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Hey guys,
I’m completely stumped. I bought a 1989 K2500, TBI 350, and it’s been running beautifully until about a week ago. I found out that the previous owner hadn’t completely tightened down the distributor hold-down bolt, so I decided to double check timing and get it buttoned up. Well, dropped a wrench out of my pocket while leaning over the passenger side of the engine bay, and the engine died. Narrowed my problem down to no spray from injectors, and further narrowed that down to a lack of grounding pulse from the ECM. Turns out the ECM wasn’t getting any power, and I figured out that a fusible link blew. Repaired that, and now ECM has power.
HOWEVER, this is where it gets weird. After all this troubleshooting, I go to start the engine, and it’s running like it’s very far out of time. Backfiring through throttle body, through exhaust, and generally being a cranky lady. I did not remove the distributor, but I double-checked firing order, made sure to set cap and #1 plug to TDC on #1 cylinder. I disconnected electronic advance timing and tried to set to 0* BTDC. It would not run at idle at 0* BTDC, and was backfiring and running horribly.
Finally, today, after much frustration and curses, I went to start the truck, it didn’t start, and in anger I left the key in the ignition and started to walk away. I heard a clicking noise, and turned around to find the injectors firing increasingly faster and faster, with the key in the ‘ON’ posiiton and without the engine cranking.
Now, my question I guess is this: What would cause the ECM to ground the injectors without the engine turning? How is the ECM getting a false reference pulse from the Ignition Control Module? Can someone walk me through the COMPLETE operation of the circuit, starting from the reference pulse, how it’s generated, where it goes in the distributor, and how it’s sent to the ECM?
Thanks guys, I appreciate any help! Stayin dirty here
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The ECM may not be grounding the injectors, perhaps the wiring is causing grounding because of something else…
The blown fusible link is a clue… ECMs can get fried because of low injector coil resistance…
Test the injectors with a noid light and turn the key to run position (don’t start it) and see what happens… If it lights then you most likely have a ground issue or possibly a bad ECM… Also test for what happens when you are turning over the engine
-Karl
Noid light test showed same symptoms I saw, no light when ignition is in ‘ON’ position, but sometimes after cranking it will pulse as if the engine is running, but in a really weird way, like way too fast.
How would I trace the grounds to see if there’s a short somewhere? I ohmed out the grounds from the injector to the ECM, they were fine, reached behind the distributor and wiggled the wires going into the ignition control module to see if I could get em to short, and they didn’t. Also ohmed the wires from ICM to ECM and they were fine as well, couldn’t get anything to short.
Is it possible that the pickup coil or some other component of the distributor is shorting and causing this?
Thanks!
EDIT: Also, something I forgot to mention, the connector for the Crankshaft Position Sensor into the ICM is pretty messed up, is that a possible cause? If that shorted, would it tell the ICM that the engine is in the same position multiple times or something? Not trying to get ahead of diagnostics, just trying to provide all info!
Did the wrench hit anything besides the pavement when you dropped it? You put the wrench info in the post for some reason. Why? Do you suspect that there is a connection between the dropped wrench and the problem?
I’m honestly not sure if it hit anything. I looked in the general area where it fell as soon as the truck died, and didn’t see anything obvious like a wire knocked loose or something. The blown fusible link was in the area the wrench fell, I suppose it could have played a role. It seems to be a bit more than coincidence, the wrench fell and the truck died. However, I’ve not been able to substantiate any direct effect corresponding to the dropped wrench, so I’m not sure if it was coincidence or not.
There’s not much on the passenger side of the engine bay in the way of electrical, there is a series of hot 12V connections mounted on the firewall, a braided ground running to the frame underneath, and miscellaneous engine bay lighting circuits. Most of the harnesses run from the firewall directly behind the engine, and were not in the path of the falling wrench.
In my experience there is usually more than one fusible link located in the same area. If the wrench fell and shorted them out, you could have more than one that’s bad. You might check them all with an ohm meter to see if they’re good. If they are, then the process begins to figure out the signals coming into the computer. At this point, based on your description, you either have bad inputs to the computer, bad wiring somewhere, or the computer is bad. Most times you eliminate all the input and wiring issues and you’re then left with the computer being the issue if everything else checks out.
Keep us posted.
:woohoo:
Sorry got excited when I saw that ETCG responded. I will be off work tomorrow, and will be working on the truck. Will start by going through all those fusible links, will double check all grounds, and I’m going to install a different distributor assembly I have laying around. I still have to take out the current distributor and inspect it as well.
On a side note, asked a GM mechanic about it and he said he had never heard of these symptoms. He recommended trying to get it to run long enough to generate trouble codes, which I will attempt. Maybe that will help narrow down the problem as well.
:silly:
EDIT: Also, something I forgot to mention, the connector for the Crankshaft Position Sensor into the ICM is pretty messed up, is that a possible cause? If that shorted, would it tell the ICM that the engine is in the same position multiple times or something? Not trying to get ahead of diagnostics, just trying to provide all info!
Repair all electrical connections first.
+1 yea, if you have bad connections this could be the cause of the issue. Especially if it’s at the ICM.
Keep us posted.
Alright guys, just giving you an update. Replaced the distributor, which solved many of the aforementioned problems. However, the truck was still running rich and bogging. Replaced the Coolant Temp Sensor, since it’s cheap and why not (I know, I know, don’t throw parts at a problem.. This one was sorta maintenance. Should have seen the one that came out…). I also replaced the Fuel Pressure Regulator, which I suspect was a part of the reason the injectors were dumping fuel with the key in the ‘On’ position. Pump primes, fuel leaks past regulator, not good.
Also, per your suggestions, double-checked all grounds and, lo and behold, the previous owners had decided, in their infinite wisdom, to simply stick the broken ends of the grounds between two washers held onto the thermostat housing stud by two different-sized rusty nuts. This poor girl has been put through hell, I keep finding strange janky repairs made previously.
Anyway, repaired that, and the truck ran around the block. Power was there at >1600rpm, no hesitation or backfiring. Below that there was a bit of stuttering; in all honesty, what with my shenanigans with the fuel system and such, the fuel level may be a bit low, which may contribute to that problem.:whistle:
I have noticed that the injectors do not spray a pleasant pattern, so a injector rebuild is in the very near future. As in, Wednesday. This truck needs to be running before winter so that I can get to work the days the minivan can’t. Crunch time.
Thanks all for the suggestions, I will continue to update on my progress. In a few thousand that 350 is coming out anyway but I’m hoping I can get it to last another year or so.
EDIT: Oh, forgot to mention, I will also be replacing the plug wires. I have not replaced them since I have owned the truck, and Lord knows when/if the previous owners did.
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