3.2 cranking but no spark

[patz]

[ischmitz]

The relay in the video is a simple single stage relay. The Bosch Motronic DME relay has two relays inside the package and bench testing is a little more involved than what is shown in the video.

However as Sal stated we are much further along in this case. If all the information given by the OP is correct he has spark but no fuel pulses when cranking. So sensors and relay work fine.The possible causes are

- damaged DME (fuel driver)
- no power to harness for injectors
- shorted injector(s)

As next steps:
- check permanent +12V on injector harness
- check continuity from 35 pin DME connector to injector plug
- check for fuel plus with NOID light and all injectors disconnected

This will ID the faulty component(s)

Ingo

[gremlin]

Thank you Patz I found that a while back, but as we know if you have spark those two sensors are good. Why does the noid have to be the LED kind? I got one from amazon that I listed a few posts backs and it's not LED, I'll have to figure out if it works anyway.

So there's been a sort of success, the NOID light didn't light up so I assumed I wasn't getting injector pulses, I checked the injector harness up against the firewall (the 4 pin one at the front or non-fan side of the motor) and sure enough it was unplugged, trying to plug it back in was a bit of a stretch so I'm thinking when the engine was out something got mis-routed and put a little stress on it. I plugged that back in and he started right up!

However not all is rosey, keep in mind he was running just fine prior to this falling off, so it's doubtful something failed, more likely I caused an alternative issue. First the idle was super rough and surged a lot as I pulled out of the garage, so I stopped and looked around, found the CHT sensor disconnected, plugged that back in and it settle right down. So I started driving. It quit again less than a mile from the house I got it started finally by giving it some throttle, and it ran okay below 2500 or so RPM but as soon as I put a foot in it, it just falls flat and then wants to stall out.

I checked the flywheel sensors and CHT wire again, I checked the injector plug to make sure it was seated well (Can this plug be installed backwards? I thought not as it has a plastic pin in it). So any ideas on what else, I also checked the obvious plugs near the front (ISV, airbox etc...) could I have damaged the plugs with all the cranking where I had spark but no fuel when I was trying to figure this out?

[mysocal911]

Originally Posted by gremlin

Why does the noid have to be the LED kind? I got one from amazon that I listed a few posts backs and it's not LED, I'll have to figure out if it works anyway.

You don't. A simple 12 volt incandescent bulb will work!

Originally Posted by gremlin

It quit again less than a mile from the house I got it started finally by giving it some throttle, and it ran okay below 2500 or so RPM but as soon as I put a foot in it, it just falls flat and then wants to stall out.

could I have damaged the plugs with all the cranking where I had spark but no fuel when I was trying to figure this out?

Very unlikely.

Mostly likely you no longer have an electrical problem but possibly an air leak (the AFM
boot loose), low fuel pressure, etc. Check all hoses and connections in the engine
compartment and check the fuel pressure with a gauge.

[gremlin]

Good thoughts for sure, but I think I'll start with connectors, the only thing I undid from when it ran perfectly was a fuel rail connection to check for fuel, I'm assuming the injectors not running can't damage the pump as it has as return circuit. So when I get home tonight it'll be a methodical check through all the connectors on the engine. I'm also going to check the CHT connector again and see if that got damaged.

[mysocal911]

Originally Posted by gremlin

I'm also going to check the CHT connector again and see if that got damaged.

You indicated before that the CHT was not connected when you first got the engine
to run after finding the injector harness not connected and the engine ran better
once the CHT was connected, so most likely it's O.K. You can very easily bypass the
CHT using a paperclip when the engine is warm, i.e. bypassing with a cold engine
results in hard starting and possible backfiring.

You can use the NOID light once the engine is running to see that the fuel injected
is increasing by seeing how the NOID brightness increases with RPM. Just pull one
injector connector and connect the NOID light.

Don't assume that some system elements are O.K. now because they were O.K.
before your problems began, i.e. remember you found the injector harness not
fully connected as being your "no injection" problem after doing engine work.

[gremlin]

Originally Posted by mysocal911

You indicated before that the CHT was not connected when you first got the engine
to run after finding the injector harness not connected and the engine ran better
once the CHT was connected, so most likely it's O.K. You can very easily bypass the
CHT using a paperclip when the engine is warm, i.e. bypassing with a cold engine
results in hard starting and possible backfiring.

You can use the NOID light once the engine is running to see that the fuel injected
is increasing by seeing how the NOID brightness increases with RPM. Just pull one
injector connector and connect the NOID light.

Don't assume that some system elements are O.K. now because they were O.K.
before your problems began, i.e. remember you found the injector harness not
connected as being your "no injection" problem after doing engine work.

Great thoughts will do that. on the last one, yes but after the engine work I did two full track days and drove the car for at least 6 other hours on the road and It was fine. And prior to it quitting on me on the road it was running great, I really feel like if I had figured out the injection harness right away and plugged it back in everything would be fine, but I could be wrong...

[88911coupe]

Sorry if someone has mentioned this before but in my case the reference sensor would test fine and then later fail until it failed enough times for me to catch it. Drove me nuts putting it back in thinking it was fine.

[gremlin]

Had one of the sensors replaced last year, but I'll double check that. I went over all the connectors tonight and found only one sort of odd thing. There was washer on top of the coil on the black wire, side, but I couldn't find anything else that looked like it should be there, are there two separate wires meant to be on one side of the coil? I did check the CHT and it OHMd out at 1.7K.

Both rails of fuel injection showed lights with the NOID light. Reving it got brighter and flashed at a higher rate, and there's no way to plug the injector harness in backwards. I haven't yet pulled the plugs to see if one or more has been damaged.

[scarceller]

OK, so you do have spark and coil pulses? If that's the case and it won't start then you need to check fuel pressure and delivery rate of fuel. Start with pressure first, just force the FP on and you need to have about 36PSI at the rail with pump on engine off. Check this next.

[gremlin]

Just to be clear with everyone the thread has evolved, the car starts now, the issue I had the first time (the start of the thread) was a flywheel sensor. The second time we had spark and no fuel injection pulses turned out to be a disconnected fuel injection harness. Now I found that it starts but runs poorly. Idles fine, but when you get on the gas in completely falls flat around 3K and up bogs down like its stalling. IT ran awesome before it quit two weeks ago, the failure of whih was the injection harness. I'm pretty sure that first I need to find something that I might have done in diagnosing the other failures which caused it to run poorly.

Because of this we know it's not the DME Relay.

[rick-l]

subscribed ..... anxious to see what this turns up

Could be fuel, could be sensor, could be anything

[mysocal911]

Originally Posted by gremlin

Idles fine, but when you get on the gas in completely falls flat around 3K and up bogs down like its stalling. IT ran awesome before it quit two weeks ago, the failure of whih was the injection harness. I'm pretty sure that first I need to find something that I might have done in diagnosing the other failures which caused it to run poorly.

1. Check the intake boot from the AFM to the plenum and make sure it's fully
connected. Any air leak there will cause the AFM flapper to not properly enrich
the fuel as the throttle is opened causing the engine to bog.

2. Check the fuel pressure. You can easily determine if it's a fuel problem when it
bogs by just momentarily disconnecting the CHT and seeing if the bog is eliminated.

3. There may be a problem in the DME control unit not allowing proper fuel enrichment
when the throttle is opened. The control unit needs to be tested in another car to
eliminate it as a problem source.

[jrhb]

Rough running, possible vacuum leak? The back side of the Carrera intake has several vacuum ports for brake assist, etc. Are yours hooked up or capped off? During the engine work or driving could one of them become dislodged?

-Jeff

[ischmitz]

Has anyone mentioned fuel delivery rate. Porsche WSM specifies a pressure test to ensure the regular works and a delivery rate spec. I would test both. Maybe clogged filter.

[scarceller]

Agree with others, next step check fuel pressure and delivery rates.

[gremlin]

Originally Posted by mysocal911

2. Check the fuel pressure. You can easily determine if it's a fuel problem when it
bogs by just momentarily disconnecting the CHT and seeing if the bog is eliminated.

Tell me more about how I do this? Warm it up and then disconnect? What would it tell me about the fuel pressure by disconnecting the CHT? I measure the sensor last night and it seemed within spec.

The vacuum ports on the back are worth looking at I could've knocked something out when I was crawling around back there, gotta get an inspection mirror and crawl around a bit checking for that.

Today I did only one small thing on the car as I'm too annoyed about it right now to really try, and a bit busy. I did print out the page on checking connections for the DME and I'll do that as well in the next day or so.

The one thing I did today was finally identify the wire which should be controlling the fuel pump, at the moment it's wired to come on when the ignition is on, always been that way but I found the wire now, and I have a relay to put it through so eventually I'll make that and the headlight relay change happen. For now I'd like it to run though

[rick-l]

Originally Posted by gremlin

Tell me more about how I do this? Warm it up and then disconnect? What would it tell me about the fuel pressure by disconnecting the CHT?

I think this is what he meant.

When it bogs down at 3000 RPM with the engine warm disconnect the Temperature sensor and short it with a paper clip (or wire) and see if it still bogs down. The short will force it into its warmed up running temperature fuel map.

[mysocal911]

Originally Posted by rick-l

I think this is what he meant.

When it bogs down at 3000 RPM with the engine warm disconnect the Temperature sensor and short it with a paper clip (or wire) and see if it still bogs down. The short will force it into its warmed up running temperature fuel map.

Actually, no paper clip is used. Disconnecting the CHT momentarily will cause the control
unit to enter the full fuel enrichment mode overcoming the bogging, if it's a lack of fuel
enrichment causing the bogging. Normally when this is done with a warm engine,
the engine will increase RPMs and begin to make black smoke (an overly rich condition).
This condition usually occurs when the CHT fails 'open' on a good running engine.

Doing this test should help to determine if the problem is fuel related. A further test requires
an actual checking/monitoring of the fuel pressure when the bogging occurs.

[scarceller]

When it bogs at 3000RPMs does it do this with car parked and in neutral, while you simply free run it up to 3000RPMs? Or does it bog under load while driving? This is important for us to know because if it bogs parked at 3000RPMs it's likely not a fueling issue because a free running (no load) motor at 3000RPMs consumes about the same amount of fuel as a engine at idle. The injection pulse width actually drops significantly (about 3 times smaller) than free running at 1000RPMs.