EJZero1

My tach bounce associated with a no-start condition a couple of years back was up to about the 2K mark. After much trouble shooting per Clark’s Garage, mine turned out to be a bad DME. Any chance you can borrow a known good one in your area to swap out? Easy 5 minute check to confirm or rule out.

Seattle 993

All good ideas guys, thank you. Well, glad there’s a well worn path here. Got to pick up a few things (like a noid light - had no idea what that was until yesterday).

The scary part is Tom M,Guinn’s “etc”... �� ... Oh boy

Tom M'Guinn

Noid lights have a so-so track record on these cars. The pulse is odd and doesn't always get picked up by noid lights. I'd start by just pulling a couple of spark plugs and crank the engine for 10-15 seconds. If the injectors are pumping in gas, you will smell it. If no fuel, then I'd try rattling the injector connectors while cranking and/or using a multimeter to see if the two leads on any of the connectors are shorted together -- remove all the injector connectors, then test if the two leads in any one connector are shorted together. Also confirm they aren't constantly getting 12v across the two terminals.

I'd also check for spark regardless of what's going on with fuel. If you aren't getting either, then that points away from isolated under hood issues like bad coils or injector connectors... If you are getting one but not the other, than that can save time chasing ghosts (e.g., no point changing/testing coils/rotors/speed/ref/relays etc. if the plugs give a good spark when you crank it).

Seattle 993

Alright guys.

i have fuel. Removed the fuel rail, bagged each injector, cranked and I have fuel in each baggie.

i think I have spark. Installed a brand new Bosch coil. Hooked up a spark tester on cylinder 4 at the distributor cap, and the light flashed. So I assume I have good spark but am going to run out and get a test light to check the ground on the coil. No change from the original coil. Maybe the ground coil is not pulsing and is causing a weak spark that is enough to flash the light but not enough voltage to jump a spark plug gap? Is that the way this works?

So, let me ask, if the ground wire to the coil pulses, and I have a new coil, and I have fuel at the injectors, and it still doesn’t start...what does that suggest?

BTW, the battery tests at 12.5 volts at rest and fully charged with a charger.

Thanks again fellas.

PS - also going to go back and retest speed and reference sensor at the DME harness for ohms and voltage according to ECU doctors video.

jimbo1111

Doesn't sound like a speed and reference sensor. if you have fuel and spark I would check the afm.

Chapman951

Did you attach the ground wire at the back of the intake manifold? I know you stated the car started previously but thought I would throw it out there.

GPA951s

Tried and true Method, Stick a Spark plug in the Wire and let it sit on the engine while you crank... you should see spark, Weak or otherwise it should be enough to ignite fuel even just enough for the engine to sputter. (Just think how little of a spark it took tom hanks to ignite that bundle of crap in cast away) .. Assuming you have good fuel... So you have fuel spark air.... Timing... next issue... Check the Rotor under the cap, Check the marks on the belts...

On a side note, I had a no start one time and as soon as I Removed one of the injector plugs it fired on three..... as It was running and I attempted to plug the injector in and it stalled... Just be aware that meerly moving the harness can "Mask" a Direct short..

Moving wires around while someone is cranking sounds so simple but it has helped me find issues SEVERAL times...

Seattle 993

Ok, now we’re getting somewhere.

Checked speed and reference sensor output to DME at harness. 1,600 ohms for both, so I have continuity. 1.6V for speed sensor and 0.5 volts for reference sensor which is in spec according to ECU Doctors on a 1988 944 turbo.

Fuel at the injectors.

However, 1 volt at the ignition coil when multi meter is on the terminals on the coil with ignition switch on. When I remove the negative probe and ground it to the engine block, I have 12.65 volts, same as my battery which is currently on a charger.

So, ignition coil is not getting full voltage while engine is not turning. Back to researching this.

Does this point to DME trigger or perhaps ignition switch (I’m thinking a long shot).

Seattle 993

PS - AFM harness connection cleaned and reconnected just to be sure, and the grounds on the back of the block are very clean (while I was doing the other work).

Thanks again guys.

GPA951s

You wont get voltage across the coil with the engine ignition on steady state... The ignition coil only sees momentary voltage, and The Secondary winding has to charge and collapse... what you are seeing on the coil is Normal the way you tested it... My car is running.... and as a 'Sanity check" I checked mine... I Get ZERO volts with the ignition switched on. and 12V with Reference to ground on the car.

You would need to crank the car and look at it with a scope to see the input...

Seattle 993

Yup, ok thanks GPA. As Droops suggested, connected a test light to the green wire on the coil and it does indeed flash (rather pulses) when cranking. I don’t see distinct off/on - it’s pretty fast - seems more like pulsating to me.

Ok, now I’m really stumped because it looks like the sensors are talking to the DME, the DME is allowing fuel and spark, I have fuel at the injectors (pulled the rail and bagged the injectors and cranked), the DME is causing the new coil to fire (seeing a pulsing test light - hooked up a halogen bulb man is that bright in the garage), the engine cranks fine, AFM connection cleaned and reconnected. I suppose the only thing left to check are the speed and reference sensors physically (which I loathe to touch again).

I did did not do any independent tests of the AFM.

ideas?

jimbo1111

I dont think you would be getting the injectors to pulse if you were having problems with the speed and reference sensors. Have you tried swapping out the afm?

GPA951s

bad fuel, Timing, .... Take a plug out and toss some gas in the cyl and see if it studders.....not enough Fuel pressure? pull individual injectors .. When testing in the bags they might have been fine.... Are the plugs wet after cranking? The rotor under the distributor cap may have come loose (still on but trying to fire at the wrong time) Recently I had a fuel line leak and no fire... but obviously I smelled fuel.... Without putting my hands on it its very difficult...

Tom M'Guinn

Does the car stumble or sputter or anything? "Usually," if you have gas in the cylinder and a spark at the plug, you'll get at least an occasional sputter or back fire or some sign that the fuel is igniting (sometimes in the intake or exhaust) when conditions are just right. I'd go back to basics and do GPA's spark plug test. Pull a wire off a plug and put it on a good spark plug that is well grounded. Do you see a good spark pulse as you crank the motor? That test alone will virtually eliminate a ton of issues. Testing upstream at the coil and so-forth is best done if/when you confirm you don't have a spark at the plug, as a way to help you figure out where the ignition problem is. As an aside, if you test the coil terminals when both have 12+ volts to ground, the meter will report 0 volts, since the meter is designed to show voltage differentials across its probes. Its essentially the same as putting both probes on the positive battery terminal. No voltage will flow and the meter will show 0. Back to the issue.... if the spark plug DOES fire, then I'd do the fuel injector test again and confirm that they are pulsing and not just constantly on. If you have pulsing spark at the plugs, and pulsing fuel at the injectors, and still getting absolutely no sputter or backfires, then I'd check the timing belt (if you haven't already!) and run a compression test.

Seattle 993

Nope. No stumble, no stutter, just crank. That’s what has me baffled. I think I start from square 1, maybe check the ref/speed sensors physically.