New 347 EFI stroker won’t idle

hankth18

Member
Feb 24, 2021
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Rock Hill, SC
I recently got the motor/tranny installed in my 1994 Mustang GT. Short run down of the engine build:
-347 Eagle stroker kit (N/A)
-F303 cam
-Flotek 205 heads
-TFS 1.6 RR
-TFS 24 lb/hr injectors
-TFS street heat intake
-Stock fuel rail/lines from tank (plans to upgrade in the future, trying to run for now)
I got the engine/trans installed as well as driveshaft & all electronics/vacuum lines hooked up except EGR electrical plug (deleted system from car). Got timing set on TDC of cylinder 1 & she fired right up but struggled to stay running. Got it running long enough to set timing (14 deg BTDC) then encountered a surge issue & shut off. Now this issue is set in. The engine will fire right up & idle for ~5-10 seconds then begin to surge & finally die. I measured fuel pressure, got 38-40 psi when priming, then goes down to 30 psi at idle where it holds until the surge starts, then pressure begins to drop. I will say I’m unsure of what fuel pump size is as I’m still running what was in the car when I got it (car had been previously modified but unsure of specs of everything). I would think even a stock fuel pump would allow the engine to idle but I could be wrong. I checked engine codes & the only active code is for the EGR. What can cause my issue? Where should I start?
 
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General karthief

wonder how much it would cost to ship you a pair
Mod Dude
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Start here paying attention to the highlighted stuff for the 94-5 specific differences.
 

revhead347

Apparently my ex-husband made that mistake.
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Not with the new engine, only whatever is on the computer alresdy
I'd start with that. In general an SN computer is going to struggle to run an engine modified that much. First off, how are you compensating for the higher flow injectors.

Kurt
 
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hankth18

Member
Feb 24, 2021
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Rock Hill, SC
I'd start with that. In general an SN computer is going to struggle to run an engine modified that much. First off, how are you compensating for the higher flow injectors.

Kurt
Honestly haven’t done anything. Was hoping it would at least idle so I could take it to a tuner. Big hopes, I know.
 

revhead347

Apparently my ex-husband made that mistake.
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Honestly haven’t done anything. Was hoping it would at least idle so I could take it to a tuner. Big hopes, I know.
You might be able to get it to run, even poorly with a calibrated MAF for the injectors, but with larger injectors on a factory MAF it's hopeless. I bet if you pull the plugs, they are all fuel fouled. With 20% more fuel that stoich, it's going to foul the plugs out in about 20 seconds.

Kurt
 
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96pushrod

I think they're a little easier to get off
May 15, 2018
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Honestly haven’t done anything. Was hoping it would at least idle so I could take it to a tuner. Big hopes, I know.
With larger injectors and a 19lb injector calibrated MAF it’s going to be adding a good bit more fuel than needed. If you have an adjustable fpr you could try knocking the pressure down and see if it idles better, but the sn95 computers just run like ass when modded if not tuned.
 

Monkeybutt2000

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Aug 11, 2019
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Not sure why you thought it was going to run correctly without the right mass air meter? And you are going to be close to the limit on 24#s with that combo.
 
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Chythar

Recently finished repairing my rear
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While factory 94-95 GT's have 19lb injectors and Cobras have 24's, they have the exact same Mass Air Meter. The Cobra tune compensates for the Mass Air Meter difference. So if OP is willing to get a chip and have the J4J1 tune burned on it, he may have better luck idling.
 

hankth18

Member
Feb 24, 2021
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Rock Hill, SC
Not sure why you thought it was going to run correctly without the right mass air meter? And you are going to be close to the limit on 24#s with that combo.
I'm still learning about how all this stuff communicates & works together and frankly didn't know that would throw it off that bad. I've built engines before but never an EFI system. I know I'm at the limit of the 24 lbers, but I had them on the shelf brand new from a friend so I plugged them in to get me going for now. I plan on upgrading to 30 or 36 lb later.

I ended up finding a new in the box Pro-M 24 lb MAF that I put on the car & it took away my idle surge. I know the car still needs to be tuned, but it's at least running better which is a step in the right direction. I also have a wbo2 sensor that I am installing to monitor my AFR.

I appreciate all the help!
 
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revhead347

Apparently my ex-husband made that mistake.
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Jun 14, 2004
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I'm still learning about how all this stuff communicates & works together and frankly didn't know that would throw it off that bad. I've built engines before but never an EFI system. I know I'm at the limit of the 24 lbers, but I had them on the shelf brand new from a friend so I plugged them in to get me going for now. I plan on upgrading to 30 or 36 lb later.

I ended up finding a new in the box Pro-M 24 lb MAF that I put on the car & it took away my idle surge. I know the car still needs to be tuned, but it's at least running better which is a step in the right direction. I also have a wbo2 sensor that I am installing to monitor my AFR.

I appreciate all the help!
Good deal. I'm glad we got you in the ballpark.

Kurt