then tried to duplicate, but it didn't die after that, the RPMs would take a serious dip (400rpm?) and then even back out at ~900 RPM and the tach would not even show the cycling I could feel when attempting to drive the car (seemed smooth at idle even with AC on). But the lack of power was such that if you try to take of from a stop and are on the low power beat, the car dies.
Then on just about the last stop light before home, the car does die again, and I'm at a stop. I turn off the AC and try to start the car, engine turns over but refuses to start. I push the car to the side of the road, check fuses, etc. get back in and it starts right back up. Drive the rest of the way home without AC.
Possibly related: my car normally shows just over the 12 volt line on the volt meter guage. when runnig the AC it dips below 12 volts.
So, what can cause such a loss of power, especially that you can count beats to? And more importantly how do I fix it

-Brett

RajDatta

Normally the ECU is supposed to compensate for your AC compressor coming on by increasing the revs but seems like that function is not working on your car. Let me think about it a little more.
Raj

Brett951

Could the ECU be faulty? It *is* compensating at no-load idle, I think. but cycling when the engine has driving load. Weird. And weird that when it died at the stop sign and didn't re-start back up, which is the first time that has happened... Any advice appreciated

flash968

check your voltage at the battery - if your alternator is not putting out the necessary current, this can happen when the ac kicks in, especially at idle when the alternator is putting out its least current - what could be happening is that there isn't enough amperage left to run properly - regardless of what the guage reads (and they usually read about a half volt low) you should have a fairly steady 13.5 or so at the battery regardless of accessories or rpm

RajDatta

Start with DME relay. Its the cheapest test and something thats long overdue. Also, do you have any other 968 owners around that can let you try their ECU?
Check for your ground on the bellhousing and make sure there is no corrosion as well. Same for both battery leads.
Raj

BruceWard

The voltage gauges in our dashes are not accurate. As others have said a multimeter across the battery terminals should read 12+ with the engine off and 13.5 - 14.5 when running. Otherwise it is time to have the battery tested and if thats not where the problem is repair or replace the alternator.

I suggest that you leave the car idling in place with the AC off. When the temp guage gets to the 10 oclock mark the fans will kick on high and cool it down. This will put a load on the electrical system similar to but perhaps not as much as having the AC on. If the stuble happens when the fans switch to high then that may help you debug the problem.

The DME relay is a good suggestion, you should have a spare of these either way. When it fails you will have trouble starting the car when it is warm or not be able to start at all. If Raj thinks this could be it then I suggest you order one right away!

flash968

the fans won't put nearly as much load on as the ac, but bruce is definitely onto something there - you can further increase the load by turning on all your lights

it may be as simple as a bad regulator inside the alternator - they are easy to change, and can be as little as 15 bucks for the aftermarket one - once you do the voltage testing, and if you determine that it is low, remove the alternator belt and spin the alternator by hand - it should not make any noise, nor spin very fast

RedlineMan

I'm lazy, and I'll stick my neck out, too.

Lazy because I didn't read every word of the thread. Sticking my neck out because I don't know for sure if they have one. 968s, that is. But I think you guys may be missing something obvious, and common. Most Series II 944s have an idle stabilizer valve that senses the vacuum drop caused by the draw on the engine, and will bump the idle a tad to compensate.

I assume the 968 has one, and it is likely under the intake where all other late 44's have them. It is VERY common for this to get sticky and not work right. Many take them out and clean them.

You could be getting fooled by the voltmeter dropping. This should be normal as the electric load increases. If you have any other idle irregularities, I'd look for that valve!

flash968

they do have one - we were just going for the "easy to test yourself" stuff first - how would you test that valve?

Brett951

So, for grins I tried running the AC tonight on the way home as well. It did not repeat the performance of yesterday. The engine would still pulse somewhat in that the power was not as smooth as consistent with the AC on as it is with the AC off, but the inconsistency of power was slight and at times I wondered if it was more my imagination then actual fluctuations in power, and it was not nearly so rythmic as yesterday.
idle stabalizer valve - given the expertise on the board, I don't discount this, but the engine is seemingly more stable at idle then when under driving load. Other then the two times the engine actually died (once while still moving and once at the stop sign) the idle has always been very stable, AC or no - well other then the dip it takes when the AC is on and returning to idle - which also seemed less tonight then yesterday, but I wasn't watching that as close and didn't get into stop-and-go traffic to test it. Even yesterday when it was behaving so badly, as soon as it came back up from the dip in RPMs it would be rock solid at ~900. Also, since it does idle so well, I can't make it stumble while doing idle testing in the garage as per some of the advice above.
DME relay - what all can this little thing affect? I know it can keep the car from starting - I replaced one in my 951 when I was having trouble with the car dying at stop signs and not restarting right away. And since that can be a major pain, I agree that I will go get one from the dealer, which is my nearest part source, at the next opportunity. However, the car has only failed to immediately start that one time. And it can't have an affect to cause power fluctuations can it?
Testing/Inspection results at home, car parked -
Volts with engine not running = 12.57
Volts with engine running, minimum load = 13.76
Volts with engine running, ligts, AC, fans, radio = as low as 12.8 - this seemed to drop as it ran, though it never went lower the 12.8
there is little corrosion at terminals or bell housing, what little corrosion there is is mostly inside the ground wire insulation as it leaves the battery (perhaps a new negative wire is in order soon)
Possibly related? the belts, alternator, or something is nosiy in a non-consistent way. I've heard that the belts can be noisy, but something up there that seems like the belts makes a nosie that comes and goes with no discernable pattern that sounds faintly like the noise a tea kettle makes before it whistles but softer - a kind of almost whistle. It is there at least as often as not, but random on/off. I would think belt noises would be more consistent.
So - any ideas given the volt readings and additional details? Given how it drove tonight I problably wouldn't have posted - but I also don't want last night's perfomance to come back...
Oh, and we have a discepency in the advice so far about whether volt drops with load or not. I thought some volt dropage is normal and should be corrected/contained by the voltage regulator. I don't know whats normal for a Porsche though :-)

Thanks, you guys are the best.

flash968

ok - first off, voltage should not drop more than a tenth of a volt or two under load at the battery - that is what the regulator is for - nothing special about porsche systems

you may be on to something with the ground wire - it is as important as the power wire

a few things can be checked in your charging system:

check your battery and main ground connections - tighten them and not just a visual check
make sure the battery is topped up with distilled water - do not use tap water
check your belt tension - a loose belt can do that
loosen the belt up and spin the alternator by hand - it should make no noise and shouldn't spin very fast and free by hand - bad bearings are common causes of your symptoms

if you fail to find your problem, then take your car to pep boys and have the charging system load tested - they will do this for free - make sure they hydrometer check the battery too

Brett951

The stumbling came back - the severity does seem to fluctuate though
I took it to the dealer last night since it was on my way and I wanted to get pricing for negative battery cable etc...
THe service manager did a test I hadn't done yet; fired the car up and let it idle, was still stable, ran the engine to about 2000 rpms then turned the AC back on. Standing beside the car with the hood shut you could hear a pretty loud click everytime the power of the engine changed, and the rpms were bouncing up and down though he was holding his foot still. He suspects that either the AC compressor is overcharged or is failing. Anyone had a compressor overcharged or fail?
Does the clicking symptom give anyone any other ideas, being new information on this problem?
Thanks,
-Brett

sm

Brett,

I suspect it is the A/C compressor as well. If I read correctly (just skimmed the previous replies), the A/C has not been used in a while, but is still blowing cold. However, has the A/C performance decreased? If so, it may be that some of the refridgerant has leaked out, bringing some of the refridgerant oil with it. That will then increase the friction inside the A/C compressor everytime it spins, eventually leading to failure. I suspect it's the increase in friction that's causing the rpms to drop like they do.

I had the same problem after overhauling my A/C system and adding a little bit of oil to the A/C system seems to have quieted the compressor. Now I just need to drive the car to see if the revs drop below 500 rpms like they used to when the A/C was on.

BTW, do your revs drop quicker between shifts, while the A/C is on? Mine did...

RajDatta

SM, I had an AC compressor jam on me and one day it just went and seized. It did not drop my rpm at all. As a matter of fact, when it jammed, it took the alternator belt with it as the compressor was in use. I am not sure if friction would cause such a drop.
Raj

flash968

mine clicks when the ac kicks on and off - that is normal

seriousl - do the tests i indicated - it will help you narrow things down and not be shotgunning at odd less likely problems - simple and obvious stuff first

remember occam's razor