So I've been pulling my hair out over my '97 powerstroke missing and idling rough after the engine has warmed up.

So far I have done:

-Valve cover gaskets and UVC harnesses
-Injector O-Rings
-Installed Aeroforce Scangauge to monitor ICP and ICP duty cycle (both perfectly normal)
-Did a handful of CCT's, usually coming back with no faults but sometimes it flagged cyl. #4
-Did a compression test, everything returned 410-420psi (thank god)
-Oil is fresh Rotella T 15W40 with a motorcraft filter, brand new microguard fuel filter
-I've run it with the valve covers off a bunch of times, can't pinpoint a single cylinder that is causing the problem. No matter which injector is unplugged it will only miss on that cylinder.

The latest theory, my fuel pressure is pegging my tire gauge at 120psi at idle. I don't know how it is even possible for the stock pump to build this much pressure. The return screen is clean and that didn't help. I took the regulator apart and everything moved freely and passed the visual inspection. Maybe the problem is somewhere outside the regulator? I'm at a loss for whats happening right now.

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I played with the regulator some more and it is sitting at 50-55psi at idle now. It must have been stuck since I had it apart last.

No change in the way it runs though, I don't want to start throwing injectors at it if it is something else. Any ideas from anyone?

 

??? Tire Gauge ??? Are you serious? And then you lowered your pressure down? You probably don't realize that an air gauge, meant to check an air pressure, wouldn't read accurate on a liquid right? So now you might have caused a problem that wasn't there before? Just saying... I have a diesel fuel gauges... but never tried to check it with a tire gauge. I know it's a schraeder valve but... I always figure the right tool for the right job... But read further and I throw out some tests you can do with low-cost tools.

While you had it on a scanner, what was the ICP pressure (HPOP)? The one you borrowed... could it read live data?

Have you tried running a clear hose (swap out one near the regulator) to check for air bubbles and to see if you have any oil residue in your return?

Did you do a resistance check of the injectors at the IDM? (Especially to see if #4 is reading high or low?) If you need info on that, I could post the tech doc. There is actually a lot of things that you can check on it with just a VOM. Sometimes, that ends up being a loose connection or a pushrod that rubbed the insulation off one of the injector wires.

Next (while running) is to unplug/plug the 4 injector connectors at the rocker covers, one at a time. Then pop off the rocker covers and do the same to the individual injectors. Normal, the rpm should fall when disconnected. If it doesn't, then that needs to be investigated, because that one (or group of) is not pulling it's weight. Also look under the oil deflectors of each injector and make sure they are squirting the same amount of oil for each.

Another way is to let it get cold and take a reading at each cyl. at the exhaust manifold w/ an IR temp gauge. Start it and take reading for each cyl. as it warms up. A dead cyl will not throw heat and will be lower than the others.

 

Checking pressure with a tire gauge is fine on these trucks. Bad for the gauge, but it won't hurt the truck. Its the accepted method, I have a fuel pressure gauge and I was just making sure they agree since due to the nature of these mechanical pumps the needle flutters around like crazy. The tire gauge works better since it will show you a max pressure, instead of bouncing around everywhere like a typical dial gauge.

The gauge I checked the HPOP with is an aeroforce scangauge. Its showing ~600psi at idle with a duty cycle of 12%, which is perfectly normal. It builds about 3000psi when at WOT.

The problem is that it doesn't seem to have a dead cylinder. No matter which injector is unplugged, it seems to run fine on the remaining 7 cylinders. The number 4 cylinder seems to be contributing the least, so I grabbed an injector for it today to swap in.

I figured out the fuel pressure problem though, the regulator must have been stuck closed. I took the cap off and fiddled with it, now its completely normal at ~55 psi idling.

After ruling out fuel pressure causing my issue, I think it has to be either worn out injectors (they have over 300,000 miles on them), or the IDM is on its way out (doubtful since it runs fine at WOT). I'm guessing that I just have several weak injectors, this number 4 one seems to be the worst. Unplugging it pretty much doesn't change the idle at all. The number 6 doesn't affect it that much either, which has me scratching my head as it is only missing on one cylinder. Unplugging number 2 or 8 makes the engine fall on its face so I know they are working. Same deal on the other side, 2 of them worsen the idle quite a bit when unplugged, and the other 2 don't have much effect. It may just be the nature of the engine though, unplugging certain injectors is going to throw the engine off balance more than others. It would be easier to diagnose if the thing was supposed to idle smooth like a cummins.

 

The way you describe that, it sounds like you really have 4 weak injectors (2 each side)... which would explain why it falls on it's face so bad when a good injector is dropped out.

It would be interesting to hear a buzz test on that one...

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There is a point in doing CCT's where dropping a cylinder out should drop the RPM. A good cylinder should drop RPM, the show that there is a change, to show it was doing something.

If you drop a cylinder and there is no or little change, then it was not contributing much.

If you drop a good cylinder and the others are struggling to keep running (like you described), then it may point to there not being much good strong cylinders left to keep those going...

I feel lucky in that most people say injectors last about 150,000 miles. I have about 260,000 miles and am doing well. Your's at over 300,000... It may just be that time.