7.3 Power Stroke Miss

Chad Technician Winter park, Florida Posted   Latest  
Question
Driveability
1999 Ford F-350 Super Duty XLT 7.3L (F) 4-spd (4R100)
Misfire

Hey guys Im ashamed to be asking such basics. But I need help and don’t work on diesels. This truck feels like it has a misfire after 2k rpm. No fault codes. 

I use a snap-on modis and the data it provides looks “good” and when the misfire happens I don’t see any real changes in anything. I think it has a failing injector IMO. 

Question is can I use low amp clamp to determine a faulty injector? The scanner says to ohm the injectors but I see they all share the same feed wire, which confuses me because wouldn’t that affect the resistance?? 

I tried to access generic obd2 to find a misfire counter but the scanner would not communicate Key in engine off. So I tried to access while running and it would shut the truck down. 

Im lost guys any help appreciated thanks much

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Chris Diagnostician
Lansdale, Pennsylvania
Chris Default
   

Never be ashamed to ask for help. We all have areas of strength and of weakness. We work together to learn and improve. To quickly answer your question, yes. You can use a low amp clamp to compare current ramps between injectors at 2000 rpm and look for the odd man out. The ohms test would be done with the injector unplugged (ohm the injector itself out), though I rarely trust the results of…

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Chad Technician
Winter park, Florida
Chad Default
 

Thanks for the reply Chris. I was gonna attempt the ohm test at the valve cover connector but didn’t like the idea and didn’t think was carrying out the procedure correctly, as simple as it is. Customer states oil is good even tho it is black as always with these engines. I also was without my amp clamp at the time so unable to do that and wasn’t sure if it were possible. But I think that’s…

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David Diagnostician
Independence, Louisiana
David Default
 

You can use a amp probe to test the electrical health of the injectors. If it has the old type black cam sensor you can access the percent of deceleration pid. If it has the gray sensor, that test is inaccurate. When you say miss, is it a hard "bump" that is intermittent or is it more of a vibration that is consistent?

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Chad Technician
Winter park, Florida
Chad Default
 

Hello David, yes it is a hard bump that is intermittent only after 2000rpm. So I was figuring the load was the issue kinda like a failing ignition coil. I mean it feels just like a bad coil but it does not have coils lol so I was leaning towards injector. I’m the 4th person looking at this for him and after many parts I would like to prove to the gentleman the injector is bad or that we need to…

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David Diagnostician
Independence, Louisiana
David Default
 

Normally, 7.3 injectors do not illustrate this symptom. They normally fail at low RPM when the voltage is low on the vehicle. I would scope the cam sensor first. If it is bad replace it only with an OEM, the aftermarket sensors are known to cause similar issues.

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Chad Technician
Winter park, Florida
Chad Default
 

Ok David I will investigate this. He has many new parts which I’m sure are aftermarket.

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Daryll Diagnostician
Seymour, Tennessee
Daryll Default
 

On the 7.3 it is very common to have the engine shut off at various times while doing test with the scan tool, whether you’re using the OE IDS or aftermarket. You can ohm the injectors plugged in or not, easy to do it at the UVCH (under valve cover harness) connector. Would be a good idea to perform a test with the low amp probe around all 8 injector control circuits at the same time, both…

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Chad Technician
Winter park, Florida
Chad Default
 

Hello Daryl, I do recall that pid . I was looking at waveforms and looking for drops or spikes when the event occurred and did not see anything obvious. I will try this alternator check regardless as that will be a good learning experience. I do not know what you mean by connecting low amp clamp to all 8 control wires at the same time. My understanding is that current clamps must only be on…

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Daryll Diagnostician
Seymour, Tennessee
Daryll Default
 

Chad, in the image here you will see a common wire between 4 injectors and another for the other 4. Do your resistance testing from this circuit to each other circuit for each injector. The individual circuits are the control wires, and you can go to the IDM and collect all 8 within the low amp probe at the same time, or go around the 2 common wires out of the IDM, just remember your current…

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Chad Technician
Winter park, Florida
Chad Default
 

Thank you for cleaning that up about the ohm test Daryll. I thought that would be the way to do it but wasn’t sure. The problem happens in the stall while revving and under load. I did not test drive it as this was a house call and it was hooked to a trailer.

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Daryll Diagnostician
Seymour, Tennessee
Daryll Default
 

So this “load” is while in gear sitting still, correct? It would have to be a major injector problem to show up that way. I would be scoping the CMP signal next. It’s a 24x signal with a wide pulse and narrow pulse nearly 180 degrees apart. If everything looks ok there, next would be to scope the signals from the pcm to idm for injector command and delivery

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Chad Technician
Winter park, Florida
Chad Default
 

Thank you Daryll. Yes I’m one to always test drive but customer was a little frustrated and I was able to make it happen while in park so I left it at that. Scoping the cam signal is what will be happening. Thank you. Abe he did just say it has been changed already lol

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Daryll Diagnostician
Seymour, Tennessee
Daryll Default
 

New parts mean nothing until verified for proper operation

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Zachary Mobile Technician
Austin, Texas
Zachary Default
 

Exactly Daryll. New is just an acronym for Never Ever Works. 😂

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Chad Technician
Winter park, Florida
Chad Default
 

Daryll it turns out to be cam sensor. I spoke with customer and told them I would do several checks first and for him to figure out if it was an oem cam sensor. I wanted to do some learning with this but he ran out and bought a cam sensor and now his truck is fixed. You guys saved me on this and I appreciate all the help and information. I’m a little disappointed that I can’t make an attempt to…

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Albin Diagnostician
Leavenworth, Washington
Albin Default
   

Hi Chad, I feel your pain. The reason this vehicle will not comm on OBD@ Generic, is because it is not an OBD2 vehicle, and yes, trying to comm with the engine running will shut the engine off. It is not normal, for this engine to misfire at high speed from a defective injector, but don't rule this out. Over the years I have found the scan tool testing on the injection system is pretty hard…

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Chad Technician
Winter park, Florida
Chad Default
 

Hey Albin, honestly I didn’t do the buzz test because I figured all the injectors were working, i just had a hunch one was failing. But I will do this test. Seeing that the plug in the harness can come out really raises an eyebrow. That connector was very crudy, I had to clean the harness very well to see what wires were what. Lots of degrees on them. It sadens me to hear that the scope…

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Zachary Mobile Technician
Austin, Texas
Zachary Default
   

Everyone's given great information so far but I figured I'd throw in my two cents. From what you're describing, sounds like a similar issue I had on a 7.3 of that vintage. Whether driving it down the road or revving the engine in the bay, once you got around 1800 RPMs or so you could feel it dropping cylinders sporadically. Very similar to what you would feel on a Triton Ford when a coil or…

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Chad Technician
Winter park, Florida
Chad Default
 

Zachary, Your description is exactly what I observed and I was relating it to ford gasoline engine misfires with coils braking down on highway travels. I just got a reply from customer and he did replace the cam sensor has been Replaced. I will do a visual to see what it looks like. I am going to spend some time with the scope first just for peace of mind that I tried before I suggest…

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Zachary Mobile Technician
Austin, Texas
Zachary Default
 

I'm right there with you Chad. I prefer to have a smoking gun before suggesting any parts replacement. Ask him if the sensor he put in was aftermarket. If it is, that makes it that much more suspect. The aftermarket sensors for these engines are absolute junk in my experience.

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Glen Owner/Technician
Arthur, Illinois
Glen Default
   

I would run the Vin and make sure the cam sensor recall isn't still open, or post it and I can run it for you.

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Chad Technician
Winter park, Florida
Chad Default
 

Zachary thank you for your input. I spoke with the customer and gave them this info and they ran out an bought an oem sensor. I really wanted the opportunity to use my scope. It’s a Modis and I know you said you couldn’t capture it and I’m sure I would not be able to either but it’s just the point of it being very fun and good learning experience. I lack diesel knowledge and maybe I’ll sign up…

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Zachary Mobile Technician
Austin, Texas
Zachary Default
 

No problem Chad. Glad the information was helpful. I'm with you 100% on getting to scope the signal and actually see the fault. Like I said before, my assumption is that the scope I used way back when didn't have a high enough sample rate and/or record buffer to capture the fault. I would think that the ATS Scope I have now or one of the Pico Scopes would've been able to catch the glitch. I'm…

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