Went to take the car for a drive today and it wouldn't start.

Turned the key, it cranked fine, fired a couple times but never started. Gave it a minute or two and tried again. Cranked over fine but this time didn't seem to fire at all. Tried this a couple of times then I could smell gas. Opened the hood and looked around but didn't see any leaks or anything unusual.

Left it sit for a few hours and tried again but same thing happened. Cranked over fine but doesn't seem to be firing at all. Sounds like the fuel pump in the tank is turning on and I smell fuel vapor.

What/where is a good place to start checking?

 

Have you verified that you have fire?

 

check to see if you have spark, check the fuses, ohm out the injectors, ect...

if the engines been swapped before check the grounds on the back of the heads to see if theyve come loose.. i had that problem before

 

Check to see if your getting spark with a spark plug tester, if thats fine, check for a clog in your fuel filter. could also be the net in your gas tank fuel pump.

 

No spark. No fault codes.

Did the HEI module checks and that seemed to point to the coil, but the coil checks out spot on for primary and secondary resistance and no shorts to ground. Not sure why, but for some reason I checked the coil wire resistance when I was toying with the multimeter and found that it was open circuit.

Dug into that a bit more and found the end of the coil wire, carbon core and jacket/insulation, were literally burnt away from the terminal inside the boot, nearly an inch, almost to the other end of the boot where the coil wire enters. I would have expected at least some resistance, albeit it high, from the burnt/carbonized material but apparently burnt silicone is non-conductive and isn't carbonized, per se.

Took out the module anyway to have it tested but hopefully I get by with just a new coil wire. Guessing this might explain the intermittent rough idle and such when it last ran that I'd almost forgotten about (don't drive this car very often).

 

Funny, I was just going to ask if you had checked the coil and wire ends. When doing a spark check on an ext coil distributor engine, always check at atleast 2 plugs first then at the coil to be sure it's not just a bad coil wire, cap ,or rotor. Good job finding the bad coil wire.

 

This is what I like to see. Good job!

 

Not so lucky on two counts.
1. Local auto parts store can't test the module "it's too new". 1991, new, who'd a though?
2. New coil wire itself didn't fix it, even though that was absolutely hosed. I suppose the stress of trying to spark though that bad wire made something work too hard and burn out.

Fault troubleshooting tree at this point is basically flip of the coin for coil or ignition module.

Note the ignition coil is not OEM. It's an Accel Super Coil (140008, I believe) for remote mount HEI. This coil has been on the car for roughly 20 years...bought it when I was younger and more impressionable. Unfortunately, I am currently unable to find the OEM coil and OEM coil bracket or I'd try that before buying a module.

So, at this point, any thoughts on which I should try first...coil or module?

 

if you have injector pulse the icm is fine.

 

I don't believe the fault tree ever directed me to check injector pulse but that's easy enough to confirm. I do smell fuel though when cranking and I hear clicking (have those noisy Bosch(?) Mustang SVO injectors in it).

Also, the fault tree said to check RPM with scan tool when cranking. This showed/shows 0 RPM. I don't have the service manual here, it's out with the car, but is that what would have guided me to check injector pulse or not to check it?

 

the icm sends a tach signal to the ecm and the ecm commands the injectors to fire. you could check it with a noid light or led and 1k resister. the tach signal your tach gets is pulled off the ignition coil not the icm signal. If your hearing the injectors click I think you have a bad coil, you might check or call another parts store because I have had icms checked.

 

I can and will check injector pulse either with duty cycle on multimeter or scopemeter, or some sort of test light.

Small town...options on auto parts stores are rather limited, and no auto dealers in town.

If I get pulse I'll go the coil route.

 

There are two sides to the ICM. The digital converter that converts the magnetic pulse of the pick up coil to a digital signal and sends it to the ECM, and the amplifier that converts the signal into the primary ignition pulse to energize the coil. At crank speed, the ICM fires the coil based on the crank reference pulse plus it's own built in timing map. When EST is activated at 400rpm, the ICM fires the coil on command from the ECM.

I have seen a few cases where the ICM sent the crank reference signal to the ECM, causing the injectors to fire, but would not support coil firing. Remember, firing the coil is the ICM's toughest job and places alot of load on the small module. This is what heats it up. In a case where the coil may well be faulty from firing through high resistance, I would not be at all surprised to see that the amplifier side of the module has been damaged.

Go ahead and check injector pulse but then check ignition primary pulse. You can do this with a test light. A good coil and ICM will produce a bright flash at the B- (white wire) when cranking with the light's ground clip grounded. If the coil is faulty, or if the ICM is bad, you will only get a dim wavering light. Then, disconnect the coil connector and connect the light's ground clip to batt+ and check for a flashing light at the white wire while cranking. A flashing light tells you that the ICM is grounding and opening the coil primary circuit as it should. This generally means that the ICM is okay. However, once you've installed a new coil, if the test light does'nt flash bright with the coil connected, testing between B- and ground. It is likely that the ICM's amplifier is damaged and will not support coil firing.

 

well said doc

 

Yup, the fault tree took me to this place and I got the dim wavering light. Problem is, this is also the point where the fault tree said (paraphrased) flip a coin, try one or the other and if that one doesn't do it then replace the other.

So, this

sounds like as good or better a way as any to determine the coin toss.

Now, if I could only find the OEM coil and bracket it would make the next step so much easier.

 

post a ad in the classified section here or ask this guy don, he's a great guy. The coil and bracket was used on 87-92 tpi f-bodys.

https://www.thirdgen.org/forums/engi...ake-setup.html

 

Thanks for the lead on that coil and bracket.

Alright...so I borrow the coil from my pickup (wrong bracket but I use a heavy jumper to ground it to the engine) cross my fingers and hit the key, but...no start.

Next I check for injector pulse using a generic test light...not sure what it should look like, but I'd describe it as sorta weak light and seems kind of erratic pulsing.

So, now I do this (because previously I got dim wavering light with coil connected)

Test light is "on" even with key off, then when I crank it light goes back to the dim wavering.

Guess that means I buy an ICM (or try to find a used one), eh?

 

Are all the 8-pin ICMs the same?

Meaning, from a functional standpoint doesn't matter if it came from a truck or whatever?

 

I think you must mean you mean a 7 pin ICM? And yes, except for Cadillac, they are all the same.

 

you can see the application listing of the D1943A here.

http://www.ebay.com/itm/ACDELCO-OE-S...5e7afb&vxp=mtr

 

I guess I didn't check how many pin locations were actually filled but going from memory there is a 4-pin connector and a 2-pin connector that stick out of the distributor, then a 2-pin connector internally that connects to the trigger(?) coil.

It's like the one in the lower left:



The salvage yard up the road has a complete distributor from a 95 pickup for $30 so that's why I was asking, which is about 1/2 the price of getting a module locally. Yeah, I could order for less, but right now I just want it fixed.

 

Oh yes, that's right. The EST, external coil dist has an 8 pin module. For some reason, I was thinking your 91 used the early style pictured on the lower right. The module on the lower left is very common as it was used in the Vortec truck motors through the late 90s. You can buy a cheap version at any parts store for around $26. A premium unit goes for around $50. I wouldn't personally bank on a used dist because of the common issues with the pick up coil. I would tend towards either rebuilding the dist I have or buying a reman.

 

It's alive! Put in the GM/Delco ICM from that used truck distributor and it lit right off.

Just went for a short drive, this thing hasn't run this good for a while. First and foremost it runs again, secondarily the rough idle and occasional misfire I had this summer and almost forgot about are gone.

Thanks for all the help with the diagnostic tips and system info!