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06-08-2006
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#11 |
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Re: BIG No start problem, please help
Unplugging the cam sensor will not keep the car from starting. It will just not receive the sequence for corrct firing and run in batch mode instead of sequential.
Just out of curiosity, when you pulled the original crank sensor out, did you notice if you had a small shim? |
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06-08-2006
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#12 |
 Join Date: May 2005
Location: Nichols, FL
Posts: 615
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Re: BIG No start problem, please help
Do you have a service manual? Its the best place to start. Having chased a bad dis for 2 1/2 years I can understand your frustration. You need to check the crank sensor function, depending on which you have there could be too much gap. I do not remember what it should be but this is a known problem. You can put a timing light on it to see if you are in fact getting spark at #1 tdc.
My dis would give good spark at <400 rpm but start falling apart over that rpm. Finally found a dis checker that checks the whole dis system including the cam and crank signals. The tester allows you to simulate the crank and cam signals through crank and running. I put a scope on the crank signal and cam signal and they were the the same as what was being simulated. I then ran the simulation through the dis and you could see the spark fall apart on a scope and at the pulgs. It is called an ISS (Ignition system simulator), made by OTC model # 3575. I bought mine off ebay for $130 and it should be a tester everyone doing LT5 troubleshooting should have. It can also be used to check injectors and coil/wire problems. IE put a plug in each wire, lay them on the plenum and watch the spark across the tip. Any plug with weak spark would indicate bad coil or wire. Again easy to check, replace wire or coil and run the check again. Tyler
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55 Polo White (Project) 61 Jewel Blue (Sold to buy above) 66 Nassau Blue 68 Torch Red 88 ZR1 Blue 90 ZR1 White |
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06-08-2006
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#13 |
Join Date: Jun 2006
Location: Thunderbolt, Ga.
Posts: 253
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Re: BIG No start problem, please help
Yes the crank sensor has a small shim, which I kept in place on the new sensor. When I tried the old sensor to see if it worked I left the shim out and still no luck. I have put the plugs on the manifold and they all fire, nice big sparks, very pretty. I guess I will try this OTC ISS device, it sounds like a great diagnostic tool. I thought the cam sensor was used to tell the ECU when cylinder #1 is on compression stroke, for ignition timing, at least that is how Ford does it. Thanks for the input.
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IT IS THE FATE OF THE INNOCENT TO MEET WITH DEATH!!! Last edited by 8upZR1; 06-08-2006 at 07:35 AM. |
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06-08-2006
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#14 |
Join Date: Dec 2004
Location: Jacksonville, FL USA
Posts: 4,645
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Re: BIG No start problem, please help
Hi, it would be a very unfortunate coincidence on the ECM. I have a 90 and I have a spare, good ECM that I bought...it is really a 91 ECM as I think the 90's were NLA...anyway it works in my car fine so I know it's functioning correctly...if you can't get a known good ECM hit me up & I'll ship my spare to you if you think it will help? Oh, it has a standard EPROM for a 90 in it, but you could swap your EPROM into it.
You can PM or here is my e-mail, ThomasTom72@aol.com.  Tom sorry I keep posting with no answers to your situation. I'm new to EEC motors...last vette was a 72!
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1990 ZR-1, Black/grey, #2233, stock. ZR-1 Net Reg Founding Member #316 & NCM member |
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06-08-2006
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#16 |
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Join Date: Jan 2004
Location: Southern CA
Posts: 108
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Re: BIG No start problem, please help
A couple years ago I tried three different crank sensors from my local dealership and all three had the magnet installed backwards. This reverses the polarity of the signal generated by the crank sensor and the car will not start. You can tell if the new sensor has the reversed magnet by holding the magnet ends of the new and old sensors near each other, if they repel each other the magnet is correctly installed, if they attract each other the magnet is reversed.
I made a couple of jumper wires to reverse the polarity of the signal and the car would start and run with the sensors with the backward magnets. The engine had to crank longer than normal to start any time I changed out the crank sensor. I talked to one of the service people at the dealership when I found the reversed magnets and he checked and found GM had other reports of reversed magnets. |
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06-08-2006
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#17 |
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Join Date: Jun 2006
Location: Thunderbolt, Ga.
Posts: 253
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Re: BIG No start problem, please help
Thankyou so much for the DIS diagnostic info. I appreciate the offer of using your ECU, maybe if nothing else works I will take you up on it. I will be going over the car today, hopefully I will make some sort of progress. While I am in the garage I will also check the magnet polarity on the crank sensor. Does anybody know what function if any the oxygen sensors play during cranking? I would think that they only come on line once the engine is running but if they are used during start up maybe I should check them out.
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IT IS THE FATE OF THE INNOCENT TO MEET WITH DEATH!!! |
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06-08-2006
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#18 |
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Join Date: Jun 2006
Location: Thunderbolt, Ga.
Posts: 253
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Re: BIG No start problem, please help
I have been playing with the vette all day today and have found some what I belive is the cause of my problem. I tested the spark plugs again by removing them and placing them on the plenum. Everything OK as usual. Then I cranked the car and no start. Removed the plugs again and retested them to see if the fuel on the plugs was hindering their work. To my astonishment, cylinders 1 and 6 and 2 and 3 fired while the others barely produced spark. I cleaned the fuel off of the plugs and then retested. This time 4 and 7 and 5 and 8 did not spark at all. In all previous tests they gave good spark as did the others. What I really do not understand is that the way I have the MSD box set up 1,6,4 and 7 are run by channel 1 and 2,3,5,and 8 are run by channel 2. I then unplugged the MSD and still the same deal, no firing on 4,7,5,and 8. So it is not the MSD. But what could it possibly be? Wiring? Ignition module? Tonight I will replace the ignition module under the manifold with a spare I keep around. It seems strange to me that it could be the module because I replaced it about a month before the head gasket failure and everything was running top notch. If anybody knows what effects only 4,7,5, and 8 please let me know.
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IT IS THE FATE OF THE INNOCENT TO MEET WITH DEATH!!! |
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06-08-2006
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#19 |
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Join Date: Jun 2006
Location: Thunderbolt, Ga.
Posts: 253
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Re: BIG No start problem, please help
Well, that was a waste of time, all plugs are firing again but no start up.
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IT IS THE FATE OF THE INNOCENT TO MEET WITH DEATH!!! |
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06-09-2006
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#20 |
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Join Date: Dec 2004
Location: Jacksonville, FL USA
Posts: 4,645
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Re: BIG No start problem, please help
On the O2 sensors, I don't think that they are part of the system until the "closed Loop" cycle has been reached. On my scan tool they don't send any data till the system gets to "closed loop".
On the ECM thing...it's here & let me know if you want to try it!
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1990 ZR-1, Black/grey, #2233, stock. ZR-1 Net Reg Founding Member #316 & NCM member |
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Linear Mode
