Interesting voltage problem? any thoughts

[mikeran]

My 87 starts right up no problems but sometimes it runs real rough and lags badly at idle, and the large warning light is on and the voltage guage is pegged to the bottom.

If I rev it a couple times the guage will pop back to 14 volts the warning light will go out and the idle will smooth out.

it does not do this everytime and can happen warm or cold start.

Any thoughts?

[John_AZ]

1. Unplug the O2 sensor and see if the symptoms happen. Replace if over 60K.

2. Clean the ICV/ISV. I use Gumout fuel injection and throttle body cleaner--Not carburator cleaner.

3. Check for bad FPR & FPD by pulling the vac hoseS. Look real carefully for drips of gas.

4. AFM needs to be cleaned--under the black cap where the contacts are. Clarks.

5. Porsche has a TSB that says: Clean all engine electrical contacts on top of the engine if the car runs rough.

6. Many mechanics think it is caused by a bad DME engine temp sensor.

7. Rebuild the throttle body with the Arnnworx kit ($20)

8. I do not know.

GL
John

[sparky4017]

Check the alt. sounds like the regulator is sticking causing low voltage till you give it gas to speed up the alt.

[matty89]

why check the o2 sensor if it is pegging low voltage ? John!

[John_AZ]

matty,

There was no order to the numbers of the ideas.

You & I just posted on thread and the O2 sensor was bad.
Got one for you guys

A bad O2 sensor can cause early idle problems & emmisions issues by keeping the engine running rich.

Actually no real reason I guess.

John

[mikeran]

The idle is only bad while the voltage is pegged low, once the guage reads correct the idle is very smooth.

[sparky4017]

I still think it may be a alt. issue if the regulator is staying closed when the car starts you will have low battery voltage which robes voltage from the DME and runs like crap when you rev
the motor the regulator opens letting voltage to the electrics of the car like the DME if you test
the alt. at like auto zone FREE it may show bad or not if you have a good shop near you they can change the regulator only i think i saw one for sale on the form rebuilt 70.00 Just my 2cents

[spec guy]

I have had an 87 924s for three years and it will not charge until it gets to 3000 rpm. Every time i start the car it's the same. I don't know why but it works and charges. I have been told it could be a faulty voltage reg. but i have never changed it to see if there is a difference

[John_AZ]

Since many think it may be the alt or VR...

Clean the connections at the back of the alt. If the alt is covered with oil, remove it and clean the mounting to the block.

Spin the pully, Do you hear grit and noise. Change or repack the bearings.

Pull the VR out and if the carbon feet are 1/4"-5/16" change the VR.
Change the VR if one of the feet is really shorter than the other.
If you change the VR you should "dress" the brass slip ring or it will eat the new VR "feet"

Check the specific gravity of the battery with a hydrometer. If all cells are low recharge. In a couple of days driving, if the cells are all low again it may be the alt.

An alt may be the fault even if the local FLAPS checks it OK. This is why you do the hydrometer test. It may show OK even though a diode is faulty.

GL
John

[mikeran]

Ok sorry for a really stupid question, but honestly in 40 years of driving I have never had a alternator go bad.

Do I have to remove the Alternator for Autozone to test it?

Thanks

[John_AZ]

If you just want to check the VR, you do not remove the alt or the belts. It is just 2 screws to unscrew it. Disconnect the battey cables first. To get to it you have to remove the air filter box. Easy.

You can check the alt at home with a cheap HF (harbor freight) volt-ohm-amp meter. Set the VOA meter at about 20 DC volts. Put the red wire on the + POS, and the black wire -NEG. Do not disconnect the battery cables. Start the car. You should get a reading of about 13.8V if the alternator is charging correctly.

If it is less than 13.8 volts take the alternator out of the car and go to the FLAPS to test.

Before you do, I suggest the hydrometer battery test as well. If the cells show low, charge the battery and retest in a couple of days. It could be the battery going bad instead of the alternator.

GL
John

[sparky4017]

Nope they can test it on the car, but the best way is to take it off my only thought is once you have started the car and the regulator sets it may not happen again until first start in the morning
another test you can do yourself is when you start it and it is showing no charge it to pull the
neg. battery cable if it dies alt. is most likely the problem ck your grounds

[sparky4017]

John is right it could be a battery going bad maybe a broken plate in the battery going to a ground
And trust me there is no stupid question if it wasn't for John and Sol mine would still be in the
barn.