I've been searching every forum I can find for the last two days. I haven't come up with any answers yet. I need some experienced or very well educated input. Long story short is that I have a solid miss on cylinder 7 with no explanation. Everything I've checked has been good, or swapped and not changed the problem. I have a background in auto/diesel technology, and have done a lot of work on gas/diesel engines, but not professionally for the last 15 years. I'm at a loss on this one, which is my own truck. I may be missing something simple due to my own physical problems. I bought it at an auction in March last year, and thought I got a good deal until I got it home. Then found so many things had been swapped with junk. Such as switches, speakers, wheels, seats, seat belts, heater control module, etc. I've replaced so much little stuff that it has driven me crazy. But I kept telling myself that it runs and drives well, and I still have less into it than book value.
Brief History-

Anyway I have a permanent code P0300 and P0307. Had a rough idle sometimes until before Christmas. It would randomly present a code for a lean condition. Despite only putting regular fuel in, (up here in MN that is up to 10% ethanol) the fuel alcohol content reading was up to 86%. After reading a bit about it, and checking to find the long term fuel trims maxed out at 29, I determined that there was either an intake leak or bad injectors. This caused the truck to either run rough and throw a code, or start very hard while after I reset the fuel alcohol mixture in the ecm to 5%. When I got the truck I had a code that indicated a bad throttle body, so I replaced it with an ACDelco. Code went away. I would get a random misfire code along with the lean fuel condition code, and while checking found my fuel pressure would drop rather quickly after shutting the truck off (5psi drop in 20 seconds). So I replaced the fuel pump (acdelco). I replaced the MAF with a delphi too when cleaning the existing one did nothing. Beginning of December (with the random miss and periodic lean condition codes) I took the truck on a trip down to SD hunting. 1,500 miles total, highway out and back, lots of fields and back roads while out there. Can't complain one bit. Always started, mileage wasn't worse than expected, rough idle wasn't anything more than annoying to me, friend that rode with never noticed it. We even tried a tank of E85 to see what would happen. It ran great on the E85, smoothed the rough idle a bit, increased the power, dropped the mileage to almost 10mpg.

The Problem-

In December after the trip I replaced injectors and intake gaskets, both known possible problems. That cured the problem, sort of. Alcohol percentage has dropped to 0% after reset (where before it would just climb) and fuel trims (at idle) level out at -20% for the RH bank and -12% for the Left. That indicates a rich condition, but the ECM is compensating by dropping the fuel delivered by 12-20%. It did not clear up the P0300 code for engine misfire, and brought about a new code for P0307 cylinder 7 misfire that was not previously a problem. The rough idle got worse, and steady. Scan tool shows an almost constant misfire on cylinder 7.

What I've done Since- (not in order tested, but grouped by system checked)

I've checked spark. Using an adjustable spark tester set to 60,000v at plug 7, cranking and running, strong consistent spark, guessing 1" long, impressive to see. I'd say there isn't an ignition system problem, but I continued. I checked the plug, no problem found, resistance around 4.5k ohms matched the other plugs that have less than 6k miles on them. So I started swapping ign parts. Swapped the coil with cyl 1, plug with cyl 3, and wire with cyl 5. Started the truck, nothing changed. Still almost consistent misfire on cylinder 7. I conclude there is not a problem with the ignition system unless the spark is coming at the wrong time, which is very unlikely.

I checked compression and valve movement (ie stuck or collapsed lifter). Twice. First time just to see if it was a likely problem, didn't see anything wrong then. After other testing produced nothing I tested again more methodically. All cylinders read 180-190psi after 4 seconds of cranking. Standard test, 4 seconds of cranking with no plugs in, repeated 3 times each and averaged. Checked cylinder 7 three separate times. Can't find a fault. After reading about the lifter problems I pulled the valve covers and observed while cranking. All rockers moved what seemed to be the same amount. After replacing the intake, not seeing any change, and doing more research, I removed the LH valve cover again after warming the engine up fully. I started the engine, and observed the rockers again. No noticeable problem. All odd cylinder valves appear to be moving to full valve opening, which would match with the good compression test. I can't find anything that would suggest a mechanical problem. Though I can't say for sure that is isn't mechanical, it is too hard to fully test the dynamic cylinder conditions.

I checked fuel delivery. First thing I did after seeing the cylinder 7 misfire was to assume a bad injector. Seen it too many times to assume I got all good injectors, although I did get GM injectors and have never seen a new GM injector be bad. So I ordered another GM injector and replaced it. When the misfire didn't change, I began to worry. I started by checking the injector pulse with a noid light. Which flashed good and strong. Than I watched it while I checked the spark output. Both flashed in time with each other. I checked power to all LH injectors, and checked ground signal to the LH injectors. Everything seemed to be working. So I swapped injectors, 7 and 1. No change. Rechecked the fuel pressure (though I don't know how that would cause a misfire on only one cylinder), still good. I had to conclude that it is not a fuel problem, though it still feels like it is.

I checked for intake air leaks. Only thing left was an air leak problem. With the LH fuel trim being a little higher than the RH, that would almost make sense. As there is no way for extra air to get into cylinder 7 except through the intake very close to cylinder 7, I had to assume that the intake was warped or cracked that I couldn't see. Did I mention that I replaced the intake gaskets? I checked the intake runners with a straight edge. I read the service bulletin that said a 3MM gap is the limit. Well, I'll be darned if there was even 1mm warp that I could see in the old intake. But I've heard the saying, "When you have eliminated all which is impossible, then whatever remains, however improbable, must be the truth." by Arthur Conan Doyle. I could not find anything, so I gave up and replaced the Intake Manifold. Which did absolutely nothing. Still a rough idle. Still an almost consistent miss on cylinder 7. I tried blocking off the vacuum booster, just incase because it is connected back by cylinder 7, though it would likely cause cylinder 8 and probably others to run lean also. But no change there either. I opened a can of propane around every part of the intake system. No change.

I tried putting sea foam in the engine oil and running it hard. Which believe it or not has helped save 2 weird engines from problems (sticking lifters and another I can't quite remember the cause of) in the last 15 years. I watched every scan tool pid that I could on a 20 mile drive into town and back. I reset the crank position learn. Reset the idle learn.

What now? Now it has been a while since college (longer since tech school), but I believe that Fuel + Air + Spark = BANG. With a little compression thrown in. Too much air or fuel will kill it, as well as not enough compression or strong enough spark. But WTH is going on here. I use to be so good at finding weird problems, and now I can't even find a solid miss. It has fuel, and air, and compression, and spark. The only two tests I can think to try are a cylinder leakage test and an injector flow test. With valves moving and good compression I can't see a leakage test showing anything more. With new injectors and swapping them I can't see an injector flow test showing anything either. That is unless somehow the #7 injector isn't getting enough current to fire, but is getting enough to light the noid light. I suppose I could pull the valve cover off the RH bank and observe those rockers, but wouldn't expect to see any problems there as there are almost no misfires reported for the even cylinders.

So, Where do I go from here?

My next step (besides asking for help) is to pull the injectors again and measure their output when commanded by the scan tool. Then maybe pull the RH Valve cover again. Check to see if the problem really is in another cylinder like 8 or 2 and the ecm is wrong (though that would be less likely as they are not AFM. Or maybe a leakage test, but I don't have a gauge to measure the volume of leakage, just could compare different cylinders. But really I'm out of ideas. I don't expect to find anything with those tests. Tomorrow I need to put the new head on my skid steer engine, and hopefully while doing that I'll come up with something.

Anyone have any bright ideas?

 

i didn't read all that. post the model year and engine size.
if you have a misfire on cylinder 7...swap a coil, wire and plug with one from a non misfiring cylinder. the misfire will either follow the coil/plug/wire or stay on #7. if it follows...replace plug/coil/wire. If it stays on 7...do a compression check on #7. If its low, you have a mechanical problem on #7. If you have an afm engine (no idea what engine you have) 7 is a afm cylinder and it may have a collapsed lifter.

 

You are correct, I did not put my truck info in.

2012 Chevy
Silverado 1500
5.3L Vin 7
With AFM/DOD

About 175,000 miles

However, I already swapped parts, checked compression, and valve movement. Which is why I'm asking for help.

I will edit to show the basic things I've checked in bold.

 

A couple of things come to mind. I look at the 1st puff of a compression test and compare to the other cylinders. Also with the engine running feel the exhaust output at the tail pipe. See if it pulls your fingers in and out, Should be a constant flow out. Can also check manifold vacuum. should be steady. It's possible you have a cam shaft issue. Have done alot of them.

 

What model vehicle and which engine are you working on ?
I had a similar issue with My 07 Silverado that ended up being collapsed lifters with hard miss / rough idle / Hard shifting / traction control issues .
I ended up putting in a remanufactured engine because after comparing costs of parts to rebuild plus machine shop costs it was less expensive to do a long block swap . My original engine (5.3) with AFM had almost 200,000 miles on it . my miss was on Cyl 4 and my code was P0304 and like you I started with a new plug then swapped coils and wires from different cylinders to see if the miss followed the coil . I also replaced the ignition coil / Injector harness on the Right side of the engine . all to no avail. I could see a definite problem with #4 intake once I pulled the rocker cover and ran the engine with the cover off at idle

 

He did all those things and still has the miss

 

does you scan tool monitor misfire data?

 

Yes I can monitor misfire data with efilive. I've read much about cams and lifters going bad, and everyone that has shown how they found the problem has found it by removing the valve cover and running the engine. Looks like one rocker stops moving, or isn't moving nearly as far as it should, which would make perfect sense for my problem. But, at an idle I can watch the misfires climb steadily on cylinder 7. Shut the truck off, pull the valve cover, and run it, and watch all the rockers moving just fine. I even took a video of it so I could look at it later. I'm not saying that I don't have a cam/lifter problem, but I would be surprised with what I'm seeing. If it were cheaper and easy, like injectors or the intake manifold, I would have just done it already on the off chance it is the cause. I tend to weigh my diagnostic time heavily against the cost of just replacing something. I will check vacuum, because I haven't yet. I will feel the exhaust, again because I haven't and it's easy. I will redo the compression test and try to compare the first puffs.

Tomorrow I'll get back out there and check some more.
Plan is;
1. Hook up vacuum gauge and feel exhaust.
2. Redo the compression test
3. Pull the injectors and pulse each one onto a clean dry rag to compare.

If I find nothing still. I might check the RH bank in case the problem really lies with cylinder 8 or 2 and isn't being reported correctly.
4. Pull the RH valve cover and observe those rockers.
5. Swap plugs, wires, coils, injectors for the RH bank.

 

After reading this again, I had a thought. Might support a bad cam diagnosis. My semi-rough idle before replacing injectors got worse and brought a steady misfire on cylinder 7 after I replaced the injectors. If cylinder 7 was lacking proper airflow, it would fire more consistently before when it wasn't getting enough fuel. And now that it is getting more fuel, it could be too rich with poor airflow to fire.

In other words before it was getting less fuel, and if it has an airflow problem, that would actually help. Now it is getting more fuel, and with an airflow problem, it would be a bigger problem especially at an idle.

Does that make sense? If my cam still has lobes for cyl 7, but they are diminished. I should be able to put a dial indicator on the rocker to measure overall lift and duration compared to the other cylinders.

 

It is sounding more and more like an electrical issue than Fuel /Air or Mechanical .
Especially since it so strongly isolated to Cylinder 7 . And Having moved known good components into that location and the Miss remains leads me to believe you have an issue in the wiring itself or an internal fault inside the ECM that grows in intensity as the ecm heats up . I know this is not a Ford but Ford has had numerous issues in their PCM on the 2.5 L Eco-Tec engines with their internal grounds and the only fix is a new PCM AND All Coils and plugs have to be replaced .
Numerous people have tried replacing only the PCM and Instantly fried the new PCM .
Just something to keep in mind This sounds very similar to an ignition coil breaking down when it gets warmer during operation . Electrical connections and wiring can and do , Have this happen . The harnesses are not really that expensive and far less expensive than tearing down the engine to physically Mic. the components . This sounds electrical to me and since the system works on only 5 vdc connections are critical . as are all the ground terminations and conductors .