Dropping cylinders under boost

I have bought the GT500 (2014) about a month ago. I was tuned and modded. It has a 2.4 pulley, long tubes and off-road X and a super big hydrocarbon JLT cold air 148mm. I did not get a tuner with the car and still have not been able to get in contact with the previous owner.

It turns out it must have been tuned for E85 and as I started driving 93 gas it started not running so well. This whole time I have been babying it to stay out of boost. I now have a good tune on the car. The dyno that had been posted before was on the same fuel that I bought it.

I have changed the plugs fuel filter and air filter. The plugs I am using now are NGK TR7IX 3690 @ .028 gap. Just bought these. If there is another plug that you would suggest please do.

The car seems to drop cylinders under boost. This seems to start around 12lbs WOT. I have had it up to 17lbs as well just to see if it cleared up. Other then this the car seems to be very strong. We have verified the tune is good and looks good other then this issue.

Since there is more knowledgeable people here with this type of car please share your thoughts.


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Compression test needed.

What does drop cylinders mean?

gimmie11s said:

Compression test needed.

What does drop cylinders mean?

Click to expand...

When I say dropping cylinders. As I have not encountered a spark blowout from boost before but sounds logical. It seems to start missing under boost 12lbs and above.

I had posted a dyno run with the car but had the fuel from when I bought it to see if it was a safe tune. It started to go lean at 5100. I know now it was running E85. This was 1/2 tank and I had just filled up with 93.

I have done the compression test when I put new plugs in. The gauge that I used seemed to read low but I did get a consistent reading on all cylinders. Not sure what it is supposed to be for being a dry cylinder. I was getting readings 118 to 125.


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LavaSlide said:

When I say dropping cylinders. As I have not encountered a spark blowout from boost before but sounds logical. It seems to start missing under boost 12lbs and above.

I had posted a dyno run with the car but had the fuel from when I bought it to see if it was a safe tune. It started to go lean at 5100. I know now it was running E85. This was 1/2 tank and I had just filled up with 93.

I have done the compression test when I put new plugs in. The gauge that I used seemed to read low but I did get a consistent reading on all cylinders. Not sure what it is supposed to be for being a dry cylinder. I was getting readings 118 to 125.


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Click to expand...

Meant to say the last 3k miles I have driven there is no oil consumption and drives great other then getting into boost.


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Have you verified the intercooler pump is working? Without it, CAC temps are going to be high enough to start pulling significant amounts of timing.

Yes the pump is working. avg driving is about 30+ ambient and while in traffic I have seen it about 145ish

Catmonkey said:

Have you verified the intercooler pump is working? Without it, CAC temps are going to be high enough to start pulling significant amounts of timing.

Click to expand...

LavaSlide said:

When I say dropping cylinders. As I have not encountered a spark blowout from boost before but sounds logical. It seems to start missing under boost 12lbs and above.

I had posted a dyno run with the car but had the fuel from when I bought it to see if it was a safe tune. It started to go lean at 5100. I know now it was running E85. This was 1/2 tank and I had just filled up with 93.

I have done the compression test when I put new plugs in. The gauge that I used seemed to read low but I did get a consistent reading on all cylinders. Not sure what it is supposed to be for being a dry cylinder. I was getting readings 118 to 125.


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Click to expand...

Gotcha. Ok if the car is tuned for e85, but you are running 93, you are now EXTREMELY rich and it makes sense the car is breaking up under boost.

Did you get a new tune for 93?

gimmie11s said:

Gotcha. Ok if the car is tuned for e85, but you are running 93, you are now EXTREMELY rich and it makes sense the car is breaking up under boost.

Did you get a new tune for 93?

Click to expand...

Yes I do have a new tune from Lund. They have looked at the logs and something else is going on.

I have a friend that has a dyno and we did a pull just after I bought it. It made 639hp/701tq letting out at 5100. It came with 1/2 tank and they put 93 in it. At the time the dealer and I did not know it was E85. I am now guessing it was E85 because after 2 tanks of 93 the car really did not run well. Here is my post just after buying. https://www.svtperformance.com/threads/new-to-me-shelby-gt500.1191274/

The car runs great until I hit 12lbs boost and above.

The orange stripes are gone now!

If the car was running E85, you probably don't have stock injectors in the car. Have you pulled one to make sure you're using the right data for the injectors?

The joys of buying a modded car.
Good choice on Lund tuning it. You're in good hands there.
And as stated, if the car was on E85 than you need to find out what injectors you have and does it have a BAP.

I just sent Lund a messages about this as your post came in.

I am going to pull a fuel rail tonight to see what they are. They look stock but can not see any numbers on them. They are just black.

2011 gtcs said:

The joys of buying a modded car.
Good choice on Lund tuning it. You're in good hands there.
And as stated, if the car was on E85 than you need to find out what injectors you have and does it have a BAP.

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I have used Lund for years now. They are great people.
for the BAP there is not anything mounted in the trunk. I will pull a fuel rail tonight to see what I can find out about the injectors.

LavaSlide said:

I am going to pull a fuel rail tonight to see what they are. They look stock but can not see any numbers on them. They are just black.

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The older Injector Dynamic injectors didn't have any external means of identification, but had unique serial numbers etched into the body. The first set I had were like that. If you see something like that, ID can help you identify them. You would have certainly needed something like an ID1000 to survive on E85.

You may also need to look for signs of a return style fuel system. That will make a difference too if they're tuning it for returnless style pumps.

Catmonkey said:

The older Injector Dynamic injectors didn't have any external means of identification, but had unique serial numbers etched into the body. The first set I had were like that. If you see something like that, ID can help you identify them. You would have certainly needed something like an ID1000 to survive on E85.

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Thank you for the information. I am only guessing that it was E85. This just makes sense as it ran great until 2nd and 3rd tank of 93.

Catmonkey said:

The older Injector Dynamic injectors didn't have any external means of identification, but had unique serial numbers etched into the body. The first set I had were like that. If you see something like that, ID can help you identify them. You would have certainly needed something like an ID1000 to survive on E85.

You may also need to look for signs of a return style fuel system. That will make a difference too if they're tuning it for returnless style pumps.

Click to expand...

When I did the fuel filter last week there did not appear to be any lines that were not stock. Would there have been something added for this?

Does it have aftermarket fuel rails? You may want to trace the fuel hoses and see if they meet up to a regulator. Placement of the regulator is usually under the hood on the driver's side fender/firewall. Return style will have a regulator in the loop to allow excessive pressure to bleed off to the gas tank.

It should have started running pig rich if you did go to from e85 to pump gas with the old tune. What did the plugs look like?

Catmonkey said:

Does it have aftermarket fuel rails? You may want to trace the fuel hoses and see if they meet up to a regulator. Placement of the regulator is usually under the hood on the driver's side fender/firewall. Return style will have a regulator in the loop to allow excessive pressure to bleed off to the gas tank.

It should have started running pig rich if you did go to from e85 to pump gas with the old tune. What did the plugs look like?

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They are stock fuel rails and stock injectors. Picture attached of the injectors. The plugs are new and look good. They are gapped at .028 wonder if I should open thme up to .030. I wonder if there is a vacuum leak or something somewhere.

Ur plug gap is fine at .28 but I would go .30. Sounds like ur TPS sensor is going bad or u have a bad coil.

When I first got my 04 it started missing under boost full boost. It slowly got worse, but never threw a code because it only happened at WOT. I ended up throwing in a set of low mile spare COPs I had and it's been fine since.

I will try to find out if the electronic part of the FRPP TB I have can be bought and If I order a set of COPs just the OEM Ford would be fine?