Double Check There Are No Fuel Leaks
As Charles and I were talking over the phone, I began to mentally go over all of the symptoms he was describing (rough idle, misfire code, raw gasoline smell out the tail-pipe) and started to think what things could cause them.
He had mentioned that everything he had replaced had not helped the problem but had not made it worse. This told me that all the new parts he had installed, had been installed correctly. Still, I asked him to double check his work.
He needed to check that:
- He had installed the ‘Spider’ assembly correctly and was not leaking fuel inside the plenum.
- He had not cracked or broken a spark plug as he was tightening them (in their spark plug holes).
- That the spark plug wires were in their correct firing order on the distributor cap.
He got back to me later on in the day and mentioned that he had double and triple checked everything and everything was installed correctly.
I made up my mind then to go over to his place and troubleshoot the problem on his Suburban in person for him.
Once I got there, the thing that really helped, to get the diagnostic underway, was that we had a specific misfire code (the P0306). I knew that most of my testing would be focused on cylinder #6 and that one of three things had to be missing: fuel or spark or air (as in low cylinder compression).
Even though Charles had already double-checked his work, I needed to check everything myself. This is what I did:
- I did the fuel pressure leak down test (even though Charles had already done one) just to make sure that the fuel pressure regulator was not leaking.
- You can find the fuel pressure leak down test here: How To Test The ‘Spider’ Fuel Injector Assembly (4.3L, 5.0L, 5.7L).
- I tested for spark:
- I did this by connecting an HEI spark tester to the end of the spark plug cable (even though they were new) and had Charles crank the engine while I checked for spark (I then tested for spark on the remaining 7 spark plug cables).
- I checked all of the spark plugs (in case Charles had accidentally dropped one or cracked the porcelain insulator as they were being installed).
- You can find the spark tests here: How To Test A Misfire / No Spark-No Start Condition (at: easyautodiagnostics.com).
- I did a compression test of cylinders #4, #6, and #8 and compared the compression values to one another and although they didn't have the exact same pressure values (this is normal), the difference was still within specification.
- You can find the engine compression test here: How To Test The Engine Compression (GM 4.3L, 5.0L, 5.7L).
- I swapped spark plugs.
- I put spark plug #8 in the place of #6 and #6 in the place of #8.
- It's happened to me before that there have been internal problems with brand new spark plugs (and the spark plug(s) would not fire) and the only way I found out was by swapping them. This made the misfire move to another cylinder and when this happened (the misfire moves), then I knew that the brand new spark plug was/is bad.
- I banged the catalytic converter with the palm of my hand (to see if the filter element inside had broken into pieces and thus was choking the engine).
All of the above tests were passed with flying colors, and with these results, I was able to eliminate the most common causes of a misfire on ‘Spider’ equipped GM vehicles.
I want to emphasize this point, the results of the above tests told me I could forget about spending more time testing anything to do with the ignition system (spark), or anything to do with the internal engine mechanics (air) and exhaust system (air).
The really interesting thing was that up until this time, the engine was still cranking (and starting) and had not hydrolocked yet.
‘Spider’ Injector Spraying Fuel All Of The Time
After having done the 5 tests in the section above, I had a feeling that the problem might be the #6 fuel injector (inside the CSFI ‘Spider’ assembly).
Specifically, that the fuel injector was spraying fuel all of the time once it got power from the ignition switch (even with the engine off). Why? Because I've seen this happen quite a bit on other vehicles and the symptoms on Charles' Suburban matched the cases I had diagnosed before.
To get into more details about this type of problem, what happens is that the PCM fries or the injector's return circuit shorts to Ground. When this happens, the injector activates and sprays fuel continually whether the engine is running or not (with the Key in the On position).
Let me explain this a bit further.
- The fuel injector is just a solenoid.
- As such, it gets power in the form of 12 Volts from the ignition switch (via the fuse box) and the PCM supplies it with a Ground only when the engine is cranking or running.
- When the PCM applies Ground to the injector, it opens its pintle and since the fuel is under pressure, it starts to spray out of the injector.
- This Ground is applied in an On/Off manner as the engine is cranking or running.
- By varying the On time (when the PCM is Grounding the injector), the PCM is able to control the amount of fuel the injector sprays.
- If the circuit inside the PCM that controls this Ground shorts to Ground (internally) the injector will come On as soon as you turn the ignition switch.
So, getting back to the Charles' GMC Suburban, what I did next was:
- Removed the upper plastic intake manifold plenum.
- Pulled out the nylon fuel line for cylinder #6 from its cavity in the intake manifold.
- I then had Charles turn the Key to the On position (He did NOT crank the engine).
- And the #6 fuel line immediately started spraying fuel!
I had found what was causing the #6 misfire code, the rough idle, and the smell of raw gasoline coming out of the tail-pipe.
But I wasn't done yet. I still had to find out if the fuel injector itself was bad (doubtful since Charles had just bought and installed a brand new ‘Spider’ assembly) or if the PCM was fried or if there was a short in the wiring between the PCM and the fuel injector.