darklordftt

So the other day, my '91 ZR1 startup and then stalled about 7 seconds later. Now it will just crank over and never start. I checked both fuel pump fuses and they are OK. It has 43 to 45 PSI while trying to start as well. I took video of the spark gap testing we did. Is this bad spark? I was expecting it to me more blue. Also after awhile of cranking the car, I started to notice a knocking sound coming from the engine. Does anyone know what may be causing that? I pulled the plenum last year and did the injector swap, and I'm prepared to pull it again should it be the ignition control modules. There is no check engine light, and I used a paperclip to check the OBD I system for any codes... none. It just repeated code 12 for diagnostic mode and never changed after the third time (after 3x any codes are then be flashed).




Also, here is a full review of the car I did for our
Please help us get her started and running again.

mhobtr

You need to check all the plugs, not just two. There are four coils, each fires two plugs, one in each bank.

Mine ran with one bad coil but not well. I think another was marginal. I replaced all four with MSD's and have been very pleased with the results.
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darklordftt

we did 3 on the left side, and 2 on the right. I didn't want to crank it over any more due to that knocking sound. It has fuel and air, it has to be the coils, right? Could it possibly be anything else?

mhobtr

If this were a carb car, I would try squirting some starting fluid (ether) into the intake to see if it fires. Some of the more experienced LT5 guys might nix this so I would research it some before I tried it. If it cranks when starter fluid is applied, you most likely have a fuel problem.

The coils can give false readings when tested like you did. I'm not 100% sure about this on the LT5 but Harley Davidson uses a similar set up in that both plugs fire at the same time but the cylinder in compression gets a high discharge while the cylinder in exhaust gets a small spark. This due to the higher impedance in the cylinder under compression. When you have the plugs pulled, the cylinder impedance differential is not there so you may be seeing what appears to be a strong spark when it isn't.

All of my plugs sparked as yours but turned out I had a bad coil. The best way to check them is with an ohmmeter across the primary and secondary terminals. If they are not in spec, toss them and install new ones. You can replace a bad unit if you want, but I would replace all four unless they are fairly new.

mhobtr

BTW, if you install new coils, check their primary and secondary impedances befor you install them. Sometimes a new one can be bad.

darklordftt

I'll do that and get all 4, thanks.

mhobtr

If you buy off ebay, be careful. There are sellers on there that clean used coils and sell them as new.

I wanted to go with MSD so I bought mine from Jegs. If you want OEM, you can't go wrong with Jerry's if he has some in stock.

darklordftt

thanks, I'll check jerry's and compare with jeggs and summit for msd

Z51JEFF

darklordftt

I missed this post earlier. I'll be sure to check that fuse. I'm in Boston for a few days and will check it when I get back on Wed. Your link also gave me the MSD coil part number I needed, thanks. MSD 8224 Coils.

darklordftt

I pulled the fuel pump fuses individually, and get 45 to 50 psi for both pumps when I give 12v to the fuel pump test lead near the widow washer motor. I tested the coils thru the wires, and they are all around 19k ohms. And I'm sure that knocking sound is the starter. It still cranks.

What wold cause the injectors to not spray? How do I test if they are spraying? I never got that flooded smell when I cranked it over and over before(only did it once, I know it's bad to do). The tps looks like a nightmare to test with how the electrical connector is bent towards the plenum. It's there a test adaptor?
How do I trouble shoot the map sensor? Still no codes, just 12 over and over again for diagnostic mode.

Tom400CFI

You need to pull the INJECTOR fuses...not the FP fuses.

You can test if they're spraying by charging the fuel rails, disable the fuel pump, then crank and watch your FP gauge. It should drop...fast as the injectors fire.

darklordftt

I'll just prime the fuel system with the test lead, pull both pump fuses, and try and start the car. No need to mess with the injector fuses... which are both fine btw, already checked those. Going to try this now, more to come.

darklordftt

Starting at about 45psi. While cranking, the fuel pressure dropped in 5 to 10psi chunks every 3 to 5 seconds. It was not a steady decline.

Tom400CFI

That seems like odd behavior to me. LT5 is Sequential...right? Should drop every compression, or 4x per revolution of the crank.

If it were MY car, I'd pull the injectors out w/the fuel rail, crank the engine and watch 'em spray.

Another easer but more costly test, b/c you need to buy a tool, is you can buy a tool that plugs into one injector. You can charge the fuel system, then press a button and the injector fires for a precise amount of time. You write down the pressure drop and move on to the next injector, re-charge fuel system and test again, write down the result. Repeat for all 8 primary injectors (secondaries aren't causing your issue).


So there are two options you could pursue.

darklordftt

What would cause the injectors to screw up? I'll post a video of the fuel pressure gauge as the engine cranks:

darklordftt

mhobtr

As I said earlier, I would try a shot of starting fluid to see if it fires.

Aaron71771

Have you checked the fuel injector relays? Do you have a service manual?

Tom400CFI

What are fuel injector relays? Never heard of such a thing.


That is definitely not how it should behave. Looks like at most, one injector is firing.