Very Interesting Carrera running problem and the solution!

GH85Carrera · GH85Carrera's Garage

I have a test for the Pelican brain trust of Carrera strange running problems. This is no hypothetical test. It is what happened to me and my 1985 Carrera this weekend. There are lots of other possible solutions and I will only address what happened to me.

To set up the test and the reality of what happened to me, I will try to keep it short. The car was running at 100% normal all last week. I drove it every day. On Saturday morning after 10 minutes of running on a cool wet day as left a stop light, the car just died like I did not know how to drive a manual. Start up, and everything is normal. Drive for several short trips, off to eat lunch. As I pull into the parking lot it dies. After lunch I go for home and I have no problems. I figure it is the classic DME relay problem. Replace DME relay, drive around the block and all is well.

Sunday morning is Autocross day. As I head to the autocross site at 70 MPH on the interstate there is a momentary shut off of the engine, like someone turned off the ignition for a hundredth of a second. At the site as I am warming up the engine to get to 185 degrees the engine just dies several times. I make my run at full engine temp and the engine does that same quick off-on. I pack up and head for home. On the way home it does it twice more.

My first thought was the DME itself. I pull the DME and look for bad solder joints. No problems that I could see. I then call a friend that works at a shop so he shall remain nameless. He brings over a known good DME. No difference. We swap the air flow meter. No difference.

All of that points to the tach and speed sensors. The good news is that I have a set of new sensors in my garage but I have just not done the work to replace them. We did that and still no cure.

I will leave it there. What would you try next?

I will post the answer to what fixed my problem tomorrow. I want to see what you would suggest next.

McLovin · 10-12-2009, 07:21 PM

I'd use a multimeter to test the cylinder head temperature sensor. Check its resistance at cold, warm and hot to see if it is in spec.

jcunning · 10-12-2009, 07:27 PM

I would first check the battery terminals and all of the main ground points...
My next check would be the coil and fuel pump wiring.

DRACO A5OG · DRACO A5OG's Garage

Sounds like an electrical issue.

Bad Ground or Loose Battery Connection?

88-diamondblue · 10-12-2009, 09:44 PM

Another thing to check is to make sure all the plug wires are connected tightly on the spark plugs. That sound like what happened to me this last weekend. #6 wire had come loose at the track before and last weekend I had a miss which would cause the tach to drop off momentarily then everything was fine for short while, then another miss. It was the #6 wire loose.

GH85Carrera · GH85Carrera's Garage

I was sure it was a electrical issue, fuel issues usually don't cause the instant on/off running.

I replaced the head temp sensor a year ago with the two wire style. At that time I also replaced the 02 sensor. While the rubber grommet was off to replace the tach & speed sensors we checked that the head temp sensor was tight.

The battery is only 6 months old. The posts are solid and the connections are tight.

While the air flow meter was off we replaced the thermostat O-ring. That had nothing to do with the problem but it was leaking.

I will post the solution to my problem this afternoon.

MURF · 10-13-2009, 04:41 AM

loose intake bolts=air leak

dtw · 10-13-2009, 04:46 AM

After the stuff you looked at, other low-hanging fruit for testing are alarm, grounds, coil, and ignition switch.

techman1 · techman1's Garage

Coil wire loose? Electrical portion of ignition switch, lotta keys on keychain?

JeremyD · JeremyD's Garage

coil, loose ground, spark plug wires loose.

Talewinds · Talewinds's Garage

Marmots?

ClickClickBoom · ClickClickBoom's Garage

Primary/points wire!
eric

nhromyak · nhromyak's Garage

Overcharging alternator...
I think this killed my DME a few years ago. Same symptoms. Then it got worse.

MURF · 10-13-2009, 08:51 AM

The signs of overcharging are tach jumping and erratic and seat belt warning lite will come on intermitently

nhromyak · nhromyak's Garage

I didn't get those, but I lost a radio amp. Then I checked the volts - over 17.

A month later, weird engine running issues, then I finally replaced my DME after all sensor checks at the sensors, then sensor checks at the DME wiring and engine speed sensor replacements.

db_cooper · db_cooper's Garage

Ignition switch, electrical contact side.

GH85Carrera · GH85Carrera's Garage

The answer is......

The coil. All the connections were tight with just very light corrosion.

I have owned the car for 14 years. I know the coil is older than that. That is why I had a new replacement in my garage. In all the years I have driven I have never personally had a bad coil. I have seen reports on this board about the new Bosch coils made in Brazil are not as good as the German Bosch coils. All that Bosch sells is the made in Brazil versions. I may go shopping for a high quality replacement.

teenerted1 · 10-13-2009, 10:50 AM

yes a hot old coil could do that...my teener had that problem till a full tune up INCLUDING the coil.

kidrock · 10-13-2009, 11:35 AM

As long as we're on the subject of poorly-made Bazillion coils ('cause there's a bazillion of 'em)...

Let's solicit the experts on their choice(s) of well-made alternatives. Do any exist, or are we stuck with these POS?

wilke3169 · 10-13-2009, 12:06 PM

I would have won the prize. I know the SC is a different beast but my problems started much the same. Actually shut down several times during DEs. (There are several posts by me requesting help.) First I disconnected the rev limiter. No help. Then I replaced the electrical side of the ignition switch. Finally I saw in a random post that a coil that is on the way out would act like this. I replaced mine a few months ago and haven't had any trouble since.