Fun with injection - Idling halfway good...

Quicksilver · 04-15-2012, 11:05 AM

I knew the cap and rotor needed replacing as they have about 50K miles on them and I had cleaned the contacts instead of replacing. (out of pure laziness...)
So I finally replaced the cap and rotor and the crappy cold idle didn't improve so I'm finally starting some real troubleshooting.

Got out the pyrometer and cylinders 1, 4 and 6 are cold. 108° to 156° compared to 400°+. Can anyone see a reason why 2, 3, and 5 would be idling correctly but not 1, 4 and 6?
I know that the injector harness is split into 2 halves but unless my diagram is wrong it is split 1, 2, 3 and 4, 5, 6.

To add to the head scratching...
Once the engine is hot it seems to go away. Even when it is cold if I get into the throttle it runs fine, Blip it to clean it out and get on the throttle and it is happy. It has fresh NGKs in it, the Magnecors test out perfect, and it had new injectors less then 20k miles ago. Only theory I have is that maybe the injector timing doesn't favor those cylinders.

Anybody have ideas?

Targalid · 04-15-2012, 12:51 PM

Check your spark plugs and see if they are igniting properly. If the plugs are black and covered with soot, or just plaln wet, I would suspect your ignition system. Otherwise your injectors are not opening properly. Try running some injector cleaner through your fuel system.

Quicksilver · 02:11 PM

The plugs definitely aren't making a fire at cold idle and they will be wet cause there is no fire. I'm leaning to the fuel delivery side because at idle it is the easiest time to get a spark to jump. High vacuum and a long time between sparks means a spark is easy. If it didn't spark at idle there is no way it would spark at higher pressures and higher rpm. The car pulls hard at full throttle all the way past redline.
I have always wondered why they run all the injectors on a side together instead of timing each one so it goes when the valve is open. Doesn't that mean that the timing of the injector pulse would favor some cylinders?

I still have the 200k mile factory injectors that I removed. Is there anyone here in socal known for doing a good injector rebuild?

Quicksilver · 05-01-2012, 07:52 AM

Checked the injectors and they are good. I did get them tweaked so they are all flowing spot on. (One was 1% off and another was 2%...)

But while I was in there I discovered that the intake manifold bolts weren't to torque. #4, #5, and #6 were probably at about 5ft/lbs. Torqued them up and had a good think about why this would effect a port injected car like this. My knee jerk reaction was that the gas was squirted in that intake runner so it shouldn't make much difference if the air comes from the butterfly or from the port leak...

...
Discussed the injector timing with Loren and learned some surprising things. I knew that #1, #2, #3 and #4, #5, #6 were on 2 separate wiring circuits which always made me wonder how the timing of the injector worked out. Turns out that ALL of the injectors fire at the same time and there is no timing of them, other then to insure that they go off once per cycle.
A little more noodling and I finally realized that a vacuum leak would allow the air/fuel charge to be drawn out of the intake runner into one of the other cylinders.
Voila... If the timing happens to be right for that cylinder and the injector fires quite a ways before the valve opens, you can get a massively lean cylinder at full vacuum while an adjoining cylinder is rich.

Cylinder #1 still isn't operating at idle so I suspect the gasket is just plain toast. Checked the wiring and the harness has continuity. I guess I'm going to get the perfect opportunity to steam clean the top of the engine and to replace the fuel lines...