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Lt1 No Start Diagnostic

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#1 HBHRacing



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Posted 26 January 2004 - 07:18 PM

This article will help to diagnose and repair an LT1 No start condition.


Specialty tools you will need...
12Volt Test light
Digital Volt Meter (DVM)
Noid Light (auto zone Part number OEM 25143)
Fuel Pressure Gauge with GM fuel rail fitting
A friend to help you out


If possible It's a good idea to Get the Car scanned for Diagnostic Trouble Codes to help pinpoint an easy problem.

Before we begin lets have a quick overview on the use of a test light. A test light is a very simple and easy to use diagnostic tool. You can pick one up at any parts store or tool store. The test light consists of 4 basic parts; a clear body, a light bulb inside the body, a sharp metal probe on one end of the body, and a wire with a clip on the other end. When you are trying to find Positive voltage place the clip on a good ground source (usually the Negative battery terminal) however any motor or chassis ground will work. When you are looking for a ground signal you must place the clip on a positive feed (usually the positive battery terminal) but the alternator wire and starter wire are other choices. That’s all there is to it.

We also need to get the theory of Back probing out of the way. The idea behind back probing is to probe the actual wire itself as near to the component that your testing. Using a test light or multi meter you will have to pierce through the wire to get a reading. The reason for back probing is to test the system with the component plugged in and active.

First thing we will do is check for Fuel.

Install the fuel pressure gauge onto the shreader valve on the fuel rail. a picture can be found here
Once you have the gauge installed turn the key on but leave the engine off. this is the first key position BEFORE start and is referred to as KOEO (Key On Engine Off)

With KOEO the fuel pressure should immediately go to 43.5psi +/- 2 PSI

The next important thing to do is watch the gauge. The pressure should hold right about there. Any bleed off could cause no start, hard start or bad performance.

If the pressure drops slowly you could have a leak in a line or fitting or possibly a faulty fuel pressure regulator. Usually if you pull off the vacuum line to the regulator it will be wet if it is faulty.

Another cause of slow bleed down is a stuck or shorted injector. this will usually cause a hard start and can be very damaging to the motor since fuel is filling that cylinder when ever there is pressure in the lines.

If the pressure bleeds down fast it is usually due to a faulty fuel pump check valve and the fuel pump will need to be replaced.

Now that we know if you have fuel lets see if you have spark

remove the spark plug wire from the ignition coil. Clip your test light to the negative post of the battery. Hold the test light probe close to the coil where the plug wire attaches to the coil. Have someone crank the car and spark should ark to the test light probe. It should be a strong blue color and arc a gap of around an inch. No we know if you have spark.

No fuel Spark ok Diagnostic.

You have a problem with your fuel system to where either the pump is not working or it's not getting to the fuel rail. With your pressure gauge attached to the rail, take a wire lead from the pos battery post to the fuel pump prim connector. This connector runs directly to the fuel pump and will activate it and let you know if you have wiring problems in the fuel system. A picture of the location of the connector can be found here

With battery voltage do you se 43.5psi on the fuel gauge?

If yes, see Fuel System electrical diagnostics.
If no, see Fuel pump testing

Fuel pump testing

The only real good way to make sure the pump is bad is to check for power at the sending unit. The sending unit witch holds the fuel pump is located inside the fuel tank. The other option is to gain access to the electrical harness block at the rear of the car. It is located on the driver’s side sub floor in front of the rear end. Locate the gray wire in the harness block. Unplug the harness and probe the terminal that corresponds to the gray wire. With the prime connector jumped with battery voltage, probe the Grey wire terminal at the harness block. Does the test-light light up?

No, Fix short in wire between the fuel pump relay and pump

yes, check for ground on the black wire.

Does your test-light light up?

No, repair ground wire

Yes, Replace fuel pump

Fuel system electrical diagnostics

First we need to get at the fuel pump relay. Here is a picture of it *note 93-95 and 96-97 systems are slightly different*.

With your test light at the relay test for ground on the black/white wire. Do you have a ground?

Yes, continue to next step
no, repair ground wire.

Next with your test light and the key in the Key on engine off (KOEO) position test for hot into the relay at the orange or pink wire depending on year. Do you have hot?

Yes, continue to next step
No, check inner panel and fuse panel fuses and replace any bad fuses and recheck

next check for hot on the Dark Green and white wire while cranking. This is a hot from the PCM to trigger the relay. Do you have hot?

Yes, continue
no, check wire back to PCM. If still not hot directly out of the PCM possible faulty PCM, or PCM power feed problem.

If you have all hot and ground at the relay next check the function of the relay itself. Do this by checking for hot at the gray Wire with your test light with the engine cranking. Do you have hot?

Yes, relay and wiring is working correctly. verify battery voltage with a Multi meter and for a break in the wire between the relay and the pump.
No. Unplug relay. With key on engine off use a fused jumper wire to apply voltage from the pink or orange wire terminal to the gray wire terminal and recheck for hot at the gray wire. if you have hot replace the relay and recheck gray wire with the relay plugged in

No Spark Fuel ok Diagnostics

The first thing we will do is verify the optical sensor is working correctly. And here is how..

Unhook 1 fuel injector connector. With your test light check for pulsing ground on the NON PINK wire while cranking the engine (different years use different color wires but all year have pink wires for the positive voltage side of the connectors) The other choice you have is to use the noid light (noid light AZ OEM part number 25143), plug the noid light into the connector and crank the car. The light should flash..

Do you have a flashing ground while cranking?

Yes, the optical sensor is sending the signal and should be ok, continue to test the ignition system.

No, the optical sensor is not working see testing opti section.

Next we will see if the coil and ICM are causing the no spark.
First on the ICM connector with KOEO check for hot (93-95) on the Pink/Black and white/black wires or (96-97) on the Pink and Dark green wires. Do you have hot?

Yes, next step
No, (93-95) Check for hot at the pink wire a the ign coil. If you have power at the coil, but not at the ICM, The coil is shorted and preventing power from reaching the ICM. Replace the coil and retest. If you do not have hot at the coils pink wire, check the 10 amp Ign fuse. Repair as needed and retest.
(96-97) If you are missing hot on both wires at the module, check the 10amp ign fuse, Repair and retest. If you are missing hot only on the dark green wire, but not he pink wire, the coil has shorted and is preventing the voltage from reaching the module and must be replaced.

Next check for ground on the black wire. do you have ground?

Yes, next step
no, repair ground

next test for a pulsing ground at the dark green wire(96-97) or the White/black wire (93-95) at the coil coming from the ICM. Do you have a pulsing ground?

Yes, next step
No Replace the ICM *note, I have seen a bad coil burn up an ICM. It might be a good time to replace or upgrade your coil*

next with your test light check for hot on the Pink wire at the coil. do you have hot?
Yes, Replace your coil *note, High resistance from bad plug wires can burn out coils. You can Ohm test plug wires or replace them if wanted*

No, Check and repair ign fuse and retest

OPTI testing

If you failed the injector pulse test we must make sure that all the power is getting to the opti.

With the test light and the car on KOEO, unplug the opti connector at the optispark and test for hot at the red wire at the opti. do you have hot?

If yes. Next step

if no, the pos feed comes from the computer. Check that the PCM is sending the signal, if not then you might have a faulty PCM

Next, using a multi meter on volts scale, test the red/black and purple/white wires for 5 volts. Do you have 5 Volts?
Yes, next step
No, The 5 volt reference is provided by the PCM, possible PCM failure or PCM power feed problem

Next, test the pink/black wire for ground. Do you have ground?
Yes, then the optical sensor is not working and is probably bad. based on the other tests run this is causing your no spark condition. It’s time to replace your opti. You can also check for corrosion or damage to the opti connector and replace as needed.

No, the ground is provided by the PCM, possible PCM failure or PCM power feed problem.

No Start, Spark ok Fuel OK diagnostics

If you have fuel pressure and you have spark at the coil there are only a few more reasons the car will not start.

First and most popular is Rotor damage in the opti. to check for this make sure you have spark at the OPTI end of the coil wire. Use your grounded test light to check that. if you have spark at the coil but not through the coil wire replace the wire. if you do have spark going in check for spark coming out of the opti. if you have spark in and none out the rotor was damaged and needs to be replaced.

Something that I have run into that causes a no start is a bad coil wire. Sometimes the opti end of the wire will crack and distort causing a lot of internal arcing and bad contact. If you look into the opti end there should be a plastic insert that keeps the wire centered and making good contact. if it’s cracked or broken replace it.

another no start problem could be no injector pulse. Use a noid light to verify injector pulse. if you have none check for the ground signal from the pcm. and check for the KOEO hot signal on the pink wire. They go back to 2 fuses in the fuse box.

Another point to consider is compression. a cyl will run on about 70 pounds of compression but it will not run well.

I am still amending and correcting this article. please bear with me

Please feel free to view the rest of the site that this article is posted on. anyone is free to join. we have several car forums and welcome new members http://mainstreamtop...dex.php?act=idx

I would like to thank Rob AKA Shoebox for the use of his pictures, you can find more at his site

This article writtcn by Harlan Hynes. All content is protected. The reproduction of this article is expressly forbidden and is only to be used for personal use. If you wish to quote or use a part or all of this article on another site, please email [email protected]
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#2 Sean


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Posted 26 January 2004 - 07:38 PM

Well done man.. Good job

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#3 GreenGoblin

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Posted 29 July 2004 - 04:28 PM

hmm i am throwing 372 but my car starts?

#4 HBHRacing



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Posted 29 July 2004 - 09:18 PM

yes. thats cause your missing the high res refference. that means that your gonna get bad performance and fuel especialy at high RPM or load. chances are the opti is bunk. but you can inspect the elec harnnes to make sure that it's ok Extreme Performance your number 1 source for Fbody manual fan switch's and more

#5 brandboZ28

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Posted 29 July 2004 - 11:12 PM

Ive used that before, multiple times laugh.gif

#6 badazzss

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Posted 21 April 2012 - 06:02 PM

I'd like to make a quick addition to this article. I swapped out the opti in my 94 LT1. After doing so, we wanted to quickly test the new opti and make sure it was functional.

We decided to start the car for a minute without the radiator / water pump. It fired up immediately. However, it died quickly and then would not start anymore.

After some quick reading, we tossed just the water pump back on in order to plug in the coolant temp sensor ON the water pump. Car fired right up perfectly.

In my case the no start was just because it wasn't plugged in; but the coolant temperature sensor on the water pump could be bad and cause the car to just crank because of a faulty reading.

#7 RoAdRaGe912


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Posted 26 April 2012 - 08:43 PM

Note first that it's an almost 8 year old thread. It wasn't meant to be complete, nor to address issues like the one you mention. It was aimed mostly at no spark or no fuel issues, with a few "other" ideas tossed in at the end.

A coolant temp sensor can definitely cause hard starts and it's pretty well known to those of us that work on cars, especially LT1s. I've started up LT1s without the water pump (and therefore without the coolant temp sensor plugged in). At the time, the engines had been apart and without fuel pressure, so a hard start would go unnoticed. But they did start, and if they ran ok (mainly checking proper opti install), then we shut them down quickly and then added the water pump.

BTW, It's good to see people still working on the LT1. My latest has been sitting neglected for about a year now.

#8 Heake1978

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Posted 12 June 2012 - 03:18 AM

Ive used that before, multiple times Posted Image

but it is still a big help to everyone that are new to this. like me

Edit- Link removed

Edited by RoAdRaGe912, 06 July 2012 - 08:53 PM.
Spam link

#9 doublez28

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Posted 30 March 2013 - 07:38 PM

i have a similar problem with a 1994 lt-1 camaro and a no start condition. i have 50 lbs fuel pressure on fuel rails. have spark at the coil and to the spark plugs. have 170 lbs of compression and have checked fuel injector pulse with noid light and have fuel injector pulse. with these 3 conditions what else could cause my no start problem. engine will turn over all day long. no spit no pops thru the exhaust or the throttle body. will not fire up even when carb cleaner is sprayed into the throttle body whrn trying to start the engine. i have no codes in computer except code 15 which says in actron tool scanner that coolant temperature is low (cold). no other codes. what am i missing. i had an accel 300+ digital ignition box connected thru a wiring harness and with it connected no spark at all to the spark plugs after taking this out of the equation still will not start up. sounds like it is not even trying to fire up.  what else can i do ? thanks, reply please doublez28.

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