PCM Shorted To Ground Internally
So far, in my troubleshooting of Charles' 97 GMC Suburban I had eliminated the following:
- Ignition system problems.
- Fuel pressure regulator leaking fuel.
- Low engine compression in cylinder #6.
- A short to Ground in injector #6's wiring.
The next step was to see if the circuit inside the PCM, that controlled injector #6, was shorted to Ground. This is what I did to find out and make sure:
- Disconnected the battery negative cable from the battery.
- Reconnected all four of the PCM connectors (C1, C2, C3, and C4).
- All 4 PCM connectors were disconnected from the previous test.
- Reconnected the battery negative terminal to the battery.
- The ‘Spider’ injector assembly connector remained disconnected (and needed to remain disconnected for this test).
- Turned the Key to the On position.
- Probed the wire piercing probe still attached to the y with black stripe wire of the C2 PCM connector.
- The 12 Volt test light's alligator clip connected to the battery's positive (+) terminal.
- Checked to see if the 12 Volt test light came On.
- And the test light DID illuminate.
- Just to make sure, I disconnected the battery's negative terminal, disconnected all of the PCM's connectors, reconnected the battery negative terminal and repeated the test light test on the y with black stripe wire and the test light remained Off (when I turned the Key On).
- I then repeated test steps 1-6 (of this test) and the test light came on again.
So, by now, I had no doubt that the PCM was fried and was the source of the problem. Why? Because the 12 Volt test light found a path to Ground (and thus illuminated) only when the PCM C2 connector was plugged into the PCM.
Hydrolock And Repair Solution
Charles waited a day or two to buy and replace the PCM, by this time cylinder #6 got filled up with enough gasoline to hydrolock the engine.
He found out the engine was hydro-locked after replacing the PCM with the new one (rebuilt one he bought from O'reilly Auto Parts) and trying to crank the engine. The Suburban simply would NOT crank.
Of course the solution was to remove the spark plug from cylinder #6 and hand turn the engine to get the piston to pump out the fuel (and replace the engine oil and filter too).
The Suburban has been running marvelously since then, not bad for a vehicle with almost 200,000 miles on it!
Safety And Repair Precautions
The one thing I've always noticed, when something like what this case study describes happens, is that what usually fries the injector circuit inside the PCM is the fuel injector itself going bad.
The fuel injector will short out inside and pass the full amount of Voltage/Current it gets (from its power circuit) to the PCM. So, if you do see/have this problem on your particular vehicle, you need to test the resistance of the fuel injector (in question) and compare this resistance reading to at least three or four others.
Although the resistance won't be exactly the same but it will be similar. It's when the resistance is totally different (from the others) that you do have a problem and need to replace the injector or the entire ‘Spider’ assembly.
Also, there are so many ways to test for a bad PCM activating a fuel injector as soon as the Key is turned on.
The way I did it, in helping Charles out, is just one way and it may not be the way you'd do it. If anything else, the purpose of this case study is to help you be aware that this type of problem does occur and the possible ways to troubleshoot it.
Here are important precautions you should take when removing the intake manifold plenum:
- Prevent objects from falling into the lower intake manifold.
- Since the upper plenum has to be removed (for this test), this opens up the possibility of something falling into the open lower intake manifold ports.
- If something does fall into them, it'll usually end up inside the engine cylinder and usually the only way to retrieve it is by removing the cylinder heads.
- Fuel is extremely flammable.
- You need to have the nylon fuel line pointing to a rag (shop towel). The shop towel's job is to get hit by the fuel being sprayed and keep the fuel from soaking the intake or the valve covers.
- You need to test them one at a time.
- Disable the ignition system.
- This will prevent spark from reaching the spark plugs and causing a back-fire thru' the intake manifold.
- Never crank the engine with the intake manifold plenum removed!.
If this info saved the day, buy me a beer!