3.2 pulsing RPM when idle

[thisisgoodnews]

tried searching the forum, but maybe the idling pattern is recognizable and someone will be able to give me a shortcut where to look for an issue.

service history of the car is unknown, changed oil, filters, transmission and brake fluids. planning to replace all the ignition components (plugs, wires, distributor, ignition coil, etc..) are to be replaced to address the issue.

hot or cold engine seems to have same idling. sometimes rpm stabilizes at 1k rpm and hold there, but rarely.

video here:
https://youtu.be/WNyRnYqqMSM

[jlex]

I think your Idle Air Control valve is trying to compensate for the idle being set too high to begin with. What year is your car? Idle maybe should be around 880. Search the forum on how to adjust the idle. You have to jumper the connection on the left side of the engine first, then turn the idle screw to get it in spec... Turning the idle screw without jumpering the IAC (to turn it off) won't work. Have fun!

[danco_]

are you able to test your o2 sensor?

I just replaced mine last week for a similar idle issue. The new o2 sensor cleared up everything and idle was smooth again.

[Nick Triesch]

Check for vacuum leaks. Check every hose connected. Unmetered air will make idle go all
Over the place . Older 3.2 should be at 800 rpm. Newer 3.2 at 850.

[old man neri]

I believe the term you are looking for is 'surging', the idle is 'surging'. It should return quite a few search results.

[Nick Triesch]

Tighten both intake runners to 18 ft pounds.

[ischmitz]

One thing to check is the idle switch is getting actuated. It's a micro-switch on the throttle body that tells the DME to go into idle mode. Check with the engine off. Pull the throttle valve open and let it go back to its resting position. I believe you can hear the click when the lever presses the switch. To make sure you could measure in the 55-pin DME connector with an Ohm meter.

Vacuum leaks are another common issue as other mentioned

Ingo

[larrym]

common complaint - dozens of threads - rarely a "for sure" solution

often related to some "improvement" we've done

sometimes ya just gotta live with it - a bit of tweaking of the idle air-mix screw can often help

- i keep a 5mm 1/4 deep socket "adjuster" in the glove box for when it gets too obnoxious when i change altitudes &/or go from HOT to moderate climes for an extended period

it might help to put a rubber cap over the adjuster screw housing if one suspects air leakage around the screw is a culprit

[larrym]

Originally Posted by Nick Triesch

Tighten both intake runners to 18 ft pounds.

yep - lot of threads about that too - but if the gaskets have gone south, (which is apparently all too common) won't make much difference

- you can first test for intake leakage there with various fluids to see if the tightening is worth effort

if you discover leakage - it is a major hassle to remove the runners & replace the gskts with engine in car

[Nick Triesch]

I did it only on the passenger side. Tightening the bolts did not make a difference do I had to replace the gaskets. A pain. But I was lucky on the other side, tightening those stopped the leaks and idle is now fine at 800 . But, a pain.

[thisisgoodnews]

thank you guys for all the responses. a number of possible causes have been pointed out, now just to search the forum for the details and try to address them.

the car itself is a 3.2 911 Coupe, manufactured in 1985, first registered in 1986. bought it last year in IAAI auction with front fender damage. mileage is high, but now unclear, due odo not running anymore. prior history is not known, apart from the carfax report, which shows somewhat around 100k miles done between 2015 and 2017. i also found many engine parts to be quite fresh, still having clean manufacturer stickers, so, highly likely engine even could have been rebuilt not too long ago.

the engine, apart from the surging issue, seems to run quite well from what I can tell.

..and yes, the strong smell of gas inside, when driving must be addressed

[larrym]

Originally Posted by thisisgoodnews

..and yes, the strong smell of gas inside, when driving must be addressed

OK - now you give us some important info - gas smell could be that the vacuum line from tank to intake is pinched or cut or ?

off - - which could also cause surging since it would be an "air leak"

go look at the PET diagrams for where all those vac lines etc run & hook up

on a wrecked-salvage car ya never know what the last guy did nor what the tow-shop-et-al animals might have done to it

[Quicksilver]

First thing: The idle on the Motronic cars is controlled by the computer. Your year should have the idle at 800 RPM. The later 28 pin chip cars have the idle set at 880 RPM. (The later chip is a good upgrade for the earlier cars.)
As mentioned in the above posts, really obvious things to check for are intake manifold gasket leaks, hose leaks, idle air control valve, and the idle switch. Also you can check if the idle is set correctly and if the DME has cracked solder joints.
In general all the original rubber items in the engine compartment should be replaced at the age of this car. The quantity of cracks in the hoses of a running car can be shocking. (The fuel lines to the injector rails should definitely be replaced just to keep the car from lighting off. WYAIT the front rubber fuel lines should be replaced too.)

- Check if all the intake manifold bolts are tight. If any are loose you can make an improvement by tightening them but if they are loose the gaskets will have certainly cracked and they should all be replaced (12) along the plastic spacers. (Another way you can test if you have leaking intake manifold gaskets is to put the car up on stands and get an IR thermometer gun. While dead cold fire it up and use the IR gun to read the exhaust port temps and see if any heat up slower then the others. If there is a marked difference on a port it probably has leaking intake gaskets.)
- Check that the idle position micro switch is working and being engaged while the car is at idle.
- Check all the hoses to see if they are flexible. If they are old they will be hardened and that leads to cracks. A lot of the hoses that are crimped to steep lines are insanely expensive but it is pretty easy to use a Dremel at a 45° angle to cut off the crimp sleeve without damaging the steel lines. There is one tapered hose section that can be replaced by forcing a smaller hose in one end to seal the taper. There is another 135° bend hose that you can get a replacement from Len (BoxsterGT) on this board. I do recommend getting Oetiker clamps and pliers to clamp the lines.
- The Idle Air Control valve can be cleaned and tested. Just search the board for the procedure.
- The DME can be opened and visually inspected with a 10x loupe or put into a know good working car to see if the issue follows the DME.
- You can search on the procedure to correctly set the idle but if there are other issues (such as any of the above) you won't be able to set it correctly.

BTW - To replace the fuel lines to the injector rails, go to a hydraulic hose shop and have them replace the rubber with something more permanent. (I recommend teflon lined, steel braided, with a rubber sheath to stop abrasion from the steel braid.)

[jlex]

Good, comprehensive write-up, Quicksilver...

[stlrj]

Originally Posted by thisisgoodnews

the car itself is a 3.2 911 Coupe, manufactured in 1985, first registered in 1986.

The base idle speed for this year is 800rpm. Unfortunately, your idle speed has been set too high as your tach is indicating, so your idle will pulsate as the computer is struggling but unable to get it down to 800. The solution is simply to turn down the idle bypass screw and help the engine computer do it's job.

Cheers,

Joe

[larrym]

i have a Word doc with links to about 100 Pelican threads on this subject

- PM me with your target email address if you want it

it's too long to post here


also see this
new ICV solves problems - http://forums.pelicanparts.com/porsche-911-technical-forum/996238-3-2-died-constantly-when-shifting-into-neutral.html