Hi Guys, Great forum.
I have seen a number of posts here similar to my problem but different enough that I don't think they are the problem.
96 F250 7.3 PS automatic trans
while driving the other day I took my foot off the throttle and the SES light came on and it started idling rough. Stepped on the throttle and no power and stumbling badly like it wasn't firing on all cylinders. took my foot off the throttle again the SES went out and idle smoothed out, back to full power.
Drove further and crested a hill, foot off the throttle, same problem. few seconds later, foot off throttle and restored to normal. So far it has restored every time within a few seconds, but always starts and restores within a few seconds when off throttle (but not necessarily every time).
No codes found by a super chips reader/programmer. Truck was in stock programming when it started, reprogrammed to tow-safe without change.
Replaced CPS 10k ago.
I've seen the posts about UVC harnesses and wiring, but I'm not convinced that is the issue because it is so reliable that it starts and stops at idle.(2 connectors / gasket)
Anybody have any ideas?
Thanks.

 

Welcome to FTE. Something's wrong if you're not seeing any codes. If the light came on, there's a code stored. Did you get no codes at all, or just nothing relevant?

UVCH is a possibility. You can test the connections with an ohm meter without pulling the valve covers, but keep in mind that if the connection is intermittent, then the test may or may not catch the problem.

To ohm test the connection, unplug it from outside the valve cover. Check the forward and rear pin on each connector against battery ground. A reading of less than 2 ohms shows a good connection and good glow plugs. Then check the 2nd & 4th pin at each connector against the center pin. A reading of less than 5 ohms indicates a good connection and an injector solenoid that should be in working order.

 

Hi Don, Welcome!

First of all I am not 100% sure that your Superchips reader will "speak PSD" to read teh codes stored in your truck's PCM (computer). If the light was on there IS a code stored and if your reader isn't seeing it, it is probably because the reader isn't communicating fully with the PCM. These trucks weren't fully OBDII compliant in '96 so a lot of the less expensive readers won't read them. If you have someone you know that has AutoEnginuity or a higher quality reader they should be able to pull the codes. Having those codes will help A TON in diagnosing what is going on.

Second, Do I understand that this only happens when you completely let off the throttle? And when you say the truck had no power would it rev the engine at all or just do nothing? I guess what I am trying to ask is, does it try to go faster and just can't or does it just sit there and idle and not even try?

 

Another welcome to yo Don.

Id check the pedal, maybe the IVS or TPS is not working well.

 

Thanks for the replies!
Rotengeep: I will take a look at the TPS. what is the IVS and how would I evaluate it?

DIY Mechanic: I am a little surprised the super chips unit won't read it as it is a tuner specifically for Ford 7.3 PSDs with an OBD reader included, it only shows a code that indicates no codes stored. It will pull some codes as it diagnosed the bad TPS and a few other things.
I also have OBDWiz on my laptop. that indicates 121 codes stored but won't retrieve or clear them.
Second, yes it only has happened when my foot is off the pedal and restores when foot comes completely off the pedal. with the SES on it idles very rough and when I step on the pedal it responds but is very rough and accelerates very slowly. even in neutral it is slow to rev up. I think it is not firing all cylinders.

F350-6: I ohmed 3 of the 4 connectors(didn't want to disconnect he turbo air tube) and all injectors were about 3 ohms. I am having trouble believing it to be UVCH because it only starts and stops when off the pedal completely. I would expect it to happen when the engine was revved up and causing more vibrations if it was UVCH.

Thanks for the ideas

 

I had a superchip on my 96 and it didn't have any problems reading codes. It would be my guess that your UVCH are bad.

 

Don, when you replaced the CPS was it a Ford or International part or one from a big box parts store like Auto Zone, Advance or Oriley's? If it wasn't a genuine Ford or International part, then I would start with that. The sensors sold in the parts stores just aren't up to snuff with the genuine OEM parts.

If the superchip is specifically for the 7.3 you're right, it should read your codes. Try unplugging the MAP sensor (located near the heater box and has a small tube that leads to the intake manifold on the passenger side) which will set a check engine light when you start the truck, then read it again and see whether you see any codes then. That should tell you whether the reader is actually working or not.

The IVS is a little switch on the throttle pedal that acts as a safety when you aren't pressing the pedal. I don't think the IVS or TPS is your issue. When the IVS goes bad, pressing the pedal makes absolutely nothing happen. When the TPS goes bad, you get inconsistent throttle application or a dead spot in the pedal. With all due respect to those who suggested them, since you are having an issue regardless of whether (or where) you are pressing the throttle pedal, I don't think it is either of those issues.

I would tend to start with the CPS and the UVCHW's (as Glenn has suggested). The CPS isn't all that expensive (should be less than $40) and it is a good idea to have a spare anyway since that thing will leave you stranded if it goes bad. The UVCWH's (Under Valve Cover Wiring Harnesses) are also a known weakness in the design of these first gen 7.3's. On the side of the VC (valve Cover) nearest the valley there are two wiring connectors on each valve cover. Those are the connections that feed through the valve cover gaskets and into the UVCWH's. Those connections are known to go bad. Pop the plugs loose on the outside (be very careful not to break the plastic tab off the connector) and give them a careful look. If you see and burned or scorched connections there, that is likely your issue. Even if you don't see any bad connections, look carefully at the pins in the VC gasket as well to see if any of them look burned. Pop the oil fill cap and smell (I'm not kidding) inside there. If you smell anything that smells like burned electrical connections, you may have to pull the valve cover and take a peek inside. In fact, it may be worth your while to just plan to pull the VC's and give the UVCWH's a good looking over. That will definitely give you a rough running truck if those are bad.

That should be enough to keep you busy for a minute, lol. Good luck!

 

I used a magifine glass to see my burnt pins, they are hard to see sometimes. This problem, if it is your problem, can be intermittent. Mine would run great, then it would run like crap. If it was me and went to the trouble of pulling the VC's, I would just replace the harness.

 

I took off the drivers side VC this afternoon and found 1 glow plug pin melted in the inner harness connector and also the gasket connector. I'll try to get the passenger side off in the next day or 2 to see if that one is bad also.
My next question is where to get parts. I usually have good luck with napa and they have the gasket and internal harnesses and connectors (one side) for about $70. thoughts on these or do you have other sources?
thanks

 

Clay at riffraff has good prices on OEM parts

 

I have the same sorta problem.but it only does it of I back up a steep hill. I got the cel, and it goes away after a few secs, no codes off my edge when it was the cel was on

 

Could you also tell me what all the abbreviations mean. Like VC means valve cover.

 

My post lists the meanings of most of the abbreviations that we are using, but just in case I missed a couple, here is a quick list.

VC=Valve Cover
UVCWH=Under Valve Cover Wiring Harness
GP=Glow Plug
CPS=Camshaft Position Sensor
TPS=Throttle Position Sensor
IVS=Idle Validation Switch
MAP=Manifold Absolute Pressure

I always try to spell out the meaning of an abbreviation before I use it a lot since I never know what someone's level of familiarity with the abbreviations is. There are certainly enough of them for these trucks (and forums) to make you head spin.

 

Ok, Thanks I'll have to reread that when I have time to understand it better now.

 

I took off the VC and found 1 pin on the drivers side for a Glow plug that was melted. replaced the gasket and harnesses got it back together tonight. played with the throttle in the yard for a while then went out and drove about a mile without a problem. don't know if it was the harness or one of the other thousands or wires I moved and wiggled looking for a short.
got my fingers crossed. thanks for the advice.