2008 Mustang GT manual - throttle pulsing & high rpm stalling

[Kuplex]

Hopefully someone can give me a few ideas on what to check here. I've got a 08 GT manual.

Basically, over the last couple weeks, when holding speed I get a light bucking/hesitation feeling. It's almost like you're lightly letting off the pedal and tapping it. It's not major but feels like a random pulse. The RPMs do not change at all that I can see.

This may be related or not, but I was trying to see if a noise was engine related or tire so I let off the gas and threw the car into neutral at around 60mph, the rpms dropped, fluctuated a little from 300-500, then dropped to zero, and then the engine died. I don't remember it ever doing that before but it's been a long time since I've done something like that. Putting it back into gear jumped the engine back to life just like starting a car without a working clutch. This was about a week ago.

Anyway, I have changed the throttle body (which did help with overall smoothness), fuel filter, air filter, coil boots, spark plugs (all looked fine), and looked all over for any kind of vacuum leaks. No coolant leaks. No knocking. No engine sputtering (other than the light throttle pulsing). No weird smells or sounds that I have noticed.

I decided to datalog a short drive today and see if I could catch anything. Sure enough it started pulsing again when holding speed around the 2m mark (sorry forgot to time it). On the second datalog, around the 3m 52s mark, I gave it about half throttle up to 60mph, let off the gas, threw it in neutral and the engine died. Maybe that's expected behavior though?

My fuel pressure and duty cycles seem to be fine, although I'm not an expert at reading these data logs. Any advice? Thanks.

Cliff

 

[Dino Dino Bambino]

I noticed that you're getting intermittent cam position sensor readings above zero. Since both are equal, that makes me suspect that you could have a bad alternator diode (rectifier) that's allowing a small AC current to leak into the main wiring harness and playing havoc with the VCT solenoids.
You could perform an AC ripple test (YouTube has several videos) to either confirm this or rule it out. Another thing you could try is disconnecting the wiring harness to the VCT solenoids and see if the engine no longer stalls. It's just a shame you spent so much money throwing parts at the problem first without properly troubleshooting it.

 

[Kuplex]

Thanks, I’ll give it a shot.

I was getting intermittent idle hangs and a little bit of a lazy throttle even after cleaning the TB (for the second time) so I decided to just go ahead and replace it and hoped it would be two birds with one stone. It did fix my throttle response and hanging idle so thankfully I only threw $150 more at it blindly before coming to my senses and data logging. I’ll report back what I find with the AC ripple and VCT tests.

 

[Kuplex]

I need to get something to test for ripple. Decided to go ahead and unplug the VCT solenoids and see what happened. No change unfortunately. Still pulses around 45-60 and still dies if you pop it into neutral from a higher speed. I was going about 40 and as soon as I put it in neutral the rpm’s immediately quickly dropped to zero and the engine died. If I put in neutral at like 15mph, it acts like normal.

 

[GlassTop09]

I need to get something to test for ripple. Decided to go ahead and unplug the VCT solenoids and see what happened. No change unfortunately. Still pulses around 45-60 and still dies if you pop it into neutral from a higher speed. I was going about 40 and as soon as I put it in neutral the rpm’s immediately quickly dropped to zero and the engine died. If I put in neutral at like 15mph, it acts like normal.

OP, this is normal behavior from PCM (hate to say this but is abnormal driving technique that is causing this) as PCM is performing DFCO (decel fuel cut off) when foot is off APP while car is moving at speed (PCM is looking at VSS off trans output shaft for MPH & at engine RPM's as well during decel) w\ intention of engine staying coupled to drivetrain until either MPH or engine RPM's drop below a certain threshold OR you press the APP again to then reactivate the fuel injectors to return engine to running operation. This is designed to save fuel during a decel event while engine is pumping air for no benefit (pumping losses). When you shift trans into neutral at speed uncoupling engine from drive train, your engine RPM's are dropping BUT your VSS is telling PCM to stay in DFCO since your foot is still off the APP so you're losing injector function thus engine will die from no fuel being sprayed. Once VSS speed drops below the DFCO low MPH threshold the PCM then returns the fuel injectors back to operation so all operates just fine again.
You can keep doing this if you want, you'll just keep seeing your engine die. Your choice.

Normal PCM function based off abnormal driving technique. All as set in your tune file.

This doesn't have any cause\effect to your suspected engine pulsing around 40-65 MPH.............

Follow what Dino has suggested since your datalog (reading Dino's post........I haven't looked at it) is showing intermittent cam position sensor signal dropout especially if the signal dropouts are occurring around the specified MPH areas. Also suggest to check the CMP connectors\wiring as well as your CKP sensor\connectors & wiring while you're at it (engine needs all these operating clean to attain proper signal synch to run clean).

Hope this helps.

 

[Kuplex]

I’ll check those out, too, thanks

Yeah I was kinda iffy on the neutral situation. I drive this car so infrequently now (maybe 500 miles year), I feel like I’ve forgotten some basics over the last couple years lol.

 

[Juice]

OP, this is normal behavior from PCM (hate to say this but is abnormal driving technique that is causing this) as PCM is performing DFCO (decel fuel cut off) when foot is off APP while car is moving at speed (PCM is looking at VSS off trans output shaft for MPH & at engine RPM's as well during decel) w\ intention of engine staying coupled to drivetrain until either MPH or engine RPM's drop below a certain threshold OR you press the APP again to then reactivate the fuel injectors to return engine to running operation. This is designed to save fuel during a decel event while engine is pumping air for no benefit (pumping losses). When you shift trans into neutral at speed uncoupling engine from drive train, your engine RPM's are dropping BUT your VSS is telling PCM to stay in DFCO since your foot is still off the APP so you're losing injector function thus engine will die from no fuel being sprayed. Once VSS speed drops below the DFCO low MPH threshold the PCM then returns the fuel injectors back to operation so all operates just fine again.
You can keep doing this if you want, you'll just keep seeing your engine die. Your choice.

Normal PCM function based off abnormal driving technique. All as set in your tune file.

This doesn't have any cause\effect to your suspected engine pulsing around 40-65 MPH.............

Follow what Dino has suggested since your datalog (reading Dino's post........I haven't looked at it) is showing intermittent cam position sensor signal dropout especially if the signal dropouts are occurring around the specified MPH areas. Also suggest to check the CMP connectors\wiring as well as your CKP sensor\connectors & wiring while you're at it (engine needs all these operating clean to attain proper signal synch to run clean).

Hope this helps.

Not sure I fully agree here.
By putting car in neutral while still moving at speed, the pcm should recalculate gear position based on engine rpm and driveshaft rpm and exit DFCO. Assuming ofcourse that the transmission ratios are correct in the tune.

The behavior the OP is describing is almos exactly what I was initially experiencing with mine (yea, copperhead vs SO pcm I know) Mine would never enter DFCO until I fixed my gear ratios.
Neutral coasting to stop is a hypermiling technique

 

[GlassTop09]

Not sure I fully agree here.
By putting car in neutral while still moving at speed, the pcm should recalculate gear position based on engine rpm and driveshaft rpm and exit DFCO. Assuming ofcourse that the transmission ratios are correct in the tune.

The behavior the OP is describing is almos exactly what I was initially experiencing with mine (yea, copperhead vs SO pcm I know) Mine would never enter DFCO until I fixed my gear ratios.
Neutral coasting to stop is a hypermiling technique

Here are 2 pictures I took of Ford's OEM DFCO tuning, 1 from a 2009 Spanish Oaks PCM tune file, 1 from a 2011 Copperhead PCM tune file:

To clarify here concerning this particular issue, please note the DFCO setting "Disable N\V" in both tune file's DFCO control. How this works is the N\V base setting that is located in the Speedo section (this is calculating a ratio using engine RPM's vs MPH as read off trans output shaft thru the VSS.....both PCM's use the VSS sensor off trans output shaft to calc MPH. What determines engine RPM's is using the trans 1:1 gear ratio in accordance w\ the gear ratio used & tire revs\mi to determine the base N\V ratio the PCM then uses to determine the MPH using any other trans gear ratio's other than the 1:1 trans gear ratio......this is what Juice is referring to).

Now look at the Disable N\V setting in DFCO. When the N\V base ratio in Speedo section is above what ever N\V base ratio is set in the DFCO section (for manuals this is used as set in Speedo control since manuals don't use a TQ converter like an automatic trans does.....autos will automatically recalc this N\V base setting based on the current TQ converter's slip ratio) is set here, the PCM will then disable DFCO control. If this is reversed then the PCM will keep DFCO in operation regardless.

As stock from Ford, this is set in SO PCM's to retain DFCO operation off N\V control, in Copperhead PCM's this is set to disable DFCO control.

So.......if your tuner hasn't touched these settings under DFCO control (most leave this stuff alone) in a SO tune file to disable DFCO off N\V base Speedo setting, then what I described in my prior posting is correct........OP's car is a 2008 using a SO PCM.................

Hope this helps.

PS--Forgot to mention in either PCM's tune file in the Speedo section, the normal hi\lo N\V range used is set between 30-60 N\V ratio for either manual or auto trans, based off the VSS reluctor gear tooth count used on trans output shaft (this count tells the PCM when the trans output shaft has made 1 rev). Need this to get more context to better understand what I typed here.

 

[Juice]

I did not change the default dfco setting in either strategy I have available. Stock 13 gt KWC4, or the Crate_6 control pack strategy. Once I fixed the manual transmission gear ratios for the magnm, dfco startet working. Confirmed by the wideband readout going full lean, and clearly audible decel. (Got rid ofany decel burble) Also confirmed by having no DFCO in 5th gear only, until I fixed the gear ratio min/max values of 5th. While 5th ratio was correct, it was outside the range of 5th.
As far as N/V, I believe it only affects automatic transmission operation. I could be wrong, but changing the N/V values had zero effect on mine.

Ps, my tuner was taking too long with updates, so I bought the PRP software. (Already had the x4, just made sense to go Advantage iii) I should have purchased it before I ever fired the engine! lol

 

[Kuplex]

I noticed that you're getting intermittent cam position sensor readings above zero. Since both are equal, that makes me suspect that you could have a bad alternator diode (rectifier) that's allowing a small AC current to leak into the main wiring harness and playing havoc with the VCT solenoids.
You could perform an AC ripple test (YouTube has several videos) to either confirm this or rule it out. Another thing you could try is disconnecting the wiring harness to the VCT solenoids and see if the engine no longer stalls. It's just a shame you spent so much money throwing parts at the problem first without properly troubleshooting it.

AC ripple testing shows voltage bouncing between 50 and 60mV with some random split second jumps to as high as 80mV. Disconnected the alternator positive cable and tested against it and the battery negative. This was done at idle.

Disconnecting both VCT solenoids has no effect.

 

[Dino Dino Bambino]

AC ripple testing shows voltage bouncing between 50 and 60mV with some random split second jumps to as high as 80mV. Disconnected the alternator positive cable and tested against it and the battery negative. This was done at idle.

Disconnecting both VCT solenoids has no effect.

At least we've ruled out the VCT solenoids as being the problem, but your alternator does sound suspect. Repeat the AC ripple test on a warm engine at a fast idle of 1500rpm with the air conditioning (the other AC) and the headlights on. If the ripple voltage is much higher with these electrical loads, your alternator will need to be replaced.

 

[Kuplex]

At least we've ruled out the VCT solenoids as being the problem, but your alternator does sound suspect. Repeat the AC ripple test on a warm engine at a fast idle of 1500rpm with the air conditioning (the other AC) and the headlights on. If the ripple voltage is much higher with these electrical loads, your alternator will need to be replaced.

Thanks Dino. I’ll do that today.