need help debugging total instantaneous cutouts while driving
 

Hi, I have a 1970 911S with MFI (of course). I've had some problems that I thought were related to a loose distributor cap (the clips were too slack so I bent them to provide more pressure). The symptom was I would be driving along, engine warmed up, everything sounding good and... boom, the engine completely cuts out. All other electrical stuff works. In trying to start, it cranks over but doesn't startup. I then pop the hood and jiggle spark plug wires around. Since I'm alone, I can't test for a spark. Anyway, after a few attempts of going back and forth it starts up and runs fine. I've also had it cut out and fire back up immediately before I could get the clutch in to coast.

Yesterday, I replaced the cap, bent those clips so it stays on firmly and went to work. Made it to work (18 miles) with no issues so I thought I was golden. However, on the way home, the engine cut out again, but this time it fired back up before I got the clutch in to coast and I heard a backfire. (I think that might mean that I was getting fuel during the cutout and that it is probably electrical?)

So it didn't appear to be that loose cap after all. Anyone have any ideas or a proceedure to diagnose? Could it be a faulty distributor?

thanks,
Michael

I have a sc and had simular problems,. i dont know if you have a cdi box but the connetor going in was loose its a six pin and i had to tighten and clean each one, my car is fine know, also my lights would flicker but they dfont any longer, just thought i would post, Kevin

My car had the same symptoms. Ended up being the CDI box and coil. Coil went bad and fried my CDS box. Replaced both and it's been smooth sailing ever since. Check the bottom of your coil for fluid leakage. If it's leaking replace it.

Michael,

How frequent are these cutouts? Are they complete loss of power, or just a stumble/surge of a second or two duration? Is high ambient temperature, say above 85°F, a factor in the cutouts?

It would be helpful to know what the system Voltage was doing at the time of these cutouts! Sears or Radio Shack (and other sources, too) have digital multimeters with data logging software to PC's over a serial cable. That would be a useful tool for diagnosing this problem, IMHO. Either high or low Voltage can cause loss of CDI-output.

If corrosion on the CDI-unit plug wiring is a factor, you might consider installing a ground strap from under the lower right Allen bolt directly to the fender sheet metal ground location for the relay panel.

If temps above 85°F are always present when the cutouts occur, be sure to check all of your spark plug wires for end-to-end continuity ... all should read in the 3000 - 4000 Ohm range, and any opens are indicative of a bad Beru connector.

Yes, the distributor could be a factor if the centrifugal weights are rusted or sticking. Check the advance curve from idle to 4000 rpm by 500 rpm increments and then 5000 rpm and 6000 rpm. My suggestion is to remove and clean the mechanism by doing a partial stripdown ... per the following thread:

http://forums.pelicanparts.com/porsche-911-technical-forum/5432-distributor-lube.html

Answers for Early_S_Man!
 

The cut outs started 2 weeks ago on the way home from work I completely stalled (>85 degrees outside). I jiggled some wires several times and about 5 minutes elapsed before it started up. No more problems on the way home. Several days later the same thing happened on a hot day. Complete stall. Again I took off the dizzycap jiggled some wires and it finally started (after about 5 minutes). This time I had a stumble about a minute after I got started but did not stall - I saw te tack drop to "0". It did this 1 more time about 10 mnutes later. I had 1 more complete stall on a hot afternoon again. This week I've only had stumbles, they were in the evening but I don't think it was that hot out. I can't recall ever having the problem in the morning drive in to work.

I will definately check the plug wires (how does high heat affect this?). Hopefully I can get back from a weekend trip early enough to check the advance curve - I've had that on my list to do for several weeks!

I should also mention that the car idles nicely when just warming up but idles much too high at temperature (1600 RPM or so). The other day it rained like he!! in Boston and I drove about 30 miles in the downpour and the car ran like new! I was majorly surprised. The idle was down to 800, accleration was smooth and responsive and now drop outs or stalls (thank goodness!). I don't know how to interpret that.

Again thanks for the suggestions!

Michael

Hey Michael,
Haven't heard from you lately, sorry to hear that your car is giving you grief... It sounds like an intermittent ground might be the fault, and that could be any of the major grounding points.

If it started recently, you might think back over work recently done, and inspect those areas for any loose connections, etc.

There is also an over rev ignition cutout built into the disty rotor right? Not sure what fail modes this little gem might have.. Maybe some of the experts could comment here..:cool:

Good Luck,

another possibitlity...COIL....
If you just sit by the side of the road...and the car will start after sitting for 10-20 mins, perhaps you have a bad coil.
Nice and cheap to fix..just replace (mine cost about $25)
Hope this helps
Bob

IMO, Warren is right on 110%..after doing what he suggests/or before listen closely to exhaust pulse/tone...put the ear close to exhaust pipe...if you have the "feeling" on what it sounds like w/in good running condition a miniscule difference in pulse may be detected if a weak coil/CDI .....................Ron

When mine did that, it was the Permatune CDI going out.

An easy way to check for spark if you are by yourself is with an inductive timing light. With it hooked up, the cord is usually long enough to reach the key.

Tom

thanks! and some more questions!
 

Thanks all for the suggestions. I spent some time on Sunday and removed the coil and cleaned all the contacts. Also cleaned the fuses/contacts in the little 3-fuse fuse box on the left wall of the engine compartment - they were badly corroded and tarnished. I also pulled every other connector and did my best to clean them.

So, I've put 75 miles on since doing these things and have not had 1 stall (I would expected to have had at least 2 in this time). However, the air temp has only been 75-80 so it might still raise its ugly head this evening on the way home from work.

Now to the questions:

1) I am trying to profile the distributor per the thread Warren linked to (thanks, great reference!). Several issues I have are that my warmed-up idle speed is 1600RPM so doing the low end is impossible. Also, what proceedure do folks use for doing this?! I can't figure out how to hold the RPMs steady at a target (like 3000RPM) while simultaneously measuring the advance with a timing light (especially 1 with an advance dial). Is this a 2 person job?

2) I tied to lower my idle speed by balancing and adjusting the air adjustment screws for each of the throttles. I've read the Porsche manuals, Haynes and a few other sources that indicated 2.5 turns open is a reasonable starting place. Mine were 2-3 turns and after I averaged the air flow and adjusted them they were still within this range but the idle speed is still around 1600. If I go in and systematically screw in each 1/2 turn I lower the idle speed but not by much. At 1 turn in the idle speed is still high 1400RM and I get a lot of caughing (rich mixture?). What should I look for to determine why my idle is so high?

thanks again,

Michael

Michael,

Good news on the electrical system connections cleaning!

To check the advance curve while watching the dwell tach and adjusting the throttle and checking the advance setting on the timing light and where the pulley notch is lining up is a trial-and-error process, at best, by yourself! Back off throttle, readjust timing advance adjustment on the timing light, back to specific rpm with the throttle lever, etc.

It sounds like a Gunston Gas Tester is the next piece of test equipment you need!

Have you found, downloaded, and read the MFI documents at Pelican? A few hours work with the computer and printer ... and a week or so of reading and rereading may be in order to attack the idle speed problem! It doesn't sound like you are too far away from having the car in great shape!

http://www.pelicanparts.com/techarticles/911_MFI/TipMFI.htm

BTW ... the miss at higher-than-85°F problem is due to a slightly diminished output (perfectly normal -- a result of higher Voltage drops by silicon semiconductors at elevated underhood temps) by the CDI-system at elevated temps, compared to room temperature ... and inability for the spark to jump across an open connection inside a BERU connector on the stock spark plug wires. One, two, or three bad BERU connectors have been encountered simultaneously in an older wire set, an the miss/surge and dramatic power loss is mind-boggling if you don't know what is happening ... especially in a traffic jam at 5:30 PM amongst several thousand ill-tempered commuters!

All of the above advice sounds good to me...Warren, as usual, excellent! Just a funny aside...I had the same problem some years back. The problem kept appearing, even after going to a trusted shop where everything worked/checked fine. Then, even though the rotor looked fine, I changed the rotor. More out of just wanting to do something than actually thinking it would help. The problem hasn't returned since. Warren? Any ideas why or if a rotor could cause these symptoms? Was my experience just a coincidence?
Ron, are you back in LB now, or did you make the post somewhere along the way? Good to see your words! :)

Paul,

Because of the rev-limiting rotor shape (essentially round) ... some people have difficulty aligning and installing them. I have seen cracked rotors that were forced into place, misaligned, with a hammer or mallet, and because there is a resistance element that can be cracked by the use of excessive force ... a convenient path to the distributor shaft ground can be found by the spark ... if the crack happens in just the 'right' place! Another good reason to have a spare rotor in your glove box at all times!

oh well, still have the problem
 

On the way home tonight it was actually cool (low 70s) and about 1/2 way (8 miles) to home, I had a stall and when it fired back up a second later - KABLOOM, a huge backfire (I had this happen on Friday too). So, it doesn't appear to be temperature related (unless sitting outside in the hot sun all day warmed everything up - it did get to 85 and only cooled off in the last hr before I left work.

Anyway, I'm going to dig around a bit more - I have replaced the cap and rotor (it is the centrifugal cutout type - and I'm sure it is seated properly) but I haven't replaced the points yet.

I do have 1 tangental question - there is a small 8" long by abot 1/2" diameter braided hose coming out of the middle of the bottom of the air filter housing. The other end is not attached to anything and I can't find this in any photos or description anywhere. Does anyone know where it goes?

Thanks,
Michael

My guess is still the CDI box, if I were you I would try to get my hands on other working CDI box and drive with that. Ooor, start the car (if it starts) and the give these little sharp hits on the side of the CDI box. It won't hurt the box if its in ok condition, but if the fault is in it, this shows you where the flaw is.

The small tube is propably for the surplus oil that comes from the oil breather tubing. On my -72T the small tube that comes out from the airfilter was attached to this metal tubing and from there it went straight on the ground.

My story with that kinda cutouts: My car ran just fine and suddenly without warning it just stalled (regardless of speed) and it sometimes fired right back up with this shotgun bang.

-> Out of the car, open the engine lid, give the CDi a little whack (sometimes my knuckles were all red from hitting it) and try to start the car.

I actually had to rewire the CDI box to the co-driver seat that this way I could hear when the CDI box was gonna go out (it started to whine irregularly) and the I had to whack it. This way I got home from a trip of 200km, even though the box turned off the engine like every 5-10 kilometers.

great story Zendalar! and more info on my problem
 

Thanks for the info and what a great story about wiring the CDI to move it to the passenger seat Zendalar! Desparate times call for desparate measures!

Other than the high idle, my drive in t owork this morning was uneventful and great on those backcountry New England roads! I'm starting to think that maybe my driving pattern is affecting it. In the morning I'm relaxed and just enjoying the fresh air and scenery, in the evening going home I'm typically a little more aggessive, rev higher and enjoy the sound of the engine. Maybe that coupled with being warmer causes some component (CDI?) to fail.

I took a look at my points and distributor last night and although i didn't measure the dwell, the points looked good and the inside of the dizzy was shiny and clean. I popped out the felt plug and oiled the advance and from what I could see of it, it looked clean.

Warren, I've had my eyes on a Gunston Gas Tester for a while now and after reading some posts on them here i've bit the bullet and ordered the model with the dwell and rpm meters (I didn't have a dwell meter either). Should be in this week so I can start to get a better handle on things.

As for that little tube Zendalar, I do see a short length of stainless tubing coming out of the firewall (just a bit up from the bottom of the pan) that the hose could both reach and fit. That's probably it. I'll check where it leads out n the undersidebut it probably is a drain of some sort. Thanks!

Anyone in the Boston/Nasua area reading this that has a CDI that I could borrow temporarily to diagnose my CDI? Is there a definitive test for these?

thanks again,
Michael

Big test tomorrow
 

Ok, after much diagnosing, reading past posts, etc I decided to swap out my coil (blue Bosch) and CDI (Permatune - many, many strange and intermittent problems reported with this unit) for stock Bosch CDI and coil. PartsHeaven had a good used Bosch CDI and I bought a new Bosch coil to go with. AS I was installing, I cleaned every connector I could reach. The car fired right up and sounded about the same as with the Permatune. Tomorrow I am taking it out for a long drive and see what happens.

The problem most certainly seems to be more prevalent at higher ambient temperatures. It always stalls if the temp is >85 shortly after getting up to operating temps. On cooler days it may or may not happen. On rainy days (like today) it doesn't happen at all.

On the funny side, when I was coming home on Wednesday night I took a long detour to enjoy the day (hot) and just as I was going past a crowded little league field, the car stalled and backfired twice quickly. Must have scared the he!! out of the players! Hope I didn't wreck any games!

So, wish me luck tomorrow!

Michael

rats, rats and more rats!
 

Well, the stock Bosch CDi and new coil are even worse. The symptoms are a bit different - rather than completely stalling out for seconds (or sometimes minutes) it "flickered" out briefly but many times in a row. It even started doing this as soon as I started driving before the car was warmed up. I ended up putting the Permatune and blue Bosch coil back in and the situation has reverted back to complete stalls.

Back to the drawing board.

Michael,

That suggests a grounding and power supply problem to me!

I suggest running a new ground wire directly to the coil clamp bolt, and new wires to temporarily replace the 3-pin harness connected to the CDI-unit. All of the new/temporary wires (which have a way of becoming PERMANENT) be 14 ga, and color-coded as follows:

Ground -- Brown

'B' Terminal to 1st fuse junction, but not a fused supply, +14 Volts -- Red

'A' Terminal to coil -- Blue

'C' Terminal to distributor -- Black

Special note -- be sure to clean the three A-B-C terminals on both Bosch and Permatune unit with vinegar applied with Q-tip for five minutes, followed by rinse with isopropyl alcohol! Corrosion present on those terminals could be the source of your problem!

thanks & will do!
 

Thanks Warren! I agree that it sounds like a supply/ground problem too. Are you suggesting I remove the wires from the 3 pin harness' terminal connector (at the CDI end) and replace them with the new wires? If not, how do I connect the wires to the connector on the CDI?

thanks,
Michael