1986 911 No Start
 

Hello

I'm working on a 86 911 that fires, runs for a few seconds then immediately dies. Common issues cht sensor, fuel pressure regulator and dme relay seem ok. When running for a few seconds the car sounds solid, no backfire or odd sounds or missing.

cht 4.2 ohms measured at the connector on the intake manifold (white connector on top)

fuel pressure is 40psi when I manually actuate the pump at the dme relay connector, ~30psi residual (after ~20min)

replaced dme relay

I was told the injectors have been serviced.

The car has been sitting for a long time. I replaced a bad breather hose (engine to oil tank) and cleaned tons of black crap from the air flow meter and throttle body. Tracing a vacuum leak I found the charcoal canister is missing. I plugged the large and small vacuum lines coming from the engine.

I'm basing tests and results on Bentley #s and everything passes. I went through the test in the Bentley and no smoking gun.

What am I missing? thanks for your help

how old it the gasoline
next
when it does not run check for spark
if there is a spark check for fuel
was the temp cyl head sensor replaced
report after

Ivan

Too low, disconnect it for a cold test start. Once warm, it can be connected at the very low resistance value.

Quick and easy test:
Squirt starting spray in the intake. If it fires every time you add starting spray, then you have spark and are missing fuel. If it does not fire with starting spray, you almost certainly are lacking spark. Report your results.

CHT indeed if it really tests at 4.2 ohms and not 4.2 kilo-ohms, as you may have misread which meter range you are using.

Squirting starting spray is probably not needed here, as the engine starts fine and dies a few seconds later. That means spark and fuel are present right from the beginning.

I think Ivan's question of "how old is the gas" is a good starting point [excuse the pun] then make sure you have constant flow of pressurised fuel to the injector bar, there could be a blockage somewhere, also the CHT resistance is not good.
If all checks out with the fuel side, move on to the ignition, and check the obvious first i.e.; leads, rotor arm etc before moving on further.
Ant.

Fair enough, skip the starting spray.

How’s the Distributor? Was it wet….? Ask me how I know. Make sure it’s free of all moister under the cap.

Maybe check the coil… ask me how I know…

Thanks folks

I'll add some more fresh gas, I havent checked but it smells good. Volume seems fine(I tested by running the pump manually but for 15 seconds not 30), filter is new. Unlikely but good idea.

I havent tested the coil. Throwing me off maybe since it starts every time, runs for a few seconds and dies. Every time. I pulled the plugs and they are black, new plugs.

I replaced the distributor rotor, old one was a little crusty and had too much resistance.

Forgot to share results from coil test

according to the bentley (famous last words?) post 1 > 15 should be .4-.7 ohms. I tested 1 ohm. Seems close enough but I bought a new bosch coil to be sure. New coil is same 1 ohm.

Yes, under the cap is dry and no obvious cracks or damage.

Car starts, runs, takes a little throttle then dies.

If this continues to drive you nuts, you could put one of the those inline blinking spark plug testers in, then have someone run the engine while you watch the tester to see if it stops blinking when the engine starts to die.... At least then you know whether it is possibly ignition related.

Your meter probes have their own contact and wire resistance, which should be around .3 - .5 ohms. That resistance should be taken into account and added to the tested device resistance when testing very small resistances like those of a coil primary. Your original coil was probably OK.

^ yes thank you. What I thought but a very frustrating issue and I didnt want to miss something obvious.

So... progress. I cant tell you how many times I read the theory of the system. Not an expert but I think I understand what to measure and expect. But it wasnt working as expected. Maybe grasping I swapped the connectors for the crank sensors. And it started right up and ran. So chasing my tail turns out to be a preexisting condition I wasnt expecting.

But now its running rich. It idles a little rough then when given full throttle it stumbles then black smoke. Maybe some fuel in the oil?

thanks again for your help, getting closer

Good progress.
I supppose the wires to the reference and speed sensor are the culprit(s). If one of them fails (reference sensor is only necessary for engine start!) engine won't start at all.

Black smoke is uberrich mixture, burnt oil looks blueish or grey...don't hink that so much fuel is in the engine oil. More likely there's still unburned fuel in the exhaust or so...

There may be still another problem, most likely an electricial issue. Keep that in mind, that two or more issues may interfere and mimic a complete different behaviour which may confuse you!

Vincent, pne thing that puzzles me, unless I misunderstood your last post, you swapped the ref and speed sensor connectors at the vertical ramp and the engine is now running? Is that right?

But in your very first post in this thread, you described your issue as car running for a few seconds then dying. How is that possible?

Damages in the speed and reference sensors wire isolation can cause electrical interferences in the signal generated by the sensors. Did you check the conditions of the sensor insulation? With age, the wire isolation gets brittle and starts falling apart. I had to replace my two sensors two months ago.

I would also check the ground points. There is a critical one that is located at the intake manifold. Remove the bolt and use a metal brush or sandpaper to clean the connectors. Poor ground can also cause that type of problem. I hope it helps.

Hmm, I was thinking the same thing, as for the car running rich, maybe there was more than one issue, can't remember, but did he check the CHT Sender for the correct resistance reading ?
Ant.

If it's running but running rich it could be a faulty CHT sensor.
These sensors are inverse temp to ohm spec.
Initially at cold start the sensor should read 1800-3000 ohms depending on cold temp. Point is they read high resistance in this range on a dead cold engine.
Once the engine is fully warm the sensor MUST read less than 200ohms.
I'd start be testing the CHT resistance on engine dead cold, report this number back here and the ambient air temp.

Another little trick, once these engines are warm you can unplug the CHT sensor and jumper the 2 female pins in the harness with a piece of wire. What this does is set the resistance at 0 ohms and this tricks the DME into thinking the engine is fully warm. You can actually run the car once warm like this.

Black smoke at the tail pipe is way super rich! Start by determining if the CHT sensor is good or faulty.