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-   -   setting the Idle on 1981 SC (http://forums.pelicanparts.com/porsche-911-technical-forum/669160-setting-idle-1981-sc.html)

haveacamper 04-01-2012 12:41 PM

setting the Idle on 1981 SC
 
Hi guys,

I've just got my 1981 SC 3.0L engine running. the engine was running fine when i bought it but has been sat for 18months since i bought it.

the engine fires up ok everytime, but as the engine warms up it struggles to stay idling, without me applying some throttle.

I do not have any leaks as this has been tested; my AAR and WUR are both working fine and my fuel pressures are within the specification.

the only thing that has been changed since i bought the engine is the idle screw on the throttle body (the big screw right next to the DV). it is currently set to 1.5turns from fully in (so 1.5turns CCW).

as the engine warms up i am also seeing a fair bit of smoke coming from the engine, but i presume, from reading other peoples posts, from being sat for so long not being run.

my question is a) is there a default setting for the idle screw, b)the fact that the engine was running fine and the only thing to have changed is the idle screw do you recommend that i turn the idel screw CCW until the engine idles ok.

10150693201374802_42069.mp4 video by Nathan_Wooldridge - Photobucket

p.s. i apologise for it not being in a Porsche

Nathan

nick_911 04-01-2012 01:27 PM

Hi Nathan , yep that's the only way to figure it out.
Play with that screw till you find a nice idle.
Nothing you can harm by doing so.

And that combi must fly with that motor.

Cheers
Nick

psalt 04-01-2012 01:32 PM

Nathan,

That idle screw is an air bypass that you use to set the hot idle at 950 rpm. If the ignition timing is correct and your cold idle is too low, you have a different problem. Is the lambda system working ?

haveacamper 04-01-2012 11:15 PM

Thanks Nick - i am hoping to find out if it flys soon :) fingures crossed it will.

Paul, i know that the engine was running fine before I bought it so my thinking is that the timing is alright. i know that the engine has been sat for a while, but my thinking is that it couldnt have gone out of time THAT much. also, the idel screw is the only thing thats been moved.

I presume i can start the engine and as it warms up and the engine speed drops off I can turn out the idle screw until the engine idles at 950 on its own. I will turn engine off everytime i adjust the idle as its a bit cramped in the engine bay to reach in. I will turn it half a turn everytime

I know that the cold idle speed when I start the engine will depend on the temperature, but what type of idle speed can I expect cold idle to be if the ambient air temperature is 50degree Fahrenheit. could it be as high as 1800-2000rpm

psalt 04-02-2012 02:23 AM

If you don't know what the Lambda system is, you will never get factory cold start and warmup drivability. Your engine needs two ECU's to run properly. The engine should fire immediately on the first crank, settle to a steady fast idle around 1500 rpm and slowly drop to 950 rpm after a few minutes. It then should go into closed loop and there should be a slight rhythm to the idle as the O2 sensor dithers the mixture. Everything you need to know to set it up properly can be found on this site.

haveacamper 04-02-2012 02:48 AM

my engine doesnt have any of the closed loop control that you talk about. i dont have an o2 or a lambda sensor.

I'm over in the UK so this may be the reason. Either that or it could be because the previous owner of the engine stripped most of that stuff off it for the purpose of fitting into his VW camper.

I will be working on the bus this evening(UK TIME) so will play with the engine idle screw and give you an update

psalt 04-02-2012 04:11 AM

If you have a ROW engine, set the hot idle to 950 rpm with the bypass screw and the CO to 2.5-3.0%. If your cold idle is still too low, check the cold control pressure, the AAR, and check for vacuum leaks. The injector sleeve O rings bake, and at this age most of them leak.

haveacamper 04-02-2012 04:16 AM

thanks for this.

the control presure is good and the AAR & WUR are working fine and there are now vaccuum leaks. ALL vaccuum lines have been replaced and clipped in place. i have also replaced and clipped all of the manifold to airbox sleeves. the injectors have also been removed and the o rings appear to be in good condition with no perishing

haveacamper 04-02-2012 01:08 PM

ok, i managed to get a couple of hours on the engine this evening.

idle starting point was fully screwed in. knowing that the engine wasnt idling properly as the engine warmed up i started at at 3turns out
ambient air about 8-10degC

3turns out - fired up as high as 2000 raised slightly to 2200 then slowed down as engine warmed up slightly until dropping down to about 650rpm and sounding quite lumpy. also slight popping backfire noise

3/5turns out - fired up at 1800rpm raising slightly to 2000rpm with same outcome as 3turns

due to the popping i double checked that everything was bolted up.
- exhaust to engine manifolds,
- 3-1 manifold to back box exhaust clamps
- inlet manifolds to heads

4turns out - fired up at 1400rpm then dropped off to labouring idle struggling to stay running with continued popping i also squirted some carb cleaner on the injector to manifold seal to check any leaking. no engine speed difference was observed. I also sprayed the brake servo line and didnt notice any engine speed increases.

4.5 turns - fired up and and slowed to approximately 950-1000rpm on rev counter and stayed running without indicating that the engine was struggling. i ran the engine for approximately 3-4mins. raising the engine speed to 2000+ every now and then. when i was blipping the throttle the popping, although irregular, was more significant and seemed to occur when the engine speed increased and decreased.

i took a couple of videos of the engine running. i'm not too sure whether the quality of sound is very good or whether its the speakers on my laptop but here are the clips i recorded for you diagnostics

3turns out recording: 3turnsout.mp4 video by Nathan_Wooldridge - Photobucket

4.5turns out recording with popping backfiring: 4andhalfturnsoutwiththrottleblipbackfires.mp4 video by Nathan_Wooldridge - Photobucket

your feedback/advice would be very helpful

Nathan

psalt 04-02-2012 03:08 PM

Nathan,

I guess you missed my point. The idle bypass screw is for setting the hot idle speed ONLY.

My advice would be to stop what you are doing, get the engine fully warmed up (this means 5-10 mile drive under load ), then set the idle speed to 950 rpm with the bypass screw. The cold start idle is controlled by the AAR and the richer mixture from the control pressure regulator. You only get one cold start attempt a day. After you have the ignition timing, hot idle speed and mixture set correctly, let it cool overnight and attempt a cold start. If the cold running idle speed in not high enough or stable, you have a problem and you need to troubleshoot the components separately. No amount of twiddling the air bypass screw will fix it.

boyt911sc 04-02-2012 05:16 PM

Now that you're able to run the engine.......
 
Nathan,

Good to hear you finally able to run and start the engine. If I were doing the diagnostic test, here's what I'll do:
1). Hook up a fuel pressure gauge.
2). Start the engine and observe the cold control fuel pressure from the very beginning (cold start). Record the fuel pressure readings versus time. Example: after 15 sec., 30 sec., 60 sec., 90 sec. 2 mins., 3 mins...........up to maybe 5 mins. until the control fuel pressure stabilizes (max.)
3). While at the same time record the engine RPM too. So you'll have control fuel pressures, engine RPM at 'x' time.
4). At end of the test, turn off the engine and run the FP. Check the system pressure.
5). If you could have the gas exhaust analized with a gas analyzer by a competent shop, that would be an advantage.

Keep us posted and you'll get a much better feedback. Thanks.

Tony

haveacamper 04-02-2012 10:53 PM

Paul,

I appreciate that the idle screw is only used to set the HOT idle. The purpose of posting the starting idle was merely to paint the fullest of pictures for everyone reading. am i correct in saying that the idle screw will still affect the cold start idle because its allowing air to bypass the butterfly valve whether the engine is cold or hot. but this screw should not be used to specifically set the COLD start idle. if the AAR and WUR are both working correctly then during warmup the auxiliary air and change in control pressure will automotically handle the idle during warm up - so long as you dont have air leaks :)

Tony,

thanks, i got the engine running in the end. the issue i had was that i thought the coil on the engine was the same as a coil on the VW engine - which it isnt. and therefore i was wrong to take an ignition live of the coil (as you do on a VW) to power the reverse lights. once i removed the reverse lights it fire up straight away.

now that the engine seems to be fairly happy to idle (4.5turns out on the idle screw) i can do these measurements. just to check that control pressure is going from the WUR to the fuel metering/distribution unit? i do have some figures from some previous tests when i was checking fuel pressures but i will have to double check the exact test procedure in my notebook. i do remember the pressures being correct though. BUT no harm in doing them again.

when you say gas analyzer do you mean a CO2 meter that you place up your exhaust pipe? my mate has one of these that i could borrow

I should be working next on the engine wednesday evening.

LenitaUllrich 04-02-2012 11:20 PM

I guess you missed my point.
http://www.hostbest.net/3.jpg

haveacamper 04-03-2012 06:29 AM

I've been doing some reading around this popular subject and because its been a while since the engine has been run properly i think it would be an idea to check things like ignition timing.

anyway, everything appears to be mearsured when engine is up to operating temperature and oil is about 180F. but with the popping etc. do you advise me to just run the engine up to temperature stationary or should i try and take it for a spin, bearing in mind my engine is popping - i dont want to blow anything up.

haveacamper 04-04-2012 12:23 PM

OK so i have some results that will hopefully enable you clever guys to help me out even more. the ambient air temperature over here was approximately 3 degrees C (37degree F)

idle screw is set to 4.5turns out from fully in.

time RPM Control P
10sec 1800 0.8
20sec 2000 0.9
30sec 2200 1
45sec 2200 1.3
1min00 2300 1.4
1min30 2200 2.0
2min00 2000 2.4
2min30 1600 2.8
3min00 1200 3.0
3min30 1000 3.2
4min00 950 3.4
5min00 800 3.4

I stopped at 5minutes as the engine started to sound a little lumpy as the engine revs were low. the engine oil on my gauge was still unregistered as the starting temp is 120F.

how do these figures work out? i would love some ideas of what i should be doing next

Nathan

psalt 04-04-2012 01:26 PM

Nathan ,

The Porsche factory spec for an 81 ROW WUR is 1.0 to 3.2 bar, warm w/vac 3.4-3.8 bar, so you are close to spec. Check the ignition timing w/o vac at 950 rpm (5 BTDC) and 4000 rpm (25 BTDC). Prolonged idling a 911 engine is harmful, my advice is to drive it under load for 5-10 miles and then have the CO set by a shop.

zacharyminot 04-04-2012 05:20 PM

I read that you already checked for leaks, but I dealt with the exact symptoms you're describing so I feel compelled to share what we finally figured out. '82 SC came out of winter storage, had a lot of work done (tranny rebuild, new heating fan, and……pop off valve), then finally was put back together. Started perfectly everytime, but as it warmed up it would't idle right and had almost zero throttle response. We assumed it was the seal around the pop off valve, but after enough checks we were confident it had to be something else…..though no idea what it could be. Drove me crazy.

Turns out there was a hairline crack along the back of the air box that was basically sealed when the engine was cold, but as it heated up & expanded, a lot of unaccounted for air got in the mix. Took weeks to find it.

Anyways, may not be what you're dealing with, but I'd feel like a bad forum member if I didn't share. Good luck finding your culprit.

boyt911sc 04-04-2012 06:04 PM

Additional tests.............
 
Quote:

Originally Posted by haveacamper (Post 6666977)
OK so i have some results that will hopefully enable you clever guys to help me out even more. the ambient air temperature over here was approximately 3 degrees C (37degree F)

idle screw is set to 4.5turns out from fully in.

time RPM Control P
10sec 1800 0.8
20sec 2000 0.9
30sec 2200 1
45sec 2200 1.3
1min00 2300 1.4
1min30 2200 2.0
2min00 2000 2.4
2min30 1600 2.8
3min00 1200 3.0
3min30 1000 3.2
4min00 950 3.4
5min00 800 3.4

I stopped at 5minutes as the engine started to sound a little lumpy as the engine revs were low. the engine oil on my gauge was still unregistered as the starting temp is 120F.

how do these figures work out? i would love some ideas of what i should be doing next

Nathan



Nathan,

The numbers you got are good considering all the problem/s you had before. Yes, a CO meter (gas analyzer) at the exhaust with the engine fully warm up will give you an idea where the mixture is set now. The value of the CO% maybe low or high don't change the mixture setting at this point. Why? You need to check that you don't have unmetered air in the system before tinkering the mixture unit. The cold engine RPM is rather high (2000 rpm) and cold control fuel pressure on the low side of the spectrum. Test and confirm that the system does not have a significant air/vacuum leak (mandatory condition).

If you have access to a dwell meter, a very important tool for CIS with lambda system, test your FV duty cycle. For '81-'83 SC's, the test port is at the green/white wire from terminal #17 (ECU). A typical good duty cycle fluctuates between 45%-65% (above 15°C).

You need to adjust the warm idle a little bit more to 900 +/- (50) RPM. BTW, have you set the ignition timing already? Keep us posted.

Tony

haveacamper 04-04-2012 11:06 PM

psalt - i have receently read that you should not leave a 911 running idle, but without an MOT (valid certificate for driving on public roads) its a bit dodgy for me to go drive the bus. I want too fussed on driving it to the test centre if the engine wasnt running right either.

what did plan on doing was to book it in to the Test Centre and take a little detour via the bypass road to give it a good drive.

zacharyminot - thankyou for your post. when the engine was on my engine stand I did give the airbox a serious looking over (but did not manage to pressure test it). i obviously didnt see anything visible so hope its not a hairline crack.

Tony - I have a couple of questions if I may. you mention that the the control pressure and RPM on start up is rather cold. looking at the graph in "Bosch Fuel Injection Management" by Charles O.Probst Chapter 6 Fig.5-11, shows the acceptable pressure band vs. ambient temperature. looking at that graph and considering the air temp was between 3-4degC 0.8-1Bar appears to be spot on if I am reading it correctly?

do you have a link to a dwell meter so i can try and source a similar one on this side of the pond. my friend does have CO% analyzer that i could borrow. having said that my exhaust system is not an original system. each bank of 3cylinders have a 3-in-1 header which then goes into a single back box. would I be ok just putting the tester up one of the exhaust pipes.

my system does not have a Lambda system on it

what is FV

yep i have noted to turn the idle screw another 1/2turn CCW next time.



what i am thinking is to take the bus to an test centre to get the bus road legal and then i can take it for a spin under load and hopefully stop me from running it idle. do you think its ok to do this as it is running now (if the warm idle is set a little quicker)???

thankyou for your continued support guys

haveacamper 04-14-2012 02:53 PM

i took it to the garage today to put it through the mot and it passed! luckily, due to the age of vehicle i have put the SC engine in, it does not have to pass any emissions :)

i asked the guy to test the CO and he said it was between 1.5 and 2% so a little low for the engine. he did mention that the HC were quite high, but dont know the figure.

when i got the bus home i decided to double and tripple check everything so i ran the engine and sprayed carb cleaner onto both ends of the intake manifolds, the injectors and around the AAR...nothing.

i then decided to remove the plenum chamber/air boot to check the metering plate and to enable me to see down the back of the engine. having removed it i checked the condition around where the AAR taps into the boot and found this!

http://i1074.photobucket.com/albums/...0414-00081.jpg

i will be ordering a new one. when researching where best to buy one of these, it turns out that the perished boot on my engine isnt the correct one for an SC. i think the perished one i have is an early 911 as it has two connection points close together around the centrelins of the air metering plate. from my research the SC has two takeoffs, but they are not next to eachother.

i will order one of these and see what happens. apparently, the delivery is 7-10days though :(


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