Bill Lynch

Encountered a series of engine stall and no start's (cranks but no fire), so I started the troubleshooting normally associated with this failure signature.

What I know:
Fuel:
pressure - 35-40 psi Got this by removeing the DME and bridging 30 to 87b
pressure - 35-40 psi got his while cranking the engine
Leak Down - 2 bar after 20 minutes
Flow - 40 Fluid Oz/30 sec. taken from after the FPR (bridged relay)

Coil
0.6 ohms resistance across the terminals
6k ohms resstance from terminals to secondary
Spark - was unsuccessful in producing a spark, although may be a technique error

Spark - from spark plug wire: plug 1 out and laying on the cam tower got sparking

Compression - not yet checked

Bill Lynch

Sorry - fat fingers hit submit before I finished

Reinstalled spark plug in cyl 1

Pulled the injector electrical plug from cylinder 4 - Engine starts.

I've read many of the threads and performed these troubleshooting steps based on the inputs and experiences of others. What I did not see was what may be causing the electrical loading problem which causes the no start when all 4 injector plugs are installed. What is this failure signature truly indicative of: Bad DME, Bad Ground, Bad Injector plug\harness or potentially all of the above

Bill

V2Rocket

the injectors are paired in wiring but all fired at the same time.
if the driver was bad youd have 2 injectors not working.

im thinking wiring issue with #4 injector?

divil

Are you sure a bad driver would only affect 2 injectors? The schematic I have shows DME pins 14 and 15 forked from each other after the driver stage...wouldn't that mean all 4 are affected equally?

Also, OP: when it cranks but doesn't fire, do you see tach bounce?

KevinGross

For the 944 and 944 Turbo, they are paired. You can for example diagnose a bad driver or injector by pulling one injector plug at a time -- the engine will run on three cylinders but not one.

For the 944S and S2, separate drivers per injector because the newer Bosch DME was sequential.

Bill Lynch

The diagnosis on my 87 NA continues

I charged the battery last night just to make sure that I had enough amps to continue the tests which require cranking

Replaced both fuel filters; in the tank and after the pump (I had planned on this and had the parts before I started having problems). The pump cover is the worst part of the job, a real PITA

- With everything assembled it cranks, builds fuel pressure and has fuel flow
- I pulled the fuel rail assembly and verified that I am getting spray from all 4 injectors while cranking
- I pulled the plug out of cyl 1 again and checked for spark - no spark
- I attempted to repeat the coil test, by my confidence in the method is low. I got no spark though
Testing the Coil
--- 12 volts at both terminals measuring from the terminals to the negative post of the battery
--- 0.6 ohms resistance between the two terminal posts
--- 6k ohms resistance between the center plug (secondary coil) and either terminal

Coil spark test (per pelican parts tech article)
--- disconnected the green lead from its terminal
--- pulled the coil lead from the distributor center post
--- pulled the spark plug from cylinder 1, plugged it into the lead from the coil (just disconnected from the distributor
--- wrapped a length of 14 gage bare wire around the threads of the spark plug
--- connected the opposite end of the 14 gage wire to the negative battery lead
--- connected a second length of 14 gage wire to the negative post of the battery
--- tapped the coil terminal which the green lead was removed from with the opposite end of the 14 gage wire tied to the battery
--- At this time I am supposed to be seeing spark - but I'm not - I saw a week spark 2 days ago but not now.
While lack of spark points to a bad coil, the nominal readings make me think otherwise.

Any thoughts on whether the issue is coil or DME unit?

ARe there any other tests or measurements I can take which will help me discriminate between the two source candidates?

Bill

Bill Lynch

Divil - Yes I am seeing about 1/16" tach bounce while cranking

blade7

Change the Fuel Pressure Regulator.

divil

Hmm...with good fuel pressure/flow and a no spark condition, I wouldn't worry about the FPR

Bill it sounds like you have diagnosed a bad coil. Your fuel system is working and and your DME seems to be working (tach bounce=base ignition signal, plus you have an injector signal).

I can't verify that your coil test is correct, but if you compare it to the Clark's Garage procedure here that might help.

blade7

Worked for me.

blade7

Alternatively check the track in the AFM for damage.

Bill Lynch

Blade7 - I'll pull the cover later today and check the track.

Also on the list;
- Clean all the grounds that I can find
- disconnect\connect all the leads in the starting circuit (ref Sensor\speed sensor\temp sensor)
- inspect temp sensor connector
- repeat coil test
- resistance and voltage tests on teh ref and speed sensors. Looking for a o-scope to complete the Ref and Speed sensor test

Seems like I still got a pretty healthy list still to do - like I've read others do, I'm quickly turning a 3 grand car into a 5 grand car. It'll be nice when its done though and its quite a thrill when you get the rpm's up.

More later

StoogeMoe

From your coil test, you should have seen the spark. That's what the DME does when it's running. It grounds the green wire momentarily to create the spark. Do you have another coil to try?

Bill Lynch

StoogeMoe - Unfortunately I do not - no spares of anything (yet). Besides work, I have a number of family things to attend to, so I likely won't get back to testing until Tuesday night. Trying to get an o-scope from work to test the ref sensors. Would like to scratch that one off the list.

By the way, has anyone else seen this video (RPM and TDC Ref Sensor testing)? Thought the only way to test the voltage was an o-scope (unliess that is a specialized volt meter he has). Comments on the Video would be appreciated


Bill

divil

The test in the video is better than nothing, but not as definitive as testing with a scope. It shows you that they're generating *something*, and that it is in the right ballpark range for the peak to peak voltage.

But, since you are seeing your tach needle bounce, that probably means your speed/ref sensors are fine (what makes the tachometer bounce is the ignition signal generated by the DME, for which it needs the speed/ref sensor signals).

Still if you can get a scope then by all means use that to make 100% sure. The ultimate test of the sensors is via the DME connector *while it's connected*. (i.e. remove the plug cover and insert wires or probes through the back of the pins). They can test just fine with an open connector, but fail when when the DME plug is attached, if the signal lead is internally shorted to the shield.