Seattle 993

Guys, after doing a bunch of work to my 88 turbo S (new Bosch reference and speed sensors, rod bearings, front engine seal, oil pan gasket, etc) started and ran successfully with multiple starts for about 30 minutes 2 weeks ago. Except for a small coolant leak which led to a 2 week hiatus while parts arrived.

Now I have a no start and need some help. Battery is good, DME relay swapped (still no start), Reference and Speed sensor within spec at the DME harness (checked with multi-meter for resistance and voltage at start - 1,000 ohms on both, 1.6V on speed sensor when cranked, 0.6V on reference sensor), fuel rail has pressure, starter cranks fine, I have 12+V at the coil when ignition switch is on, haven’t yet tested power at the plug but will tomorrow.

What I can’t figure out is why I have a no start when it started 2 weeks ago without issue. I initially thought maybe I had misadjusted the sensors (I recall seeing huge tach bounce on the first start up which I thought was unusual, now it’s small bounce which I’m more accustomed to), but they seems to check out fine according to Clark’s Garage and ECU Doctors video...

I’m now wondering if I need to inspect the DME.

Any bright ideas guys? Is the attached tach bounce normal?

Attached is video with my tach bounce...

markl951

Rotor come loose underneath the distributor cap?

Seattle 993

Nope. Cap, rotor checked. All in place. Would have jumped for joy at that one, unless of course I lost that socket set screw for the rotor!

GPA951s

Keep it simple dont over-think it.... what changed? Dot you have spark where you need it? Pull a plug see if it sparks when you crank it.... Do the SAME for the Fuel injector... Does it spray? how much? 1 Spark 2 Fuel 3.Air Narrow it down where its supposed to happen, Plugs/ Injectors...The Last thing to check is time if all Three of the other things check out...

Chapman951

Pull or pop a fuse?

jerome951

That tach bounce looks unusually low to me. I'd suggest start by readjusting the reference sensor and check those connections.

Humboldtgrin

Do you have the updated reference sensor and speed sensor mount with the sleeve? I would focus on the reference sensor and speed sensors being your first culprit, you may have a bad wire in you engine harness side. It’s not unheard of, that’s why they make replacement sub-harnesses.

GJB928

Tach bounce looks just like mine which is currently starting fine.

jimbo1111

I just recently went through the same issue only I had no tach bounce. It turned out to be the dme. There is a procedure to jump out the klr computer to eliminate it as a failure point as well. Should be on Clark's. Echoing others, I would inspect the dme side of the reference and speed sensors.

Seattle 993

Yes, I took ohm and voltage readings at the DME, checked out within spec. Leads me to believe that the sensors and harness are passing information through to the DME. My car came with the updated sleeve and has previously started and run with the speed and reference sensors, so I’ll check that only if I have to (really don’t want to remove the fuel rail and intake if I don’t have to). My own suspicion is that the coil failed (maybe just hoping it’s an easy one) so I’ll test that tonight at the spark plug then go from there.

I know I have fuel at the rail, voltage to the coil, but as GPA said, I’m gonna keep it simple (hopefully). Funny thing is NOTHING has changed since the last start except I fixed a small coolant leak at the gooseneck and I moved the FQS switch to position 1 (very gently I might add).

Baffling. I’ll let you guys know what I find.

dme

I had a similar problem many years ago after replacing the clutch. Started up and ran fine for awhile. Couple days later it died on me. Turned out I routed the O2 sensor wire too close to the exhaust where it shorted out, blew the fuse and lost power to the fuel pump.

Droops83

Check the coil negative switch signal while cranking. The easiest way is to hook up a test light to the negative coil terminal and see if the light flashes while cranking, which indicates that the DME transistor is providing a switching signal to the coil. This tells you that the speed/ref sensor signals are likely OK, and that you should probably have spark at the coil. If the voltage at the coil negative terminal stays at 12V or zero while cranking, then you either have a speed/ref sensor problem, or the driver in the DME is bad. Since you had the DME out to access the FQS switch, make sure the connector is good and tight!

Humboldtgrin

Another item to check would be the power wires coming off the positive battery terminal, make sure they are not corroded and secure. They send power to the power distribution block. Also check all the fuses and as always, and put a noid light on the injector connectors to make sure you have injector pulse.

Tom M'Guinn

Looks like your DME has power and is picking up the speed and ref sensors. I'd open the hood and check for fuel and spark. If no fuel, check for a shorted fuel injector connector. If no spark, check coil, coil pulse, bypass KLR, etc.

GJB928

I second the shorted fuel injector connector. Those same symptoms hit me last year. If one shorts they all fail. I had to cut the connector boot back to find it. Fairly easy to rebuild the connectors and boots. It showed up after a cam tower reseal. Took me awhile to narrow down the cause.