Got fuel, got spark, but no start. 1986 3.2 911

[andyt11]

I'm trying to help a friend with his 86 Carrera. It ran great when parked for the winter six months ago. Come spring, the car won't start. It cranks like crazy but will not fire until you get off the starter and then you get a tiny cough / splutter as the engine stops spinning. We can smell fuel at this point.

At first, just to rule it out, we put some fresh fuel in the car on the off chance that somehow the fuel stabilizer he used did not work. Still no start.

We tried spraying fresh fuel directly into the intake, and did get it to start, but it ran terrible. I think it was very rich and it would stall when throttle was applied quickly but you could build revs if you slowly increased the throttle. Exhaust fumes smelled strong of fuel.

That was the last time it ran. Now we are back to a no start condition.

So we started investigating.

So far we have confirmed the following:

• We defiantly have a strong spark.
• DME relay clicks twice as it should, as did a replacement relay.
• Fuel pump is running.
• Injectors are clicking.
• ICV is vibrating.
• Crank speed and position sensor are within spec (checked resistance)
• As is the cylinder head temp sensor.
• The AFM shows smooth voltage increase as the barn door moves, no dead spots or wonky readings. (Used the 9v battery method).
• Intake air temp sensor is also within spec when we check resistance.

Whats left? Fuel pump and regulator? Bad DME control unit?

What could possibly have changed over the winter? There is no evidence of mice or anything like that.

Any help appreciated.

[patz]

First choice is the DME relay. URO manufactured items suck.

[dienstuhr]

Just adding a bit more (it's my car that we're trying to start)

The car is an '85, mostly stock with cat delete, m&k muffler, Steve Wong chip.

After our first attempt to start the car when it ran poorly, a couple days later I started the car and went for a drive. It ran well, but was somewhat hard starting. The trip had several stops with engine off but restarted OK. It also ran well the next day too. Then it sat for a week and wouldn't start yesterday.

Thanks,

d.

[andyt11]

Oh yeah, I forgot, he has driven the car since we got it running roughly that first day. So it can run.

It just doesn't right now!

[brianlay]

I think you should take a closer look at fuel delivery.
If spraying fuel into the intake caused it to start then you're lacking fuel
Pump may be running but is it delivering enough fuel or making sufficient pressure?
There is a pump delivery test you can do as well as measuring pressure
Could be a clogged screen or filter

[GH85Carrera]

Originally Posted by brianlay

I think you should take a closer look at fuel delivery.
If spraying fuel into the intake caused it to start then you're lacking fuel
Pump may be running but is it delivering enough fuel or making sufficient pressure?
There is a pump delivery test you can do as well as measuring pressure
Could be a clogged screen or filter

My guess as well. If the pump is the original 32 year old pump is could well be just weak or a clogged screen. I went ahead and replaced my working pump when it was 28 years old. I go on long road trips and a fuel pump problem is not fun.

[andyt11]

Where is the screen that you are talking about? We should check it.

There is a procedure in the Bentley manual for checking volume of fuel, we could check to see if enough is delivered.

Checking pressure required making up hoses / gauges etc.... was hoping to avoid that.

We have not changed the filter yet.

[brianlay]

The screen is on the fuel pickup in the tank.
Check delivery rate and pressure before tackling the screen

[john walker's workshop]

Check spark quality by seeing how far the spark jumps from the coil wire. Should jump at least 1/2" and be bluish, not orange.

[andyt11]

ok. we'll do that next. i"ll have to get the bentley out and read up on what is required. I think we have to make up a hose with a gauge on it though.

Flow test seemed simpler if I remember right.

Is there any merit to putting his DME in my car ( i have an 88) and seeing if the no start problem transfers to my car? Or is that daft since we clearly have fuel and a spark? Or daft since they are different years?

[andyt11]

Originally Posted by john walker's workshop

Check spark quality by seeing how far the spark jumps from the coil wire. Should jump at least 1/2" and be bluish, not orange.

That's easy to do. Is there such a thing as not enough spark? It was blue if I remember right. And seemed strong too, but I didn't try to make it jump 1/2". Will do that too.

[ischmitz]

Originally Posted by andyt11

ok. we'll do that next. i"ll have to get the bentley out and read up on what is required. I think we have to make up a hose with a gauge on it though.

Flow test seemed simpler if I remember right.

Is there any merit to putting his DME in my car ( i have an 88) and seeing if the no start problem transfers to my car? Or is that daft since we clearly have fuel and a spark? Or daft since they are different years?

yes - it's a no-brainer and fully confirms it's no the DME.

I agree with others here. As long as you have spark and fuel pulse signals it's probably lack of fuel pressure in the rails. Pump, fuel filter, fuel accumulator etc. are suspects.

Ingo

[dienstuhr]

Okay, on a tip from andy911t I unplugged the AFM and the car started! It ran for a bit then stalled. I plugged in the AFM and it ran ok. When I stopped the engine it would not restart with AFM plugged in.

[ischmitz]

It might point to a worn track in the AFM. There is a test where you power it by a 9V block battery and measure its output with a Digital voltmeter while moving the barn door. Google 944 AFM testing.

[brianlay]

The original post said AFM was already checked

[dienstuhr]

We did that exact test and the voltage increased smoothly.

[ischmitz]

In that case either a poor connector or harness (not very common) or the observation the car started was not related to the AFM being unplugged. All the AFM does is adjust the fuel pulse duration and timing according to the barn door position. The integrated IAT sensor has marginal effect on mixture.

Ingo

[dienstuhr]

I tried multiple times. Car would start and run (not great) (rich) with AFM unplugged. Then with it plugged in, starts and runs well. Then will not start again until AFM unplugged again. Should clean contacts perhaps?

[ischmitz]

Have you done the DME swap yet to confirm its not the DME improperly reacting to the AFM? If not do that. It won't hurt and is another data point.

[andyt11]

We can try that this weekend perhaps.

After that, think we should look at his AFM more closely , perhaps move the slider onto a different, new part of the copper rail.

After that I think we are back to fuel pressure and flow rates.