I picked up a Fiero with a 92ish 4.9 Caddy swap in stalled progress. Since the wiring was half done, I've had some trouble picking up where the previous owner left off. I have the Caddy computer wired into the Fiero harness, and hooked up to the engine. About half of the distributor was missing, so I picked up a new one. I've been able to get it to run on starting fluid, but haven't gotten the injectors to fire. That's where my problem begins! It started and ran, then the next time I tried it wouldn't spark. Changed to a different coil and it ran again, the next day it wouldn't. Thought it might be a ground. When looking for the possible ground problem, I discovered that when I put in the new distributor, I connected the main connector upside down. That meant that all 5 wires were reversed and the ground wire was grounding to a signal wire to the computer. Corrected my mistake and had high hopes of solving my problem, but no spark or injector pulses. Any ideas of what damage I might have done to the coil, distributor or computer?? Not sure what to try next.
any ideas? Sorry for the long winded post!!
Incorrect wire to the ECM fried it?
They're super simple critters. First thing I check when I have an HEI that isn't working is I replace the module in the base with a known good one. Don't forget the dielectric grease on the bottom. Never made sense to me, but it's a thing.
Check your gap between the rotor and contacts on the cap. Some cheap rotors mold the rotor contact in the wrong spot leaving too much gap. The way I check it is to put a piece of duct tape over the rotor contact and one of the points in the cap. If you don't get rubbing when you turn it, they're not close enough.
Also, put your coil in a toaster oven at 225 degrees for a half hour or so. Re-install it after it cools. If it runs better, it's a bad coil. The oven just cooked out moisture, meaning the sealant between the bits inside has failed allowing air/moisture to make it operate poorly.
I highly doubt this is the issue, but check voltage going to the pink wire while cranking. Sometimes you get an incompatibility between the donor engine and the host vehicle. Most early HEI got power from the ign circuit which is dead during crank, so there was a separate wire and diode from the starter solenoid that provided the 12v during cranking. On later EFI versions that used an HEI, that 12v during crank came from the ECM grounding a relay.
I'm not sure that I've damaged the ECU, but I'm pretty sure that the coil had to be grounding through it. I'll check the voltage. It has had spark with 2 different caps and coils, but both stopped firing after I let it sit for a while. Can't figure out why.
If it's a 7747 ecm they have interchangeable chips with the programing, i have an extra ecm you can borrow to test with.
In reply to Patrick (Forum Supporter) :
Thanks. I have 2 computers , I don't know if either are any good and don't know if they are 7747. No wonder I'm having problems!
On closer inspection, it looks like the black wire that I thought was a ground for the coil was a signal wire. So even though all five wires were wrong, the ECU shouldn't have grounded out the coil. Still doesn't explain my problem, but hopefully I didn't hurt the ECU.
I still need to figure out why I have had and lost spark. The coil checks out, so I need to check the Grounds and ignition module.
Well, I have spark again! Turns out my brand new distributor was crap. Spent 32 whole dollars on it! Guess you get what you pay for.
Put the original back in with anew coil and module, and so Far so good.
In reply to Stampie :
No not that one, I want to see this 4.9 V8 swapped Fiero
In reply to maschinenbau :
I will start a build thread on this one. I'm hoping that I can make this a Challenge car. I bought it with the engine already in it for $600. So far I have spent less than $100 on parts and I have parts to sell!
NICE. 2022 year of the mid-engine Challenge cars...
Mr_Asa
PowerDork
1/6/22 12:16 p.m.
Curtis73 (Forum Supporter) said:
They're super simple critters. First thing I check when I have an HEI that isn't working is I replace the module in the base with a known good one. Don't forget the dielectric grease on the bottom. Never made sense to me, but it's a thing.
The GM HEI ignition modules are super prone to heat destroying them. I had multiple ones die on me and finally got fed up and got a full on heatsink I sourced from a trashed computer at school. Very similar to this one
In reply to Mr_Asa :
I hope that I can get mine to run long enough for heat to become a problem!
Finally got back to trying to get the 4.9 running without starter fluid. I'm not getting any pulse at the injectors. I have power at all of the injector plugs, but no flashing noids. It could still be the VATS in the computer, I have a Vats delete module installed and have tried all of its settings, but I suppose that it could be bad.
my question is... does the reliuctor wheel / Hall Effect pick up in the distributor play a part in controlling the injector pulse? On my distributor the tab that passes through the pickup has surface rust that causes ne to be concerned about its health, but I do have spark.
In reply to Sofa King :
VATS will disable fuel pulse (NOT spark) if it is not deleted properly. I don't think the distributor is at fault if you have spark, unless it's the wrong distributor for this ECM or not wired correctly. Does the ECM have all the appropriate signals from the distributor? In addition to the normal 2 wires every HEI has - 12V Ignition-On and Tach, yours should have a 5-wire plug going to the ECM.
https://www.cadillacforums.com/threads/l26-4-9l-pcm-wiring-diagram.1107072/
If a rusty stator was a problem there would be a whole lot of dead GMs.
In reply to maschinenbau :
It is the correct distributor, it does have the 5-wire plug. I plan to double check that they are routed properly to the ECM.
In reply to bentwrench :
Thanks, I expected that it might be common, I know that it only needs to break the magnetic field. I'm not sure if the sensor is working properly as it looks pretty old also. Would a faulty sensor prevent an injector pulse?
Finally got it to run on its own! Ran it for about 15 seconds and the VATS shut off and the injectors took over. Ran on the injectors for about a minute before I shut it off! Smokes, but I think it is just because the injectors are probably fouled.
Now that I know it will run, the fun can begin!
