Well dang. I installed all new reman fuel injectors this evening and fired it up. It had a very rough idle at the start and I noticed something from the exhaust marking the concrete on the driveway. I cleared the codes and took it for a short drive. By the time I got back the SES light was on.

Codes are pretty much the same P0308 (#8 misfire), P0304 (#4 misfire), and P1300 (misfire sufficient for cat damage).

At this point I've replaced everything I can think of. New plugs, wires, coilpacks, injectors. No change to the codes. I'll pull the plugs on the misfiring cylinders tomorrow to see what they look like. About the only thing I could think of would be seafoam in the oil in case its a valve problem? I'm not sure how quickly that would help if it were even that.

Any ideas?

 

Could be due to vacuum leak. It seems like you have replaced stuff related but re check hoses if properly fitted. Also the hose that connects to the IAVC

 

I'm not 100% on this vacuum leak test. I got some carb cleaner and sprayed it at all the intake hose connections and I did not notice any significant RPM response. I kind of thought with a vacuum leak the misfires would be more random, but mine appear pretty consistent.

I also stopped by HF and picked up some tools/toys. I got an inline spark detector and the cylinders do appear to be firing correctly. I also got an inspection camera and stuck it in the misfiring cylinders. None had any coolant pooled in them from overnight and the 3 that I checked appeared to look similar (I didn't notice one cleaner than another). The colors were a little off though and that was hard to tell. Again no coolant pooling is inline with my previous coolant pressure test results.

I pulled the #8, #6, and #4 plugs. Since the #4 misfire is somewhat new, the plug was still dirty and doesn't appear pressure washed. However #8 is was still clean. We thought that the clean plug was possibly a clogged injector but replacing those didn't change the outcome.

So I've verified spark on the cylinders, I've replaced injectors to rule out a clogged injector. I think I've tested for vacuum leaks. I'm thinking that would leave a valve problem? Since I'm not finding coolant in the cylinders, I'm wondering if a stuck valve on #8 is preventing the fuel from reaching the cylinder and resulting in a clean plug and misfire?

Any thoughts?

 

have you checked voltage to the injector on #8

 

No I haven't check voltage to the injector. How do you test that? I thought it didn't get power until it fired?

 

Normally, on most EFI systems...there is power/voltage to the injector constantly...the ECM controls firing the injector by grounding the circuit/injector, using the crank/cam sensors as its reference. You have two wires going to each injector, positive on one terminal and ground on the other.

Also, a bent valve would have nothing to do with fuel getting into the cylinder, one valve lets air in (intake) and the other lets air out (exhaust). Fuel enters the combustion chamber through the fuel injector. A bent valve would cause low compression in the cylinder.

And you are absolutely sure you got the plug wires right? Just asking, didn't read entire topic, if you've checked them...sorry about the question.

Brian.

 

Thanks for that info about the cylinder and valves. I wasnt sure exactly what that looked like. I have put an inline spark check and I know #8 is getting spark, Ive verified the cables are run correctly to the coil packs.

A little update, I put about 7oz of seafoam in the oil. It almost immediately smoothed out the rough idle. I let it idle for almost 20 minutes, cleared the codes, and took it for a drive. When I got back the SES light came on, but only with a P0308 this time. No misfire in #4 this time.

I had swapped the plugs around when I put the inspection camera in. I plan on checking again to see if the dirty plug I put in #8 got cleaned up.

Also how should I check the injector voltage? Is it ok to turn the vehicle on with the manifold removed?

 

Alright, I found a generic article about checking the injector voltage. The #8 injector does have voltage at plug/harness. It was a little less than 12V on one side and about 5V on the other. This was the same on other injectors.

Also when I previously swapped a dirty plug into the misfiring #8 cylinder, it has not been power washed. I feel confident (finally) that its not a coolant leak.

Any more ideas as to what could cause a single cylinder misfire?

 

Compression maybe?

 

I will do a compression test after I put the manifold back on. Is there anything I should check while I still have the manifold off? I've removed this thing almost 6 times haha.

I think checking the injectors was on the right track. When I put a new plug in #8, it doesn't get the soot/carbon build up on it like the other cylinders. I have replaced the injector and verified the injector harness has power. Would something else give a similar result besides the injector? One thing I'm not sure about is plug wires routing. When I tore things down for the HG job I didn't pay attention to the routing of the cables so when I re-installed them (twice now) I just ran them however fit. How sensitive are the cables to crossing/interference? What should you do with slack in the longer cables?

I wanted to thank everyone who has input so far.