1993 LT1 Intermittent Crank-No Start

[Edward J.]

Re: 1993 LT1 Intermittent Crank-No Start

Edward,

I also expected greater fuel pressure than a dribble. I didn't attach the fuel pressure gauge while driving (as someone earlier suggested) because I was concerned about causing a leak. But in the next day or two, I'll check pressure and spark and report back. I'm actually hoping it won't start like nothing is wrong so I have a chance to figure this out.

Mark my years at the dealer I. Drove many cars with the pressure gauge to help with dia. As long as no leaks at fitting, if your going to be driving more than 15 minutes,stop and look under hood for leaks, another thing you can do is leave the a/c on outside air if you smell fuel pull over and remove gauge.

[Mark E.]

Re: 1993 LT1 Intermittent Crank-No Start

I spent the morning diagnosing this. The flowchart in the FSM highlights three main reasons for crank-no start:

1- No fuel
2- No signal to the fuel injectors
3- No ignition

I have a fuel pressure gauge and in-line spark plug tester for 1 and 3. I got this "Noid Light Set" for #2:

20230114_141437.jpg

The idea is to unplug a fuel injector, plug the right sized "noid light" into the harness, then crank the engine. A flashing light is what you want to see.

Here's the engine with the fuel pressure gauge, noid light attached to #4 injector harness, and in-line spark tester attached to #4 spark plug cable:

20230114_125940 (1).jpg

You can see there's 42 pounds of fuel pressure which held steady for an hour and counting. But while cranking, neither the noid light nor the spark plug tester flashed. It surprise me that there are two simultaneous failures.

Following the diagnostic flowchart, I checked for voltage at the fuel injector harness terminals (2), then at the two harness leads to the ECM that control each bank of injectors. There is voltage in all cases which means the fuel injection circuit is working until it gets to the ECM. The ECM switches ground on/off to energize/de-energize the injectors. So I've concluded this function of the ECM isn't working.

Instead of proceeding with diagnosing the ignition, I stopped there. I was getting hungry :-) and I figured that the most likely cause of two simultaneous failures is a common component (the ECM) and I need to have the ECM repaired anyway.

Can anyone recommend a service that tests and repairs these ECMs? I thought Dave Perry posted with a link to one but I don't see it.

Here's the car with the ECM removed and harness connectors taped up:

20230114_140609.jpg


Comments by neophyte with computer controlled cars:

1. The FSM is invaluable. The schematics and diagnostic flowcharts really help to understand where to look and how to go through a diagnostic.

2. YouTube is invaluable. The FSM doesn't answer dummy questions. For example, I could not figure out how to unplug the harness connector to the fuel injector. I gently pried the locking clip in different directions, but no go. I found a video that showed how to press on the clip to release the connector. (Obvious now; not at the time.)

3. "Modern" cars require some tools that an experienced "analog" mechanic does not have. For example a fuel gauge with a schrader connection, and a fuel injector test light set. Luckily they're not expensive.

3. OBD cars have trouble codes to help with diagnosing the problem. No codes were triggered in this case even though there are two failures (no spark and no injector signal). My theory is codes point to a failure to a component attached to the ECM, but do not detect a failure of the ECM itself. I'm interested in comments about this.

So if you can read a schematic and flowchart, working on these cars is not so scary (I'm saying confidently even though the car isn't fixed yet).

I'll keep you posted with progress (or set-backs).

[Edward J.]

Re: 1993 LT1 Intermittent Crank-No Start

Mark, the computer is looking for a Rpm signal to to fire spark plugs and injectors. When you look at data on scan tool do you see a RPM signal.or on dash the Tach should see something register when cranking.

[Edward J.]

[Floyd B.]

Re: 1993 LT1 Intermittent Crank-No Start

Is there ANY chance the Opti is getting wet. If it does, the car won't start until it dries out.

[Douglas C.]

Re: 1993 LT1 Intermittent Crank-No Start

Mark,

Fellow C4 owner in DFW here. I had a similar issue about 5 years ago. I replaced the starter relay, had the starter re-built, installed a rew battery checked ground wires, fuses, etc. and the problem persisted. The car finally ended up not even getting the "click" out of the starter solenoid.

After about 3 months of this - I looked closely at the positive battery cable, especially in the unreachable location between the engine and the firewall, and found it to be completely corroded. After replacing the positive cable the car fired right up.

Possibly something else to look at.

[Joe L.]

Re: 1993 LT1 Intermittent Crank-No Start

Mark,

Fellow C4 owner in DFW here. I had a similar issue about 5 years ago. I replaced the starter relay, had the starter re-built, installed a rew battery checked ground wires, fuses, etc. and the problem persisted. The car finally ended up not even getting the "click" out of the starter solenoid.

After about 3 months of this - I looked closely at the positive battery cable, especially in the unreachable location between the engine and the firewall, and found it to be completely corroded. After replacing the positive cable the car fired right up.

Possibly something else to look at.

Douglas------


This is a problem that's more common than many people think.

[Mark E.]

Re: 1993 LT1 Intermittent Crank-No Start

Mark,

Fellow C4 owner in DFW here. I had a similar issue about 5 years ago. I replaced the starter relay, had the starter re-built, installed a rew battery checked ground wires, fuses, etc. and the problem persisted. The car finally ended up not even getting the "click" out of the starter solenoid.

After about 3 months of this - I looked closely at the positive battery cable, especially in the unreachable location between the engine and the firewall, and found it to be completely corroded. After replacing the positive cable the car fired right up.

Possibly something else to look at.

The engine cranks fine but does not start. So the battery and starter system are working.

[Mark E.]

Re: 1993 LT1 Intermittent Crank-No Start

^^ THIS ^^
The ECM won't fire either the injectors or ignition if it doesn't know that the engine is turning. This signal comes from the optispark.

Anyone that has spent more than 2 minutes on Corvette Forum knows that the 92 and 93 optisparks never cause trouble. /SARC (My recently purchased 1992 had its opti replaced at 3,200 miles and 4 months. The early (92-93) optis have a terrible reputation for reliability.)

While the 92 and 93 ECM are problematic, the opti is more likely to cause intermittent crank/no-start. When the ECM shoots craps it never runs again until it is repaired.

EDIT:

Having said all the above, the fuel pump should run for 2 seconds and stop when the key is first turned on. You mention "don't hear the fuel pump", and "gas just dribbles out". If the pump runs for 2 seconds, there should be 40 psi. The opti has nothing to do with the 2 second run on power-up. The ECM does, however.

Begin with the basics. Is the fuel pump running or not with power-up? Is there fuel pressure or not after the power-up?

Get your diagnostics correct, or you're going to replace good parts with Never Ever Worked (NEW) introducing the possibility of bad new parts, and become really frustrated.

As mentioned earlier, fuel pressure is normal (a bit above 40 psi) and holds for over an hour.

[Mark E.]

Re: 1993 LT1 Intermittent Crank-No Start

Is there ANY chance the Opti is getting wet. If it does, the car won't start until it dries out.

No evidence of moisture or coolant loss.

[Mark E.]

Re: 1993 LT1 Intermittent Crank-No Start

Mark, the computer is looking for a Rpm signal to to fire spark plugs and injectors. When you look at data on scan tool do you see a RPM signal.or on dash the Tach should see something register when cranking.

Edward,

I don't have a Tech 1 (the scanner the FSM mentions). For this and future diagnostics, can a modern scanner be used with an adapter? Or do I need to find a 30 year old Tech 1?

While cranking, the tach needle flutters a bit (even goes a bit below zero) but does not register engine crank speed.

The FSM says to disconnect the tach during diagnostics, which I did. Going through the chart:

- NO secondary voltage at the plugs or coil.
- There IS primary voltage at the Coil and Ignition Coil Module (CKT 39 and 121)
- NO voltage from the ECM to Ignition Coil Module (CKT 423)

The chart then says to check for "low resolution signal" using the Tech 1. Is this the same as the RPM signal? (I know the distributor has a low and high resolution signal it sends to the ECM.)

If there IS a low resolution signal, this points to a bad ECM.

If there is NO signal, then I need to check voltage at Terminal C on “Ignition System Test Connector”. This is the lead from the ECM that provides power to the distributor. This will help determine if it's a bad ECM or distributor.

So I need to somehow determine if there's a signal, going back to my question about what type of scanner I need to get.

Thanks again!

[Floyd B.]

Re: 1993 LT1 Intermittent Crank-No Start

Mark,

I highly recommend borrowing a Tech II from a fellow Texas member to see if it supports your '93. I'm pretty sure the Tech II supports GM cars back to '91 (i.e. anything with an OBDII port). Original/used Tech I's are available on Ebay sometimes but they have limited functionality and would only support '91 - '95. I have never seen a Tech I clone. The Chinese Tech II knock offs are plentiful and cheap and I think they are backwards compatible with Tech I. I bought one from a US seller and it has been one of the best tool investments I have ever made. I have used it for diagnostics and maintenance on my '96 and my wife's '98 on many occasions. There are certain jobs that can only be done with a Tech II, for example, cycling the secondary valves in the ABS module as the final step when bleeding the system. Plus they support most GM cars up through 2013.

I was worried about spending $250 on a Chinese knock off so I did a lot of research before making the purchase, because I wasn't going to spend $1000+ for a genuine used Vertronix. Repair shops that use these things daily and own both original Vertronix and Chinese knock off versions have reported no differences in functionality or quality. The knock offs come with all the attachments and TIS2000 software. (I've never found a need to use the software)

Perhaps others can chime in with experience using the Tech II with pre-OBDII (i.e. 1996) Corvettes.

Floyd

[Mark E.]

Re: 1993 LT1 Intermittent Crank-No Start

Dave,

Responses below. Thanks again for your help.


Sometimes you have fuel pressure and sometimes you don't? This can also be a diagnostic hint.

Let me clarify. I believe the car has had fuel pressure consistently. When I press the schrader valve, it dribbles. But when I attach the pressure gauge immediately afterwards, it shows 40 psi. Maybe I'm not pressing the valve down enough to release the pressure. I'm new to FI so this may be mechanic error.

The 1992 and 1993 is susceptible to two issues that can cause intermittent crank/no-start. Internal ECM problems. And opti-spark problems. There are other possibilities as well such as wiring connections, but things like fuel pump, ICM, are usually good or bad, not "intermittent".

Here's my dumb-ass's thought train for your symptoms:

The ECM controls the fuel pump for the 2-second prime. If there is no 2-second prime, it could be the ECM, or something with relay, wiring, and the pump itself.

The car has had two fuel pumps, the original and a new Delphi. I can't hear the original pump but have confirmed fuel pressure goes from zero to 40 psi almost immediately when the ignition is turned on. With the Delpi, I can hear it for a couple of seconds, as expected. So I believe the fuel system is working normally, albeit the original pump is very quiet. Currently, the original is installed. I have a second fuel pump because I got a new fuel sending assembly complete with a pump. I did this because of fuel gauge issues; the pump works.

After the 2 second prime, the ECM will again activate the fuel pump if it receives reference pulses. These pulses come from the opti. After the fuel pump stops, give a brief crank and stop but leave key ON. Does the fuel run for 2 seconds and stop? If = YES, the opti sent pulses, the ECM can control the fuel pump, and the pump functions. (Hint: Injector pulses or ignition.)

Will the pump activate again if fuel pressure remains at its set pressure? Doesn't it need to drop before the ECM activates the pump?

Reference pulses are necessary for the ECM to control spark and injector operation.

Something we haven't talked about is "Spark". The ECM controls the Ignition Control Module (ICM) which fires the coil. The ECM of course requires reference pulses, but the ECM has an output to the ICM for spark control. If this circuit or the ICM is faulty, there can be fuel pressure, and injection, but no spark. (I didn't look it up, but I believe Code 41 in Module 4 (ECM) is associated with spark control and ICM.)

There's NO voltage from the ECM to Ignition Coil Module (CKT 423). See my prior post to Edward for more detail with primary ignition circuit.

According to the FSM, DTC 41 sets if an IC circuit (CKT 423 from the ECM to the IC Module) problem occurs during cranking. Oddly, there's no ECM or CCM codes showing.
There IS fuel pressure.
NO voltage at fuel injector connectors.

Pull a couple of spark plugs. Are they wet or dry? Gives a hint as to whether there is fuel being injected or not.

I'll check the plugs, but since there's no voltage at the fuel injectors, I suspect there isn't any fuel flowing.

That's my thinking: Use the fuel pump "prime" to verify reference pulses, and if pulses present, it is not the opti. Check the plugs for wet or dry. If wet, ignition. If dry, injector control, points to the ECM.

Will check. But please confirm the fuel pump should prime during a second crank even though fuel pressure is already 42 psi. If so, this is an easy way to confirm low resolution crank signal from the distributor.

Interested in your suggestions regarding what scanner can be used... I have a fairly new OBD2 scanner. Can it be used with the right software and adapter? Or can you recommend another device?

[Mark E.]

Re: 1993 LT1 Intermittent Crank-No Start

Mark,

I highly recommend borrowing a Tech II from a fellow Texas member to see if it supports your '93. I'm pretty sure the Tech II supports GM cars back to '91 (i.e. anything with an OBDII port). Original/used Tech I's are available on Ebay sometimes but they have limited functionality and would only support '91 - '95. I have never seen a Tech I clone. The Chinese Tech II knock offs are plentiful and cheap and I think they are backwards compatible with Tech I. I bought one from a US seller and it has been one of the best tool investments I have ever made. I have used it for diagnostics and maintenance on my '96 and my wife's '98 on many occasions. There are certain jobs that can only be done with a Tech II, for example, cycling the secondary valves in the ABS module as the final step when bleeding the system. Plus they support most GM cars up through 2013.

I was worried about spending $250 on a Chinese knock off so I did a lot of research before making the purchase, because I wasn't going to spend $1000+ for a genuine used Vertronix. Repair shops that use these things daily and own both original Vertronix and Chinese knock off versions have reported no differences in functionality or quality. The knock offs come with all the attachments and TIS2000 software. (I've never found a need to use the software)

Perhaps others can chime in with experience using the Tech II with pre-OBDII (i.e. 1996) Corvettes.

Floyd

Floyd,

I would gladly purchase a scanner that works with this car. This car has a 12 pin OBD1 port which is different than the 16 pin OBD2 port. So will a Tech 2 with OBD2 port work somehow?

I have a "modern" Foxwell scanner (about 5 years old) for our OBD2 cars. I called Foxwell and they said I might be able to find a Tech 2 software module to work with GM OBD2 cars. But nothing to emulate a Tech 1 scanner.

So if anyone knows- know what will work other than an original Tech 1 scanner? I know some technicians use SnapOn diagnostic tools that can do many diagnostics, but I haven't researched this.

[Keith B.]