Lower RPM hesitation and RPM fluctuation

tampasnake

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All, any help and or suggestions on where to go next with my car? Other than the new crank position sensor. Details below. Sorry for the long post. Typing everything out helps my troubleshooting thoughts as well.

It has been on jack stands for almost 2 months now trying to figure out what the problem is. It started having slight hesitation under load. For example if I was in 5th going 40mph so low RPM. It was just slightly bucking. I thought I hadn’t changed the plugs in awhile and I have run TR6s since I bought the car in 2010. I installed the new plugs gapped at .030 and disconnected the battery to clean the terminals. The next time out my car died multiple times when pulling up to stop lights. It ran fine other than that. I was actually going to the track made 3 runs and drove it home.

I did some reading and it seemed the stalling could be related to a vaccum leak. This is when I started down the rabbit hole. The symptoms slowly got worse leading up to more often than not when I start, leave the car in neutral, press the gas slightly to 1500rpm it doesn’t hold the rpm. If fluctuates between 1000-1500. It will rev higher and rev to for example 4000rpm and hold while in neutral. I haven’t taken the car off of jack stands to attempt to drive it since this started. Maybe 1 out of 4 times I start it and it doesn’t act up. Shut the car off, start it again and the problem is back. I began troubleshooting and replacing parts, but haven’t seemed to make any progress with it. It is not throwing any codes. This is the frustrating part.

Here is a video of the problem

2010 Shelby GT500
Lund tune Jan 2019. No performance upgrades since tune.
2.3 TVS with VMP pulley
2013/14 injectors
62.5mm CJ Throttle Body
I installed my stock throttle body and electronics, same issue.
JLT 123mm cold air
VMP boost a pumps
I unplugged the pumps and connected stock wiring to eliminate the boost a pump and still had the same issue.
Kooks long tubes catted mid pipes

NGK TR7-IX plugs gapped at .028
I spoke to John at Lund initially and he suggested using the TR7s gapped at .028
All OEM Ford replacements done as part of my troubleshooting.
New fuel filter
New MAF
New TPMS
New EGR
New PCV
New O2 sensors
Crank position sensor going in this weekend. I didn’t realize I have to unbolt the a/c to remove it. From everything I have read it seems this could be a probable cause of my issue being the issue primarily occurs at low rpm.
Smoke tested for vaccum leaks. None found
Reloaded the tune

Thank you for any suggestions
 

Catmonkey

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What did the plugs look like? How long did it take before it started running bad after you loaded the tune (time and mileage)? Have you checked your voltage? Low load is generally low rpm and if you're not putting out adequate voltage these cars have issues. Have you considered running a compression test for mechanical problems?
 

tampasnake

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This was the only picture I took of the first set of TR6 plugs. They all looked pretty much the same. There was anti-seize on the threads.
The car was originally tuned in June 2019. I don't drive the car that much but I drove it from Pa to Florida spring 2019. My brother was storing it for my while I was in Germany. Its probably got 3000 miles on it from the time it was originally tuned Jan 2019 to now when it started acting up. Pretty much the only times I drove it this year was to test and tune nights at the track. I would run 2-3 passes and drive it home.
I have checked the voltage a few times. 14.4 volts constant when running. I also cleaned the couple ground points. I was hoping for something like that after researching the issue.
I haven't run a compression test. It is something I'll have to add to the list of things to check. My assumption is if it was something mechanical the issue would be constant at all rpms and each time I start it.
In some of my experiments if I unplug the MAF the rpm doesn't fluctuate and will hold 1500 rpms. I also removed the air filter and taped a portion of the intake tube off which resulted in it going into limp mode. In limp mode it didn't fluctuate or hesitate.
 

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XP900

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I am going through somewhat of the same issue...2010 GT500 with 42K.....started before I upgraded but went ahead with my VMP 2.3 Budget upgrade anyway with the VMP tiune. My issue is rpm fluctuations and stalling sometimes stopping at a light. It is not the plugs, coils, or MAF - swapped them all out. Found no air leaks but I suspect it is excessive air somewhere. I checked on the engine when I did the SC swap and everything on top looked good. Changed PVC ( valve did not seem to be working correctly) and doubled checked all hoses and my oil separators. Also tried the honeycomb air diffuser in the intake - nothing really made any difference - same issues still. I have having the car tuned in a week so this is not good.

It seems if I unplug the battery and let it sit the PCM does a much better job of controlling the idle descent until it finishes its learning process...then back to rough idle at times and stalling coasting down or stopping at a light. If I start it it can sometimes run fine and idle correctly forever until I start to drive it.

NO CODES ever listed.

I read that the purge and vent solenoids in the rear of the car can cause this issue...esp on higher mileage cars so I ordered both...built in solenoids in the valves are simple two wires connections to the PCM....so I am thinking they could fail and the PCM would not really even know if a solenoid picked or hung open....so maybe you might get a code from an O2 sensor but maybe not???

Here is what really bothered me....that purge valve is very hard to get and everywhere I tried to get it from told me they were out of stock and it would take days to weeks to get in. It is used on a LOT of ford and lincoln vehicles --- I am hoping my purge valve solenoid is sticking or binding and letting too much air into the intake at idle conditions. Will replace once it stops pouring here.


I ordered the WVE parts....Dorman and a few others make the replacement parts too or you can ordered FORD parts too. (Used Rockauto)
I am hoping these parts help since this issue is a real bummer. Reloading tunes doesn't fix my issue. Thought it did but then realized I had the battery unplugged for hours and so the PCM was relearning and somehow stopping the issue.

WVE 2M1146
WVE 2M1150
 
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tampasnake

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Mine just turned 50K miles when all this started. I visually inspected the vapor canister but hadn't researched that. I didn't think of that maybe a cause of unmetered air. It makes sense. I'll add that to the list as well.
I am hesitant on non-Ford electrical parts from past experience. I have ordered a few of my recent parts from rock auto due to faster shipping.
Maybe having the issue and going to tune isn't so bad. They maybe able to diagnose the problem from the data logs. Being a 2010 and not having widebands the closest shop to me that Lund works with is 150 miles away. I'm hesitant to take it to a local guy for a data log and them wanting to start all over with a tune.
 

XP900

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I already have a tuned scheduled so it costs money to back out and I rescheduled twice already. Your bug may be different then mine....mine is more of a idle issue and right now my low end power isn't there but that may be the VMP that I just put on since it is very different than a BAMA tune.

My tuner says he will only use his shop widebands regardless of what I have (and I don't have them anyway). He pulls the OS2 sensors out for the dyno tests and installs his WBands in their place.

You also didn't mention the plug coils....which I replaced at 30K just to eliminate and possible issues I had last year - and my new coils didn't make any difference so I kept the old ones for future use. Easy to change but several hundred dollars. My tuner said he would be able to tell if I have any coil issues when he is testing it. If I can't figure out my stalling issue his shop can but it will cost me $$ ----- but it may be cheaper than swapping every part in the car. It would be nice if I could get a code but my car rarely puts out any codes when I look for them.

If and when I find out what my solution is I will let you know.
 

tampasnake

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I haven't changed the coils. I did ohm test them and they all were identical. My thoughts were if it were a bad coil pak it would have misfires throughout the rpm range, but I never experienced a bad coil pak in any of my mod motors to know.

I know what you mean about having a tuner look at it. That probably my next step before I spend a bunch more money throwing parts at it in hopes I find the cause.
 

tampasnake

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Update for anyone following. I installed the new crank position sensor this weekend. I took it for a ride after that and the first 1/2 mile it stumbled a bit and smoothed out. Then both traction control lights came on for a 1/2 mile and went off.
I drove the car for 20 minutes probably about 15 miles various stop lights and straight line driving. The car ran good. Even with the a/c on pulling up to lights and no RPM issues.
I'm not sure I trust it that it is fixed yet but it felt good to have it out for the first time in 6 weeks and not have drivability issues. I'll drive it some more next weekend.
I have to read more about traction control and how it operates. When the car stumbled and hesitated it acted like it does when traction control kicks in, but no lights would come on or flash when the car wasn't running correctly.
 

XP900

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Did you ever get any codes for the crank sensor ?? I would also test with the A/C off for stalling since it affects idle rpm.

Update in a week or two after a few hundred miles and let me know if it fixed your issue. My rpm has seemed to settle down after driving for a while. No more stalling at lights but stalls when I first start it...second time I start it idles high and then slowly comes down as it warms up. I changed my sensor a long time ago due to a leak but wouldn't hesitate to replace it again even though the AC makes it a PITA.
 

tampasnake

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No I never got any codes. That is what made it challenging. The only time I could get it to throw a code is when I unplugged something while running which ultimately forced a code. I tried all kinds of stuff.
 

XP900

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My idle issues are gone...
I had the car tuned with the new VMP Gen2R budget kit (VMP send me a tune for the kit but my idle issues were still present after swapping in the new SC) Anyway after Speidell Supercars tuned it the idle issues are completed gone...during startup and even slowing down at lights. My car is currently running better than it ever did but I don't know if it will last - idling issus always seem to just pop up out of the blue and I changed a lot of parts (now all spare parts) trying to figure it out with no error codes popping. Either my tunes were messed up (an I reloaded them several times and used different tunes and still had the same idling issues) or the PCM itself settled down and fixed the issue. I don't know...tuner said he had no real issues tuning it...and took it for a long test drive too when he was done. So maybe that was enough time for it to recalibrate?? The tuner had a 2009 Shelby himself so he was pretty familiar with the engine and tuning it. He definitely tuned it the way I like ...smooth, no jerking as pedal increases - only a steady increase in power and torque. Better than the VMP tune they sent and better than all my BAMA tunes.
 

XP900

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By the way my car just turned over 43K.....and I had no vacuum leaks when they were tuning the car. Wasted my time looking for an intermittent vacuum leak in the rear end and purge valves.
 

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