This artical 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 teh Car scaned for Diagnostic Trouble Codes to help pinpoint an easy problem.


Before we begain lets have a quick overview on the use of a test light. A testlight is a very simple and easy to use diagnostic tool. You can pick one up at any parts store or tool store. The testlight consits of 4 basic parts. a clear body, a light bulb inide 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 soruce (usualy the Negitive 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 (usualy the positive battery terminal) but the alternator wire and starter wire are other choices. Thats all there is to it.


We also need to get the thory 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. Usuing a test light or multi meter you will have to peirce through the wire to get a reading. The reason for back probing is to test the system with the component pluged 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 http://shbox.com/1/fuel_schrader_valve.jpg
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 refered to as KOEO (Key On Engine Off)

With KOEO the fuel pressure should imediatly 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 perfromance.

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

Another cause of slow bleed down is a stuck or shorted injector. this will usualy cause a hard start and can be very very damadgeing 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 usualy 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 plug wire from the ignition coil. Clip your test light to the neg 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 wireing problems in the fuel system. a picture of the location of the connector can be found here http://shbox.com/1/fuel_pump_prime.jpg

With battery volatge 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 drivers 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 at the tank or harness block. Does the test-light light up?

No, Fix short in wire between 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 http://shbox.com/1/fuel_pump_relay.jpg *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 ahve a ground?

Yes, contunie 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 teh orange or pink wire depending on year. do you have hot?

yes, continue to next step
No, check iner panal and fuse panal 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 teh PCM possible faulty PCM.

if you have all hots and ground at teh relay next check the function of the relay itself. do this by checking for hot at the gray Wire with your testlight with the engine cranking. Do you have hot?

yes, relay and wireing is working correctly. veryify battery voltage with a Multi meter
No. unplug relay. with key on engine off use a fused jumper wire to aplly 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 pluged in





No Spark Fuel ok Diagnostics


The first thing we will do is verify the optical sesnor is working correctly. and here is how..

unhook 1 fuel injector connector. with your testlight 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 teh connector and crank the car. The light should flash..

Do you have a flashing ground while cranking?

Yes, the optical sesnor is sending the signal and should be ok, continue to to test the ign 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 on the pink wire. do you ahve hot?

yes, next step
No, check ign fuse, repalce and retest.

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

yes, next step
no, repair ground

next test for a pusing ground at the green wire 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 testlight check for hot on the Pink wire at the oil. do you have hot?
yes, Replace your coil *note, High resistence 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




OPTI testing

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

With teh testlight and the car on KOEO test for hot at the red wire at the opti. do you have hot?

if yes. then the optical sensor is not working and is porobably bad. based on the other tests run this is causing your no spark condition. it;s time to replace your opti.

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




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 damagde in the opti. to check for this make sure you have spark at the OPTI end of the coil wire. Use your grounded testlight to check that. if you have spark at teh 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 ahve spark in and non out the rotor was damaged and needs to be repalced.


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 compresstion but it will not run well.



I am still amending and correcting this artical. please bear with m

This article can be found at:

http://mainstreamtopics.com/forums/i...p?showtopic=78

 

Wow, that's a lot of work and will be extremely helpful to a lot of people when it's done; good luck and keep at it!

 

Excellent post!

 

now what? car still doesn't start....seriously

 

Seriously? Everything is getting power/spark/fuel? Are you getting spark at all the plugs?

Did you do a compression test? Your plugs might be really fouled, but i cant really see plugs causing that much of a problem. Is all the wiring right? Did it just die or did you rebuild it and put everything back together?

 

no compression test...I made a thread about it, but basically I put on some long tubes and ran open headers for like 30 miles and the car developed a bad hesitation when it got real warm, then the idle was off, then the car wouldn't start, it went downhill fast, I beat a C5 the weak before. No bad sounds or anything, replaced the opti, put new plugs in it, I have spark at the 2 plugs I tested and I get some backfires on startup and got smoke out the intake last night...ugh...went over the plug wires 3 times and everything is installed right, makes zero sense.

 

Do you have a spare coil to try? Did you try the spark test by using a spark checker or making sure it jumps a substantial gap; i.e. about an inch as posted above? A weak coil can cause problems and is often overlooked because people see spark and rule it out.

 

Just thought I would bump this for refrence

 

Why not just provide the link to the article you quoted, so the guy that wrote it can get the credit?

 

My bad... found this article and was like WOW.. gotta share this.. fixed