3.2 Hunting / Surging Idle Question
 

Hi folks,

I have just started experiencing a hunting / surging idle with my transplated 1987 3.2L engine.

I have done several searches but most of the time the issue seems to point to either the ICV or sometimes a cylinder head temp sensor. Today I swapped in a known good ICV and the car continued to idle eratically.

It seems just recently that the idle has gotten worse and at times will osccilate until the car actually stalls out. It also seems like the issue can happen when the car is cold or warm.

The other issue I found odd was that, when I took the oil cap off the idle steadies right out at 850ish. I would have thought this would have made the problem worse. Maybe someone can comment on why this is happening?

Also maybe someone else who has done a 2.7 to 3.2L transplant can comment if maybe I may have missed something in the vacuum line system that could cause the issue.

I do have one small braided hose from the front drivers fender (recirculation tank) that use to go to the charcoal box, but since I removed the charcoal box I normally just push it on the back of the throttle body where there appears to be an extra fitting about the same size. This seems to fall off alot, but even if I put it back on when the idle is hunting it has no effect.

I think that covers about everything I can recall about the issue.

Suggestions?

http://forums.pelicanparts.com/porsche-911-technical-forum/250438-idle-bounce.html

ICV usually seems to be the logical and reasonable place to start, but in actuality there are about 8 specific and solid possibilities that can cause it. Unfortunately you just have to start isolating and working away at them.
Can you describe your symptoms further with more detail?
And, here's a thread http://forums.pelicanparts.com/showthread.php?t=250438 , in it details how I fixed my particular issue with idle bounce.

IIRC, a "rich" condition in your air/fuel mixture will make your idle "hunt" as you've described. When you remove the oil cap, it creates a vacuum, which sounds as if you are "leaning out" a possibly rich mixture.

Although there are several conditions which will make the idle hunt in your 3.2, I think this may be your problem. But, you want to try the easy remedies first.

x2 on the rich mixture.

You will need a wide band oxygen sensor or CO tester.

The adjustment screw is in the front left corner of the AFM (underside). The screw as a 3mm hex socket and is hiding at the bottom of a well. Originally there was a plug but mostly these are long gone.

IIRC the adjustment is counter intuitive: CW rich, CCW lean.

There is an idle speed switch.

I would think the O2 sensor would keep it within appropriate CO, unless of course, you're not running one. It could also be so far out, the O2 can't compensate.

I am not running an O2 sensor.

If I adjust the mixture screw does that not alter the fuel map used by the computer?

Check that your idle microswitch engages properly. A miss-adjusted throttle linkage will cause the throttle to stay a little shy of its resting and not engage the microswitch properly. The DME needs this switch to be closed to go into idle mode.

You can try to pull the throttle all the way shut and see if that improves the idle while the engine is running.

Also try do do this with the engine off. You should hear a faint click when the switch closes.

The idle bypass screw is to bleed air into the engine. It allows the ICV to stay close to its center positon to maintain a stable idle. The procedure is to center the ICV by putting a little jumper into the diagnostics port (same as closing the WOT switch). This will make the DME move the ICV to its center. Then you adjust for proper CO value with that screw.

I don't think this idle bypass screw is your issue. It doesn't become unadjusted over time. And since you apear to run rich you either have a bad CHT or some other problem. Removing the oil cap just lets more air into the intake through the oil tank.

Ingo

OK, here goes...experts, please correct me if I'm off base.

If I understand correctly, the computer used by the 3.2 (unlike the newer generation ECU's) does not "adapt" or change it's mixture according to temperature, humidity or altitude changes. If the 3.2 mixture is affecting operation, 2 things will need to be checked and/or corrected:

1) check/adjust the air-fuel mixture, and
2) adjust the base idle.

This link should explain things a bit better for you:

http://forums.pelicanparts.com/porsche-911-technical-forum/250438-idle-bounce.html?highlight=idle+bump

good luck.

Hi Guys,

I appreciate your inputs.

I am quite certain the microswitch works properly as I hear it click when it closes.

I am going to work through the idle and mixture procedure that is listed in the threads above.

I do have on concern tha tI would like to address. I believe my alternator is on its last legs. It is fairly noisy and I think the bearings are worn. I have a rebuilt one just waiting to go in but I have been putting its installation off. Could this have an affect on a hunting idle that is temperature independant?

Thanks,

I don't think so. But if it's on it's last legs, why don't you just go ahead and replace it? Put your mind at ease.

Hi Folks,

I jumpered the B&C test connection and the car idles perfectly (no hunting) but at 790-800 RPM. My understnading for an 87 is that this is far to low. I want to increase the idle, but I can't figure out how to turn the idle nut. What size wrench or tool does everyone use. Is it simply the nut that turns or the post as well?

I'm not sure why no one ever mentions it but, you might want to put a vacuum pump/tester on your fuel pressure regulators. Dashpots are not known for being reliable and when these fail, not only do the regulators no longer reduce fuel pressure properly but they also pump more fuel dirrectly into the intake via the vacuum line.

It'll take you 5 minutes and if they don't hold a vacuum you very likely found your problem.

Jee, why does my car idle so rich!?

Ok so my idle set screw is very badly stripped, and it was quite dirty. I have hit it with a few sprays of WD40, but a 7 mm socket just spins on it. Even when I use vice grips I cant seem to get it to move without having the tool slip. Any thoughts on what I should do?

Ok now I'm really screwed. The 7mm nut was so badly stripped you couldn't even get a pair of vice grips on it. I tried to drill it out and broke and easy out and a drill bit in there, so now I have two peices of harden steel in the soft brass fitting. I honestly have no idea what I can do now to get it out. I have removed the idle set screw/fixture from another running car so I know what the entire peice looks like.

remove the AFM as a unit, and have a professional drill and re-tap if necessary. Replace damaged screw.

I think he is referring to the idle speed adjustment. This is the screw on the throttle body and is for idle speed. Still, take the throttle body out and take it to a machine shop. Let them do the repair. Worst case you need a new throttle body but I think a competent machine shop can fix that. You don't want to drill it out with the unit in the car anyway. Metal shavings are the last thing you want in your intake.

The idle mixture screw is a bypass allen screw on the AFM. On a 3.2 with the AFM upside down it is easy to get to. It is on the underside of the AFM on the boss on the right side. There is usually a small blue plastic cap covering access. When you tighten it the air bypassing the flapper is reduced. It makes the flapper open more and hence the DME introduces more fuel (richer). If you loosen it more air bypasses the flapper and less fuel is delivered by the DME.

Ingo

Ok the machine shop was able to remove the idle screw and I have one on order. Does anyone know what torque the throttle body (4 bolts) should be tighened to? THey were quite tight when I took them off and appeared to have a white colored loctite material in the threads.

I don't know if there is a specific torque specification for these. They are M6, correct. I would simply tighten them with reason. Use a small rachet and twist it with thumb and index finger - don't crank like crazy. And if you want you could use blue Locktide to secure them.

+1. There really shouldn't be any specific torque value for these things. Snug it down, don't bust your balls doing it, and you should be OK. Personally, I have a little bit of an issue with using blue loctite. But I've been wrong before!!

BTW Dennis, thanks for the nice IM...sorry I wasn't able to respond, my computer is giving my family the fits. I just re-installed a Norton Security program, and it's acting like Hal2000 :mad:

OK so I got the throttle body back from the shop and started reinstalling it. I then noticed that the metal air line in the picture below had broken in its rubber bushing side. I don't know if this was like this before I removed the throttle body or was caused when I was moving things around.

http://forums.pelicanparts.com/uploa...1247525285.jpg

I have temporarily replaced the rubber part of the hose with a short peice of plastic vacuum hose. Can someone comment on what this line is used for? It looks like it hooks up to the brake booster line which I have plugged. Does it go anywhere else?

Anyway, I put the throttle body back together and hooked up all the lines. I adjusted the idle screw to 880 rpm, but the car still seemed to occasionally surge up and down just at a higher idle now.

I then turned the mixture screw CCW while the car was surging. I turned it about 3/4 of the way around and it completly smoothed out the idle. The car now does not surge. Could it have been that the mixture was just set too rich, or am I just masking the problem and simulating a similar condition to when I remove the oil cap?

My engine is a euro version so it doesn't have the 02 sensor, so I am not certain what my fuel/air mixture is now. Can a shop do this by just putting a sensor up the exhaust passed the cat bypass?

I think you are fine now. The procedure is to adjust CO to 0.4 - 1.2 % at idle with the mixture screw for cars with no O2 sensor. It is measured before the catalytic converter. Since you have no O2 sensor is is critical to get this setting right for it to idle properly. I guees you could also use a wide-band O2 sensor to check A/F ratio directly. These just weren't available when Porsche wrote the procedures.

On my engine (3.6 but same principle) idle becomes really rough when I take the oil cap off and I even get a fault code (Oxygen regulation stop). That means the DME can not adjust the fuel mixture to make the O2 sensor happy.

The line is the vacum supply for the break booster - you should be fine.

Ingo

Check the vaccum lines behind your air meter.

Ok, so the car started hunting again even after adjusting the A/F mixture. So I am back to where I started. I am going to do my best to describe what I am seeing.

I started working through all the vacuum lines. As I mentioned previously the vacuum line that goes from the Brake Booster loop to the back of the MAF Elbow piece was cracked and so I tried to replace it with a piece of plastic vacuum hose and two hose clamps. I was worried that this wasn't making a good seal so I tried to cap both sides of it off not realizing that the booster line was hooked up to the intake and had a vacuum. When I capped both sides as in the picture the car runs absolutely perfectly! No hunting whatsoever.

http://forums.pelicanparts.com/uploa...1249246265.jpg

I tired to replace the vacuum line with a new hard vacuum line in the picture.
http://forums.pelicanparts.com/uploa...1249246716.jpg

The car went right back to hunting at idle.

I am starting to think that I am creating too much of a vacuum up near Airbox elbow area.

Keep in mind I have plugged off the brake booster line, as I don't have vacuum assisted brakes,

http://forums.pelicanparts.com/uploa...1249247050.jpg

I also have the hose that goes from the back of the throttle body to the air valve blocked off as well. From what I can tell on most 3.2 Carrera's this line hooks up to the air valve then to some valve which hooks up to the oil tank. I don't have the hook up on my oil tank for this hose.

Can anyone comment on how this vacuum system works and why removing the vacuum line from the back of the airbox elbow section completely makes the car run perfect. I would also like to reiterate that If the idle is hunting if I remove the oil cap the idle steadies right out. Does the air valve or vacuum limiter do this same task as I am doing manually?

Thoughts?

Thanks,

Let me try to interpret what you said; Removing the oil cap straigtens out your idle (while the break booster is hooked up) and capping off the brake booster vacuum port makes the idle steady, too.

When you cap the brake booster outlet most likely eliminate a vacuum leak in the break booster circuit. When you remove the oil filler cap you introduce a leak. Apparently both eliminating a leak and introducing a new leak straigthen out your idle. So my best guess is that you current adjustment is out of whack. Cap the brake booster, put the jumper into the diagnostics port to center the ICV and adjust your idle to be steady. That should give you a non-hunting condition. Then isolate the leak in the break booster circuit.

ingo

Dennis,

Did you check these for correct location(s)?

I encountered the very same issue a few years back.

It was a loosely fitted vaccum line.

All were replaced.

Gerry

http://forums.pelicanparts.com/uploa...1249341061.jpg

Ischmitz,

I believe you are interpreting me somewhat correctly. The only thing I am confused/somewhat disagree with you on, is when you mention " capping off the brake booster vacuum port makes the idle steady". What I would like to add to this statement to make sure we are both on the same page is that I always have the brake booster line capped off as seen in my 3rd picture, and if left like this the car will hunt. If I block off the brass hose vacuum line that you see in my 2nd picture (the one of the part) on both sides the car runs perfectly (the brake booster line is still blocked off). This line is drawing a vacuum from the intake. Does this make sense?

After looking at the vacuum hose routing diagram, I also believe I must have a leak in the brake booster loop between the venturi tube and the actual splitter in the engie bay that I have capped (shown in the third picture) When I am back at the car I will look for leaks there.

Ok,

So now I am really confused.

Today I removed the entire brake booster loop from the engine to inspect it for air leaks. It was a little dirty inside but other than that it cleaned up and all the rubber bits seemed fine. I also checked the vacuum lines from from the Y to the fuel regulator and damper, they seemed good as well.

I put the brake booster vacuum lines back on the car and reconnected the hose from the venturi to the airbox.

I then started playing around with the CO mixture on the AFM box. I was worried that it was far from factory set so I went on pelican to search the topic. I found in a post where Draco mentioned that the factory setting for the CO screw is to turn it in all the way (clockwise) and then back it out two turns. He also mentioned that you adjust it by backing it out another 0.5 to 1 turn, anymore and something else is out of wack.

Well first of all when I removed the AFM box I relized that my CO screw was about 10turns out. I went through the procedure of turning the screw clockwise to tighen it down until it seemed to bottom out. I then backed it out counterclockwise two turns.

I started the car and it hunted worse than ever before, swinging from 2k down to an almost stall condition over and over again. Taking the oil cap off barely changed anything. I tried to adjust the CO screw by turning it out 1 turn but it made no difference.

So now I am under the impression that the car is massively rich. Also when I took the oil cap off it sounded like there was a much louder hissing coming from the oil tank. Does that make sense? I have an older style oil cap that just does the half twist on. Is it possible that it is not making a seal?

What would cause my settings to be so out of whack? Where should I look next?

Also did I turn the CO screw the right way? The screw was very deeply set into the AFM when it bottomed out. I was scared to go to the other extreme and back it out all the way for fear that it would come out and let loose something inside the AFM box that I wouldn't be able to get to. Is this possible?

I don't have an O2 sensor as it is a euro engine, but is there some way to get a reading out of the AFM box itself, so I can try and adjust it?

Sorry for all the questions but I am really confused now as to where to go?

Put a hand vacuum pump on both of the fuel pressure regulators and see if one doesn't hold a vacuum.

sorry whats a hand vacuum pump? do you mean like a rubber bulb? would i be watching to see if it stayed sucked in?

Also i jsut read in a different post that some say the factory AFR mixture setting was 7-8 turns out. Can someone comment on which one is correct 2, or 7-8?

http://horizon-education.com/images/VACUUM_PUMP.jpg
Available pretty cheap at any local parts store. And yes, watch the gauge - a good regulator should be able to hold a vacuum more or less indefinitely.

ok after reading searching for the last few hours I would like to verify something:

The CO screw under the AFM, does this simply change the mixture of the idle or does it affect the mixture over the entire rpm range?

Also, I have seen a lot of variance as to how many turns out this screw should be. Does anyone actually have an idea of where a good starting point is?

My understanding is that the idle screw affects how much air is able to bypass the barn door. It thus affects the barn door position when the engine is at idle. The amount of air ingested into the engine at idle is NOT affected by that screw. The screw just influences how much of the air is metered by the door.

When you turn it all the way in you block the bypass passage around the barn door completely and the barn door will have to open a tad more during idle and the mixture will become richer. You can completely remove that screw but it is a little tricky. There is an O-ring on that screw and it makes you think that it is still engaged in its thread when you loosen it. Getting it out requires a strong magnet or some tapping.

So in other words the screw adjusts the zero offset of the barn door output signal. And this signal changes the mixture. The DME adjusts the fuel pulse length at idle according to this signal. Once the engine is off idle and the barn door opens more the screw effect is negligible.

On the 3.6 (same AFM) the starting point I heard is 4 turns out FWIR. I can see a situation where your AFM is worn around its idle position. The carbon tracks on the thick film resistor are worn through. That could cause poor idle since the idle AFM signal fluctuates when the wiper hits the worn spot.

Hi GUys,

I thought I would provide a quick update.

KTL was kind enough to send me a few of the adjustment microfiche slides form the factory manual. I was under the impression originally that an 87 3.2 idle speed was suppose to be set to 880. I had mine set at about 900. I have come to realize that this is incorrect for a Euro version of the 3.2. My engine spec is for 800rpm idle.

I went home and went through the bosch procedure for setting the CO mixture and idle again. As mentioned above I had the CO screw at 3 turns out, and it was hunting wildly. I backed it out two more turns (5 turns out) and the hunting got better. I then jumpered the test port and dropped my idle down to 800 rpm. The car idled perfectly.

I drove the car for a bit to let it warm up, and for some reason I thought it may still be a bit rich, so I turned it a quarter turn and adjusted the idle. It still ran ok, but it did do a minor hunt (2 small variations up and down) before settling in at 800. By this point the car was getting pretty warm from idling, which I believe the bosch procedure warns about. Why is that? Does the computer alter the mixture/idle if it idles to long? Anyway, I think I will just turn it back a little to 5 turns and readjust the next time I am at the car. I have also ordered a new oil cap and gasket as my old one was missing it and I believe this was causing a slight vacuum leak. The good news is the car no longer hunts wildly and is drivable again.

Thanks again for all the advice and help everyone provided me.

Update: While I really tweaked my idle adjustment and idle speed to try and eliminate my hunting it was still mildly there and dropping the idle speed made the car more susceptible to sub idle dipping which often caused the car to stall out when I clutched in.

OK so today I decided to completly swap out my entire throttle body and up, from a known good car to my car. However the hunting issue and sub dipping persisted.

I then swapped out the DME and guess what the hunting completly stopped and there was no more sub dipping of the rpms. It would catch at 1080 and then slowly control the idle descent.

I did some reading and noticed in a thread that Steve Wong had commented that on deceleration the DME is programmed to cut out the fuel until 1080 RPM and then slowly roll it down. I noticed on the good DME that the computer did this and on my DME the computer would let it drop out below idle which then initiated the hunting.

Since my engine was a 3.2 transplant, I decided to do a bit more reading. It now looks as though the DME that I have for my 1987 3.2L 930/20 engine is the wrong one. My engine serial number is 63J00096 which verifies it as a 1987, however my DME part number is 911.618.111.08, which appears to correspond with an 83-85 Euro engine.

So now on to my question:

Was there any major change between the Euro 83-85 DME to the 87 DME that may control that fuel cutoff? As I mentioned I put a good DME from a North American 88 3.2 in (DME #911.618.111.20 that had a SW Chip) and the engine worked better than it ever has through the full RPM range and no hunting at idle.

I have contacted the person I bought the engine off of in the hopes that I could swap DME's for the proper one which I believe should be 911.618.111.18. Is this the right part number?

If the seller is not willing to do this, is there anything I can do to the DME to change it over to the proper settings? I am a electrical engineer so I am not afraid of using a soldering iron and swapping parts if need be. Just need advice on whether this can be done or not, and if so what is required.

It's all in the chip/programming - there is no difference on the DME itself if you have the O2 sensor circuitry installed. Some earlier DME's come without the O2 sensor electronics. If that is the case you are out of luck and might be better off getting a replacement DME.

The later DME's had different idle code and different idle speed settings. You can convert your DME to the later style and that might require soldering. You need to expand the IC socket from 24 pins to 28 pins, disable the 8059 internal ROM by setting jumper S700 and then get a late stock EPROM. You want to get the 1-267-355-358 EPROM code revision with is a late US version.

Cheers,
Ingo

Hi Ingo,

I was hoping you would chime in. I just saw your post on your DME tester, quite impressive.

My engine doesn't have the O2 sensor so I are you saying I may need to just update the chip? Steve mentioned that may be the issue. The other possibility is the idle circuitry in the DME has malfunctioned. I am thinking I may want to update the chip. How difficult is it to add the 28 pin connector?

Dennis,

putting in 4 additional pins is pretty straight forward if you can solder. You don't want to de-solder the existing 24 pin socket. Just add 4 pin sockets.

There is no dedicated idle circuitry in the DME. All it does is sense the RPM and drive the ICV through a set of transistors. A simple test for the ICV circuit is to turn the igniton to ON (don't start the engine) and feel the ICV. It should vibrate with 80Hz and jerk ever so slightly once per second. If that is the case all is well inside you DME.

Then get the later chip, put a jumper wire into location S700 and you should be fine.

Ingo

I do plan on eventually buying a SW chip however in the meantime do you think I could just add the 4 pin socket and use a chip from a 1988 North American 3.2 engine in my 1987 3.2 Euro Engine (930/20). The reason I ask is I have this 1988 chip from my dads 911 when we added the SW chip. I know his would have the lamda data for the 02 sensor, but wouldn't my car just ignore those maps?

Also any suggestions on a part number/manufacturer for the 4 pin socket?

PS. The ICV and everthing was switched over, and both worked fine. The problem was completly isolated to the DME.

This is a part that I am using http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=ED56286-ND just cut off the rest to end up with 4 pins and make it fit. Be careful - the material is very brittle when you use dykes cutting it. Make a test cut and get a feel for it and use a file to machine it down to fit next to the existing socket.

The NA chip should work similar to how the other DME worked that you swapped in. The good news is that if it doesn't work you can go back to your chip without much trouble. So it is worht a try as far as I am concerned.

Ingo