1986 3.2 intermittent non starting issues

[moush993]

If anyone can help. I have been having some strange issues recently with my '86 911 turning over but not starting. It happens occasionally if I take it on short errands. I will come out from shopping only to find she wont start up. Engine cranks strong but not catch. I switched out the DME at the time but no change. I have done quite a bit of work changing out various parts that typically go out with age. The thing is once the car sits for about 1 hr it will finally start up. Happened about 5 times in the past 2 months usually need to wait around until something resets then its good to go. When the car is good and hot it doesn't seem to have this issue or when starting up for the first time.
If anyone has similar issue or might know what this might be Id appreciate any insights.

Thanks

[Tea Tray]

Sounds electrical and temperature related.
Check all your ground connections; battery, the groups near the fuel filler and near the fuel filter.
Check your fuses for cleanliness and spring tension.
There is an injector connector back by the firewall.
The two flywheel sensors and the corresponding connectors behind the heater fan.

[proporsche]

..usually start with the cylinder head temperature sensor....also check your fuel pump fuse No 6 from the -loose windshield for any problems...

Ivan
here is a little help

[cabmandone]

When it isn't starting, what things are you checking? Is it losing spark or fuel?

[darrin]

Did you switch out the DME computer (big box) or DME relay? Your symptoms are typical of a cold soldier joint in a failing DME relay (among other things referenced above)

[RobFrost]

Faulty flywheel position sensor gives total absence of firing. I'd guess the connection could be dodgy on that.

Sent from my SM-G988B using Tapatalk

[moush993]

Thanks guys for the inputs I did suspect temperature sensor or one of the 3 connectors there. I just did my intake gaskets and had the top intakes out. Maybe I messed with these when putting them back so Ill check and see if there is an issue there. I did replace the timing and temp sensors at the fly wheel last year.

[pmax]

There are two flywheel sensors, one of them's used only during startup is what I understand, the other, the CPS, is in use all the time. If the CPS's dodgy, I would expect fuel cutoffs, abrupt and can't be missed I will add, on the road.

[Tea Tray]

…just did intake gaskets…
Check the 3 connectors that the sensors plug into on the bracket behind the heater blower.
Mine was missing one of the retainer wires.

[GH85Carrera]

I don't know how long you have had your 911, but if the flywheel sensors have never been replaced, they should be replaced as a matter of maintenance. If they have not failed yet, they will soon, and at a bad time. Of course, do the head temp sensor "while you are in there" as well.

Also, as a matter of routine maintenance, go from the batter ground to the back of the car and double check every ground. That does not mean just look at them ans see if they look good. Disassemble, and clean every ground connector and be 100% certain the are solid grounds.

You can never have too good of a ground. More is always better for grounds.

[911obgyn]

If, as you say crank sensors and cht sensor have been replaced, then grounds, fuel pump fuse and connectors at left front of the motor. I use a bit of dielectric grease on all the connections once they are clean and bright so i dont have to do it again next year. If it still happens then a solder joint on the circuit board in the DME could be the culprit.

[Rodsrsr]

I've seen the connectors for the sensors (in the engine bay) cause this problem several times over the years. They're not necessarily dirty, they just tend to come loose.

[MAH77]

Is the tach moving when you crank and it will not start? Also, my understanding of the CHT is the car will start with a bad one, but it will not run well when cold.

[mysocal911]

Read here; 911 3.2 No-Start Troubleshooting

[wjdunham]

My guess would be the cylinder head temp sensor. One way to check is to pull the plug and stick a 270 Ohm resistor across the contacts on the side that goes to the DME. If that sensor is bad it will cause an overly rich condition which will prevent the engine from starting. Since your problem seems to be temperature related, it's an easy thing to try.

Bill

[mysocal911]

Originally Posted by wjdunham

My guess would be the cylinder head temp sensor. One way to check is to pull the plug and stick a 270 Ohm resistor across the contacts on the side that goes to the DME. If that sensor is bad it will cause an overly rich condition which will prevent the engine from starting. Since your problem seems to be temperature related, it's an easy thing to try.

Bill

You need to restate your post this way:

Originally Posted by wjdunham

My guess would be the cylinder head temp sensor.
If that sensor is bad it will cause an overly rich condition which will prevent the engine from starting.

Because bad temp sensors typically fail in the open state (very high resistance), causing a very rich no-start condition.

Originally Posted by wjdunham

One way to check is to pull the plug

And bypass a bad sensor by using a paper clip jumper or low resistance resistor (270 ohms).

[moush993]

Thanks for the inputs. I ran the ohm test at the temp sensor plug at 70deg it read 2.5. I assume its ok.
if car fails to start and I unplug sensor and jump the connectors will it at least start?

The car eventually starts after it sits a while. I did clean the pins on connectors so hopefully that was the problem. Also cleaned grounds.
Ill have to see if it does it again when out and about. It only does it occasionally so I assume temperature conditions are just right to not let it start.

I did replace the these three sensors 1-2 years ago so I know they are new. The wiring was disintegrating and I read these will eventually be a problem so I just did them all.

[Rodsrsr]

As someone else mentioned, you need to start the troubleshooting the next time it occurs. Next time it fails to start, check for spark and fuel, very straightforward.

Its possible there could be a failed solder connection in the DME itself which may cause an intermittent problem but no sense going down the rabbit hole until you determine if you're getting fuel and spark.

[ischmitz]

To the OP: when you state you swapped the DME are you referring to the DME relay or the DME itself. The former is a dual-relay that has a track record of failures and folks advise to carry a spare. The latter is the engine control unit (ECU) or brain that drives the injectors and the coil based on sensor inputs.

In reading your first post what you see (intermittent no-starts) is very consistent with either one of the sensors or the DME being bad. And given that you replaced the sensors the DME could be your culprit. The next step is to find out whether you loose spark only, fuel only or both when the no-start happens. Spark-only is a strong indicator for the DME. Fuel-only would point towards the fuel pump. Loosing both intermittently is very likely a sensor issue.

[moush993]

Thanks, Yes I do have an extra DME in the car and did try that the last time it did not fire up. It was dark at the time and I couldn't troubleshoot. The good thing is if I wait enough time the engine will cool to a point it will eventually start. I just have to kill some time until that point. If car is good and hot and was driven awhile it will start up when on the road. It seems to be a temperatures issue sweet spot.
Im hoping the connector pins were not contacting properly and they were influenced by the temperature and were not touching lead? I just went through and cleaned pins on all three. Need to try and drive around and see if that solved it.