engine cuts out.

[Fl Ted]

I've just spent 4 hrs searching this forum and found
similar problems to mine , but none quite the same:
I just bought a 86 cab, 3.2. Runs great, then the engine cuts out intermittently, but keeps on going , bucking wildly, yanking me fore and aft in the seat like some teenager on crack jiving to some rap crap! Very embarassing in a covertible with the top down!
I'm sure it's fuel related, and I just replaced the DME/fuel pump relay, no joy
Here's the really strange part(s):
1. You have to drive it approx 10 mi before it acts up.
2. When it starts bucking, moderate gas pedal action, (more or less) won't stop the craziness, but STOMP on the gas (WOT) and she accelerates without any hesitation!! Back down on the gas, and the bucking is back.
3. Drove daughter to TIA yesterday, about 8 mi out of town, the thing starts cutting out, and I'm looking at going up the Sunshine Skyway Bridge It's still cutting out like mad, sometimes for a millisecond, sometimes for so long that I'm thinking that I'll have to pull over to the shoulder, but it never dies completely

So there we are, bucking along in 5TH gear, starting our ascent of the bridge thinking that if I can just make it to the top,, maybe we can coast the 25 mi to the airport, and my daughter wont miss her flight to N.O. and get fired for missing work.
Well, the stupid car goes up the bridge in 5th at 2k (lugging the engine somewhat) and never misses a beat!!!
Rest of trip to TIA uneventful.
Trip back home it runs perfectly UNTIL I hit city traffic some 50 miles from airport, then it's "ride m cowboy all the way home
I'ts not ignition, because if it were, the engine would run worse under load, and thats not the case!
I've seen on other threads that the o2 sensor or alarm system will shut down the fuel pump. I would like to bypass the alarm system as I dont have the key for it anyway, and there's a relay behind the dash on the drivers side that clicks madly whenever I open the drivers door, or if i'm just sitting in the drivers seat, and ignition is OFF. Alarm motion sensor??
Anyway, any help is appreciated. Sorry for the long story, just trying to pass on all the clues
Thanks,
Ted

[MysticAlpaca]

Alternator/Battery?? I had a similar problem but more severe where the car just died after about10 minutes of driving, then after it sat it drove fine again. It was real lurchy/luggy under low RPM's. Turned out the Alternator was bad and the battery to boot...I think the battery took the Alternator with it.

Fortunately i was only a block from Redmond European and limped it into their shop.

--Bll

[JTL]

I had similar problems last summer on my -84 targa. My problems were more severe and started with a surging rpm while driving. I found a bad fly wheel sensor. This can be easily tested by a voltmeter. I ended up needing AAA help because the motor finally cut out and would not start. Intermittent problems would of course be harder to diagnose.

jt

[kqw]

When it starts bucking...what happens if you depress the clutch?

Does the car rev. smoothly?

[Mookster]

The rev limiter relay. It's mounted behind the fuel gauge. Unhook it and see if your problem goes away.

Diagnosis cheap, replacement $200+.

[mede8er]

I also believe its a fuel prob...

I had the same symptom but mine was the fuel pump relay. Reseated it and no more bucking....

[derkpitt]

Don't forget to ck the DME relay. Some say to carry a spare.

Could it be a temperature related problem?
When was the CHT sense last replace? Has it been updated?

[Fl Ted]

Guys:
Thanks for the tips!

Derk; Dme/fuel pump relay was replaced yesterday, no change.

Mede8er, Same as above.

Mookster, JTL, MysticAlpaca, I will be checking your suggestions tonight/tomorrow.

kqw: Haven't tried that, will try next drive.

I'll post things as I find them, this seems to be a very common problem with lots of possible manifestations and solutions. Thats what happens when everthing is electronic and multple systems are interconnected.
Can accomplish amazing things, but is amazingly complex to troubleshoot.
Ted

[Early_S_Man]

Ted,

There is no rev-limiter relay on a '84 - '89 911!!! No need to go off on a rabbit trail looking for one!

Might be a good time to get a Bentley manual ...

[Fl Ted]

S-man: thanks for the tip!
Read on another thread that the alarm system will shut down fuel pump, and that relay was forward of the left gauges. Mine has two relays in that area, one is 10 amp, and other is 40amp. Anybody know what these two relays are for??
Also, I can't seem to locate the CHT probe or the crank sensor.
Early-S, why would I want a Bently manual?? I have a Porsche!! ;-)
Actually a friend has the factory manuals. I'll have to borrow it!
This electrical system is terrible !! Makes Lucas look good!!

[JeremyD]

Where in Florida are you Ted?

[Fl Ted]

Jeremy:
I'm in West Bradenton.

[kjb]

Originally posted by Fl Ted

This electrical system is terrible !! Makes Lucas look good!!

You need some documentation: either a Bentley manual or the factory service manuals. There are simple ways to test each sensor and at least deduce where the problem isn't coming from. You'd be surprised how far you can get with a DMM.

Our host also sells a Bosch fuel injection manual which explains Motronic in detail. It costs a lot less than the Bentley manual or factory service manuals.

Hang in there and keep the board posted on what you find.

/ J

[JeremyD]

Ah - know the area, Not many Carreras down that way that I know of - Nigel911 has an early 911 - 72 T that is having some engine work -

I have heard of a few folks that have had similiar symptoms have either a bad flywheel sensor or a bad cyl head temp sensor. my 02 cents

[JeremyD]

not very good pictures - here's the DME end of the flywheel sensor (part 18)



But since you said it's doing it after it warms up - my bet is cht sensor... (part #19)



This is assuming fuel system has already checked out - filters changed, etc.

[kqw]

Ted...

What happens when it starts bucking and you depress the clutch????

Can you rev. the engine freely or is there hesitation or missing????

[Mysterytrain]

I'm not sure how modern cars work..SC owner here...but since we all think this is a fuel issue you might want to look for the circuit that cuts off fuel in the event of an accident. On CIS cars there is a micro switch that is triggered when the airflow sensor plate is in the rest position...not sure how it works with your year car but I know it must be there. Just a thought.

[Fl Ted]

Just got back in operation yesterday after a week of searching for items that might be causing my engine to cut-out. Car was up on jack stands for shift rod seal and boot replacement. Also installed short-shift kit and all new bushings (Pelican brass in back U-joint). Found crank sensor (i think) up fwd of sheet metal in bell housing area, but did't do anything with it. Still cannot find cyl head temp probe!
Inspected DME connector for corroded contacts and found it clean.
Opened connector to check for broken/bad wires, all in good shape. Noticed corrosion around base of DME, so pulled it out and opened it up. Found botom board was very corroded on both sides, thought I had found the problem for sure!!
Cleaned it all up and reinstalled it.
Someone said there was an alarm relay in the instrument area, fwd of the fuel/oil quan gauge. I found two of them in that location, a 10 amp and a 40 amp. My owners manual shows only one, for the power windows, which i figured was the 10 amp one , so I pulled out the 40 amp one.
Down off the stands and out for a drive. The engine felt much smoother and more responsive, but it always runs good until it gets warmed up. After the 3/4 mark of the trip ( about 8 mi) I stopped for a red light. My driving up to that point had been very conservative due to traffic. I had a clear road when the light turned green, so I got on it hard up through 3 third gear, reving to 5K each gear. Ran fine. When I backed off, it started the same old s&it !! Cutting out randomly for the rest of the trip home.
KQW I tried what you said re: pushing in the clutch when it acts up. I did, and the engine revs smoothly and freely, no cutting out with no load on the engine!!
Now that sounds like ignition to me, coil or coil wire
I'm going to try another coil.
What say the rest of ye????
Ted

[kqw]

Originally posted by Fl Ted
I tried what you said re: pushing in the clutch when it acts up. I did, and the engine revs smoothly and freely, no cutting out with no load on the engine!!
Now that sounds like ignition to me, coil or coil wire
I'm going to try another coil.
What say the rest of ye????
Ted

When was the last time the clutch was serviced? i.e., pressure plate, throwout bearing, fly wheel etc????

[ChrisBennet]

My vote is for the CHT. Because it is intermittent, a DMM isn't going to help you. The CHT is the white connector on the left side of the motor. There should be a group of 3 connectors near the heater blower motor. The top one is CHT (usually).

The CHT should read approximately 3500 ohms cold, and approx 300 ohms hot. You can "fake out" the computer by substituting a resistor for the sensor.

Go to Radio Shack and get a 330 ohm 1/2 watt resistor (or a selection).

Once the motor is warmed up:
1. remove the CHT connector,
2. bend the ends of the resistor leads over on themselves
3. shove the ends of the resistor leads into the female connector for the CHT and use some masking tape to hold it in place.

Now you should be able to drive around as long as the motor is warm. If you still get bucking, it's not the head temp sensor.

If this doesn't make sense I can take pictures.
-Chris