Misfire - test injectors?

[kcabpilot]

After doing my engine swap from my old, overheating 4.0 to a low mileage 4.6 I've been experiencing an intermittent and sporadic miss that is mostly on a cold start but seems to be getting worse. On two occasions I have gotten a SES light with a code for #8 cylinder. So my thoughts are it could be plug (new) wire (new) coil pack (original) or injector (or injector connection)

I'm going to swap in one of the old plugs today. If that doesn't work I need to pull the plenum off to check the injector connection or swap in another injector if that looks good. Then I will have to reinstall the plenum to test and if it's still not fixed I will need to remove the plenum again to replace the coil pack.

So my question:

Is there a way to electronically test or monitor the injectors with OBD2? Or is there a parameter I can look at to see if the injector is the problem. I use the Torque app for scanning.

 

[squirt]

You can monitor injector activity with a set of noid lights, but on a D2 it's a pain in the ass because of the intake.


If you're taking the plenum off anyway, swap injectors between cyls 2&8, and swap the coil packs left to right.
If the miss starts showing up on cyl 2, it's the injector. If the miss starts showing up on cyl 7, it's the coil pack.
If it stays on cyl 8, the coil pack and injector are good, so you're looking for something else.

When it's missing, can you feel it? Since you say it happens on cold startup, it's entirely possible that the injector is occasionally sticking and dumping too much fuel into the cylinder, which needs to burn up and causes a miss on startup.

 

[discostew]

The way you test injectors is by doing something called an injector balance test. You do it with a small box that produces an identical pulse width, or open time when you press a button. Mine is old and was made by OTC. Then with a fuel pressure gauge on the rail you see how much the pressure dropped for each injector. Thing is if that injector isn't really fucked up your not going to see much difference among injectors as far as the pressure drop. So I like to have the fuel rail out of the intake manifold so I can see the spray pattern of each injector. When they start to get restricted or what have you the pattern will change drastically. Like the ones I have found doing the test like this come out the end of the injector like a stream from a squirt gun and not a mist like you want to see.

I think squirt is right in advising you to move the injector to a new hole and see if the problem follows. Because its an early truck in the Bosch run, I think you need to consider stuck exhaust valves also. The way I check for that is by removing the valve spring while you have air pressure in the cylinder then remove the air supply and move the valve open and closed while rotating it in the guide. Kind of time consuming but if you don't come up with anything in checking spark and fuel it should be your next thing to check.

 

[kcabpilot]

Well I was looking for something simple. My old '91 Alfa is OBD1 so no port for a scanner just a test button but there is a function where you press the test button then turn on the ignition and it cycles through the injectors and you can hear each one so I was looking for something like that. My Disco has SAI so anything to do with the coil packs is a general PITA.

Also the misfire is a complete misfire, I can definitely feel it, so it's not some half ass thing. It really feels like a bad injector connection but no way to get my fingers on it, he'll I can't even see the damn thing under that intake plenum.

 

[discostew]

Well I was looking for something simple. My old '91 Alfa is OBD1 so no port for a scanner just a test button but there is a function where you press the test button then turn on the ignition and it cycles through the injectors and you can hear each one so I was looking for something like that. My Disco has SAI so anything to do with the coil packs is a general PITA.

Also the misfire is a complete misfire, I can definitely feel it, so it's not some half ass thing. It really feels like a bad injector connection but no way to get my fingers on it, he'll I can't even see the damn thing under that intake plenum.

Check for spark at the end of the wire first, that's easiest. Then compression cause that's pretty easy. Then pull the plenum and look into injectors. Since your not from Canada you probably are willing to check the easy stuff first.

 

[kcabpilot]

Lol well it's a new plug and new ignition leads. I checked compression before I installed the engine. I'm kind of kicking my own ass for not just putting new coil packs in to begin with and I did make pretty certain that the injector connectors were well seated but the whole rail and bank were with the replacement engine. I just figured it had 100k less miles on it so I went with them. I spent all weekend installing a MaxJax in my garage for my Alfas but first thing I'm going to do tomorrow is swap in another plug. Kind of a long shot that a brand new plug would be bad. The ignition wires are new Magnacores from AB so I'm doubting that as well. My bet is injector because even though coil packs are old they were both working perfectly well before the swap.

 

[ptschram]

The fast and dirty method is to put your fuel pressure gauge on the test port and monitor the pressure with engine running and then every ten minutes after shutting the engine down.

Fuel injectors are pretty simple and by the time we got to the second generation of Bosch injectors, they are bombproof BUT, our fuel quality has deteriorated in quality due mainly to regualtory pressures. As a result, 15 year-old injectors that were thought ot resist leakage far better than the first generation as now beginning to show their age.

That said, if you have an injector-related misfire, it's probably due to over-fuelling one fo the cylinders and testing the fuel pressure (even as inconvenient as it is to measure on the DII/P38) is a good way to gain some information without incurring any parts expense and we all need a fuel pressure gauge as it is such a primary test tool.

 

[kcabpilot]

One other thing I should mention about this misfire is that it only seems to be at idle. When I step on the gas or I'm cruising along I cannot detect anything so the idea of a stuck injector or over-fueling might make sense. That and this may be unrelated, but although I figured that going from the 4 liter to the 4.6 my fuel economy would drop a bit I didn't think it would be as much as I'm seeing. The scanner is telling me I'm getting like 11 mpg. Using the odometer between fill ups I calculated just under 13. I used to get 16 to 18 mpg. Of course I've been kind of enjoying the extra oomph so maybe I'm just romping on it more (ha, ha)

 

[kcabpilot]

Swapped in another plug, no help. If anything the problem is getting progressively worse, it doesn't seem to be intermittent anymore. SES light comes on right after start with same code for misfire #8. So I pulled the intake plenum off - a real pain to be honest - and I found the pins on #8 injector exhibit a slight layer of greenish/blue corrosion. All of the injectors show 15.1 ohms resistance. I'm not sure if there's a way to test my wiring harness with everything torn apart like this.

So I'm going to swap in all of my original injectors but considering the hassle to get at them I'm going to replace all of the o-rings and nobody has the kits so I have to wait until tomorrow afternoon. Which is okay because it's about 98F out there right now and I bet it's gonna hit 100 before the afternoon is over. Plan for the rest of today is - inside, AC on, slushy Margarita and find a movie.

But I'm cautiously optimistic that I may have found the source of my problem but equally skeptical. So, again considering the amount of work getting in there I found an aftermarket coil pack - Standard UF-595 on the internet for fifty bucks. They even had them on Amazon with free one day delivery so I'm going to try that as well. Prior to this engine swap my original motor was suffering from an overheat problem and on more than one occasion it reached a temperature in excess of 260 degrees so maybe it did some damage to one of the coil packs. It's time to change them anyway, I should have done it during the swap.

 

[kcabpilot]

Well it's fixed but I can't say whether it was the injector or the coil pack since I replaced both. I can say that I tested the injector as best I could - DIY. They all were 15.1 ohms on the meter, it clicked when I put 12 volts to it and it had a normal spray pattern, as far as I could tell when I pressurized it with carb cleaner and actuated it. So I guess my bet is on the coil pack.

BTW just an FYI if you decide to buy the aftermarket coil packs that are $220 cheaper than the ones at AB - save yourself some grief and use the original mounting bracket because the one they came with is about an eighth of an inch too narrow on the bottom end where it bolts to the intake manifold - ask me how I know. The upper part that bolts to the plenum is okay but that doesn't help much.

Anyway it's running great so far, no more misfires and no more Service Engine Soon light. I'm a happy camper.