Do I need bigger injectors?

[86MustangGtRob]

Hi guys,

I have a 1985 GT that has been converted to efi with mass air. I have cold start issues with it running rough. I have checked all vacuum lines and the car has new sensors. I am wondering if the car isn't getting enough fuel. The only reason I say that is because when I got my car dyno tested, it made 271 hp to the rear wheels. I am just wondering if that is too much for the 19lb injectors? Any insight would be greatly appreciated.

[erratic50]

Without any info regarding how conversion was done, it's likely the fuel pressure regulator.

I swapped to an adjustable regulator and immediately experienced hard starts when mine has always cracked right off.

Dyno results suggest you are nearly out of injector capacity but that would cause a lean condition at WOT, not hard starts.

I'd swap to a 93 cobra ECU and 24lb injectors and a Cobra MAF, or chip your existing ECU.

[86MustangGtRob]

Thanks for the advice. I have an adjustable fuel pressure regulator and a high volume fuel pump. It just almost stalls on cold starts.

[Walking-Tall]

New sensors or not, check for trouble codes, and fuel pressure. Could be IAC, wonky TPS, or EGR flow when it's not supposed to be flowing at idle, etc...

[86MustangGtRob]

I would love to check for codes, but my scanner won't work because I don't have a clutch safety switch. My brother in law had the car before me and in the early 90's when he got the shop to do the efi conversion. They did not run the harness for the clutch switch.

[fairmontdave]

I dug up some Ford literature on injector size versus horsepower, and according to it, you're at the limit for 19lb injectors. Ford states on non-forced induction, 19lb are good for about 258, or 85% of duty cycle. If you assume 100%, which you can't, you could tack on another 36, 37 horsepower. Ford says technically, they would be good for about 300, if the injectors are perfect, they're not, so they calculate at 85% duty cycle. 24lb would be good for your present setup, but you're going to add more horsepower someday, aren't you? Have fun!
I didn't have a problem pulling codes with the stock harness without the clutch switch installed.

[86MustangGtRob]

Thanks for the advice I am going to upgrade the injectors. I'm installing aluminum cylinder heads this winter. I just wanted the car to be running right before I start ripping it apart.

[deathbypsi]

The injector size will have nothing to do with the idle issue. You would have to be idling with more than 90% duty cycle......

Look at stuff like the base fuel pressure, clean or replace the IAC, do you have some kind of cold air kit? Check the sensor inside the MAF and try clocking it.

[Ourobos]

^ Agreed.

When tuned, how was the idle tune?

As for what the 19# can handle, general consensus is 300 fwhp. With a 5-speed, that translates to about 270rwhp on a Mustang Dyno to be at full cycle on stock fuel pressure. So unless you spend most of your time at wide open throttle, I think you're fine.

[erratic50]

Adjustable fuel pressure regulators are NOTORIOUS for allowing pressure to leak back off the fuel rail on initial pre-start prime. That leads to horrible starts and stumbling at startup.

Try key on and wait 1 sec, key off, key on wait 1 sec, key off, key on wait 1 sec, key off, key on. Fire it. If it cracks right off, 99.9% it's your pressure regulator.

Get an OE stock non-adjustable one if there is any doubt as that will prove it.

mine does the exact same thing...... and I know when it started doing it.



100% duty cycle at stock fuel pressure is 320 HP _at the crank_.


I would get EV1 to EV6 wiring adapters and EV6 24lb injectors and a Cobra ECU and a 70MM Cobra MAF.

Good luck!

[Walking-Tall]

Adjustable fuel pressure regulators are NOTORIOUS for allowing pressure to leak back off the fuel rail on initial pre-start prime. That leads to horrible starts and stumbling at startup.

Try key on and wait 1 sec, key off, key on wait 1 sec, key off, key on wait 1 sec, key off, key on. Fire it. If it cracks right off, 99.9% it's your pressure regulator.

How so? And/or, they all do, it's in the design. The only difference between a non-adjustable and adjustable fuel pressure regulator, is the ability to change the physical position the diaphragm plate inside nearer (higher pressure) or further away (lower pressure) from the return line port, and for you SEFI folks, that's during no/low vacuum or WOT conditions. Cranking is basically the no/low vacuum situation (higher pressure), and idling cold, hot, or running whichever, manifold vacuum lowers the pressure the injectors see somewhat.

That does not compute... the return port is always variably open... in a situation like that I'd suspect the fuel pump or filter not getting with the program... I already detest EFI. If I had to fiddle around like that like it was the norm with the simple addition of the ability to adjust the fuel pressure, I'd really hate it. Ran an adjustable fuel pressure regulator and a$$ kickin' electric fuel pump for years on my 5.0L Thunderbird, and never had any such issues...

OP, have you tried pulling the vacuum line off of the regulator and check for fuel there? There shouldn't be any...

As well, did you get a graph of the AFR (air:fuel ratio) for that dyno run? I'd be interested to see that... and also see if your set fuel pressure happens to be cranked up to the moon, like 65+psi or something....

[erratic50]

Everyone has different experiences for sure. That's what I love about FEP!

I have a Holley regulator and that with no other changes yields the problem described here. I've put a pressure gauge on the rail before to prove it out. I don't see pressure come up sufficiently during the prime cycle until multiple on/off cycles. Swapping back to a stock regulator "fixes" it.

My problem at the moment for me is my car goes horribly lean up top with the Typhoon intake and headers and larger throttle body and MAF and 19's. It's running out of fuel up there according to the plugs and wideband too which I have worked around with a regulator and more fuel pressure.

Next time I work on it, my fix will be a stock regulator and 24lb EV6 injectors and my Cobra ECU. Eventually I'll add my Quarterhorse and A9L with 47lb injectors and E85 into the equation but not until my tired old motor gets a rebuild. Personally I am done playing with fuel pressure - it's not a good fix, just a fire hazard.

[86MustangGtRob]

Hey guys, I finally figured out my cold start issue. The fuel pressure was low. My fuel pressure gauge was not working properly. Thanks for everyone input