Tricky '86 Ignition or Fuel Problem? How to find out which one.

[triwave]

Hi,

I've been trying to solve an irritating condition on my '86 3.2 Carrera for a while now with lots of $$ and time, yet no concrete results. Looking for opinions on areas to look at and diagnose, or maybe somebody had the same problem and can point me the right way.

Symptoms: a condition where the spark or fuel seem to cut out abruptly and the car violently lurches and stutters under a certain set of conditions.

Problem Conditions: Almost always in 1st gear with greater than 1/4 throttle positions, and starting from or near idle. You can generally rev the engine freely with no issue. You can accelerate slowly from idle to red line with no issues. You can run it in 2nd gear or higher under all conditions with no issues. Generally the problem has some temp dependancy, but hard to tell if ambient air temp or block temp. In summer it would only do it briefly (2-3 min of driving) and then disappear completely; now with colder weather it will do it a solid 10-15 min and is very repeatable.

Car runs excellent otherwise and is fully tuned, passes smog, and is smooth in all conditions (when warm).

More Gory details: Mechanic really believes it is a seconday ignition problem - have tested and/or replced many parts however: throttle position switch, wide open throttle switch, Mass Air Flow Meter, ECU, oxygen sensor, rotor, caps, distributor, wires, coil, cylinder head temp sensors, fuel pressure regulator and a few other things he did ....

It seems like it is something that isn't gear related, but related to the rate of change of RPM under load. No other gear can produce such rapid changes in engine RPM under a load. Not thinking fuel system because it seems like WOT under 2nd or 3rd would consume more fuel than a quick blip in 1st. Not thinking injectors as that would likely show up under other conditions...

Help!! Looking for opinions on where to look next. All suggestions or comments appreciated.... Thanks!

Warren

[indigowhale]

Please look at the Thread titled Carrera Balkiness Season, I had similar symptoms and some of the experts on this board helped me solve it without replacing any parts. In summary, your mix may be too lean. (I went the part replacement route first too)

[bigrubberjeep]

I'm no expert, but I would assume your wrench would have already replaced your DME relay, if not, then do it, it's only a $35 part.

It worked for me 1 year ago when I had a similar issue.

[nhromyak]

This sounds similar to what had happened to me. However, mine didn’t depend on how fast the RPM was accelerating. For about a month or two I would intermittently lose power during warm-up. I thought it was spark. Then if I leave it in gear, it would fire up again and be OK the rest of the day.

It seems like it is something that isn't gear related, but related to the rate of change of RPM under load. No other gear can produce such rapid changes in engine RPM under a load. ,

I suspect you have already replaced the DME relay and your mechanic had checked it too...

Because of the above quote, I tend to think it may be one of your flywheel speed sensors. I found one of mine to be in REALLY bad shape - externally. It still worked, but it was VERY scary looking. You can check their ohm resistance, but only a scope on it will tell you if it is referencing the flywheel correctly. At $125 or so each (there are two), I hope your mechanic has at least inspected them. If either of these fails even for a second you get no fuel and no spark....
I did this because my DME died, in a slow, agonizing death. At first your symptom was mine too. This lasted about a month or two before the computer made the car run FULL RICH at idle.


Could also be a dead spot or so in the AFM.

Could be a DME heat problem. USUALLY, the DME computers lose their ignition circuit. But mine bench tested fine, but it ran like crap in the car. I replaced it with a friends ’84 computer and it ran fine. Then I put my computer in his car, and it ran like crap.

Good luck.

[ianc]

Fuel filter been changed?

ianc

[triwave]

I'll read the suggested thread a bit later to see what that's about.

As for DME relay ... we didn't change it because of the symptoms. Don't the relay's usually quit working and shut off fuel completely? This is just under certain conditions. To be more explicit ... when the car lurches and fuel/ignition cut off, letting off on the throttle immediately brings things back to good running order and you continue on your way. The engine doesn't quit, it surges while the throttle is applied, and then corrects itself again. Sort of a bucking bronco effect when trying to take off from a stop.

nhromyak: Some very interesting comments you made. First, the flywheel speed sensors were tested, but only for Ohmic reading. I don't have available the tools to test under operation, and more importantly, I (we) can't create the problem in the shop - only while driving in 1st gear. If a flywheel problem exists I assume it would look OK in the shop. Would be difficult to monitor and capture waveforms while driving Second: the full rich condition is something I have had intermittently. You just can't lean out the car. Fiddled around, changed DME and things worked. Put my old one back in and things still worked fine. No parts were changed - I just chalked it up to a poor connection at DME.

Currently I have a friends working DME from a same year '86 to diagnose this issue and it idles better (dialed in to spec, no issue) but still lurches.

No how to better test that flywheel sensor ... anybody have a test procedure / diagram for what to look for?

Thanks, Warren

[john walker's workshop]

the dme relay is a double relay that powers both the fuel pump and the dme computer.

[rick-l]

I have been fascinated with the mechanical advance to accommodate the DME advance in my rusty distributor on my 88.

I wonder if this was sticking if it could interrupt the spark? Of course only when cold on changing RPM only under load. If you are just revving the engine you might not notice a period of missed spark.

There is a wild guess for you.

[nhromyak]

Replace the DME relay. It seems to me there have been more fixes by replacing the relay than any other part, especially intermittent problems.

As I recall, the flywheel sensors require a scope to measure the wave they create and send to the computer. I have heard these can be intermittent, although rarely. Usually these just up and die.

Of course, I have hard the same thing of the DME computer, but that was not the case for me either.

Did you or the mechanic check the AFM sweep?

Check ALL of your connections from each component to the DME computer. This was a fun job. It took about 1.5 – 2 hours as there really isn’t that many sensors on the 3.2 cars.


Of course, if you’re still getting the “lurching” (is this the intermittent sudden shut-down) with your friends computer, then there is probably a primary (DME relay, flywheel sensors) or secondary (coil) ignition problem.

If it is a slow shutdown, loss of power, then it will be a fuel delivery problem (like the fuel-pump or relay).

Good luck.

[Jeff]

Does this only happen when you are pulling away from a dead stop or moving slowly? If this is the case I would check the adjustment of the Bowden tube on the clutch cable. This can cause the car to buck wildly.

[Jeff]

Here is a thread if the above is the case...

clutch chatter

[ischmitz]

The failure mode of the DME relay is a "cold" solder connection of the relay contact on the PCB. Some say it was a batch from Hungary where they used improper solder. Vibration finally leads to microscopic cracks in the solder connection.

First it is highly intermittant and might be the cause for your issues. If you are handy with a soldering iron or soldering gun you can fix it in no time yourself.

Ingo

[triwave]

I will take out the DME and touch up the solder joint, that should be a simple enough procedure. Scratching my head as to how that would reliably show up as a 1st gear acceleration issue only however ... My mechanic said he always sees the DME problems much more severe and obvious.

I also discussed flywheel speed sensors with him. Obvious problem of working fine in the garage, and not on the street and trying to capture that. But questioned as to how this would only show up under 1st gear acceleration. Also stated they usually do a hard failure at some point where it's RPM related and thus can be duplicated free wheeling the engine and observing the sensor fail. Mine car only hesitates under one condition.

C U L8R - I have a friend (in a different state) with the same plate as yours! He has had it ~ 20 years Anyway, the car will do it from a dead stop, or rolling. It's most obvious from a stop because this occurs all the time in daily driving, but there is no doubt you can already be rolling with clutch engaged and "create" the same problem. I'll read the thread in more detail however, as I've not hear of the Bowden Tube before.

The car is currently running with a bunch of donated parts from a working '86 - the AFM, coil, wires, rotor, and DME.

Keep the ideas coming! Thanks

[ischmitz]

One comment: The DME relay is the small round or square black box the size of a match box under the drivers seat. I houses two relays that switch power to the fuel pump and to the DME box itself. The inards are a couple of metal relays that have said issues with the solder joints. It can be bought new from our host for 30$. It is always a good idea to have a spare with you.



The DME itself also has a history of problems with solder joints when getting old. Humidity compounds the problem. Before you open that one it is a verty good idea to swap in a good working one from a friend. They are a little bit more expensive. Soldering on those should be left to the experts.

As to your question about why only under hard acceleration I had another thought: Maybe a certain resonance of the chasis under hard acceleration or a certain torque load on the engine-tranny package shorts or opens something electrical: I once had a gauge light coming on in hard cornering. It turned out a bare wire was pinched in the engine bay between the rear engine mount and the chasis. It shorted to ground in hard left turns when the motor mounts moved. Failures like that are hard to track down in the shop. If you have a recording DVM or portable scope you could try and monitor signals while driving.

Cheers and good luck,
Ingo

[Jeff]

Tri,

It seems that if you are only having this problem in first gear it is not fuel related. Try adjusting the bowden tube.

A friend of mine used to have this plate (C U L8R) and ended up dieing way to early. My on line name is a tribute to him.

[PatrickB]

Tri~

I've had a myriad of "rough running" problems, however, the symptoms you have described fit in line with a faulty CHT sensor.

I experienced similar running problems soon after I purchased my car. Car would run great in all conditions, however, under hard throttle towards the upper end of the tach, the engine would mysteriously and violently cut out for maybe a quarter to a half a second. What made it violent, was that it would "cut -out" and before you could even react it would cut back in, and SLAM the C-wrap out of the drivetrain, as you would likely still be at wide open throttle... If this more accurately describes the symptoms, replacing the CHT sensor solved the problem in my car...

[triwave]

Thanks for the hint Ingo, a pinched, exposed, or bad solder joint is something I can certain see creating intermittent problems that respond with chassis flex. Tracking that one down would be a challenge!! How did you find yours - a bit of luck? or actually went through things meticulously under you found it?

CUL8TR - We have basically ruled out fuel issues due to described symptoms also. Focus has been on electrical but I'll mention the Bowden Tube to my mechanic as he's a bit stumped right now too. Thanks for suggestion.

Patrick - Appreciate your description as it sounds familiar - the harshness of the cutout and the duration are in line with my experiences, but I can make it happen in medium throttle conditions, not just full throttle. We tested the CHT sensor and it appeared good, but didn't swap a known good one in it's place. Can I ask a question - did you have the problem in all gears? i.e. Mash it in 3rd and near red line it would cut out too? I was wondering the problem might happen in higher gears but it masked by the transmission ratio, with 1st being so low you really feel it and the drive train can respond/oscillate with the loss of power. maybe in 3rd or 4th the oscillations can't really effect the car because of a flywheel/inertia type effect??

Thanks for the input guys.

[ischmitz]

I found my by chance. I remembered that I tucked that wire away in an attempt to make the engine bay look a little cleaner. Its isolation was borderline and brittle. That's how it came to me. What gave it away was the left turn thing. That clearly points towards chasis flex.

I had a similar one with my aftermarket transmission speedo transducer. I added this device when I converted to electronic gauges. The speedo would cut out under heavy acceleration. It were the spade connectors that corroded from the elements. I finally had to solder them on solid and never had an issue since.

Good luck in tracking yours down. Go methodically: tranny ground strap, DME relay, all connectors to sensors in the engine bay, wiggle the DME harness and all wires and try to reproduce the cutting during idle to check if it really is an electrical gremlin.

Ingo

[PatrickB]

Tri~

I would almost be certain that it would be the CHT sensor. It only had to happen to me twice in 3rd gear near redline, before I replaced mine. It was such a viloent hit to the driveline, I could see no reason to attempt to duplicate the problem in other gears, or rpm's. Any more than 2-3 times at higher rpm's would likely break something in the trans or maybe the cv's!

I'm not one for normally replacing a bunch of parts shotgun style, but your car is 20 years old, and likely has some miles on it. The CHT, among other sensors, don't exactly live forever. It couldn't hurt to replace it, along with the 2 flywheel sensors - speed and timing reference sensors.

[ischmitz]

Patric,
Correct me if I am wrong but I thought Warren said in his first post that all sensors (CHT, speed, reference) have already been replaced by his mechanic with no success.