Surging Idle

FA-18C · 02-09-2002, 07:00 PM

After getting my car running again it now runs rough at idle, which also surges between 1-2K continuously. Smooths out above 2K RPM, but seems rough above, obviously not running right. Had starting problems after replacing cap, rotor, points and pulling the fan shroud for cleaning. Have chased all sorts of grmelins and am frustrated since it was running fine before pulling the fan housing. Help would be greatly appreciated. Standard 70 E with MFI, not mods. thanks

FA-18C · 02-10-2002, 07:41 AM

Update - have isolated the surging to the speed switch attached to the No 1 MFI stack. After finally getting the dwell set at 38 and timing at 5 ATDC the car would surge between 1-2K continuously. Baffled I unscred the speed switch until it was no longer engaged and the idle settled out, but at 2K RPM. Drove the neighborhood and the motor seems to be pulling fine, just idling at 2K which is very annoying. Is this a problem with the fuel cutoff solenoid on the fuel pump and what can i do about that? Will pulling the therm stack and cleaning it have any effect? I am a bit intimidated by the MFI system and the $$'s associated with anything I mess up, any helps appreciated.

FA-18C · 02-10-2002, 11:14 AM

Part III - after jumping with aligator clips and running multiple combo's it appears that when the speed switch is depressed (at idle) I get a cycling voltage at the shutoff solenoid. The switch works fine, it almost seems as if the shutoff solenoid is dumping or shorting the circuit, goes from 0-14V. This appears to be what is causing the surge at idle from 1-2K. the check Measure Adjust calls for pulling the rubber boot and checking for rod actuator movement but is unclear as to what rubber boot. Is this the cover at the end of the solenoid cylinder where the spade connector attaches? Is it likely this switch is bad and where do i go from there?

dmj · dmj's Garage

Look for vacuum leaks.. I had the same problem, and it was caused by a few leaks.

Regards,
Daniel

------------
911 SC -80
Porsche Club Sweden

Dean71S · 02-10-2002, 01:35 PM

Hi,

I had similar problem with our MFI on our 71S. It turned out to be a loose wire at the rear fuse box for the power to the micro switch and fuel soleniod. Also could be worth checking the earth connections near the coil on the fan shroud.

Not sure if this could be your problem.

Regards

Dean
Australia

john walker's workshop · 02-10-2002, 01:50 PM

sounds like the idle speed is initially too high. turn all the air screws equal amounts clockwise, maybe as much as a couple turns each. the throttles may be open a bit also. did you mess with any of the stop screws?

FA-18C · 02-10-2002, 03:27 PM

Didn't mess with anything on the MFI, only changed out the cap, rotor, and points. AFter that I had a hell of a time starting again which was isolated to the batteries being shot (both 6 years old) and after a night of charging one at 10.8 one at 9.8 V. Don't have any vacuum lines to the dist. on my 70E and the thermostat line is in good shape. The desire for a better looking fan shroud started this mess! JW, thanks for the help. Is there some sort of capacitor or something in the fuel shutoff solenoid? The speed switch checks fine, but with the switch depressed (at idle condition) the voltage to the solenoid varies between 0 and 14V. If I disconnect the lead to the cutoff soleoid the fluctuating idle stops, but the idle is way up at 2-2100RPM. VERY bizarre. Am wondering if the solenoid is not maintaining the shutoff position but might be shorting the load fromt he speed switch...I am really hoping this doesnt mean a pump rebuild...

dmj · dmj's Garage

Don't you have any preassurized parts at all of the intake system on your -70?

A broken o-ring on my oil tank cap added 100rpm to the idle, a faulty vacuum hose another 200, and a cracked air intake gasket added the surge.. It runs great now!

/Daniel

FA-18C · 02-11-2002, 07:52 AM

OK, gotcha now. I guess I don't think its there since it only surges when the fuel cutoff solenoid is hooked up, when I pull the wire the surging stops and the idle jumps up to 2K...

jstobo · 02-11-2002, 08:46 AM

Chris,
With that high of an idle the surging is normal. You must lower the idle with the air screws on the throttle body as John Walker said. Here is how the circuit works. Above about 1800 rpm when you let off the accelerater, the microswitch is depressd sending current to the stop solennoid which engages and cuts off fuel. At 1300rpm the stop solenoid disengages and lets fuel flow again. This stops backfires when you decelerate. If your idle is above 1800rpm whenever the microswitch is depress it will surge between 1300 and 1800 rpm if all electronics are functioning, which it sounds as if they are. In fact, in the Check Measure Adjust book, the way to test the microswitch-stop solenoid circuit is to raise the rpm above 2000 the depress microswitch. If it surges between 1300 and 1800 all is well. Sounds like after your tune up you just need to readjust the idle with the air screws. Be sure to keep them balance by using a Unisync or other such air flow measureing device. Good luck.

jstobo · 02-11-2002, 08:57 AM

Chris,
As a follow up. Do Not repeat Do Not use the screw on the microswitch to try and adjust the idle. That will lead to just what is happening. You set that scew by turning it just past the point it engages the microswitch when the throttle is off. Its only purpose is to engage that microswitch at zero throttle. Screwing past that point will increase your idle but with the microswitch depressed at idle ( as it should be) it will lead to surging.

FA-18C · 02-11-2002, 11:32 AM

jstobo and JW, I was just reasoning through the same conclusion after JW mentioned the idle being too high. It now makes sense, should I leave the cutoff solenoid disconnected to adjust the idle and then reconnect? Havent messed with the throttle bodies at all so this will be a first, standby for mor panic'd Q's...again, to echo many otheres, thanks to all for helping..I am learning volumes and becoming more familiar and comfortable tweaking things...I think I can now gap points by sight to get the dweel nailed!

FA-18C · 02-11-2002, 02:38 PM

No huge suprise here that JW and jstobo were spot on. Cranking the air adj screws evenly at first and then using a unisyn to balance everything has solved the surge issue...thanks! In the process I have also discovered a couple of small exhaust leaks that could possibly be the casue of my backfiring issues...so looks like Ill be on the hust for a new muffler....as I have read seems that I need to stick to a stock type set-up, would like to also get something that will take SSI's for down the road..suggestions more than welcome, thanks again!